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Huan Yang

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preprint2026arXiv

A nonlinear voice from GW250114 ringdown

The detection of quadratic quasi-normal modes would provide a direct probe into black hole nonlinear perturbations. We report the first observational evidence of a set of quadratic quasi-normal modes in the gravitational-wave ringdown of a binary black hole merger. Analyzing the signal from GW250114, we detect six nonlinear modes from the quadratic coupling of the fundamental $(2,2,0)$ mode and its first two overtones. At 5 final mass ($M_\mathrm{f}$) after the merger, the evidence for these nonlinear modes reaches a Bayes factor of 74. To single out these contributions, we employ recent theoretical progress to compute the waveforms and subtract the corresponding nonlinear modes from a numerical relativity surrogate waveform. Our data analysis uses a novel method that incorporates inspiral-merger inference results as a highly constraining prior for the ringdown inference. We further perform a test allowing for phenomenological deviations for the theoretically predicted amplitudes of the quadratic modes. The results show that an amplitude of zero is excluded at $3.0~σ$ significance level, while the theoretical expectation is consistent with the inference. This detection marks a first step towards observationally characterizing nonlinear perturbations in the ringdown of a black hole.

preprint2026arXiv

Artificial Intelligence Driven Channel Coding and Resource Optimization for Wireless Networks

The ongoing evolution of 5G and its enhanced version, 5G+, has significantly transformed the telecommunications landscape, driving an unprecedented demand for ultra-high-speed data transmission, ultra-low latency, and resilient connectivity. These capabilities are essential for enabling mission-critical applications such as the Internet of Things, autonomous vehicles, and smart city infrastructures. This paper investigates the important role of Artificial Intelligence (AI) in addressing the key challenges faced by 5G/5G+ networks, including interference mitigation, dynamic resource allocation, and maintaining seamless network operation. The study particularly focuses on AI-driven innovations in coding theory, which offer advanced solutions to the limitations of conventional error correction and modulation techniques. By employing deep learning, reinforcement learning, and neural network-based approaches, this research demonstrates significant advancements in error correction performance, decoding efficiency, and adaptive transmission strategies. Additionally, the integration of AI with emerging technologies, such as massive multiple-input and multiple-output, intelligent reflecting surfaces, and privacy-enhancing mechanisms, is discussed, highlighting their potential to propel the next generation of wireless networks. This paper also provides insights into the transformative impact of AI on modern wireless communication, establishing a foundation for scalable, adaptive, and more efficient network architectures.

preprint2026arXiv

ResTok: Learning Hierarchical Residuals in 1D Visual Tokenizers for Autoregressive Image Generation

Existing 1D visual tokenizers for autoregressive (AR) generation largely follow the design principles of language modeling, as they are built directly upon transformers whose priors originate in language, yielding single-hierarchy latent tokens and treating visual data as flat sequential token streams. However, this language-like formulation overlooks key properties of vision, particularly the hierarchical and residual network designs that have long been essential for convergence and efficiency in visual models. To bring "vision" back to vision, we propose the Residual Tokenizer (ResTok), a 1D visual tokenizer that builds hierarchical residuals for both image tokens and latent tokens. The hierarchical representations obtained through progressively merging enable cross-level feature fusion at each layer, substantially enhancing representational capacity. Meanwhile, the semantic residuals between hierarchies prevent information overlap, yielding more concentrated latent distributions that are easier for AR modeling. Cross-level bindings consequently emerge without any explicit constraints. To accelerate the generation process, we further introduce a hierarchical AR generator that substantially reduces sampling steps by predicting an entire level of latent tokens at once rather than generating them strictly token-by-token. Extensive experiments demonstrate that restoring hierarchical residual priors in visual tokenization significantly improves AR image generation, achieving a gFID of 2.34 on ImageNet-256 with only 9 sampling steps. Code is available at https://github.com/Kwai-Kolors/ResTok.

preprint2026arXiv

Steering Visual Generation in Unified Multimodal Models with Understanding Supervision

Unified multimodal models are envisioned to bridge the gap between understanding and generation. Yet, to achieve competitive performance, state-of-the-art models adopt largely decoupled understanding and generation components. This design, while effective for individual tasks, weakens the connection required for mutual enhancement, leaving the potential synergy empirically uncertain. We propose to explicitly restore this synergy by introducing Understanding-Oriented Post-Training (UNO), a lightweight framework that treats understanding not only as a distinct task, but also a direct supervisory signal to steer generative representations. By incorporating objectives that encode semantic abstraction (captioning) and structural details (visual regression), we enable effective gradient flow from understanding to generation. Extensive experiments on image generation and editing demonstrate that understanding can serve as an effective catalyst for generation.

preprint2026arXiv

UniPPTBench: A Unified Benchmark for Presentation Generation Across Diverse Input Settings

Existing works typically focus on presentation generation under isolated input settings, whereas real-world use cases span diverse scenarios, including vague user prompts, long documents, multimodal materials, and multiple heterogeneous sources. Moreover, current evaluations are often insufficiently scenario-specific. They mainly rely on generic presentation-quality criteria, such as visual appeal, layout quality, and overall coherence, but fail to assess the core capabilities required by different input settings, including grounded compression, visual-text alignment, and cross-source synthesis. Consequently, the field lacks a unified benchmark and a scenario-aware evaluation framework for faithfully diagnosing presentation-generation systems across diverse real-world settings. We present UniPPTBench, a unified benchmark for presentation generation across four representative input settings: vague-prompt, long-document, multimodal-document, and multi-source generation. We further introduce UniPPTEval, a scenario-aware evaluation protocol that combines shared metrics for cross-setting comparison with scenario-specific metrics tailored to the core requirements of each setting. We also provide transparent reference baselines to support reproducible comparison. Experiments on UniPPTBench reveal substantial performance variation across settings and recurring failure modes in content grounding, multimodal integration, and cross-source synthesis. In particular, strong performance on generic presentation-quality metrics does not necessarily imply strong task fulfillment in grounded scenarios. Together, UniPPTBench and UniPPTEval provide a faithful and diagnostic foundation for evaluating presentation generation across diverse real-world scenarios. Code and data will be publicly available.

preprint2025arXiv

Beijing Normal University 12-meter Interferometric kHz GW Detector Prototype: Design and Scientific Prospects

Current gravitational-wave detectors have achieved remarkable sensitivity around 100 Hz, enabling ground-breaking discoveries. Enhancing sensitivity at higher frequencies in the kilohertz (kHz) range promises access to rich physics, particularly the extreme conditions during the merger stage of binary neutron stars. However, the high-frequency sensitivity of Michelson-based interferometers is fundamentally limited by their linear optical cavities, which are optimized for low-frequency signal enhancement. In [Phys. Rev. X 13, 021019 (2023)], a new configuration employing an L-shaped optical resonator was proposed to overcome this limitation, offering exceptional sensitivity in the kHz band. As a pathfinder, the 12-meter prototype at Beijing Normal University is designed to demonstrate the sensing and control schemes of this new kHz detector configuration and to explore its performance in the high-power regime with suspended optics. Beyond its primary scientific goal, the prototype also offers potential sensitivity in the megahertz (MHz) range, potentially enabling constraints on exotic sources. This paper presents an overview of the prototype, including its optical design and current development status of key components.

preprint2024arXiv

Incentivizing Massive Unknown Workers for Budget-Limited Crowdsensing: From Off-Line and On-Line Perspectives

How to incentivize strategic workers using limited budget is a very fundamental problem for crowdsensing systems; nevertheless, since the sensing abilities of the workers may not always be known as prior knowledge due to the diversities of their sensor devices and behaviors, it is difficult to properly select and pay the unknown workers. Although the uncertainties of the workers can be addressed by the standard Combinatorial Multi-Armed Bandit (CMAB) framework in existing proposals through a trade-off between exploration and exploitation, we may not have sufficient budget to enable the trade-off among the individual workers, especially when the number of the workers is huge while the budget is limited. Moreover, the standard CMAB usually assumes the workers always stay in the system, whereas the workers may join in or depart from the system over time, such that what we have learnt for an individual worker cannot be applied after the worker leaves. To address the above challenging issues, in this paper, we first propose an off-line Context-Aware CMAB-based Incentive (CACI) mechanism. We innovate in leveraging the exploration-exploitation trade-off in an elaborately partitioned context space instead of the individual workers, to effectively incentivize the massive unknown workers with a very limited budget. We also extend the above basic idea to the on-line setting where unknown workers may join in or depart from the systems dynamically, and propose an on-line version of the CACI mechanism. We perform rigorous theoretical analysis to reveal the upper bounds on the regrets of our CACI mechanisms and to prove their truthfulness and individual rationality, respectively. Extensive experiments on both synthetic and real datasets are also conducted to verify the efficacy of our mechanisms.

preprint2022arXiv

Advancing High-Resolution Video-Language Representation with Large-Scale Video Transcriptions

We study joint video and language (VL) pre-training to enable cross-modality learning and benefit plentiful downstream VL tasks. Existing works either extract low-quality video features or learn limited text embedding, while neglecting that high-resolution videos and diversified semantics can significantly improve cross-modality learning. In this paper, we propose a novel High-resolution and Diversified VIdeo-LAnguage pre-training model (HD-VILA) for many visual tasks. In particular, we collect a large dataset with two distinct properties: 1) the first high-resolution dataset including 371.5k hours of 720p videos, and 2) the most diversified dataset covering 15 popular YouTube categories. To enable VL pre-training, we jointly optimize the HD-VILA model by a hybrid Transformer that learns rich spatiotemporal features, and a multimodal Transformer that enforces interactions of the learned video features with diversified texts. Our pre-training model achieves new state-of-the-art results in 10 VL understanding tasks and 2 more novel text-to-visual generation tasks. For example, we outperform SOTA models with relative increases of 40.4% R@1 in zero-shot MSR-VTT text-to-video retrieval task and 55.4% in high-resolution dataset LSMDC. The learned VL embedding is also effective in generating visually pleasing and semantically relevant results in text-to-visual editing and super-resolution tasks.

preprint2022arXiv

AI Illustrator: Translating Raw Descriptions into Images by Prompt-based Cross-Modal Generation

AI illustrator aims to automatically design visually appealing images for books to provoke rich thoughts and emotions. To achieve this goal, we propose a framework for translating raw descriptions with complex semantics into semantically corresponding images. The main challenge lies in the complexity of the semantics of raw descriptions, which may be hard to be visualized (e.g., "gloomy" or "Asian"). It usually poses challenges for existing methods to handle such descriptions. To address this issue, we propose a Prompt-based Cross-Modal Generation Framework (PCM-Frame) to leverage two powerful pre-trained models, including CLIP and StyleGAN. Our framework consists of two components: a projection module from Text Embeddings to Image Embeddings based on prompts, and an adapted image generation module built on StyleGAN which takes Image Embeddings as inputs and is trained by combined semantic consistency losses. To bridge the gap between realistic images and illustration designs, we further adopt a stylization model as post-processing in our framework for better visual effects. Benefiting from the pre-trained models, our method can handle complex descriptions and does not require external paired data for training. Furthermore, we have built a benchmark that consists of 200 raw descriptions. We conduct a user study to demonstrate our superiority over the competing methods with complicated texts. We release our code at https://github.com/researchmm/AI_Illustrator.

preprint2022arXiv

Degradation-Guided Meta-Restoration Network for Blind Super-Resolution

Blind super-resolution (SR) aims to recover high-quality visual textures from a low-resolution (LR) image, which is usually degraded by down-sampling blur kernels and additive noises. This task is extremely difficult due to the challenges of complicated image degradations in the real-world. Existing SR approaches either assume a predefined blur kernel or a fixed noise, which limits these approaches in challenging cases. In this paper, we propose a Degradation-guided Meta-restoration network for blind Super-Resolution (DMSR) that facilitates image restoration for real cases. DMSR consists of a degradation extractor and meta-restoration modules. The extractor estimates the degradations in LR inputs and guides the meta-restoration modules to predict restoration parameters for different degradations on-the-fly. DMSR is jointly optimized by a novel degradation consistency loss and reconstruction losses. Through such an optimization, DMSR outperforms SOTA by a large margin on three widely-used benchmarks. A user study including 16 subjects further validates the superiority of DMSR in real-world blind SR tasks.

preprint2022arXiv

Dynamical Instability of Self-Gravitating Membranes

We show that a generic relativistic membrane with in-plane pressure and surface density having the same sign is unstable with respect to a series of warping mode instabilities with high wave numbers. We also examine the criteria of instability for commonly studied exotic compact objects with membranes, such as gravastars, AdS bubbles and thin-shell wormholes. For example, a gravastar which satisfies the weak energy condition turns out to be dynamically unstable. A thin-layer black hole mimicker is stable only if it has positive pressure and negative surface density (such as a wormhole), or vice versa.

preprint2022arXiv

Evidence for Black Holes in Green Peas from WISE colors and variability

We explore the presence of active galactic nuclei (AGN)/black holes (BH) in Green Pea galaxies (GPs), motivated by the presence of high ionization emission lines such as HeII and [NeIII] in their optical spectra. In order to identify AGN candidates, we used mid-infrared (MIR) photometric observations from the all-sky Wide-field Infrared Survey Explorer (WISE) mission for a sample of 1004 GPs. Considering only $>5σ$ detections with no contamination from neighboring sources in AllWISE, we select 31 GPs out of 134 as candidate AGN based on a stringent 3-band WISE color diagnostic. Using multi-epoch photometry in W1 and W2 bands based on time-resolved unWISE coadd images, we find two sources exhibiting variability in both the WISE bands among 112 GPs with W1$\leqslant16$ mag and no contamination from neighboring sources in unWISE. These two variable sources were selected as AGN by the WISE 3-band color diagnostic as well. Compared to variable AGN fractions observed among low-mass galaxy samples in previous studies, we find a higher fraction ($\sim1.8\%$) of MIR variable sources among GPs, which demonstrates the uniqueness and importance of studying these extreme objects. Through this work, we demonstrate that MIR diagnostics are promising tools to select AGN that may be missed by other selection techniques (including optical emission-line ratios and X-ray emission) in star-formation dominated, low-mass, low-metallicity galaxies.

preprint2022arXiv

Finding Peas in the Early Universe with JWST

The Early Release Observations (EROs) of JWST beautifully demonstrate the promise of JWST in characterizing the universe at cosmic dawn. We analyze the ERO spectra of three $z \sim 8$ galaxies to determine their metallicities, gas temperatures and ionization. These galaxies offer the first opportunity to understand the physical properties of epoch-of-reionization galaxies through detailed rest-optical emission line spectroscopy. We show that these objects have metal abundances $12+\log[O/H] \approx 6.9 - 8.2$, based on both the $T_e$ method and on a recent calibration of the $R_{23}$ metallicity indicator. Since the spectra are some of the earliest science data from JWST, we compare several line ratios with values expected from robust physics, to validate our measurement procedures. We compare the abundances and emission line ratios to a nearby sample of Green Pea galaxies -- a population of nearby emission line galaxies whose UV properties resemble epoch-of-reionization galaxies, and which often have large Lyman continuum escape fractions. The JWST data show striking further similarities between these high redshift galaxies and nearby Green Peas. The $z\sim 8$ galaxies span the metallicity range covered by Green Peas. They also show the compact morphology that is typical of emission line dominated galaxies at all redshifts. Based on these similarities with Green Peas, it is likely that these are the first rest-optical spectra of galaxies that are actively driving cosmological reionization

preprint2022arXiv

LAGER Ly$α$ Luminosity Function at $z\sim7$, Implications for Reionization

We present a new measurement of the Ly$α$ luminosity function at redshift $z=6.9$, finding moderate evolution from $z=5.7$ that is consistent with a fully or largely ionized $z\sim7$ intergalactic medium. Our result is based on four fields of the LAGER (Lyman Alpha Galaxies in the Epoch of Reionization) project. Our survey volume of $6.1\times10^{6}$ Mpc$^{3}$ is double that of the next largest $z\sim 7$ survey. We combine two new LAGER fields (WIDE12 and GAMA15A) with two previously reported LAGER fields (COSMOS and CDFS). In the new fields, we identify $N=95$ new $z=6.9$ Ly$α$ emitters (LAEs); characterize our survey's completeness and reliability; and compute Ly$α$ luminosity functions. The best-fit Schechter luminosity function parameters for all four LAGER fields are in good general agreement. Two fields (COSMOS and WIDE12) show evidence for a bright-end excess above the Schechter function fit. We find that the Ly$α$ luminosity density declines at the same rate as the UV continuum LF from $z=5.7$ to $z=6.9$. This is consistent with an intergalactic medium that was fully ionized as early as redshift $z\sim 7$, or with a volume-averaged neutral hydrogen fraction of $x_{HI} < 0.33$ at $1σ$.

preprint2022arXiv

Learning Spatiotemporal Frequency-Transformer for Compressed Video Super-Resolution

Compressed video super-resolution (VSR) aims to restore high-resolution frames from compressed low-resolution counterparts. Most recent VSR approaches often enhance an input frame by borrowing relevant textures from neighboring video frames. Although some progress has been made, there are grand challenges to effectively extract and transfer high-quality textures from compressed videos where most frames are usually highly degraded. In this paper, we propose a novel Frequency-Transformer for compressed video super-resolution (FTVSR) that conducts self-attention over a joint space-time-frequency domain. First, we divide a video frame into patches, and transform each patch into DCT spectral maps in which each channel represents a frequency band. Such a design enables a fine-grained level self-attention on each frequency band, so that real visual texture can be distinguished from artifacts, and further utilized for video frame restoration. Second, we study different self-attention schemes, and discover that a divided attention which conducts a joint space-frequency attention before applying temporal attention on each frequency band, leads to the best video enhancement quality. Experimental results on two widely-used video super-resolution benchmarks show that FTVSR outperforms state-of-the-art approaches on both uncompressed and compressed videos with clear visual margins. Code is available at https://github.com/researchmm/FTVSR.

preprint2022arXiv

Learning Spatiotemporal Frequency-Transformer for Low-Quality Video Super-Resolution

Video Super-Resolution (VSR) aims to restore high-resolution (HR) videos from low-resolution (LR) videos. Existing VSR techniques usually recover HR frames by extracting pertinent textures from nearby frames with known degradation processes. Despite significant progress, grand challenges are remained to effectively extract and transmit high-quality textures from high-degraded low-quality sequences, such as blur, additive noises, and compression artifacts. In this work, a novel Frequency-Transformer (FTVSR) is proposed for handling low-quality videos that carry out self-attention in a combined space-time-frequency domain. First, video frames are split into patches and each patch is transformed into spectral maps in which each channel represents a frequency band. It permits a fine-grained self-attention on each frequency band, so that real visual texture can be distinguished from artifacts. Second, a novel dual frequency attention (DFA) mechanism is proposed to capture the global frequency relations and local frequency relations, which can handle different complicated degradation processes in real-world scenarios. Third, we explore different self-attention schemes for video processing in the frequency domain and discover that a ``divided attention'' which conducts a joint space-frequency attention before applying temporal-frequency attention, leads to the best video enhancement quality. Extensive experiments on three widely-used VSR datasets show that FTVSR outperforms state-of-the-art methods on different low-quality videos with clear visual margins. Code and pre-trained models are available at https://github.com/researchmm/FTVSR.

preprint2022arXiv

Learning Trajectory-Aware Transformer for Video Super-Resolution

Video super-resolution (VSR) aims to restore a sequence of high-resolution (HR) frames from their low-resolution (LR) counterparts. Although some progress has been made, there are grand challenges to effectively utilize temporal dependency in entire video sequences. Existing approaches usually align and aggregate video frames from limited adjacent frames (e.g., 5 or 7 frames), which prevents these approaches from satisfactory results. In this paper, we take one step further to enable effective spatio-temporal learning in videos. We propose a novel Trajectory-aware Transformer for Video Super-Resolution (TTVSR). In particular, we formulate video frames into several pre-aligned trajectories which consist of continuous visual tokens. For a query token, self-attention is only learned on relevant visual tokens along spatio-temporal trajectories. Compared with vanilla vision Transformers, such a design significantly reduces the computational cost and enables Transformers to model long-range features. We further propose a cross-scale feature tokenization module to overcome scale-changing problems that often occur in long-range videos. Experimental results demonstrate the superiority of the proposed TTVSR over state-of-the-art models, by extensive quantitative and qualitative evaluations in four widely-used video super-resolution benchmarks. Both code and pre-trained models can be downloaded at https://github.com/researchmm/TTVSR.

preprint2022arXiv

Mass Testing and Characterization of 20-inch PMTs for JUNO

Main goal of the JUNO experiment is to determine the neutrino mass ordering using a 20kt liquid-scintillator detector. Its key feature is an excellent energy resolution of at least 3 % at 1 MeV, for which its instruments need to meet a certain quality and thus have to be fully characterized. More than 20,000 20-inch PMTs have been received and assessed by JUNO after a detailed testing program which began in 2017 and elapsed for about four years. Based on this mass characterization and a set of specific requirements, a good quality of all accepted PMTs could be ascertained. This paper presents the performed testing procedure with the designed testing systems as well as the statistical characteristics of all 20-inch PMTs intended to be used in the JUNO experiment, covering more than fifteen performance parameters including the photocathode uniformity. This constitutes the largest sample of 20-inch PMTs ever produced and studied in detail to date, i.e. 15,000 of the newly developed 20-inch MCP-PMTs from Northern Night Vision Technology Co. (NNVT) and 5,000 of dynode PMTs from Hamamatsu Photonics K. K.(HPK).

preprint2022arXiv

Mass-gap extreme mass ratio inspirals

In this work, we propose a new subclass of extreme-mass-ratio-inspirals (EMRIs): mass-gap EMRIs, consisting of a compact object in the lower mass gap $\sim (2.5-5) M_\odot$ and a massive black hole (MBH). The mass-gap object (MGO) may be a primordial black hole or produced from a delayed supernova explosion. We calculate the formation rate of mass-gap EMRIs in both the (dry) loss-cone channel and the (wet) active galactic nucleus disk channel by solving Fokker-Planck-type equations for the phase-space distribution. In the dry channel, the mass-gap EMRI rate is strongly suppressed compared to the EMRI rate of stellar-mass black holes (sBHs) as a result of mass segregation effect. In the wet channel, the suppression is roughly equal to the mass ratio of sBHs over MGOs, because the migration speed of a compact object in an active galactic nucleus disk is proportional to its mass. We find that the wet channel is much more promising to produce mass-gap EMRIs observable by spaceborne gravitation wave detectors. (Non-)detection of mass-gap EMRIs may be used to distinguish different supernova explosion mechanisms and constrainthe abundance of primordial black holes around MBHs.

preprint2022arXiv

Online Video Super-Resolution with Convolutional Kernel Bypass Graft

Deep learning-based models have achieved remarkable performance in video super-resolution (VSR) in recent years, but most of these models are less applicable to online video applications. These methods solely consider the distortion quality and ignore crucial requirements for online applications, e.g., low latency and low model complexity. In this paper, we focus on online video transmission, in which VSR algorithms are required to generate high-resolution video sequences frame by frame in real time. To address such challenges, we propose an extremely low-latency VSR algorithm based on a novel kernel knowledge transfer method, named convolutional kernel bypass graft (CKBG). First, we design a lightweight network structure that does not require future frames as inputs and saves extra time costs for caching these frames. Then, our proposed CKBG method enhances this lightweight base model by bypassing the original network with ``kernel grafts'', which are extra convolutional kernels containing the prior knowledge of external pretrained image SR models. In the testing phase, we further accelerate the grafted multi-branch network by converting it into a simple single-path structure. Experiment results show that our proposed method can process online video sequences up to 110 FPS, with very low model complexity and competitive SR performance.

preprint2022arXiv

Pseudogap and Strong Pairing Induced by Incipient and Shallow Bands in the Quasi-Two-Dimensional KCa$_{2}$Fe$_{4}$As$_{4}$F$_{2}$

The optical properties of KCa$_{2}$Fe$_{4}$As$_{4}$F$_{2}$ (K12442, $T_c = 33.5$~K) and KCa$_{2}$(Fe$_{0.95}$Ni$_{0.05}$)$_{4}$As$_{4}$F$_{2}$ (Ni-K12442, $T_c = 29$~K) have been examined at a large number of temperatures. For both samples, a nodeless superconducting gap is clearly observed in the optical conductivity at 5~K. The superconducting gap $Δ\simeq 8.7$~meV ($2Δ/k_{\text{B}}T_{c} \simeq 6.03$) in K12442, pointing towards strong-coupling Cooper pairs, but in sharp contrast, $Δ\simeq 4.6$~meV ($2Δ/k_{\text{B}}T_{c} \simeq 3.68$) in Ni-K12442, which agrees with the BCS weak-coupling pairing state. More intriguingly, below $T^{\ast} \simeq 75$~K, the optical conductivity of K12442 reveals a pseudogap that smoothly evolves into the superconducting gap below $T_{c}$, while no such behavior is detected in the electron-doped Ni-K12442. The comparison between the two samples hints that the pseudogap and strong-coupling Cooper pairs in K12442 may be intimately related to the shallow and incipient bands. We provide arguments supporting a preformed pairing mechanism of the pseudogap, but at the moment a magnetic scenario can not yet be excluded.

preprint2022arXiv

Resonant control of elastic collisions between $^{23}$Na$^{40}$K molecules and $^{40}$K atoms

We have demonstrated the resonant control of the elastic scattering cross sections in the vicinity of Feshbach resonances between $^{23}$Na$^{40}$K molecules and $^{40}$K atoms by studying the thermalization between them. The elastic scattering cross sections vary by more than two orders of magnitude close to the resonance, and can be well described by an asymmetric Fano profile. The parameters that characterize the magnetically tunable s-wave scattering length are determined from the elastic scattering cross sections. The observation of resonantly controlled elastic scattering cross sections opens up the possibility to study strongly interacting atom-molecule mixtures and improve our understanding of the complex atom-molecule Feshbach resonances.

preprint2022arXiv

Strong [OIII]λ5007 emission line compact galaxies in LAMOST DR9: Blueberries, Green Peas and Purple Grapes

Green Pea and Blueberry galaxies are well-known for their compact size, low mass, strong emission lines and analogs to high-z Lyα emitting galaxies. In this study, 1547 strong [OIII]λ5007 emission line compact galaxies with 1694 spectra are selected from LAMOST DR9 at the redshift range from 0.0 to 0.59. According to the redshift distribution, these samples can be separated into three groups: Blueberries, Green Peas and Purple Grapes. Optical [MgII]λ2800 line feature, BPT diagram, multi-wavelength SED fitting, MIR color, and MIR variability are deployed to identify 23 AGN candidates from these samples, which are excluded for the following SFR discussions. We perform the multi-wavelength SED fitting with GALEX UV and WISE MIR data. Color excess from Balmer decrement shows these strong [OIII]λ5007 emission line compact galaxies are not highly reddened. The stellar mass of the galaxies is obtained by fitting LAMOST calibrated spectra with the emission lines masked. We find that the SFR is increasing with the increase of redshift, while for the sources within the same redshift bin, the SFR increases with mass with a similar slope as the SFMS. These samples have a median metallicity of 12+log(O/H) of 8.10. The metallicity increases with mass, and all the sources are below the mass-metallicity relation. The direct-derived Te-based metallicity from the [OIII]λ4363 line agrees with the empirical N2-based empirical gas-phase metallicity. Moreover, these compact strong [OIII]λ5007 are mostly in a less dense environment.

preprint2022arXiv

Tick-Tock: The Imminent Merger of a Supermassive Black Hole Binary

Supermassive black hole binaries (SMBHs) are a fascinating byproduct of galaxy mergers in the hierarchical universe. In the last stage of their orbital evolution, gravitational wave radiation drives the binary inspiral and produces the loudest siren awaiting to be detected by gravitational wave observatories. Periodically varying emission from active galactic nuclei has been proposed as a powerful approach to probe such systems, although none of the identified candidates are close to their final coalescence such that the observed periods stay constant in time. In this work, we report on the first system with rapid decaying periods revealed by its optical and X-ray light curves, which has decreased from about one year to one month in three years. Together with its optical hydrogen line spectroscopy, we propose that the system is an uneven mass-ratio, highly eccentric SMBH binary which will merge within three years, as predicted by the trajectory evolution model. If the interpretation is true, coordinated, multi-band electromagnetic campaign should be planned for this first binary SMBH merger event observed in human history, together with possible neutrino measurements. Gravitational wave memory from this event may also be detectable by Pulsar Timing Array with additional five-to-ten year observation.

preprint2022arXiv

TTVFI: Learning Trajectory-Aware Transformer for Video Frame Interpolation

Video frame interpolation (VFI) aims to synthesize an intermediate frame between two consecutive frames. State-of-the-art approaches usually adopt a two-step solution, which includes 1) generating locally-warped pixels by flow-based motion estimations, 2) blending the warped pixels to form a full frame through deep neural synthesis networks. However, due to the inconsistent warping from the two consecutive frames, the warped features for new frames are usually not aligned, which leads to distorted and blurred frames, especially when large and complex motions occur. To solve this issue, in this paper we propose a novel Trajectory-aware Transformer for Video Frame Interpolation (TTVFI). In particular, we formulate the warped features with inconsistent motions as query tokens, and formulate relevant regions in a motion trajectory from two original consecutive frames into keys and values. Self-attention is learned on relevant tokens along the trajectory to blend the pristine features into intermediate frames through end-to-end training. Experimental results demonstrate that our method outperforms other state-of-the-art methods in four widely-used VFI benchmarks. Both code and pre-trained models will be released soon.

preprint2022arXiv

UID2021: An Underwater Image Dataset for Evaluation of No-reference Quality Assessment Metrics

Achieving subjective and objective quality assessment of underwater images is of high significance in underwater visual perception and image/video processing. However, the development of underwater image quality assessment (UIQA) is limited for the lack of comprehensive human subjective user study with publicly available dataset and reliable objective UIQA metric. To address this issue, we establish a large-scale underwater image dataset, dubbed UID2021, for evaluating no-reference UIQA metrics. The constructed dataset contains 60 multiply degraded underwater images collected from various sources, covering six common underwater scenes (i.e. bluish scene, bluish-green scene, greenish scene, hazy scene, low-light scene, and turbid scene), and their corresponding 900 quality improved versions generated by employing fifteen state-of-the-art underwater image enhancement and restoration algorithms. Mean opinion scores (MOS) for UID2021 are also obtained by using the pair comparison sorting method with 52 observers. Both in-air NR-IQA and underwater-specific algorithms are tested on our constructed dataset to fairly compare the performance and analyze their strengths and weaknesses. Our proposed UID2021 dataset enables ones to evaluate NR UIQA algorithms comprehensively and paves the way for further research on UIQA. Our UID2021 will be a free download and utilized for research purposes at: https://github.com/Hou-Guojia/UID2021.

preprint2022arXiv

Using machine learning to parametrize postmerger signals from binary neutron stars

There is growing interest in the detection and characterization of gravitational waves from postmerger oscillations of binary neutron stars. These signals contain information about the nature of the remnant and the high-density and out-of-equilibrium physics of the postmerger processes, which would complement any electromagnetic signal. However, the construction of binary neutron star postmerger waveforms is much more complicated than for binary black holes: (i) there are theoretical uncertainties in the neutron-star equation of state and other aspects of the high-density physics, (ii) numerical simulations are expensive and available ones only cover a small fraction of the parameter space with limited numerical accuracy, and (iii) it is unclear how to parametrize the theoretical uncertainties and interpolate across parameter space. In this work, we describe the use of a machine-learning method called a conditional variational autoencoder (CVAE) to construct postmerger models for hyper/massive neutron star remnant signals based on numerical-relativity simulations. The CVAE provides a probabilistic model, which encodes uncertainties in the training data within a set of latent parameters. We estimate that training such a model will ultimately require $\sim 10^4$ waveforms. However, using synthetic training waveforms as a proof-of-principle, we show that the CVAE can be used as an accurate generative model and that it encodes the equation of state in a useful latent representation.

preprint2022arXiv

X-ray view of a merging supermassive black hole binary candidate SDSSJ1430+2303: Results from the first ~200 days of observations

Recently we discovered an unprecedented supermassive black hole binary (SMBHB) candidate in the nearby Seyfert galaxy SDSS J1430+2303, which is predicted to merge within three years. X-ray spectroscopy may bring unique kinematic evidence for the last inspiraling stage, when the binary is too close to allow each of them to hold an individual broad line region. We try to confirm the unique SMBHB merger event and understand the associated high-energy processes from a comprehensive X-ray view. We observed SDSS J1430+2303 with XMM-Newton, NuSTAR, Chandra, and Swift spanning the first ~200 days since its discovery. X-ray variability, up to a factor of 7, has been detected on a timescale of a few days. The broadband spectrum from 0.2-70 keV can be well fitted with a model consisting of a power law and a relativistic reflection covered by a warm absorber. The properties of the warm absorber changed dramatically, for example, with a decrease in the line-of-sight velocity from ~0.2c to ~0.02c, between the two XMM-Newton observations separated by only 19 days, which can be naturally understood in the context of the SMBHB; although, the clumpy wind scenario cannot be completely excluded. Broad Fe Kalpha emission has been robustly detected, though its velocity shift or profile change is not yet measurable. Further longer X-ray observations are highly encouraged to detect the expected orbital motion of the binary.

preprint2021arXiv

A low-temperature scanning probe microscopy system with molecular beam epitaxy and optical access

A low-temperature ultra-high vacuum scanning probe microscopy (SPM) system with molecular beam epitaxy capability and optical access was conceived, built, and tested in our lab. The design of the whole system is discussed here, with special emphasis on some critical parts. We made an SPM scanner head with a modified Pan-type design, enclosed by a double-layer cold room under a bath type cryostat. The scanner head is very rigid, compatible with optical access paths, and can accommodate both scanning tunneling microscope (STM) tips and atomic force sensors. Two piezo-actuated focus-lens stages are mounted on the two sides of the cold room to couple light in and out. To demonstrate the system performance, we performed STM and scanning tunneling spectroscopy studies. The herringbone reconstruction and atomic structure of Au(111) surface were clearly resolved. The dI/dV spectra of an Au(111) surface were obtained at 5 K. In addition, a periodic 2D tellurium (Te) structure was grown on Au(111) surface using MBE.

preprint2021arXiv

A Lyman-α protocluster at redshift 6.9

Protoclusters, the progenitors of the most massive structures in the Universe, have been identified at redshifts of up to 6.6. Besides exploring early structure formation, searching for protoclusters at even higher redshifts is particularly useful to probe the reionization. Here we report the discovery of the protocluster LAGER-z7OD1 at a redshift of 6.93, when the Universe was only 770 million years old and could be experiencing rapid evolution of the neutral hydrogen fraction in the intergalactic medium. The protocluster is identified by an overdensity of 6 times the average galaxy density, and with 21 narrowband selected Lyman-$α$ galaxies, among which 16 have been spectroscopically confirmed. At redshifts similar to or above this record, smaller protogroups with fewer members have been reported. LAGER-z7OD1 shows an elongated shape and consists of two subprotoclusters, which would have merged into one massive cluster with a present-day mass of $3.7 \times 10^{15}$ solar masses. The total volume of the ionized bubbles generated by its member galaxies is found to be comparable to the volume of the protocluster itself, indicating that we are witnessing the merging of the individual bubbles and that the intergalactic medium within the protocluster is almost fully ionized. LAGER-z7OD1 thus provides a unique natural laboratory to investigate the reionization process.

preprint2021arXiv

Black-Hole Perturbation Plus Post-Newtonian Theory: Hybrid Waveform for Neutron Star Binaries

We consider the motion of nonspinning, compact objects orbiting around a Kerr black hole with tidal couplings. The tide-induced quadrupole moment modifies both the orbital energy and outgoing fluxes, so that over the inspiral timescale there is an accumulative shift in the orbital and gravitational wave phase. Previous studies on compact object tidal effects have been carried out in the Post-Newtonian (PN) and Effective-One-Body (EOB) formalisms. In this work, within the black hole perturbation framework, we propose to characterize the tidal influence in the expansion of mass ratios, while higher-order PN corrections are naturally included. For the equatorial and circular orbit, we derive the leading order, frequency dependent tidal phase shift which agrees with the Post-Newtonian result at low frequencies but deviates at high frequencies. We also find that such phase shift has weak dependence ($\le 10\%$) on the spin of the primary black hole. Combining this black hole perturbation waveform with the Post-Newtonian waveform, we propose a frequency-domain, hybrid waveform that shows comparable accuracy as the EOB waveform in characterizing the tidal effects, as calibrated by numerical relativity simulations. Further improvement is expected as the next-leading order in mass ratio and the higher-PN tidal corrections are included. This hybrid approach is also applicable for generating binary black hole waveforms.

preprint2021arXiv

Direct Visualization of a Static Incommensurate Antiferromagnetic Order by Suppressing the Superconducting Phase Coherence in Fe-doped Bi2Sr2CaCu2O8+delta

In cuprate superconductors, due to strong electronic correlations, there are multiple intertwined orders which either coexist or compete with superconductivity. Among them the antiferromagnetic (AF) order is the most prominent one. In the region where superconductivity sets in, the long-range AF order is destroyed. Yet the residual short-range AF fluctuations are present up to a much higher doping and their role in the emergence of the superconducting phase is still highly debated. Here, by using a spin polarized scanning tunneling microscope, for the first time, we directly visualize an emergent incommensurate AF order in the nearby region of Fe impurities embedded in the optimally doped Bi2Sr2CaCu2O8+δ (Bi2212). Remarkably the Fe impurities suppress the superconducting coherence peaks with the gapped feature intact, but pin down the ubiquitous short-range incommensurate AF order. Our work shows an intimate relation between antiferromagnetism and superconductivity.

preprint2021arXiv

Distinct Properties of Vortex Bound States Driven by Temperature

We investigate the behavior of vortex bound states in the quantum limit by self-consistently solving the Bogoliubov-de Gennes equation. We find that the energies of the vortex bound states deviates from the analytical result $E_μ=μΔ^2/E_F$ with the half-integer angular momentum $μ$ in the extreme quantum limit. Specifically, the energy ratio for the first three orders is more close to $1:2:3$ instead of $1:3:5$ at extremely low temperature. The local density of states reveals an Friedel-like behavior associated with that of the pair potential in the extreme quantum limit, which will be smoothed out by thermal effect above a certain temperature even the quantum limit condition, namely $T/T_c<Δ/E_F$ is still satisfied. Our studies show that the vortex bound states can exhibit very distinct features in different temperature regimes, which provides a comprehensive understanding and should stimulate more experimental efforts for verifications.

preprint2021arXiv

Evidence for association of triatomic molecule in ultracold $^{23}$Na$^{40}$K and $^{40}$K mixture

Ultracold assembly of diatomic molecules has enabled great advances in controlled chemistry, ultracold chemical physics, and quantum simulation with molecules. Extending the ultracold association to triatomic molecules will offer many new research opportunities and challenges in these fields. A possible approach is to form triatomic molecules in the ultracold atom and diatomic molecule mixture by employing the Feshbach resonance between them. Although the ultracold atom-diatomic-molecule Feshbach resonances have been observed recently, utilizing these resonances to form triatomic molecules remains challenging. Here we report on the evidence of the association of triatomic molecules near the Feshbach resonances between $^{23}$Na$^{40}$K molecules in the rovibrational ground state and $^{40}$K atoms. We apply a radio-frequency pulse to drive the free-bound transition and monitor the loss of $^{23}$Na$^{40}$K molecules. The association of triatomic molecules manifests itself as an additional loss feature in the radio-frequency spectra, which can be distinguished from the atomic loss feature.The binding energy of triatomic molecule is estimated from the measurement. Our work is helpful to understand the complex ultracold atom-molecule Feshbach resonance and may open up an avenue towards the preparation and control of ultracold triatomic molecules.

preprint2021arXiv

JUNO Physics and Detector

The Jiangmen Underground Neutrino Observatory (JUNO) is a 20 kton LS detector at 700-m underground. An excellent energy resolution and a large fiducial volume offer exciting opportunities for addressing many important topics in neutrino and astro-particle physics. With 6 years of data, the neutrino mass ordering can be determined at 3-4 sigma and three oscillation parameters can be measured to a precision of 0.6% or better by detecting reactor antineutrinos. With 10 years of data, DSNB could be observed at 3-sigma; a lower limit of the proton lifetime of 8.34e33 years (90% C.L.) can be set by searching for p->nu_bar K^+; detection of solar neutrinos would shed new light on the solar metallicity problem and examine the vacuum-matter transition region. A core-collapse supernova at 10 kpc would lead to ~5000 IBD and ~2000 (300) all-flavor neutrino-proton (electron) scattering events. Geo-neutrinos can be detected with a rate of ~400 events/year. We also summarize the final design of the JUNO detector and the key R&D achievements. All 20-inch PMTs have been tested. The average photon detection efficiency is 28.9% for the 15,000 MCP PMTs and 28.1% for the 5,000 dynode PMTs, higher than the JUNO requirement of 27%. Together with the >20 m attenuation length of LS, we expect a yield of 1345 p.e. per MeV and an effective energy resolution of 3.02%/\sqrt{E (MeV)}$ in simulations. The underwater electronics is designed to have a loss rate <0.5% in 6 years. With degassing membranes and a micro-bubble system, the radon concentration in the 35-kton water pool could be lowered to <10 mBq/m^3. Acrylic panels of radiopurity <0.5 ppt U/Th are produced. The 20-kton LS will be purified onsite. Singles in the fiducial volume can be controlled to ~10 Hz. The JUNO experiment also features a double calorimeter system with 25,600 3-inch PMTs, a LS testing facility OSIRIS, and a near detector TAO.

preprint2021arXiv

Necessary and sufficient criterion of steering for two-qubit T states

Einstein-Podolsky-Rosen (EPR) steering is the ability that an observer persuades a distant observer to share entanglement by making local measurements. Determining a quantum state is steerable or unsteerable remains an open problem. Here, we derive a new steering inequality with infinite measurements corresponding to an arbitrary two-qubit T state, from consideration of EPR steering inequalities with N projective measurement settings for each side. In fact, the steering inequality is also a sufficient criterion for guaranteering that the T state is unsteerable. Hence, the steering inequality can be viewed as a necessary and sufficient criterion to distinguish whether the T state is steerable or unsteerable. In order to reveal the fact that the set composed of steerable states is the strict subset of the set made up of entangled states, we prove theoretically that all separable T states can not violate the steering inequality. Moreover, we put forward a method to estimate the maximum violation from concurrence for arbitrary two-qubit T states, which indicates that the T state is steerable if its concurrence exceeds 1/4.

preprint2021arXiv

New spectroscopic confirmations of Lyman-$α$ emitters at z $\sim$ 7 from the LAGER survey

We report spectroscopic confirmations of 15 Lyman-alpha galaxies at $z\sim7$, implying a spectroscopic confirmation rate of $\sim$80% on candidates selected from LAGER (Lyman-Alpha Galaxies in the Epoch of Reionization), which is the largest (24 deg$^2$) survey aimed at finding Lyman-alpha emitters (LAEs) at $z\sim7$ using deep narrow-band imaging from DECam at CTIO. LAEs at high-redshifts are sensitive probes of cosmic reionization and narrow-band imaging is a robust and effective method for selecting a large number of LAEs. In this work, we present results from the spectroscopic follow-up of LAE candidates in two LAGER fields, COSMOS and WIDE-12, using observations from Keck/LRIS. We report the successful detection of Ly$α$ emission in 15 candidates (11 in COSMOS and 4 in WIDE-12 fields). Three of these in COSMOS have matching confirmations from a previous LAGER spectroscopic follow-up and are part of the overdense region, LAGER-$z7$OD1. Additionally, two candidates that were not detected in the LRIS observations have prior spectroscopic confirmations from Magellan. Including these, we obtain a spectroscopic confirmation success rate of $\sim$$80$% for LAGER LAE candidates. Apart from Ly$α$, we do not detect any other UV nebular lines in our LRIS spectra; however, we estimate a 2$σ$ upper limit for the ratio of NV/Ly$α$, $f_{NV}/f_{Lyα} \lesssim 0.27$, which implies that ionizing emission from these sources is mostly dominated by star formation. Including confirmations from this work, a total of 33 LAE sources from LAGER are now spectroscopically confirmed. LAGER has more than doubled the sample of spectroscopically confirmed LAE sources at $z\sim7$.

preprint2021arXiv

No observation of chiral flux current in the topological kagome metal CsV$_{3}$Sb$_{5}$

Compounds with kagome lattice usually host many exotic quantum states, including the quantum spin liquid, non-trivial topological Dirac bands and a strongly renormalized flat band, etc. Recently an interesting vanadium based kagome family $A$V$_{3}$Sb$_{5}$ ($A$ = K, Rb, or Cs) was discovered, and these materials exhibit multiple interesting properties, including unconventional saddle-point driving charge density wave (CDW) state, superconductivity, etc. Furthermore, some experiments show anomalous Hall effect which inspires that there might be some chiral flux current states. Here we report scanning tunneling measurements by using spin-polarized tips. Although we have observed clearly the $2a_0\times2a_0$ CDW and $4a_0$ stripe orders, the well-designed experiments with refined spin-polarized tips do not reveal any trace of the chiral flux current phase in CsV$_3$Sb$_5$ within the limits of experimental accuracy. No observation of the local magnetic moment in our experiments may put an upper bound constraint on the magnitude of magnetic moments induced by the possible chiral loop current which has a time-reversal symmetry breaking along $c$-axis in CsV$_{3}$Sb$_{5}$.

preprint2021arXiv

Orbit Tomography of Binary Supermassive Black Holes with Very Long Baseline Interferometry

In this work, we study how to infer the orbit of a supermassive black hole binary (SMBHB) by time-dependent measurements with Very Long Baseline Interferometry (VLBI), such as the Event Horizon Telescope (EHT). Assuming a point-like luminosity image model, we show that with multiple years of observations by EHT, it is possible to recover the SMBHB orbital parameters -- eccentricity, (rescaled) semi-major axis, orbital frequency, and orbital angles -- from their time-varying visibilities even if the binaries orbital period is a few times longer than the duration of observation. Together with the future gravitational wave detections of resolved sources of SMBHBs with Pulsar Timing Array, and/or the detections of optical-band light curves, we will be able to further measure the individual mass of the binary, and also determine the Hubble constant if the total mass of the binary is measured through the light curves of the two black holes or by alternative methods.

preprint2021arXiv

Supercritical accretion of stellar-mass compact objects in active galactic nuclei

Accretion disks of active galactic nuclei (AGN) have been proposed as promising sites for producing both (stellar-mass) compact object mergers and extreme mass ratio inspirals. Along with the disk-assisted migration/evolution process, ambient gas materials inevitably accrete onto the compact objects. The description of this process is subject to significant theoretical uncertainties in previous studies. It was commonly assumed that either an Eddington accretion rate or a Bondi accretion rate (or any rate in between) takes place, although these two rates can differ from each other by several orders of magnitude. As a result, the mass and spin evolution of compact objects within AGN disks are essentially unknown. In this work, we construct a relativistic supercritical inflow-outflow model for black hole (BH) accretion. We show that the radiation efficiency of the supercritical accretion of a stellar-mass BH (sBH) is generally too low to explain the proposed electromagnetic counterpart of GW190521. Applying this model to sBHs embedded in AGN disks, we find that, although the gas inflow rates at Bondi radii of these sBHs are in general highly super-Eddington, a large fraction of inflowing gas eventually escapes as outflows so that only a small fraction accretes onto the sBH, resulting in mildly super-Eddington BH absorption in most cases. We also implement this inflow-outflow model to study the evolution of neutron stars (NS) and white dwarfs (WD) in AGN disks, taking into account corrections from star sizes and star magnetic fields. It turns out to be difficult for WDs to grow to the Chandrasekhar limit via accretion because WDs are spun up more efficiently to reach the shedding limit before the Chandrasekhar limit. For NSs the accretion-induced collapse is possible if NS magnetic fields are sufficiently strong, keeping the NS in a slow rotation state during accretion.

preprint2021arXiv

Van Hove Singularity Arising from Mexican-Hat-Shaped Inverted Bands in the Topological Insulator Sn-doped Bi$_{1.1}$Sb$_{0.9}$Te$_{2}$S

The optical properties of Sn-doped Bi$_{1.1}$Sb$_{0.9}$Te$_{2}$S, the most bulk-insulating topological insulator thus far, have been examined at different temperatures over a broad frequency range. No Drude response is detected in the low-frequency range down to 30~cm$^{-1}$, corroborating the excellent bulk-insulating property of this material. Intriguingly, we observe a sharp peak at about 2\,200~cm$^{-1}$ in the optical conductivity at 5~K. Further quantitative analyses of the line shape and temperature dependence of this sharp peak, in combination with first-principles calculations, suggest that it corresponds to a van Hove singularity arising from Mexican-hat-shaped inverted bands. Such a van Hove singularity is a pivotal ingredient of various strongly correlated phases.

preprint2020arXiv

A comprehensive study of H$α$ emitters at $z \sim$ 0.62 in the DAWN survey: the need for deep and wide regions

We present new estimates of the luminosity function (LF) and star formation rate density (SFRD) for an H$α$ selected sample at $z\sim0.62$ from the Deep And Wide Narrow-band (DAWN) survey. Our results are based on a new H$α$ sample in the extended COSMOS region (compared to Coughlin et al. 2018) with the inclusion of flanking fields, resulting in a total area coverage of $\sim$1.5 deg$^2$. A total of 241 H$α$ emitters were selected based on robust selection criteria using spectro-photometric redshifts and broadband color-color classification. We explore the effect of different dust correction prescriptions by calculating the LF and SFRD using a constant dust extinction correction, A{$_{\textrm{H}α}=1$} mag, a luminosity-dependent correction, and a stellar-mass dependent correction. The resulting H$α$ LFs are well fitted using Schechter functions with best-fit parameters: L$^*=10^{42.24}$ erg s$^{-1}$, $ϕ^*=10^{-2.85}$ Mpc$^{-3}$, $α= -1.62$ for constant dust correction, L$^*=10^{42.31}$ erg s$^{-1}$, $ϕ^*=10^{-2.8}$ Mpc$^{-3}$, $α=-1.39$ for luminosity-dependent dust correction, and L$^*=10^{42.36}$ erg s$^{-1}$, $ϕ^*=10^{-2.91}$ Mpc$^{-3}$, $α= -1.48$, for stellar mass-dependent dust correction. The deep and wide nature of the DAWN survey effectively samples H$α$ emitters over a wide range of luminosities, thereby providing better constraints on both the faint and bright end of the LF. Also, the SFRD estimates $ρ_{\textrm{SFR}}=10^{-1.39}$ M$_{\odot}$yr$^{-1}$Mpc$^{-3}$ (constant dust correction), $ρ_{\textrm{SFR}}=10^{-1.47}$ M$_{\odot}$yr$^{-1}$Mpc$^{-3}$ (luminosity-dependent dust correction), and $ρ_{\textrm{SFR}}=10^{-1.49}$ M$_{\odot}$yr$^{-1}$Mpc$^{-3}$ (stellar mass-dependent dust correction) are in good agreement with the evolution of SFRD across redshifts ($0 < z < 2$) seen from previous H$α$ surveys.

preprint2020arXiv

A large, deep 3 deg$^2$ survey of H$α$, [OIII], and [OII] emitters from LAGER: constraining luminosity functions

We present our measurements of the H$α$, [OIII], and [OII] luminosity functions as part of the Lyman Alpha Galaxies at Epoch of Reionization (LAGER) survey using our samples of 1577 $z = 0.47$ H$α$-, 3933 $z = 0.93$ [OIII]-, and 5367 $z = 1.59$ [OII]-selected emission line galaxies in a single 3 deg$^2$ CTIO/Blanco DECam pointing of the COSMOS field. Our observations reach 5$σ$ depths of $8.2\times10^{-18}$ erg s$^{-1}$ cm$^{-2}$ and comoving volumes of $(1-7)\times10^{5}$ Mpc$^3$ making our survey one of the deepest narrowband surveys. We measure the observed luminosity functions and find best-fits of $ϕ^\star = 10^{-3.16\pm0.09}$ Mpc$^{-3}$ and $L^\star = 10^{41.72\pm0.09}$ erg s$^{-1}$ for H$α$, $ϕ^\star = 10^{-2.16^{+0.10}_{-0.12}}$ Mpc$^{-3}$ and $L^\star = 10^{41.38^{+0.07}_{-0.06}}$ erg s$^{-1}$ for [OIII], and $ϕ^\star = 10^{-1.97^{+0.07}_{-0.07}}$ Mpc$^{-3}$ and $L^\star = 10^{41.66\pm0.03}$ erg s$^{-1}$ for [OII], with $α$ fixed to $-1.75$, $-1.6$, and $-1.3$, respectively. An excess of bright $> 10^{42}$ erg s$^{-1}$ [OIII] emitters is observed and may be due to AGN contamination. Dust corrections are applied assuming $A_{\rm{H}α} = 1$ mag. We also design our own empirical rest-frame $g - r$ calibration using SDSS DR12 data, test it against our $z = 0.47$ H$α$ emitters with $z$COSMOS $1$D spectra, and calibrate it for $(g - r)$ between $-0.8$ and $1.3$ mag. Dust and AGN-corrected star formation rate densities (SFRDs) are measured as $\log_{10}ρ_{\rm{SFR}}/(\rm{M}_\odot\ \rm{yr}^{-1}\ \rm{Mpc}^{-3}) = -1.63\pm0.04$, $-1.07\pm0.06$, and $-0.90\pm0.10$ for H$α$, [OIII], and [OII], respectively. We find our [OIII] and [OII] samples fully trace cosmic star formation activity at their respective redshifts in comparison to multi-wavelength SFRDs, while the H$α$ sample traces $\sim 70$ percent of the total $z = 0.47$ SFRD.

preprint2020arXiv

Application of Structural Similarity Analysis of Visually Salient Areas and Hierarchical Clustering in the Screening of Similar Wireless Capsule Endoscopic Images

Small intestinal capsule endoscopy is the mainstream method for inspecting small intestinal lesions,but a single small intestinal capsule endoscopy will produce 60,000 - 120,000 images, the majority of which are similar and have no diagnostic value. It takes 2 - 3 hours for doctors to identify lesions from these images. This is time-consuming and increase the probability of misdiagnosis and missed diagnosis since doctors are likely to experience visual fatigue while focusing on a large number of similar images for an extended period of time.In order to solve these problems, we proposed a similar wireless capsule endoscope (WCE) image screening method based on structural similarity analysis and the hierarchical clustering of visually salient sub-image blocks. The similarity clustering of images was automatically identified by hierarchical clustering based on the hue,saturation,value (HSV) spatial color characteristics of the images,and the keyframe images were extracted based on the structural similarity of the visually salient sub-image blocks, in order to accurately identify and screen out similar small intestinal capsule endoscopic images. Subsequently, the proposed method was applied to the capsule endoscope imaging workstation. After screening out similar images in the complete data gathered by the Type I OMOM Small Intestinal Capsule Endoscope from 52 cases covering 17 common types of small intestinal lesions, we obtained a lesion recall of 100% and an average similar image reduction ratio of 76%. With similar images screened out, the average play time of the OMOM image workstation was 18 minutes, which greatly reduced the time spent by doctors viewing the images.

preprint2020arXiv

Dynamic Signatures of Black Hole Binaries with Superradiant Clouds

Superradiant clouds may develop around a rotating black hole, if there is a bosonic field with Compton wavelength comparable to the size of the black hole. In this paper, we investigate the effects of the cloud on the orbits of nearby compact objects. In particular, we consider the dynamical friction and the backreaction due to level mixing. Under these interactions, the probability of a black hole dynamically capturing other compact objects, such as stellar mass black holes and neutron stars, is generally enhanced with the presence of the cloud. For extreme mass ratio inspirals and binary stellar mass binary black holes, the cloud-induced orbital modulation may be detected by observing the gravitational waveform using space borne gravitational wave detectors, such as LISA. Interestingly within certain range of boson Compton wavelength, the enhanced capture rate of stellar mass black holes could accelerate hierarchical mergers, with higher-generation merger product being more massive than the mass threshold predicted by supernova pair instability. These observational signatures provide promising ways of searching light bosons with gravitational waves.

preprint2020arXiv

Experimental demonstration of complementarity relations between quantum steering criteria

The ability that one system immediately affects another one by using local measurements is regarded as quantum steering, which can be detected by various steering criteria. Recently, Mondal et al. [Phys. Rev. A 98, 052330 (2018)] derived the complementarity relations of coherence steering criteria, and revealed that the quantum steering of system can be observed through the average coherence of subsystem. Here, we experimentally verify the complementarity relations between quantum steering criteria by employing two-photon Bell-like states and three Pauli operators. The results demonstrate that if prepared quantum states can violate two setting coherence steering criteria and turn out to be steerable states, then it cannot violate the complementary settings criteria. Three measurement settings inequality, which establish a complementarity relation between these two coherence steering criteria, always holds in experiment. Besides, we experimentally certify that the strengths of coherence steering criteria dependent on the choice of coherence measure. In comparison with two setting coherence steering criteria based on l1 norm of coherence and relative entropy of coherence, our experimental results show that the steering criterion based on skew information of coherence is more stronger in detecting the steerability of quantum states. Thus, our experimental demonstrations can deepen the understanding of the relation between the quantum steering and quantum coherence.

preprint2020arXiv

Feasibility and physics potential of detecting $^8$B solar neutrinos at JUNO

The Jiangmen Underground Neutrino Observatory~(JUNO) features a 20~kt multi-purpose underground liquid scintillator sphere as its main detector. Some of JUNO's features make it an excellent experiment for $^8$B solar neutrino measurements, such as its low-energy threshold, its high energy resolution compared to water Cherenkov detectors, and its much large target mass compared to previous liquid scintillator detectors. In this paper we present a comprehensive assessment of JUNO's potential for detecting $^8$B solar neutrinos via the neutrino-electron elastic scattering process. A reduced 2~MeV threshold on the recoil electron energy is found to be achievable assuming the intrinsic radioactive background $^{238}$U and $^{232}$Th in the liquid scintillator can be controlled to 10$^{-17}$~g/g. With ten years of data taking, about 60,000 signal and 30,000 background events are expected. This large sample will enable an examination of the distortion of the recoil electron spectrum that is dominated by the neutrino flavor transformation in the dense solar matter, which will shed new light on the tension between the measured electron spectra and the predictions of the standard three-flavor neutrino oscillation framework. If $Δm^{2}_{21}=4.8\times10^{-5}~(7.5\times10^{-5})$~eV$^{2}$, JUNO can provide evidence of neutrino oscillation in the Earth at the about 3$σ$~(2$σ$) level by measuring the non-zero signal rate variation with respect to the solar zenith angle. Moveover, JUNO can simultaneously measure $Δm^2_{21}$ using $^8$B solar neutrinos to a precision of 20\% or better depending on the central value and to sub-percent precision using reactor antineutrinos. A comparison of these two measurements from the same detector will help elucidate the current tension between the value of $Δm^2_{21}$ reported by solar neutrino experiments and the KamLAND experiment.

preprint2020arXiv

Full Reference Screen Content Image Quality Assessment by Fusing Multi-level Structure Similarity

The screen content images (SCIs) usually comprise various content types with sharp edges, in which the artifacts or distortions can be well sensed by the vanilla structure similarity measurement in a full reference manner. Nonetheless, almost all of the current SOTA structure similarity metrics are "locally" formulated in a single-level manner, while the true human visual system (HVS) follows the multi-level manner, and such mismatch could eventually prevent these metrics from achieving trustworthy quality assessment. To ameliorate, this paper advocates a novel solution to measure structure similarity "globally" from the perspective of sparse representation. To perform multi-level quality assessment in accordance with the real HVS, the above-mentioned global metric will be integrated with the conventional local ones by resorting to the newly devised selective deep fusion network. To validate its efficacy and effectiveness, we have compared our method with 12 SOTA methods over two widely-used large-scale public SCI datasets, and the quantitative results indicate that our method yields significantly higher consistency with subjective quality score than the currently leading works. Both the source code and data are also publicly available to gain widespread acceptance and facilitate new advancement and its validation.

preprint2020arXiv

Learning Texture Transformer Network for Image Super-Resolution

We study on image super-resolution (SR), which aims to recover realistic textures from a low-resolution (LR) image. Recent progress has been made by taking high-resolution images as references (Ref), so that relevant textures can be transferred to LR images. However, existing SR approaches neglect to use attention mechanisms to transfer high-resolution (HR) textures from Ref images, which limits these approaches in challenging cases. In this paper, we propose a novel Texture Transformer Network for Image Super-Resolution (TTSR), in which the LR and Ref images are formulated as queries and keys in a transformer, respectively. TTSR consists of four closely-related modules optimized for image generation tasks, including a learnable texture extractor by DNN, a relevance embedding module, a hard-attention module for texture transfer, and a soft-attention module for texture synthesis. Such a design encourages joint feature learning across LR and Ref images, in which deep feature correspondences can be discovered by attention, and thus accurate texture features can be transferred. The proposed texture transformer can be further stacked in a cross-scale way, which enables texture recovery from different levels (e.g., from 1x to 4x magnification). Extensive experiments show that TTSR achieves significant improvements over state-of-the-art approaches on both quantitative and qualitative evaluations.

preprint2020arXiv

NTIRE 2020 Challenge on Perceptual Extreme Super-Resolution: Methods and Results

This paper reviews the NTIRE 2020 challenge on perceptual extreme super-resolution with focus on proposed solutions and results. The challenge task was to super-resolve an input image with a magnification factor 16 based on a set of prior examples of low and corresponding high resolution images. The goal is to obtain a network design capable to produce high resolution results with the best perceptual quality and similar to the ground truth. The track had 280 registered participants, and 19 teams submitted the final results. They gauge the state-of-the-art in single image super-resolution.

preprint2020arXiv

Orbit-induced spin precession as an origin of periodicity in periodically-repeating fast radio bursts

FRB 180916.J0158+65 has been found to repeatedly emit fast radio bursts with the period in roughly 16 days. We propose that such periodicity comes from orbit-induced, spin precession of the emitter, which is possibly a neutron star. Depending on the mass of the companion, the binary period ranges from several hundred to thousands of seconds. Such tight binaries have a relatively short lifetime, which does not likely come from the gravitational decay of a wide binary. We comment on the relation to GW190425 and the possibility in LISA and LIGO detections.

preprint2020arXiv

Quantum correlation of light mediated by gravity

We propose to explore the quantum nature of gravity using the correlation of light between two optomechanical cavities, and the quantumness of the correlation is witnessed by squeezing. As long as the gravity between the end mirrors of two cavities is quantum in the Newtonian limit, we show that the squeezing is always nonzero and monotonically increases as the mechanical property of the mirrors is improved. The proposed scheme provides a new pathway for testing the quantum nature of gravity systematically with tabletop experiments.

preprint2020arXiv

TAO Conceptual Design Report: A Precision Measurement of the Reactor Antineutrino Spectrum with Sub-percent Energy Resolution

The Taishan Antineutrino Observatory (TAO, also known as JUNO-TAO) is a satellite experiment of the Jiangmen Underground Neutrino Observatory (JUNO). A ton-level liquid scintillator detector will be placed at about 30 m from a core of the Taishan Nuclear Power Plant. The reactor antineutrino spectrum will be measured with sub-percent energy resolution, to provide a reference spectrum for future reactor neutrino experiments, and to provide a benchmark measurement to test nuclear databases. A spherical acrylic vessel containing 2.8 ton gadolinium-doped liquid scintillator will be viewed by 10 m^2 Silicon Photomultipliers (SiPMs) of >50% photon detection efficiency with almost full coverage. The photoelectron yield is about 4500 per MeV, an order higher than any existing large-scale liquid scintillator detectors. The detector operates at -50 degree C to lower the dark noise of SiPMs to an acceptable level. The detector will measure about 2000 reactor antineutrinos per day, and is designed to be well shielded from cosmogenic backgrounds and ambient radioactivities to have about 10% background-to-signal ratio. The experiment is expected to start operation in 2022.

preprint2020arXiv

The Importance Of Star Formation Intensity In LYα Escape From Green Pea Galaxies And Lyman Break Galaxy Analogs

We have studied ultraviolet images of 40 Green Pea galaxies and 15 local Lyman Break Galaxy Analogs to understand the relation between Ly$α$ photon escape and central UV photometric properties. We measured star formation intensity (SFI, star formation rate per unit area) from the central 250 pc region ($S_{\rm 250pc}$) using COS/NUV images from the \textit{Hubble Space Telescope}. The measured $S_{\rm 250pc}$ of our sample Green Peas ranges from 2.3--46 $M_{\odot} \ \rm{year}^{-1} \ \rm{kpc^{-2}}$, with a geometric mean of $15 M_{\odot} \ \rm{year}^{-1} \ \rm{kpc^{-2}}$ and a standard deviation of 0.266 dex, forming a relatively narrow distribution. The Lyman Break Galaxy Analogs show a similarly narrow distribution of $S_{\rm 250pc}$ (0.271 dex), though with a larger mean of 28 $M_{\odot} \ \rm{year}^{-1} \ \rm{kpc^{-2}}$. We show that while the Ly$α$ equivalent width (EW(Ly$α$)) and the Ly$α$ escape fraction ($f^{Lyα}_{esc}$) are not significantly correlated with the central SFI ($S_{\rm 250pc}$), both are positively correlated with the ratio of surface brightness to galaxy stellar mass ($S_{\rm 250pc}/M_{\rm star}$), with correlation coefficients ($p$-values) of 0.702 ($1\times 10^{-8}$) and 0.529 ($5\times 10^{-4}$) with EW(Ly$α$) and $f^{Lyα}_{esc}$, respectively. These correlations suggest a scenario where intense central star formation can drive a galactic wind in galaxies with relatively shallow gravitational potential wells, thus clearing channels for the escape of Ly$α$ photons.

preprint2020arXiv

Towards observing the neutron star collapse with gravitational wave detectors

Gravitational waves from binary neutron star inspirals have been detected along with the electromagnetic transients coming from the aftermath of the merger in GW170817. However, much is still unknown about the post-merger dynamics that connects these two sets of observables. This includes if, and when, the post-merger remnant star collapses to a black hole, and what are the necessary conditions to power a short gamma-ray burst, and other observed electromagnetic counterparts. Observing the collapse of the post-merger neutron star would shed led on these questions, constraining models for the short gamma-ray burst engine and the hot neutron star equation of state. In this work, we explore the scope of using gravitational wave detectors to measure the timing of the collapse either indirectly, by establishing the shut-off of the post-merger gravitational emission, or---more challengingly---directly, by detecting the collapse signal. For the indirect approach, we consider a kilohertz high-frequency detector design that utilises a previously studied coupled arm cavity and signal recycling cavity resonance. This design would give a signal-to-noise ratio of 0.5\,-\,8.6 (depending on the variation of waveform parameters) for a collapse gravitational wave signal occurring at 10\,ms post-merger of a binary at 50\,Mpc and with total mass $2.7 M_\odot$. For the direct approach, we propose a narrow-band detector design, utilising the sensitivity around the frequency of the arm cavity free spectral range. The proposed detector achieves a signal-to-noise ratio of 0.3\,-\,1.9, independent of the collapse time. This detector is limited by both the fundamental classical and quantum noise with the arm cavity power chosen as 10\,MW.

preprint2020arXiv

Twofold symmetry of proximity-induced superconductivity in Bi$_{2}$Te$_{3}$/Bi$_{2}$Sr$_{2}$CaCu$_{2}$O$_{8+δ}$ heterostructures revealed by scanning tunneling microscopy

We observe proximity-induced superconductivity in the \textit{in situ} prepared heterostructures constructed by topological insulator Bi$_{2}$Te$_{3}$ thin films and high-temperature cuprate superconductors Bi$_{2}$Sr$_{2}$CaCu$_{2}$O$_{8+δ}$. The superconducting gap maximum is about 7.6 meV on the surface of Bi$_{2}$Te$_{3}$ thin films with a thickness of two quintuple layers, and the gap value decreases with an increase in the film thickness. Moreover, the quasiparticle interference data show a clear evidence of a twofold symmetric superconducting gap with gap minima along one pair of the principal crystalline axes of Bi$_{2}$Te$_{3}$. This gap form is consistent with the $Δ_{4y}$ notation of the topological superconductivity proposed in such systems. Our results provide fruitful information of the possible topological superconductivity induced by the proximity effect in high-temperature superconducting cuprates.

preprint2019arXiv

Excess Thermal Energy and Latent Heat in Nanocluster Collisional Growth

Nanoclusters can form and grow by nanocluster-monomer (condensation) and nanocluster-nanocluster (coagulation) collisions. During growth, product nanoclusters have elevated thermal energies due to potential and thermal energy exchange following a collision. Even though nanocluster collisional heating may be significant and strongly-size dependent, no prior theory describes such phenomenon. We derive a model to describe the excess thermal energy, the kinetic energy increase of the product cluster, and latent heat, the heat released to the background upon thermalization of the non-equilibrium cluster, of collisional growth. Both quantities are composed of an enthalpic term, related to potential energy minimum differences, and a size-dependent entropic term, which hinges upon heat capacity and energy partitioning. Example calculations using gold nanoclusters demonstrate that collisional heating can be important and strongly size dependent, particularly for reactive collisions involving nanoclusters composed of 14-20 atoms. Excessive latent heat release may have considerable implications in cluster formation and growth.

preprint2019arXiv

Experimental certification of steering criterion based on general entropic uncertainty relation

Quantum steering describes the phenomenon that one system can be immediately influenced by another with local measurements. It can be detected by the violation of a powerful and useful steering criterion from general entropic uncertainty relation. This criterion, in principle, can be evaluated straightforwardly and achieved by only probability distributions from a finite set of measurement settings. Herein, we experimentally verify the steering criterion by means of the two-photon Werner-like states and three Pauli measurements. The results indicate that quantum steering can be verified by the criterion in a convenient way. In particular, it is no need to perform the usual quantum state tomography in experiment, which reduces the required experimental resources greatly. Moreover, we demonstrate that the criterion is stronger than the linear one for the detecting quantum steering of the Werner-like states.

preprint2019arXiv

Experimental investigation of entropic uncertainty relations and coherence uncertainty relations

Uncertainty relation usually is one of the most important features in quantum mechanics, and is the backbone of quantum theory, which distinguishes from the rule in classical counterpart. Specifically, entropy-based uncertainty relations are of fundamental importance in the region of quantum information theory, offering one nontrivial bound of key rate towards quantum key distribution. In this work, we experimentally demonstrate the entropic uncertainty relations and coherence-based uncertainty relations in an all-optics platform. By means of preparing two kinds of bipartite initial states with high fidelity, i.e., Bell-like states and Bell-like diagonal states, we carry on local projective measurements over a complete set of mutually unbiased bases on the measured subsystem. In terms of quantum tomography, the density matrices of the initial states and the post-measurement states are reconstructed. It shows that our experimental results coincide with the theoretical predictions very well. Additionally, we also verify that the lower bounds of both the entropy-based and coherence-based uncertainty can be tightened by imposing the Holevo quantity and mutual information, and the entropic uncertainty is inversely correlated with the coherence. Our demonstrations might offer an insight into their uncertainty relations and their connection to quantum coherence in quantum information science, which might be applicable to the security analysis of quantum key distributions.

preprint2019arXiv

Experimental observation the Einstein-Podolsky-Rosen Steering based on the detection of entanglement

The Einstein-Podolsky-Rosen (EPR) steering is an intermediate quantum nonlocality between entanglement and Bell nonlocality, which plays an important role in quantum information processing tasks. In the past few years, the investigations concerning EPR steering have been demonstrated in a series of experiments. However, these studies rely on the relevant steering inequalities and the choices of measurement settings. Here, we experimentally verify the EPR steering via entanglement detection without using any steering inequality and measurement setting. By constructing two new states from a two-qubit target state, we observe the EPR steering by detecting the entanglement of these new states. The results show that the entanglement of the newly constructed states can be regarded as a new kind of steering witness for target states. Compared to the results of Xiao et al. [Phys. Rev. Lett. 118, 140404 (2017)], we find that the ability of detecting EPR steering in our scenario is stronger than two-setting projective measurements, which can observe more steerable states. Hence, our demonstrations can deepen the understanding of the connection between the EPR steering and entanglement.

preprint2019arXiv

Relativistic Mean Motion Resonance

Mean motion resonances are commonly seen in planetary systems, e.g., in the formation of orbital structure of Jupiter's moons and the gaps in the rings of Saturn. In this work we study their effects in fully relativistic systems. We consider a model problem with two stellar mass black holes orbiting around a supermassive black hole. By adopting a two time-scale expansion technique and averaging over the fast varying orbital variables, we derive the effective Hamiltonian for the slowly varying dynamical variables. The formalism is illustrated with a n'_phi : n'_r : n_phi= 2:1:-2 resonance in Schwarzschild spacetime, which naturally becomes the 3:2 resonance widely studied in the Newtonian limit. We also derive the multi-body Hamiltonian in the post-Newtonian regime, where the radial and azimuthal frequencies are different because of the post-Newtonian precession. The capture and breaking conditions for these relativistic mean motion resonances are also discussed. In particular, pairs of stellar mass black holes surrounding the supermassive black hole could be locked into resonances as they enter the LISA band, and this would affect their gravitational wave waveforms.

preprint2018arXiv

Directly visualizing the sign change of d-wave superconducting gap in Bi2Sr2CaCu2O8+δ by phase-referenced quasiparticle interference

The superconducting state is achieved by the condensation of Cooper pairs and is protected by the superconducting gap. The pairing interaction between the two electrons of a Cooper pair determines the superconducting gap function. Thus, it is very pivotal to detect the gap structure for understanding the mechanism of superconductivity. In cuprate superconductors, it has been well established that the superconducting gap may have a d-wave function Δ = Δ_0cos2θ. This gap function has an alternative sign change by every pi/2 in the momentum space when the in-plane azimuthal angle theta is scanned. It is very hard to visualize this sign change. Early experiments for recommending or proving this d-wave gap function were accomplished by the specially designed phase sensitive measurements based on the Josephson effect. Here we report the measurements of scanning tunneling spectroscopy in one of the model cuprate system Bi2Sr2CaCu2O8+δ and conduct the analysis of phase-referenced quasiparticle interference (QPI). Due to the unique quasiparticle excitations in the superconducting state of cuprate, we have seen the seven basic scattering vectors that connect each pair of the terminals of the banana-shaped contour of constant quasiparticle energy (CCE). The phase-referenced QPI clearly visualizes the sign change of the d-wave gap. Our results illustrate a very effective way for determining the sign change of unconventional superconductors.

preprint2016arXiv

Green Pea Galaxies Reveal Secrets of Ly$α$ Escape

We analyze archival Ly$α$ spectra of 12 "Green Pea" galaxies observed with the Hubble Space Telescope, model their Ly$α$ profiles with radiative transfer models, and explore the dependence of Ly$α$ escape fraction on various properties. Green Pea galaxies are nearby compact starburst galaxies with [OIII]$λ$5007 equivalent widths of hundreds of Å. All 12 Green Pea galaxies in our sample show Ly$α$ lines in emission, with a Ly$α$ equivalent width distribution similar to high redshift Ly$α$ emitters. Combining the optical and UV spectra of Green Pea galaxies, we estimate their Ly$α$ escape fractions and find correlations between Ly$α$ escape fraction and kinematic features of Ly$α$ profiles. The escape fraction of Ly$α$ in these galaxies ranges from 1.4% to 67%. We also find that the Ly$α$ escape fraction depends strongly on metallicity and moderately on dust extinction. We compare their high-quality Ly$α$ profiles with single HI shell radiative transfer models and find that the Ly$α$ escape fraction anti-correlates with the derived HI column densities. Single shell models fit most Ly$α$ profiles well, but not the ones with highest escape fractions of Ly$α$. Our results suggest that low HI column density and low metallicity are essential for Ly$α$ escape, and make a galaxy a Ly$α$ emitter.

preprint2016arXiv

Multi-band superconductivity and large anisotropy in FeS crystals

By using a hydrothermal method, we have successfully grown crystals of the newly discovered superconductor FeS, which has an isostructure of the iron based superconductor FeSe. The superconductivity appears at about 4.5K, as revealed by both resistive and magnetization measurements. It is found that the upper critical field is relatively low, with however an rather large anisotropy $Γ=[(dH_{c2}^{ab}/dT)/(dH_{c2}^{c}/dT)]_{T_c}\approx5.8$. A huge magnetoresistivity (290$\%$ at 9T and 10K, ${H}$ $\parallel$ c-axis) together with a non-linear behavior of Hall resistivity vs. external field are observed. A two-band model is applied to fit the magnetoresistance and non-linear transverse resistivity, yielding the basic parameters of the electron and hole bands.

preprint2016arXiv

Plasma-wave generation in a dynamic spacetime

We propose a new electromagnetic-emission mechanism in magnetized, force-free plasma, which is driven by the evolution of the underlying dynamic spacetime. In particular, the emission power and angular distribution of the emitted fast-magnetosonic and Alfvén waves are separately determined. Previous numerical simulations of binary black hole mergers occurring within magnetized plasma have recorded copious amounts of electromagnetic radiation that, in addition to collimated jets, include an unexplained, isotropic component which becomes dominant close to merger. This raises the possibility of multimessenger gravitational-wave and electromagnetic observations on binary black hole systems. The mechanism proposed here provides a candidate analytical characterization of the numerical results, and when combined with previously understood mechanisms such as the Blandford-Znajek process and kinetic-motion-driven radiation, allows us to construct a classification of different electromagnetic radiation components seen in the inspiral stage of compact-binary coalescences.

preprint2016arXiv

Possible superconducting fluctuation and pseudogap state above $T_c$ in CsFe$_2$As$_2$

Resistive, magnetization, torque, specific heat and scanning tunneling microscopy measurements are carried out on the hole heavily doped CsFe$_2$As$_2$ single crystals. A characteristic temperature $T^*\sim13$ K, which is several times higher than the superconducting transition temperature $T_c=2.15$ K, is observed and possibly related to the superconducting fluctuation or the pseudogap state. A diamagnetic signal detected by torque measurements starts from the superconducting state, keeps finite and vanishes gradually until a temperature near $T^*$. Temperature dependent resistivity and specific heat also show kinks near $T^*$. An asymmetric gap-like feature with the energy of 8.4 meV and a symmetric superconducting related gap of 2.2 meV on the scanning tunneling spectra are detected, and these pseudogap-related features disappear at temperatures up to at least 9 K. These observations by different experimental tools suggest the possible existence of superconducting fluctuation or pseudogap state in the temperature range up to 4 - 6 times of $T_c$ in CsFe$_2$As$_2$.

preprint2016arXiv

Robust superconductivity and transport properties in (Li1-xFex)OHFeSe single crystals

The recently discovered (Li${_{1-x}}$Fe${_x}$)OHFeSe superconductor with $T_c$ about 40K provides a good platform for investigating the magnetization and electrical transport properties of FeSe-based superconductors. By using a hydrothermal ion-exchange method, we have successfully grown crystals of (Li${_{1-x}}$Fe${_x}$)OHFeSe. X-ray diffraction on the sample shows the single crystalline PbO-type structure with the c-axis preferential orientation. Magnetic susceptibility and resistive measurements show an onset superconducting transition at around ${T_c}$=38.3K. Using the magnetization hysteresis loops and Bean critical state model, a large critical current ${J_s}$ is observed in low temperature region. The critical current density is suppressed exponentially with increasing magnetic field. Temperature dependencies of resistivity under various currents and fields are measured, revealing a robust superconducting current density and bulk superconductivity.

preprint2016arXiv

Strong coupling superconductivity revealed by scanning tunneling microscope in tetragonal FeS

We investigate the electronic properties of the tetragonal FeS superconductor by using scanning tunneling microscope/spectroscopy. It is found that the typical tunneling spectrum on the top layer of sulfur can be nicely fitted with an anisotropic s-wave or a combination of two superconducting components in which one may have a highly anisotropic or nodal like superconducting gap. The fittings lead to the maximum superconducting gap $Δ_{max}\approx$ 0.90$\;$meV, which yields a ratio of 2$Δ_{max}/k_BT_c\approx$ 4.65. This value is larger than that of the predicted value 3.53 by the BCS theory in the weak coupling limit, indicating a strong coupling superconductivity. Two kinds of defects are observed on the surface, which can be assigned to the defects on the S sites (four-fold image) and Fe sites (dumbbell shape). Impurity induced resonance states are found only for the former defects and stay at zero-bias energy.

preprint2016arXiv

Towards an understanding of the force-free magnetosphere of rapidly spinning black holes

The ability of a plasma surrounding spinning black holes to extract rotational energy and power energetic emissions has been recognized as a key astrophysical phenomenon. Important insights into the nature of this process are obtained through the analysis of the interplay between a force-free magnetosphere and the black hole. This task involves solving a complicated system of equations, often requiring complex numerical simulations. Recent analytical attempts at tackling this problem have exploited the fact that the near horizon region of extreme Kerr (NHEK) is endowed with an enhanced symmetry group. We continue in this direction and show that for some conformally self-similar solutions, the NHEK force-free equations reduce to a single non-linear ordinary differential equation which is difficult to solve with straightforward integration. We here introduce a new approach specifically tailored to this problem and describe how one can obtain physically meaningful solutions.

preprint2015arXiv

Anisotropic Electronic Mobilities in the Nematic State of the Parent Phase NaFeAs

Hall effect and magnetoresistance have been measured on single crystals of the parent phase NaFeAs under a uniaxial pressure. Although significant difference of the in-plane resistivity $ρ_{xx}(I\parallel a)$ and $ρ_{xx}(I\parallel b)$ with the uniaxial pressure along $b$-axis was observed, the transverse resistivity $ρ_{xy}$ shows a surprisingly isotropic behavior. Detailed analysis reveals that the Hall coefficient $R_\mathrm{H}$ measured in the two orthogonal configurations ($I\parallel a$-axis and $I\parallel b$-axis) coincide very well and exhibit a deviation from the high temperature background at around the structural transition temperature $T_{\mathrm{s}}$. Furthermore, the magnitude of $R_\mathrm{H}$ increases remarkably below the structural transition temperature. This enhanced Hall coefficient is accompanied by the non-linear transverse resistivity versus magnetic field and enhanced magnetoresistance, which can be explained very well by the two band model with anisotropic mobilities of each band. Our results together with the two band model analysis clearly show that the anisotropic in-plane resistivity in the nematic state is closely related to the distinct quasiparticle mobilities when they are moving parallel or perpendicular to the direction of the uniaxial pressure.

preprint2015arXiv

Coupled Oscillator Model for Nonlinear Gravitational Perturbations

Motivated by the gravity/fluid correspondence, we introduce a new method for characterizing nonlinear gravitational interactions. Namely we map the nonlinear perturbative form of the Einstein equation to the equations of motion of a collection of nonlinearly-coupled harmonic oscillators. These oscillators correspond to the quasinormal or normal modes of the background spacetime. We demonstrate the mechanics and the utility of this formalism within the context of perturbed asymptotically anti-de Sitter black brane spacetimes. We confirm in this case that the boundary fluid dynamics are equivalent to those of the hydrodynamic quasinormal modes of the bulk spacetime. We expect this formalism to remain valid in more general spacetimes, including those without a fluid dual. In other words, although borne out of the gravity/fluid correspondence, the formalism is fully independent and it has a much wider range of applicability. In particular, as this formalism inspires an especially transparent physical intuition, we expect its introduction to simplify the often highly technical analytical exploration of nonlinear gravitational dynamics.

preprint2015arXiv

Fully gapped superconductivity in In-doped topological crystalline insulator Pb$_{0.5}$Sn$_{0.5}$Te

Superconductors derived from topological insulators and topological crystalline insulators by doping have long been considered to be candidates as topological superconductors. Pb$_{0.5}$Sn$_{0.5}$Te is a topological crystalline insulator with mirror symmetry protected surface states on (001), (011) and (111) oriented surfaces. The superconductor (Pb$_{0.5}$Sn$_{0.5}$)$_{0.7}$In$_{0.3}$Te is induced by In doping in Pb$_{0.5}$Sn$_{0.5}$Te, and is thought to be a topological superconductor. Here we report the first scanning tunneling spectroscopy measurement of the superconducting state as well as the superconducting energy gap in (Pb$_{0.5}$Sn$_{0.5}$)$_{0.7}$In$_{0.3}$Te on a (001)-oriented surface. The spectrum can be well fitted by an anisotropic $s$-wave gap function of $Δ=0.72+0.18\cos4θ$ meV using Dynes model. The results show that the quasi-particle density of states seem to be fully gapped without any in-gap states, in contradiction with the expectation of a topological superconductor.

preprint2015arXiv

Magnetization relaxation, critical current density and vortex dynamics in a Ba$_{0.66}$K$_{0.32}$BiO$_{3+δ}$ single crystal

We have conducted extensive investigations on the magnetization and its dynamical relaxation on a Ba$_{0.66}$K$_{0.32}$BiO$_{3+δ}$ single crystal. It is found that the magnetization relaxation rate is rather weak compared with that in the cuprate superconductors, indicating a higher collective vortex pinning potential (or activation energy), although the intrinsic pinning potential $U_\mathrm{c}$ is weaker. Detailed analysis leads to the following discoveries: (1) A second-peak effect on the magnetization-hysteresis-loop was observed in a very wide temperature region, ranging from 2K to 24K. Its general behavior looks like that in YBa$_2$Cu$_3$O$_7$; (2) Associated with the second peak effect, the magnetization relaxation rate is inversely related to the transient superconducting current density $J_\mathrm{s}$ revealing a quite general and similar mechanism for the second peak effect in many high temperature superconductors; (3) A detailed analysis based on the collective creep model reveals a large glassy exponent $μ$ and a small intrinsic pinning potential $U_\mathrm{c}$; (4) Investigation on the volume pinning force density shows that the data can be scaled to the formula $F_{p}\propto b^p(1-b)^q$ with $p=2.79$ and $q=3.14$, here $b$ is the reduced magnetic field to the irreversible magnetic field. The maximum normalized pinning force density appears near $b\approx0.47$. Finally, a vortex phase diagram is drawn for showing the phase transitions or crossovers between different vortex phases.

preprint2015arXiv

Magnetosphere of a Kerr black hole immersed in magnetized plasma and its perturbative mode structure

This work studies jet-like electromagnetic configurations surrounding a slowly-spinning black-hole immersed in a uniformly magnetized force-free plasma. In the first part of this work, we present a family of stationary solutions that are jet-capable. While these solutions all satisfy the force-free equations and the appropriate boundary conditions, our numerical experiments show a unique relaxed state starting from different initial data, and so one member of the family is likely preferred over the others. In the second part of this work, we analyze the perturbations of this family of jet-like solutions, and show that the perturbative modes exhibit a similar split into the trapped and traveling categories previously found for perturbed Blandford-Znajek solutions. In the eikonal limit, the trapped modes can be identified with the fast magnetosonic waves in the force-free plasma and the traveling waves are essentially the Alfven waves. Moreover, within the scope of our analysis, we have not seen signs of unstable modes at the light-crossing timescale of the system, within which the numerical relaxation process occurs. This observation disfavors mode instability as the selection mechanism for picking out a preferred solution. Consequently, our analytical study is unable to definitively select a particular solution out of the family to serve as the aforementioned preferred final state. This remains an interesting open problem.

preprint2015arXiv

Merging Dirac electrons and correlation effect in the heterostructured Bi2Te3/Fe1+dTe

The topological insulator and strong electronic correlation effect are two important subjects in the frontier studies of modern condensed matter physics. A topological insulator exhibits a unique pair of surface conduction bands with the Dirac dispersion albeit the bulk insulating behaviour. These surface states are protected by the topological order, and thus the spin and momentum of these surface electrons are locked together demonstrating the feature of time reversal invariance. On the other hand, the electronic correlation effect becomes the very base of many novel electronic states, such as high temperature superconductivity, giant magnetoresistance etc. Here we report the discovery of merging the two important components: Dirac electrons and the correlation effect in heterostructured Bi2Te3/Fe1+dTe. By measuring the scanning tunneling spectroscopy on Bi2Te3 thin films (a typical topological insulator) thicker than 6 quintuple layers on top of the Fe1+dTe single crystal (a parent phase of the iron based superconductors FeSe1-xTex), we observed the quantum oscillation of Landau levels of the Dirac electrons and the gapped feature at the Fermi energy due to the correlation effect of Fe1+dTe. Our observation challenges the ordinary understandings and must demonstrate some unexplored territory concerning the combination of topological insulator and strong correlation effect.

preprint2015arXiv

Nodal Superconducting Gap in $β$-FeS

Low temperature specific heat has been measured in superconductor $β$-FeS with T$_c$ = 4.55 K. It is found that the low temperature electronic specific heat C$_e$/T can be fitted to a linear relation in the low temperature region, but fails to be described by an exponential relation as expected by an s-wave gap. We try fittings to the data with different gap structures and find that a model with one or two nodal gaps can fit the data. Under a magnetic field, the field induced specific heat $Δγ$=[C$_e$(H)-C$_e$(0)]/T shows the Volovik relation $Δγ_e(H)\propto \sqrt{H}$, suggesting the presence of nodal gap(s) in this material.

preprint2015arXiv

Observation of a van Hove singularity and implication for strong coupling induced Cooper pairing in KFe2As2

Scanning tunneling spectroscopy (STS) and angle-resolved photoemission spectroscopy (ARPES) have been investigated on single crystal samples of KFe2As2. A van Hove singularity (vHs) has been directly observed just a few meV below the Fermi level E_F of superconducting KFe2As2, which locates in the middle of the principle axes of the first Brillouin zone. The majority of the density-of-states at E_F, mainly contributed by the proximity effect of the saddle point to E_F, is non-gapped in the superconducting state. Our observation of nodal behavior of the momentum area close to the vHs points, while providing consistent explanations to many exotic behaviours previously observed in this material, suggests Cooper pairing induced by a strong coupling mechanism.

preprint2015arXiv

Reply to "On the branching of quasinormal resonances of near-extremal Kerr black holes" by Shahar Hod

In a study of the quasinormal mode frequencies of nearly extremal black holes, we pointed out a bifurcation of the mode spectrum into modes with finite decay and modes with vanishing decay in the extremal limit. We provided analytic and semi-analytic results identifying which families of modes bifurcated, and when modes with finite decay rates exist when approaching the extremal limit. In a recent note (arXiv:1510.05604), Hod suggests that additional modes asymptote to finite decay at extremely high spin parameter, based on past work by Detweiler. We search for these suggested modes and find no evidence of their existence. In addition, we point out an inconsistency in the derivation of the proposed modes, which further indicates that these damped modes do not exist.

preprint2015arXiv

Scrutinizing the double superconducting gaps and strong coupling pairing in (Li1-xFexOH)FeSe

In the iron based superconductors, one of the on-going frontier studies is about the pairing mechanism. The recent interest concerns the high temperature superconductivity and its intimate reason in the monolayer FeSe thin films. The challenge here is how the double superconducting gaps seen by the scanning tunnelling spectroscopy (STS) associate however to only one set of Fermi pockets seen by the angle resolved photoemission spectroscopy (ARPES). The recently discovered (Li1-xFexOH)FeSe phase with Tc=40 K provides a good platform to check the fundamental problems. Here we report the STS study on the (Li1-xFexOH)FeSe single crystals. The STS spectrum clearly indicates the presence of double anisotropic gaps with maximum magnitudes of Delta_1=14.3 meV and Delta_2=8.6 meV, and mimics that of the monolayer FeSe thin film. Further analysis based on the quasiparticle interference (QPI) allows us to rule out the d-wave gap, and for the first time assign the larger (smaller) gap to the outer (inner) hybridized Fermi pockets associating with the dxy (dxz/dyz) orbitals, respectively. The huge value Delta_1/k_BT_c = 8.7 discovered here undoubtedly proves the strong coupling mechanism in the present superconducting system.

preprint2015arXiv

Towards the Laboratory Search for Space-Time Dissipation

It has been speculated that gravity could be an emergent phenomenon, with classical general relativity as an effective, macroscopic theory, valid only for classical systems at large temporal and spatial scales. As in classical continuum dynamics, the existence of underlying microscopic degrees of freedom may lead to macroscopic dissipative behaviors. With the hope that such dissipative behaviors of gravity could be revealed by carefully designed experiments in the laboratory, we consider a phenomenological model that adds dissipations to the gravitational field, much similar to frictions in solids and fluids. Constraints to such dissipative behavior can already be imposed by astrophysical observations and existing experiments, but mostly in lower frequencies. We propose a series of experiments working in higher frequency regimes, which may potentially put more stringent bounds on these models.

preprint2015arXiv

Unsupervised Extraction of Video Highlights Via Robust Recurrent Auto-encoders

With the growing popularity of short-form video sharing platforms such as \em{Instagram} and \em{Vine}, there has been an increasing need for techniques that automatically extract highlights from video. Whereas prior works have approached this problem with heuristic rules or supervised learning, we present an unsupervised learning approach that takes advantage of the abundance of user-edited videos on social media websites such as YouTube. Based on the idea that the most significant sub-events within a video class are commonly present among edited videos while less interesting ones appear less frequently, we identify the significant sub-events via a robust recurrent auto-encoder trained on a collection of user-edited videos queried for each particular class of interest. The auto-encoder is trained using a proposed shrinking exponential loss function that makes it robust to noise in the web-crawled training data, and is configured with bidirectional long short term memory (LSTM)~\cite{LSTM:97} cells to better model the temporal structure of highlight segments. Different from supervised techniques, our method can infer highlights using only a set of downloaded edited videos, without also needing their pre-edited counterparts which are rarely available online. Extensive experiments indicate the promise of our proposed solution in this challenging unsupervised settin

preprint2014arXiv

A z ~ 5.7 Lyα Emission Line with an Ultra Broad Red Wing

Using Lyα emission line as a tracer of high redshift star forming galaxies, hundreds of Lyα emission line galaxies (LAEs) at z > 5 have been detected. These LAEs are considered to be low mass young galaxies, critical to the reionization of the universe and the metal enrichment of circumgalactic medium (CGM) and intergalactic medium (IGM). It is assumed that outflows in LAEs can help ionizing photons and Lyα photons escape out of galaxies. However we still know little about the outflows in high redshifts LAEs due to observational difficulties, especially at redshift > 5. Models of Lyα radiative transfer predict asymmetric Lyα line profiles with broad red wing in LAEs with outflows. Here we report a z ~ 5.7 Lyα emission line with a broad red wing extending to > 1000 km/s relative to the peak of Lyα line, which has been detected in only a couple of z > 5 LAEs till now. If the broad red wing is ascribed to gas outflow instead of AGN activity, the outflow velocity could be larger than the escape velocity (~ 500 km/s) of typical halo mass of z ~ 5.7 LAEs, being consistent with the picture that outflows in LAEs disperse metals to CGM and IGM.

preprint2014arXiv

Anisotropic Superconducting Gap and Elongated Vortices with Caroli-De-Gennes-Matricon States in the New Superconductor Ta4Pd3Te16

The superconducting state is formed by the condensation of a large number of Cooper pairs. The normal state electronic properties can give significant influence on the superconducting state. For usual type-II superconductors, the vortices are cylinder like with a round cross-section. For many two dimensional superconductors, such as Cuprates, 2H-NbSe$_2$ etc., albeit the in-plane anisotropy, the vortices generally have a round shape. In this paper we report results based on the scanning tunneling microscopy/spectroscopy measurements on a newly discovered superconductor Ta$_4$Pd$_3$Te$_{16}$. The chain like conducting channels of PdTe$_2$ in Ta$_4$Pd$_3$Te$_{16}$ make a significant anisotropy of the in-plane Fermi velocity. We suggest at least one anisotropic superconducting gap with gap minima or possible node exists in this multiband system. In addition, elongated vortices are observed with an anisotropy of $ξ_{\parallel b}/ξ_{\perp b}\approx 2.5$. Clear Caroli-de-Gennes-Matricon states are also observed within the vortex cores. Our results will initiate the study on the elongated vortices and superconducting mechanism in the new superconductor Ta$_4$Pd$_3$Te$_{16}$.

preprint2014arXiv

Are X-ray Emitting Coronae around Super Massive Black Holes Outflowing?

Hard X-ray emission in radio-quiet active galactic nuclei (AGNs) is believed to be produced via inverse Compton scattering by hot and compact coronae near the super massive black hole. However the origin and physical properties of the coronae, including geometry, kinematics and dynamics, yet remain poorly known. In this work, taking [OIV] 25.89um emission line as an isotropic indicator of AGN's intrinsic luminosity, we compare the intrinsic corona X-ray emission between Seyfert 1 and Compton-thin Seyfert 2 galaxies, which are viewed at different inclinations according to the unification scheme. We compile a sample of 130 Compton-thin Seyfert galaxies with both [OIV] 25.89um line luminosities measured with Spitzer-IRS and X-ray spectra observed by XMM-Newton, Chandra, Suzaku or Swift. Known radio-loud sources are excluded. We fit the X-ray spectra to obtain the absorption-corrected 2-10 keV continuum luminosities. We find that Seyfert 1 galaxies are intrinsically brighter in intrinsic 2-10 keV emission by a factor of 2.8+0.5-0.4 (2.2+0.9-0.3 in Swift-BAT 14-195 keV emission), comparing with Compton-thin Seyfert 2 galaxies. The Seyfert 1 and Compton-thin Seyfert 2 galaxies follow a statistically identical correlation between the absorption-corrected 2-10 keV luminosity and the 14-195 keV luminosity, indicating that our absorption correction to the 2-10 keV flux is sufficient. The difference in X-ray emission between the two populations is thus unlikely due to X-ray absorption, and instead implies an intrinsic anisotropy in the corona X-ray emission. This striking anisotropy of X-ray emission can be explained by a bipolar outflowing corona with a bulk velocity of ~0.3-0.5c. This would provide a natural link between the so-called coronae and weak jets in these systems. Other consequences of outflowing coronae are also discussed.

preprint2014arXiv

On the Anisotropy of Nuclei Mid-Infrared Radiation in Nearby Active Galactic Nuclei

In the center of active galactic nuclei (AGN), the dusty torus absorb the radiation from the central engine and re-emit in mid-infrared (MIR). Observations have detected moderate anisotropy in the dust MIR emission, in the way that type 1 AGNs (type1s) are mildly brighter in MIR comparing with type 2 sources (type2s). However, type1s and type2s were found to follow statistically the same tight MIR -- hard X-ray correlation, suggesting the MIR emission is highly isotropic assuming the hard X-ray radiation is inclination independent. We argue this discrepancy could be solved considering the hard X-ray emission in AGN is also mildly anisotropic as we recently discovered. To verify this diagram, we compare the sub-arcsecond 12μm flux densities of type1s and type2s using [OIV]$λ$25.89μm emission line as an isotropic luminosity indicator. We find that on average type1s are brighter in nuclei 12μm radiation by a factor of $2.6 \pm 0.6$ than type2s at given [OIV]$λ$25.89μm luminosities, confirming the mild anisotropy of the nuclei 12μm emission. We show that the anisotropy of the 12μm emission we detected is in good agreement with radiative transfer models of clumpy torus. The fact that type 1 and type 2 AGNs follow the same tight MIR -- hard X-ray correlation instead supports that both the MIR and hard X-ray emission in AGNs are mildly anisotropic.

preprint2014arXiv

Quantum Limits of Interferometer Topologies for Gravitational Radiation Detection

In order to expand the astrophysical reach of gravitational wave detectors, several interferometer topologies have been proposed to evade the thermodynamic and quantum mechanical limits in future detectors. In this work, we make a systematic comparison among them by considering their sensitivities and complexities. We numerically optimize their sensitivities by introducing a cost function that tries to maximize the broadband improvement over the sensitivity of current detectors. We find that frequency-dependent squeezed-light injection with a hundred-meter scale filter cavity yields a good broadband sensitivity, with low complexity, and good robustness against optical loss. This study gives us a guideline for the near-term experimental research programs in enhancing the performance of future gravitational-wave detectors.

preprint2014arXiv

Quasinormal Modes Beyond Kerr

The quasinormal modes (QNMs) of a black hole spacetime are the free, decaying oscillations of the spacetime, and are well understood in the case of Kerr black holes. We discuss a method for computing the QNMs of spacetimes which are slightly deformed from Kerr. We mention two example applications: the parametric, turbulent instability of scalar fields on a background which includes a gravitational QNM, and the shifts to the QNM frequencies of Kerr when the black hole is weakly charged. This method may be of use in studies of black holes which are deformed by external fields or are solutions to alternative theories of gravity.

preprint2014arXiv

Simultaneous Vanishing of the Nematic Electronic State and the Structural Orthorhombicity in NaFe$_{1-x}$Co$_x$As Single Crystals

We have carried out in-plane resistivity measurements under a uniaxial pressure in NaFe$_{1-x}$Co$_x$As single crystals. A clear distinction of the in-plane resistivity $ρ_a$ and $ρ_b$ with the uniaxial pressure along $b$-axis was discovered in the parent and underdoped regime with the doping level up to about x=0.025$\pm$0.002. From the deviating point of $ρ_a$ and $ρ_b$, and the unique kinky structure of resistivity together with the published data we determined the temperatures for the nematic, structural and antiferromagnetic transitions. It is clearly shown that the nematic electronic state vanishes simultaneously with the structural transition. The antiferromagnetic state disappears however at a lower doping level. Our results, in combination with the data in BaFe$_{2-x}$Co$_x$As$_2$, indicate a close relationship between nematicity and superconductivity.

preprint2014arXiv

Stability of Force-Free Magnetospheres

We analyze the dynamical evolution of a perturbed force-free magnetosphere of a rotating black hole, which is described by the Blandford-Znajek solution in the stationary limit. We find that the electromagnetic field perturbations can be classified into two categories: "trapped modes" and "traveling waves". The trapped modes are analogous to the vacuum (without plasma) electromagnetic quasinormal modes in rotating black hole spacetimes, but with different eigenfrequencies and wave functions, due to their coupling with the background electromagnetic field and current. The traveling waves propagate freely to infinity or the black hole horizon along specific null directions, and they are closely related to the no-scattering Poynting flux solutions discovered by Brennan, Gralla and Jacobson. Our results suggest that the Blandford-Znajek solution is mode stable, and more importantly we expect this study to illuminate the dynamical behavior of force-free magnetospheres as well as to shed light on the path to new exact solutions.

preprint2014arXiv

Strong and nonmonotonic temperature dependence of Hall coefficient in superconducting K$_x$Fe$_{2-y}$Se$_2$ single crystals

In-plane resistivity, magnetoresistance and Hall effect measurements have been conducted on quenched K$_x$Fe$_{2-y}$Se$_2$ single crystals in order to analysis the normal-state transport properties. It is found that the Kohler's rule is well obeyed below about 80 K, but clearly violated above 80 K. Measurements of the Hall coefficient reveal a strong but non-monotonic temperature dependence with a maximum at about 80 K, in contrast to any other FeAs-based superconductors. With the two-band model analysis on the Hall coefficient, we conclude that a gap may open below 65 K. The data above 65 K are interpreted as a temperature induced crossover from a metallic state at a low temperature to an orbital-selective Mott phase at a high temperature. This is consistent with the recent data of angle resolved photoemission spectroscopy. These results call for a refined theoretical understanding, especially when the hole pockets are absent or become trivial in K$_x$Fe$_{2-y}$Se$_2$ superconductors.

preprint2014arXiv

The Quasinormal Modes of Weakly Charged Kerr-Newman Spacetimes

The resonant mode spectrum of the Kerr-Newman spacetime is presently unknown. These modes, called the quasinormal modes, play a central role in determining the stability of Kerr-Newman black holes and their response to perturbations. We present a new formalism, generalized from time-independent perturbation theory in quantum mechanics, for calculating the quasinormal mode frequencies of weakly charged Kerr-Newman spacetimes of arbitrary spin. Our method makes use of an original technique for applying perturbation theory to zeroth-order solutions that are not square- integrable, and it can be applied to other problems in theoretical physics. The new formalism reveals no unstable modes, which together with previous results in the slow-rotation limit strongly indicates the modal stability of the Kerr-Newman spacetime. Our techniques and results are of interest in the areas of holographic duality, foundational problems in General Relativity, and possibly in astrophysical systems.

preprint2014arXiv

The scalar Green function of the Kerr spacetime

In this paper we study the scalar Green function in the Kerr spacetime using WKB methods. The Green function can be expressed by Fourier-transforming to its frequency-domain counterpart, and with the help of complex analysis it can be divided into parts: 1) the "direct part" which propagates on the light cone and dominates at very early times; 2) the "quasinormal-mode part" which represents the waves traveling around the photon sphere, and is important at early and intermediate times; 3) the "tail part" which is due to scattering by the Coulomb-type potential and becomes more important at later times. We focus on the "quasinormal-mode part" of the Green function and derive an approximate analytical formula for it using WKB techniques. This approximate Green function diverges at points that are connected by null geodesics, and recovers the four-fold singular structure of Green functions that are seen in Schwarzschild and other spacetimes. It also carries unique signatures of the Kerr spacetime such as frame-dragging. Along the way, we also derive approximate quasinormal mode wavefunctions and expressions for the black hole excitation factors in the Kerr spacetime. We expect this work to benefit the understanding of both wave propagation and the problem of self-force in the Kerr spacetime.

preprint2014arXiv

The spectroscopic signature of Kondo screening on single adatoms in Na(Fe0.96Co0.03Mn0.01)As

The electronic states of surface adatoms in Na(Fe0.96Co0.03Mn0.01)As have been studied by low temperature scanning tunneling spectroscopy. The spectra recorded on the adatoms display both superconducting coherence peaks and an asymmetric resonance in a larger energy scale. The Fano-type line shape of the spectra points towards a possible Kondo effect at play. The apparent energy position of the resonance peak shifts about 5 meV to the Fermi level when measured across the critical temperature, supporting that the Bogoliubov quasiparticle is responsible for the Kondo screening in the superconducting state. The tunneling spectra do not show the subgap bound states, which is explained as the weak pair breaking effect given by the weak and broad scattering potential after the Kondo screening.

preprint2014arXiv

Turbulent Black Holes

We show that rapidly-spinning black holes can display turbulent gravitational behavior which is mediated by a new type of parametric instability. This instability transfers energy from higher temporal and azimuthal spatial frequencies to lower frequencies--- a phenomenon reminiscent of the inverse energy cascade displayed by 2+1-dimensional turbulent fluids. Our finding reveals a path towards gravitational turbulence for perturbations of rapidly-spinning black holes, and provides the first evidence for gravitational turbulence in an asymptotically flat spacetime. Interestingly, this finding predicts observable gravitational wave signatures from such phenomena in black hole binaries with high spins and gives a gravitational description of turbulence relevant to the fluid-gravity duality.

preprint2014arXiv

Wavefront twisting by rotating black holes: orbital angular momentum generation and phase coherent detection

In this paper we study wave propagation and scattering near a rotating black hole. In particular, we assume a coherent emission source near the black hole and investigate the wavefront distortion as seen by a distant observer. Near the observer, the propagating wave can be decomposed using the Laguerre-Gaussian mode basis and its wavefront distortion can be characterized by the decomposition coefficient. We find that this decomposition spectrum is symmetric for wave sources located near a Schwarzschild black hole, but is generically asymmetric if the host black hole is rotating. The spectrum asymmetry, or the net orbital angular momentum carried by the wave, is intimately related to the black hole spin and mass, the wave frequency and the locations of the source and the observer. We present semi-analytical expressions and numerical results for these parameter-dependences. If the radiation is temporally coherent, our results show that the secondary images of the source can be almost as bright as its primary image. In the case of temporally-incoherent radiation, we show that the non-fundamental spectrum components could be resolved by spatially-separated telescopes, although that would be degenerate with the telescope direction. Finally, our results suggest that the black-hole-induced spectrum asymmetry is generally too weak to be observed in radio astronomy, even if the observer were located near a caustic point.

preprint2013arXiv

Close relationship between superconductivity and the bosonic mode in Ba0.6K0.4Fe2As2 and Na(Fe0.975Co0.025)As

Since the discovery of high temperature superconductivity in the iron pnictides and chalcogenides in early 2008, a central issue has been the microscopic origin of the superconducting pairing. Although previous experiments suggest that the pairing may be induced by exchanging the antiferromagnetic spin fluctuations and the superconducting order parameter has opposite signs in the electron and hole pockets as predicted by the S+- pairing model, it remains unclear whether there is a bosonic mode from the tunneling spectrum which has a close and universal relationship with superconductivity as well as the spin excitation. In this paper, based on the measurements of scanning tunneling spectroscopy, we show the clear evidence of a bosonic mode with the energy identical to that of the neutron spin resonance in two completely different systems Ba0.6K0.4Fe2As2 and Na(Fe0.975Co0.025)As with different superconducting transition temperatures. In both samples, the superconducting coherence peaks and the mode feature vanish simultaneously inside the vortex core or above Tc, indicating a close relationship between superconductivity and the bosonic mode. Our data also demonstrate a universal ratio between the mode energy and superconducting transition temperature, that is [mode energy]/kBTc ~ 4.3, which underlines the unconventional mechanism of superconductivity in the iron pnictide superconductors.

preprint2013arXiv

Impurity effect and suppression to superconductivity in Na(Fe$_{0.97-x}$Co$_{0.03}$T$_x$)As (T=Cu, Mn)

We report the successful growth and the impurity scattering effect of single crystals of Na(Fe$_{0.97-x}$Co$_{0.03}$T$_x$)As (T=Cu, Mn). The temperature dependence of DC magnetization at high magnetic fields is measured for different concentrations of Cu and Mn. Detailed analysis based on the Curie-Weiss law indicates that the Cu doping weakens the average magnetic moments, while doping Mn enhances the local magnetic moments greatly, suggesting that the former may be non- or very weak magnetic impurities, and the latter give rise to magnetic impurities. However, it is found that both doping Cu and Mn will enhance the residual resistivity and suppress the superconductivity at the same rate in the low doping region, being consistent with the prediction of the S$^{\pm}$ model. For the Cu-doped system, the superconductivity is suppressed completely at a residual resistivity $ρ_0$ = 0.87 m$Ω$ cm at which a strong localization effect is observed. However, in the case of Mn doping, the behavior of suppression to \emph{T}$_{c}$ changes from a fast speed to a slow one and keeps superconductive even up to a residual resistivity of 2.86 m$Ω$ cm. Clearly the magnetic Mn impurities are even not as detrimental as the non- or very weak magnetic Cu impurities to superconductivity in the high doping regime.

preprint2013arXiv

In-Gap Quasiparticle Excitations Induced by Non-Magnetic Cu Impurities in Na(Fe0.96Co0.03Cu0.01)As Revealed by Scanning Tunneling Spectroscopy

The pairing mechanism in the iron pnictides remains unresolved yet. One of the central issues is the structure of the superconducting order parameter which classifies the community into two different and highly disputed camps. On one hand the picture of pairing based on the magnetic origin predicts a sign reversal gap on the electron and hole Fermi pockets, leading to the S+- pairing. On the other hand, a more conventional S++ pairing gap was suggested based on the phonon or orbital fluctuation mediated pairing. In the superconducting state, the impurities may generate a unique pattern of local density of states in space and energy, which are regarded as the fingerprints for checking the structure of the pairing gap. In this study, we successfully identified the non-magnetic and magnetic impurities in Na(Fe0.97-xCo0.03Tx)As (T=Cu, Mn) and investigated the spatial resolved scanning tunneling spectroscopy. We present clear evidence of the in-gap quasiparticle states induced by the nonmagnetic Cu impurities, giving decisive evidence of the S+- pairing. This is corroborated by the consistency between the experimental data and the first-principles calculations based on the S+- pairing gap with a scalar scattering potential.

preprint2013arXiv

Macroscopic Quantum Mechanics in a Classical Spacetime

We apply the many-particle Schrödinger-Newton equation, which describes the co-evolution of an many-particle quantum wave function and a classical space-time geometry, to macroscopic mechanical objects. By averaging over motions of the objects' internal degrees of freedom, we obtain an effective Schrödinger-Newton equation for their centers of mass, which are degrees of freedom that can be monitored and manipulated at the quantum mechanical levels by state-of-the-art optoemchanics experiments. For a single macroscopic object moving quantum mechanically within a harmonic potential well, we found that its quantum uncertainty evolves in a different frequency from its classical eigenfrequency --- with a difference that depends on the internal structure of the object, and can be observable using current technology. For several objects, the Schrödinger-Newton equation predicts semiclassical motions just like Newtonian physics, yet they do not allow quantum uncertainty to be transferred from one object to another through gravity.

preprint2013arXiv

Quasinormal modes of nearly extremal Kerr spacetimes: spectrum bifurcation and power-law ringdown

We provide an in-depth investigation of quasinormal-mode oscillations of Kerr black holes with nearly extremal angular momenta. We first discuss in greater detail the two distinct types of quasinormal mode frequencies presented in a recent paper (arXiv:1212.3271). One set of modes, that we call "zero-damping modes", has vanishing imaginary part in the extremal limit, and exists for all corotating perturbations (i.e., modes with azimuthal index m being nonnegative). The other set (the "damped modes") retains a finite decay rate even for extremal Kerr black holes, and exists only for a subset of corotating modes. As the angular momentum approaches its extremal value, the frequency spectrum bifurcates into these two distinct branches when both types of modes are present. We discuss the physical reason for the mode branching by developing and using a bound-state formulation for the perturbations of generic Kerr black holes. We also numerically explore the specific case of the fundamental l=2 modes, which have the greatest astrophysical interest. Using the results of these investigations, we compute the quasinormal mode response of a nearly extremal Kerr black hole to perturbations. We show that many superimposed overtones result in a slow power-law decay of the quasinormal ringing at early times, which later gives way to exponential decay. This exceptional early-time power-law decay implies that the ringdown phase is long-lived for black holes with large angular momentum, which could provide a promising strong source for gravitational-wave detectors.

preprint2013arXiv

UM 625 Revisited: Multiwavelength Study of A Seyfert 1 Galaxy with a Low-mass Black Hole

UM 625, previously identified as a narrow-line active galactic nucleus (AGN), actually exhibits broad \ha\ and \hb\ lines whose width and luminosity indicate a low black hole mass of $1.6 \times 10^6$ \msun. We present a detailed multiwavelength study of the nuclear and host galaxy properties of UM 625. Analysis of \chandra\ and \xmm\ observations suggests that this system contains a heavily absorbed and intrinsically X-ray weak ($\aox=-1.72$) nucleus. Although not strong enough to qualify as radio-loud, UM 625 does belong to a minority of low-mass AGNs detected in the radio. The broad-band spectral energy distribution constrains the bolometric luminosity to $\lbol\approx(0.5-3)\times10^{43}$ \lum\ and $\lratio\approx0.02-0.15$. A comprehensive analysis of Sloan Digital Sky Survey and {\it Hubble Space Telescope}\ images shows that UM 625 is a nearly face-on S0 galaxy with a prominent, relatively blue pseudobulge (\sersic\ index $n = 1.60$) that accounts for $\sim$60% of the total light in the $R$ band. The extended disk is featureless, but the central $\sim150-400$ pc contains a conspicuous semi-ring of bright, blue star-forming knots, whose integrated ultraviolet luminosity suggests a star formation rate of $\sim$0.3 \msun yr$^{-1}$. The mass of the central black hole roughly agrees with the value predicted from its bulge velocity dispersion but is significantly lower than that expected from its bulge luminosity.

preprint2013arXiv

Unexpected Giant Superconducting Fluctuation and Anomalous Semiconducting Normal State in NdO1-xFxBi1-yS2 Single Crystals

The BiS2-based superconductors were discovered recently. The superconductivity has been proved by many other groups. Since the previous experiments were all done on polycrystalline samples, therefore there remains a concern whether the superconductivity is really derived from the materials intrinsically or from some secondary phases. Experiments on single crystals are highly desired. In this paper, we report the successful growth of the NdO1-xFxBi1-yS2 single crystals. Resistive and magnetic measurements reveal that the bulk superconducting transition occurs at about 5 K, while an unexpected giant superconducting fluctuation appears at temperatures as high as 2-4 kBTC. Analysis based on the anisotropic Ginzbaug-Landau theory gives an anisotropy of 30-45. Two gap features with magnitudes of about 3.5+-0.3 meV and 7.5+-1 meV were observed by scanning tunneling spectroscopy. The smaller gap is associated with the bulk superconducting transition at about 5 K yielding a huge ratio 2Delta_s/kBTc =16.8, the larger gap remains up to about 26 K. The normal state recovered by applying a high magnetic field shows an anomalous semiconducting behavior. All these suggest that the superconductivity in this newly discovered superconductor cannot be formatted into the BCS theory.

preprint2012arXiv

Branching of quasinormal modes for nearly extremal Kerr black holes

We show that nearly extremal Kerr black holes have two distinct sets of quasinormal modes, which we call zero-damping modes (ZDMs) and damped modes (DMs). The ZDMs exist for all harmonic indices $l$ and $m \ge 0$, and their frequencies cluster onto the real axis in the extremal limit. The DMs have nonzero damping for all black hole spins; they exist for all counterrotating modes ($m<0$) and for corotating modes with $0\leq μ\lesssim μ_c=0.74$ (in the eikonal limit), where $μ\equiv m/(l+1/2)$. When the two families coexist, ZDMs and DMs merge to form a single set of quasinormal modes as the black hole spin decreases. Using the effective potential for perturbations of the Kerr spacetime, we give intuitive explanations for the absence of DMs in certain areas of the spectrum and for the branching of the spectrum into ZDMs and DMs at large spins.

preprint2012arXiv

Brownian Thermal Noise in Multilayer Coated Mirrors

We analyze the Brownian thermal noise of a multi-layer dielectric coating, used in high-precision optical measurements including interferometric gravitational-wave detectors. We assume the coating material to be isotropic, and therefore study thermal noises arising from shear and bulk losses of the coating materials. We show that coating noise arises not only from layer thickness fluctuations, but also from fluctuations of the interface between the coating and substrate, driven by internal fluctuating stresses of the coating. In addition, the non-zero photoeleastic coefficients of the thin films modifies the influence of the thermal noise on the laser field. The thickness fluctuations of different layers are statistically independent, however, there exists a finite coherence between layers and the substrate-coating interface. Taking into account uncertainties in material parameters, we show that significant uncertainties still exist in estimating coating Brownian noise.

preprint2012arXiv

Distinct behaviors of suppression to superconductivity in $LaRu_3Si_2$ induced by Fe and Co dopants

In the superconductor LaRu$_3$Si$_2$ with the Kagome lattice of Ru, we have successfully doped the Ru with Fe and Co atoms. Contrasting behaviors of suppression to superconductivity is discovered between the Fe and the Co dopants: Fe-impurities can suppress the superconductivity completely at a doping level of only 3%, while the superconductivity is suppressed slowly with the Co dopants. A systematic magnetization measurements indicate that the doped Fe impurities lead to spin-polarized electrons yielding magnetic moments with the magnitude of 1.6 $μ_B$\ per Fe, while the electrons given by the Co dopants have the same density of states for spin-up and spin-down leading to much weaker magnetic moments. It is the strong local magnetic moments given by the Fe-dopants that suppress the superconductivity. The band structure calculation further supports this conclusion.

preprint2012arXiv

Evidence of a spin resonance mode in the iron-based superconductor Ba$_{0.6}$K$_{0.4}$Fe$_{2}$As$_{2}$ from scanning tunneling spectroscopy

We used high-resolution scanning tunneling spectroscopy to study the hole-doped iron pnictide superconductor Ba$_{0.6}$K$_{0.4}$Fe$_{2}$As$_{2}$ ($T_c=38$ K). Features of a bosonic excitation (mode) are observed in the measured quasiparticle density of states. The bosonic features are intimately associated with the superconducting order parameter and have a mode energy of $\sim$14 meV, similar to the spin resonance measured by inelastic neutron scattering. These results indicate a strong electron-spin excitation coupling in iron pictnide superconductors, similar to that in high-$T_c$ copper oxide superconductors.

preprint2012arXiv

First-Order Perturbative Hamiltonian Equations of Motion for a Point Particle Orbiting a Schwarzschild Black Hole

We formulate a spherical harmonically decomposed 1+1 scheme to self-consistently evolve the trajectory of a point particle and its gravitational metric perturbation to a Schwarzschild background spacetime. Following the work of Moncrief, we write down an action for perturbations in space-time geometry, combine that with the action for a point-particle, and then obtain Hamiltonian equations of motion for metric perturbations, the particle's coordinates, as well as their canonical momenta. Hamiltonian equations for the metric-perturbation and their conjugate momenta reduce to Zerilli-Moncrief and Regge-Wheeler master equations with source terms, which are gauge invariant, plus auxiliary equations that specify gauge. Hamiltonian equations for the particle, on the other hand, now include effect of metric perturbations - with these new terms derived from the same interaction Hamiltonian that had lead to those well-known source terms. In this way, space-time geometry and particle motion can be evolved in a self-consistent manner, in principle in any gauge. However, the point-particle nature of our source requires regularization, and we outline how the Detweiler-Whiting approach can be applied. In this approach, a singular field can be obtained using Hadamard decomposition of the Green's function and the regular field, which needs to be evolved numerically, is the result of subtracting the singular field from the total metric perturbation. In principle, any gauge that has the singular-regular field decomposition is suitable for our self-consistent scheme. In reality, however, this freedom is only possible if our singular field has a high enough level of smoothness. In the case of Lorenz gauge, for each l and m, we have 2 wave equations to evolve gauge invariant quantities and 8 first order differential equations to fix the gauge and determine the metric components.

preprint2012arXiv

Multi-Band Exotic Superconductivity in the New Superconductor Bi4O4S3

Resistivity, Hall effect and magnetization have been investigated on the new superconductor Bi4O4S3. A weak insulating behavior has been induced in the normal state when the superconductivity is suppressed. Hall effect measurements illustrate clearly a multiband feature dominated by electron charge carriers, which is further supported by the magnetoresistance data. Interestingly, a kink appears on the temperature dependence of resistivity at about 4 K at all high magnetic fields when the bulk superconductivity is completely suppressed. This kink can be well traced back to the upper critical field Hc2(T) in the low field region, and is explained as the possible evidence of residual Cooper pairs on the one dimensional chains.

preprint2012arXiv

Open quantum dynamics of single-photon optomechanical devices

We study the quantum dynamics of a Michelson interferometer with Fabry-Perot cavity arms and one movable end mirror, and driven by a single photon --- an optomechanical device previously studied by Marshall et al. as a device that searches for gravity decoherence. We obtain an exact analytical solution for the system's quantum mechanical equations of motion, including details about the exchange of the single photon between the cavity mode and the external continuum. The resulting time evolution of the interferometer's fringe visibility displays interesting new features when the incoming photon's frequency uncertainty is narrower or comparable to the cavity's line width --- only in the limiting case of much broader-band photon does the result return to that of Marshall et al., but in this case the photon is not very likely to enter the cavity and interact with the mirror, making the experiment less efficient and more susceptible to imperfections. In addition, we show that in the strong-coupling regime, by engineering the incoming photon's wave function, it is possible to prepare the movable mirror into an arbitrary quantum state of a multi-dimensional Hilbert space.

preprint2012arXiv

Quantum back-action in measurements of zero-point mechanical oscillations

Measurement-induced back action, a direct consequence of the Heisenberg Uncertainty Principle, is the defining feature of quantum measurements. We use quantum measurement theory to analyze the recent experiment of Safavi-Naeini et al. [Phys. Rev. Lett. {\bf 108}, 033602 (2012)], and show that results of this experiment not only characterize the zero-point fluctuation of a near-ground-state nanomechanical oscillator, but also demonstrate the existence of quantum back-action noise --- through correlations that exist between sensing noise and back-action noise. These correlations arise from the quantum coherence between the mechanical oscillator and the measuring device, which build up during the measurement process, and are key to improving sensitivities beyond the Standard Quantum Limit.

preprint2012arXiv

Quasinormal-mode spectrum of Kerr black holes and its geometric interpretation

There is a well-known, intuitive geometric correspondence between high-frequency QNMs of Schwarzschild black holes and null geodesics that reside on the light-ring : the real part of the mode's frequency relates to the geodesic's orbital frequency, and the imaginary part of the frequency corresponds to the Lyapunov exponent of the orbit. For slowly rotating black holes, the QNM real frequency is a linear combination of a the orbit's precessional and orbital frequencies, but the correspondence is otherwise unchanged. In this paper, we find a relationship between the QNM frequencies of Kerr black holes of arbitrary (astrophysical) spins and general spherical photon orbits, which is analogous to the relationship for slowly rotating holes. To derive this result, we first use the WKB approximation to compute accurate algebraic expressions for large-l QNM frequencies. Comparing our WKB calculation to the leading-order, geometric-optics approximation to scalar-wave propagation in the Kerr spacetime, we then draw a correspondence between the real parts of the parameters of a QNM and the conserved quantities of spherical photon orbits. At next-to-leading order in this comparison, we relate the imaginary parts of the QNM parameters to coefficients that modify the amplitude of the scalar wave. With this correspondence, we find a geometric interpretation to two features of the QNM spectrum of Kerr black holes: First, for Kerr holes rotating near the maximal rate, a large number of modes have nearly zero damping; we connect this characteristic to the fact that a large number of spherical photon orbits approach the horizon in this limit. Second, for black holes of any spins, the frequencies of specific sets of modes are degenerate; we find that this feature arises when the spherical photon orbits corresponding to these modes form closed (as opposed to ergodically winding) curves.

preprint2012arXiv

Transport properties, upper critical field and anisotropy of Ba(Fe$_{0.75}$Ru$_{0.25}$)$_2$As$_2$ single crystals

The temperature and angle dependent resistivity of Ba(Fe$_{0.75}$Ru$_{0.25}$)$_2$As$_2$ single crystals were measured in magnetic fields up to 14 T. The temperature dependent resistivity with the magnetic field aligned parallel to c-axis and ab-planes allow us to derive the slope of d$H_{c2}^{ab}/dT$ and d$H_{c2}^c/dT$ near $T_c$ yielding an anisotropy ratio $Γ=dH_{c2}^{ab}/dT/dH_{c2}^c/dT \approx$ 2. By scaling the curves of resistivity vs. angle measured at a fixed temperature but different magnetic fields within the framework of the anisotropic Ginzburg-Landau theory, we obtained the anisotropy in an alternative way. Again we found that the anisotropy $(m_c/m_{ab})^{1/2}$ was close to 2. This value is very similar to that in Ba$_{0.6}$K$_{0.4}$Fe$_2$As$_2$ (K-doped Ba122) and Ba(Fe$_{0.92}$Co$_{0.08}$)$_2$As$_2$ (Co-doped Ba122). This suggests that the 3D warping effect of the Fermi surface in Ru-doped samples may not be stronger than that in the K-doped or Co-doped Ba122 samples, therefore the possible nodes appearing in Ru-doped samples cannot be ascribed to the 3D warping effect of the Fermi surface.

preprint2012arXiv

Unexpected weak spatial variation of local density of sates induced by individual Co impurity atoms in Na(Fe{1-x}Cox)As as revealed by scanning tunneling spectroscopy

We use spatially resolved scanning tunneling spectroscopy in Na(Fe{1-x}Cox)As to investigate the impurity effect induced by Co dopants. The Co impurities are successfully identified, and the spatial distributions of local density of state at different energies around these impurities are investigated. It is found that the spectrum shows negligible spatial variation at different positions near the Co impurity, although there is a continuum of the in-gap states which lifts the zero-bias conductance to a finite value. Our results put constraints on the S+- and S++ models and sharpen the debate on the role of scattering potentials induced by the Co dopants.

preprint2012arXiv

Vortex images on Ba{1-x}KxFe2As2 observed directly by the magnetic force microscopy

The vortex states on optimally doped Ba0.6K0.4Fe2As2 and underdoped Ba0.77K0.23Fe2As2 single crystals are imaged by magnetic force microscopy at various magnetic fields below 100 Oe. Local triangular vortex clusters are observed in optimally doped samples. The vortices are more ordered than those in Ba(Fe{1-x}Co{x})2As2, and the calculated pinning force per unit length is about 1 order of magnitude weaker than that in optimally Co-doped 122 at the same magnetic field, indicating that the Co doping at the Fe sites induces stronger pinning. The proportion of six-neighbored vortices to the total amount increases quickly with increasing magnetic field, and the estimated value reaches 100% at several tesla. Vortex chains are also found in some local regions, which enhance the pinning force as well as the critical current density. Lines of vortex chains are observed in underdoped samples, and they may have originated from the strong pinning near the twin boundaries arising from the structural transition.

preprint2011arXiv

A unique X-ray unabsorbed Seyfert 2 galaxy IRAS F01475-0740

X-ray unabsorbed Seyfert 2 galaxies appear to have X-ray absorption column densities that are too low (NH < 10^22 cm-2) to explain the absence of broad emission lines in their optical spectra, challenging the standard AGN unification model. In this paper we report Suzaku exposure on the X-ray unabsorbed Seyfert 2 galaxy IRAS F01475-0740, in which a hidden broad line region was detected through spectropolarimetric observation. The X-ray data show rapid and significant variations on time scales down to 5 ks, indicating that we are viewing its central engine directly. A newly obtained optical spectrum and previous optical/X-ray data suggest that state transition is unlikely in this source. These make IRAS F01475-0740 a very peculiar X-ray unabsorbed Seyfert 2 galaxy which can only be explained by absorption from materials with abnormally high dust-to-gas ratio (by a factor of > 4 larger than Galactic). This is in contrast to most AGNs, which typically show dust-to-gas ratios 3 - 100 times lower than the Galactic.

preprint2011arXiv

Anomalous Properties in the Normal and Superconducting States of LaRu$_3$Si$_2$

Superconductivity in LaRu$_3$Si$_2$ with the honeycomb structure of Ru atoms has been investigated. It is found that the normal state specific heat C/T exhibits a deviation from the Debye model down to the lowest temperature. A relation $C/T = γ_n+βT^2-ATlnT$ which concerns the electron correlations can fit the data very well. The suppression to the superconductivity by the magnetic field is not the mean-field like, which is associated well with the observation of strong superconducting fluctuations. The field dependence of the induced quasiparticle density of states measured by the low temperature specific heat shows a non-linear feature, indicating the significant contributions given by the delocalized quasiparticles.

preprint2011arXiv

Gap Anisotropy in Iron-Based Superconductors: A Point-Contact Andreev Reflection Study of BaFe$_{2-x}$Ni$_{x}$As$_2$ Single Crystals

We report a systematic investigation on c-axis point-contact Andreev reflection (PCAR) in BaFe$_{2-x}$Ni$_x$As$_2$ superconducting single crystals from underdoped to overdoped regions (0.075 $\leq x\leq 0.15$). At optimal doping ($x=0.1$) the PCAR spectrum feature the structures of two superconducting gap and electron-boson coupling mode. In the $s\pm$ scenario, quantitative analysis using a generalized Blonder-Tinkham-Klapwijk (BTK) formalism with two gaps: one isotropic and another angle dependent, suggest a nodeless state in strong-coupling limit with gap minima on the Fermi surfaces. Upon crossing above the optimal doping ($x > 0.1$), the PCAR spectrum show an in-gap sharp narrow peak at low bias, in contrast to the case of underdoped samples ($x < 0.1$), signaling the onset of deepened gap minima or nodes in the superconducting gap. This result provides evidence of the modulation of the gap amplitude with doping concentration, consistent with the calculations for the orbital dependent pair interaction mediated by the antiferromagnetic spin fluctuations.

preprint2011arXiv

Multiple Magnetization Peaks and New Type of Vortex Phase Transitions in Ba0.6K0.4Fe2As2

Magnetization and its relaxation have been measured on Ba0.6K0.4Fe2As2 single crystals with Tc = 39 K. The magnetization hysteresis loops (MHLs) exhibit flux jumps in the low temperature region, and a second peak-effect in the intermediate temperature region, especially when the field sweeping rate is low. Interestingly a third magnetization peak can be easily observed on the MHLs in the high temperature region. Further analysis find that the first magnetization peak is very sharp, which is associated with the strong vortex pinning. However the first dip of the MHL corresponds to a moderate relaxation rate, then a second peak appears accompanied by a vanishing vortex motion. Finally a third magnetization peak emerges and the vortex motion becomes drastic beyond this threshold. The novel features accompanying the second magnetization peak suggest a new type of vortex phase transition.

preprint2011arXiv

Non-adiabatic elimination of auxiliary modes in continuous quantum measurements

When measuring a complex quantum system, we are often interested in only a few degrees of freedom-the plant, while the rest of them are collected as auxiliary modes-the bath. The bath can have finite memory (non-Markovian), and simply ignoring its dynamics, i.e., adiabatically eliminating it, will prevent us from predicting the true quantum behavior of the plant. We generalize the technique introduced by Strunz et. al. [Phys. Rev. Lett 82, 1801 (1999)], and develop a formalism that allows us to eliminate the bath non-adiabatically in continuous quantum measurements, and obtain a non-Markovian stochastic master equation for the plant which we focus on. We apply this formalism to three interesting examples relevant to current experiments.

preprint2011arXiv

Reveal non-Markovianity of open quantum systems via local operations

Non-Markovianity, as an important feature of general open quantum systems, is usually difficult to quantify with limited knowledge of how the plant that we are interested in interacts with its environment-the bath. It often happens that the reduced dynamics of the plant attached to a non-Markovian bath becomes indistinguishable from the one with a Markovian bath, if we left the entire system freely evolve. Here we show that non-Markovianity can be revealed via applying local unitary operations on the plant-they will influence the plant evolution at later times due to memory of the bath. This not only provides a new criterion for non-Markovianity, but also sheds light on protecting and recovering quantum coherence in non-Markovian systems, which will be useful for quantum-information processing.

preprint2011arXiv

Transport properties and asymmetric scattering in Ba$_{1-x}$K$_x$Fe$_2$As$_2$ single crystals compared to the electron doped counterparts Ba(Fe$_{1-x}$Co$_{x}$)$_{2}$As$_{2}$}

Resistivity, Hall effect and magnetoresistance have been investigated systematically on single crystals of Ba$_{1-x}$K$_x$Fe$_2$As$_2$ ranging from undoped to optimally doped regions. A systematic evolution of the quasiparticle scattering has been observed. It is found that the resistivity in the normal state of Ba$_{1-x}$K$_x$Fe$_2$As$_2$ is insensitive to the potassium doping concentration, which is very different from the electron doped counterpart Ba(Fe$_{1-x}$Co$_{x}$)$_{2}$As$_{2}$, where the resistivity at 300 K reduces to half value of the undoped one when the system is optimally doped. In stark contrast, the Hall coefficient R$_H$ changes suddenly from a negative value in the undoped sample to a positive one with slight K-doping, and it keeps lowering with further doping. We interpret this dichotomy due to the asymmetric scattering rate in the hole and the electron pockets with much higher mobility of the latter. The magnetoresistivity shows also a non-monotonic doping dependence indicating an anomalous feature at about 80 K to 100 K, even in the optimally doped sample, which is associated with a possible pseudogap feature. In the low temperature region, it seems that the resistivity has the similar values when superconductivity sets in disregarding the different T$_c$ values, which indicates a novel mechanism of the superconductivity. A linear feature of resistivity $ρ_{ab}$ vs. $T$ was observed just above $T_c$ for the optimally doped sample, suggesting a quantum criticality.

preprint2010arXiv

Low Temperature Specific Heat of Optimally Doped BaFe$_{2-x}$$TM_x$As$_{2}$ ($TM$ = Co and Ni) Single Crystals: Constraint on the Pairing Gap

Low temperature specific heat has been measured in optimally doped and highly overdoped non-superconducting BaFe$_{2-x}$TM$_x$As$_{2}$ ($TM$ = Co and Ni) single crystals. By using the data of the overdoped samples, we successfully removed the phonon contribution of the optimally doped ones, and derived the electronic specific heat coefficient $γ_e$. Remarkably, we found a continuing temperature dependent $γ_e(T)$ which follows the quadratic relation $γ_e=γ_0+αT^2$ in the low temperature limit. Together with the very small residual term $γ_0$, linear magnetic field dependence of $γ_e$, it is concluded that there are either small segments of nodal lines, or point-like nodes in these samples.

preprint2010arXiv

Multi-band effect in the noncentrosymmetric superconductors Mg_{12-δ}Ir_{19}B_{16} revealed by Hall effect and magnetoresistance measurements

We report the longitudinal resistivity and Hall effect measurements on the noncentrosymmetric superconducting Mg$_{12-δ}$Ir$_{19}$B$_{16}$ samples with different critical transition temperatures. A strong temperature dependence of the Hall coefficient $R_H$ and nonlinear magnetic field dependence of the Hall resistivity $ρ_{xy}$ in wide temperature region are observed, suggesting a strong multi-band effect in this system. Moreover, a large magnetoresistance up to 20% is found at the field of 9 T. We also observe the violation of the Kohler's rule from our magnetoresistance data, further confirming the presence of multi-band effect in our samples. A detailed analysis shows that the data can't be simply described within the two-band scenario at low temperatures, so we argue that there may be more than two bands contributing to the conduction of the samples.

preprint2010arXiv

Nernst quantum oscillations in bulk semi-metals

With a widely available magnetic field of 10 T, one can attain the quantum limit in bismuth and graphite. At zero magnetic field, these two elemental semi-metals host a dilute liquid of carriers of both signs. When the quantum limit is attained, all quasi-particles are confined to a few Landau tubes. Each time a Landau tube is squeezed before definitely leaving the Fermi surface, the Nernst response sharply peaks. In bismuth, additional Nernst peaks, unexpected in the non-interacting picture, are resolved beyond the quantum limit. The amplitude of these unexpected Nernst peaks is larger in the samples with the longest electron mean-free-path.

preprint2010arXiv

Phase diagram of bismuth in the extreme quantum limit

Elemental bismuth provides a rare opportunity to explore the fate of a three-dimensional gas of highly mobile electrons confined to their lowest Landau level. Coulomb interaction, neglected in the band picture, is expected to become significant in this extreme quantum limit with poorly understood consequences. Here, we present a study of the angular-dependent Nernst effect in bismuth, which establishes the existence of ultraquantum field scales on top of its complex single-particle spectrum. Each time a Landau level crosses the Fermi level, the Nernst response sharply peaks. All such peaks are resolved by the experiment and their complex angular-dependence is in very good agreement with the theory. Beyond the quantum limit, we resolve additional Nernst peaks signaling a cascade of additional Landau sub-levels caused by electron interaction.

preprint2010arXiv

Preparing a mechanical oscillator in non-Gaussian quantum states

We propose a protocol for coherently transferring non-Gaussian quantum states from optical field to a mechanical oscillator. The open quantum dynamics and continuous-measurement process, which can not be treated by the stochastic-master-equation formalism, are studied by a new path-integral-based approach. We obtain an elegant relation between the quantum state of the mechanical oscillator and that of the optical field, which is valid for general linear quantum dynamics. We demonstrate the experimental feasibility of such protocol by considering the cases of both large-scale gravitational-wave detectors and small-scale cavity-assisted optomechanical devices.

preprint2009arXiv

Hall plateaus at magic angles in bismuth beyond the quantum limit

We present a study of the angular dependence of the resistivity tensor up to 35 T in elemental bismuth complemented by torque magnetometry measurements in a similar configuration. For at least two particular field orientations a few degrees off the trigonal axis, the Hall resistivity was found to become field-independent within experimental resolution in a finite field window corresponding to a field which is roughly three times the frequency of quantum oscillations. The Hall plateaus rapidly vanish as the field is tilted off theses magic angles. We identify two distinct particularities of these specific orientations, which may play a role in the emergence of the Hall plateaus.

preprint2009arXiv

Nernst effect and dimensionality in the quantum limit

Nernst effect, the transverse voltage generated by a longitudinal thermal gradient in presence of magnetic field has recently emerged as a very sensitive, yet poorly understood, probe of electron organization in solids. Here we report on an experiment on graphite, a macroscopic stack of graphene layers, which establishes a fundamental link between dimensionality of an electronic system and its Nernst response. In sharp contrast with single-layer graphene, the Nernst signal sharply peaks whenever a Landau level meets the Fermi level. This points to the degrees of freedom provided by finite interlayer coupling as a source of enhanced thermoelectric response in the vicinity of the quantum limit. Since Landau quantization slices a three-dimensional Fermi surface, each intersection of a Landau level with the Fermi level modifies the Fermi surface topology. According to our results, the most prominent signature of such a topological phase transition emerges in the transverse thermoelectric response.

preprint2009arXiv

Specific-Heat Measurement of Residual Superconductivity in the Normal State of Underdoped Cuprate Superconductors

We have measured the magnetic field and temperature dependence of specific heat on $Bi_2Sr_{2-x}La_xCuO_{6+δ}$ single crystals in wide doping and temperature regions. The superconductivity related specific heat coefficient $γ_{sc}$ and entropy $S_{sc}$ are determined. It is found that $γ_{sc}$ has a hump-like anomaly at $T_c$ and behaves as a long tail which persists far into the normal state for the underdoped samples, but for the heavily overdoped samples the anomaly ends sharply just near $T_c$. Interestingly, we found that the entropy associated with superconductivity is roughly conserved when and only the long tail part in the normal state is taken into account for the underdoped samples, indicating the residual superconductivity above T$_c$.

preprint2008arXiv

Doping Dependence of Superconductivity and Lattice Constants in Hole Doped $La_{1-x}Sr_xFeAsO$

By using solid state reaction method we have fabricated the hole doped $La_{1-x}Sr_xFeAsO$ superconductors with Sr content up to 0.13. It is found that the sharp anomaly at about 150 K and the low temperature upturn of resistivity are suppressed by doping holes into the parent phase. Interestingly both the superconducting transition temperature $T_c$ and the lattice constants (a-axis and c-axis) increase monotonously with hole concentration, in sharp contrast with the electron doped side where the $T_c$ increases with a continuing shrinkage of the lattice constants either by dope more fluorine or oxygen vacancies into the system. Our data clearly illustrate that the superconductivity can be induced by doping holes via substituting the trivalent La with divalent Sr in the LaFeAsO system with single FeAs layer, and the $T_c$ in the present system exhibits a symmetric behavior at the electron and hole doped sides, as we reported previously.

preprint2008arXiv

Synthesizing and characterization of hole doped nickel based superconductor (La$_{1-x}$Sr$_{x}$)NiAsO

We report the synthesizing and characterization of the hole doped Ni-based superconductor ($La_{1-x}Sr_{x})NiAsO$. By substituting La with Sr, the superconducting transition temperature $T_c$ is increased from 2.4 K of the parent phase $LaNiAsO$ to 3.7 K at the doping levels x= 0.1 - 0.2. The curve $T_c$ versus hole concentration shows a symmetric behavior as the electron doped samples $LaNiAs(O_{1-x}F_{x})$. The normal state resistivity in Ni-based samples shows a good metallic behavior and reveals the absence of spin density wave induced anomaly which appears in the Fe-based system at about 150 K. Hall effect measurements indicate that the electron conduction in the parent phase $LaNiAsO$ is dominated by electron-like charge carriers, while with more Sr doping, a hole-like band will emerge and finally prevail over the conduction, such a phenomenon reflects that the Fermi surface of $LaNiAsO$ comprises of electron pockets and hole pockets, thus the sign of charge carriers could be changed once the contribution of hole pockets overwhelms that of electron pockets. Magnetoresistance measurements and the violation of Kohler rule provide further proof that multiband effect dominate the normal state transport of ($La_{1-x}Sr_{x})NiAsO$.

preprint2005arXiv

Effect of nano-C doping on the in-situ processed MgB2 tapes

The effect of nano-C doping on the microstructure and superconducting properties of Fe-sheathed MgB2 tapes prepared through the in-situ powder-in-tube method was studied. Heat treatment was performed at a low temperature of 650C for 1 h. Scanning electron microscopy investigation revealed that the smaller grain size of MgB2 in the samples with the C-doping. Further, the a-axis lattice parameter and transition temperature decreased monotonically with increasing doping level, which is due to the C substitution for B. High critical current density Jc values in magnetic fields were achieved in the doped samples because of the very fine-grained microstructure of the superconducting phase obtained with C doping.

Huan Yang

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132 published item(s)

A nonlinear voice from GW250114 ringdown

Artificial Intelligence Driven Channel Coding and Resource Optimization for Wireless Networks

ResTok: Learning Hierarchical Residuals in 1D Visual Tokenizers for Autoregressive Image Generation

Steering Visual Generation in Unified Multimodal Models with Understanding Supervision

UniPPTBench: A Unified Benchmark for Presentation Generation Across Diverse Input Settings

Beijing Normal University 12-meter Interferometric kHz GW Detector Prototype: Design and Scientific Prospects

Incentivizing Massive Unknown Workers for Budget-Limited Crowdsensing: From Off-Line and On-Line Perspectives

Advancing High-Resolution Video-Language Representation with Large-Scale Video Transcriptions

AI Illustrator: Translating Raw Descriptions into Images by Prompt-based Cross-Modal Generation

Degradation-Guided Meta-Restoration Network for Blind Super-Resolution

Dynamical Instability of Self-Gravitating Membranes

Evidence for Black Holes in Green Peas from WISE colors and variability

Finding Peas in the Early Universe with JWST

LAGER Ly$α$ Luminosity Function at $z\sim7$, Implications for Reionization

Learning Spatiotemporal Frequency-Transformer for Compressed Video Super-Resolution

Learning Spatiotemporal Frequency-Transformer for Low-Quality Video Super-Resolution

Learning Trajectory-Aware Transformer for Video Super-Resolution

Mass Testing and Characterization of 20-inch PMTs for JUNO

Mass-gap extreme mass ratio inspirals

Online Video Super-Resolution with Convolutional Kernel Bypass Graft

Pseudogap and Strong Pairing Induced by Incipient and Shallow Bands in the Quasi-Two-Dimensional KCa$_{2}$Fe$_{4}$As$_{4}$F$_{2}$

Resonant control of elastic collisions between $^{23}$Na$^{40}$K molecules and $^{40}$K atoms

Strong [OIII]λ5007 emission line compact galaxies in LAMOST DR9: Blueberries, Green Peas and Purple Grapes

Tick-Tock: The Imminent Merger of a Supermassive Black Hole Binary

TTVFI: Learning Trajectory-Aware Transformer for Video Frame Interpolation

UID2021: An Underwater Image Dataset for Evaluation of No-reference Quality Assessment Metrics

Using machine learning to parametrize postmerger signals from binary neutron stars

X-ray view of a merging supermassive black hole binary candidate SDSSJ1430+2303: Results from the first ~200 days of observations

A low-temperature scanning probe microscopy system with molecular beam epitaxy and optical access

A Lyman-α protocluster at redshift 6.9

Black-Hole Perturbation Plus Post-Newtonian Theory: Hybrid Waveform for Neutron Star Binaries

Direct Visualization of a Static Incommensurate Antiferromagnetic Order by Suppressing the Superconducting Phase Coherence in Fe-doped Bi2Sr2CaCu2O8+delta

Distinct Properties of Vortex Bound States Driven by Temperature

Evidence for association of triatomic molecule in ultracold $^{23}$Na$^{40}$K and $^{40}$K mixture

JUNO Physics and Detector

Necessary and sufficient criterion of steering for two-qubit T states

New spectroscopic confirmations of Lyman-$α$ emitters at z $\sim$ 7 from the LAGER survey

No observation of chiral flux current in the topological kagome metal CsV$_{3}$Sb$_{5}$

Orbit Tomography of Binary Supermassive Black Holes with Very Long Baseline Interferometry

Supercritical accretion of stellar-mass compact objects in active galactic nuclei

Van Hove Singularity Arising from Mexican-Hat-Shaped Inverted Bands in the Topological Insulator Sn-doped Bi$_{1.1}$Sb$_{0.9}$Te$_{2}$S

A comprehensive study of H$α$ emitters at $z \sim$ 0.62 in the DAWN survey: the need for deep and wide regions

A large, deep 3 deg$^2$ survey of H$α$, [OIII], and [OII] emitters from LAGER: constraining luminosity functions

Application of Structural Similarity Analysis of Visually Salient Areas and Hierarchical Clustering in the Screening of Similar Wireless Capsule Endoscopic Images

Dynamic Signatures of Black Hole Binaries with Superradiant Clouds

Experimental demonstration of complementarity relations between quantum steering criteria

Feasibility and physics potential of detecting $^8$B solar neutrinos at JUNO

Full Reference Screen Content Image Quality Assessment by Fusing Multi-level Structure Similarity

Learning Texture Transformer Network for Image Super-Resolution

NTIRE 2020 Challenge on Perceptual Extreme Super-Resolution: Methods and Results

Orbit-induced spin precession as an origin of periodicity in periodically-repeating fast radio bursts

Quantum correlation of light mediated by gravity

TAO Conceptual Design Report: A Precision Measurement of the Reactor Antineutrino Spectrum with Sub-percent Energy Resolution

The Importance Of Star Formation Intensity In LYα Escape From Green Pea Galaxies And Lyman Break Galaxy Analogs

Towards observing the neutron star collapse with gravitational wave detectors

Twofold symmetry of proximity-induced superconductivity in Bi$_{2}$Te$_{3}$/Bi$_{2}$Sr$_{2}$CaCu$_{2}$O$_{8+δ}$ heterostructures revealed by scanning tunneling microscopy

Excess Thermal Energy and Latent Heat in Nanocluster Collisional Growth

Experimental certification of steering criterion based on general entropic uncertainty relation

Experimental investigation of entropic uncertainty relations and coherence uncertainty relations

Experimental observation the Einstein-Podolsky-Rosen Steering based on the detection of entanglement

Relativistic Mean Motion Resonance

Directly visualizing the sign change of d-wave superconducting gap in Bi2Sr2CaCu2O8+δ by phase-referenced quasiparticle interference

Green Pea Galaxies Reveal Secrets of Ly$α$ Escape

Multi-band superconductivity and large anisotropy in FeS crystals

Plasma-wave generation in a dynamic spacetime

Possible superconducting fluctuation and pseudogap state above $T_c$ in CsFe$_2$As$_2$

Robust superconductivity and transport properties in (Li1-xFex)OHFeSe single crystals

Strong coupling superconductivity revealed by scanning tunneling microscope in tetragonal FeS

Towards an understanding of the force-free magnetosphere of rapidly spinning black holes

Anisotropic Electronic Mobilities in the Nematic State of the Parent Phase NaFeAs

Coupled Oscillator Model for Nonlinear Gravitational Perturbations

Fully gapped superconductivity in In-doped topological crystalline insulator Pb$_{0.5}$Sn$_{0.5}$Te

Magnetization relaxation, critical current density and vortex dynamics in a Ba$_{0.66}$K$_{0.32}$BiO$_{3+δ}$ single crystal

Magnetosphere of a Kerr black hole immersed in magnetized plasma and its perturbative mode structure