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Kai Liu

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preprint2026arXiv

Toward Better Geometric Representations for Molecule Generative Models

Geometric representation-conditioned molecule generation provides an effective paradigm that decouples molecule representation modeling from structure generation. By decoupling molecule generation into two stages-first generating a meaningful molecule representation, and then generating a 3D molecule conditioned on this representation-the efficiency and quality of the generation process can be significantly enhanced. However, its effectiveness is fundamentally limited by the quality of the representation space: pretrained molecular encoders, such as UniMol, produce representations that are non-smooth and not fully exploited during the generative training process. In this work, we propose LENSEs, a framework that better exploits the potential of molecule representations in representation-conditioned generation methods. In particular, LENSEs introduces three complementary mechanisms: (1) a representation head, simultaneously trained during generative tasks, that extracts multi-level representations from the pretrained encoder; (2) a molecule perceptual loss that optimizes the generator in a semantic-informative representation space; and (3) a node-level representation alignment (REPA) loss that explicitly aligns the generator's hidden states with encoder representations, reducing the semantic gap between pretraining and generation. We demonstrate the effectiveness of these improvements through extensive molecule generation tasks. Specifically, on the challenging molecule generation dataset GEOM-DRUG, LENSEs achieves 97.28% validity and 98.51% molecule stability, surpassing existing advanced methods. Further analyses through Lipschitz constant reduction (4.6x) and QM9 probing tasks also demonstrate the smoother, more informative refined representations, establishing generative training with alignment objectives as a potential pretraining paradigm for molecular encoders.

preprint2024arXiv

An eigenvalue problem for self-similar patterns in Hele-Shaw flows

Hele-Shaw problems are prototypes to study the interface dynamics. Linear theory suggests the existence of self-similar patterns in a Hele-Shaw flow. That is, with a specific injection flux the interface shape remains unchanged while its size increases. In this paper, we explore the existence of self-similar patterns in the nonlinear regime and develop a rigorous nonlinear theory characterizing their fundamental features. Using a boundary integral formulation, we pose the question of self-similarity as a generalized nonlinear eigenvalue problem, involving two nonlinear integral operators. The flux constant $C$ is the eigenvalue and the corresponding self-similar pattern $\mathbf{x}$ is the eigenvector. We develop a quasi-Newton method to solve the problem and show the existence of nonlinear shapes with $k$-fold dominated symmetries. The influence of initial guesses on the self-similar patterns is investigated. We are able to obtain a desired self-similar shape once the initial guess is properly chosen. Our results go beyond the predictions of linear theory and establish a bridge between the linear theory and simulations.

preprint2024arXiv

Fourier neural operator based fluid-structure interaction for predicting the vesicle dynamics

Solving complex fluid-structure interaction (FSI) problems, characterized by nonlinear partial differential equations, is crucial in various scientific and engineering applications. Traditional computational fluid dynamics (CFD) solvers are insufficient to meet the growing requirements for large-scale and long-period simulations. Fortunately, the rapid advancement in neural networks, especially neural operator learning mappings between function spaces, has introduced novel approaches to tackle these challenges via data-driven modeling. In this paper, we propose a Fourier neural operator-based fluid-structure interaction solver (FNO-based FSI solver) for efficient simulation of FSI problems, where the solid solver based on the finite difference method is seamlessly integrated with the Fourier neural operator to predict incompressible flow using the immersed boundary method. We analyze the performance of the FNO-based FSI solver in the following three situations: training data with or without the steady state, training method with one-step label or multi-step labels, and prediction in interpolation or extrapolation. We find that the best performance for interpolation is achieved by training the operator with multi-step labels using steady-state data. Finally, we train the FNO-based FSI solver using this optimal training method and apply it to vesicle dynamics. The results show that the FNO-based FSI solver is capable of capturing the variations in the fluid and the vesicle.

preprint2024arXiv

Interference of Two-Dimensional Bose-Einstein Condensates in Micro-Gravity

We investigate the interference of two-dimensional Bose-Einstein condensates in micro-gravity, which influenced by the interaction strength, initial momentum, gravitational potential and phase difference. We demonstrate that the gravitational potential from the Earth can change the density distribution and phase distribution of the condensate's wave function. As time evolves, a portion of the gravitational potential energy of the microscopic particles can be converted into kinetic energy, which changes the motion of the microscopic particles, and leads to the varying of the density and phase distribution of the wave function. Nevertheless, the influences of the Earth's gravity on the wave function can be eliminated by the micro-gravity environment, which confirmed by many micro-gravity cold atom experiments. Our results present the influences of gravity and other parameters on interference of Bose-Einstein condensates, which help us to reveal the intrinsic natures of the related theoretical predictions and experimental phenomena. Furthermore, our work builds a bridge between the related physical phenomena and our physical intuition about the Bose-Einstein condensates in micro-gravity environment.

preprint2023arXiv

Deep Biological Pathway Informed Pathology-Genomic Multimodal Survival Prediction

The integration of multi-modal data, such as pathological images and genomic data, is essential for understanding cancer heterogeneity and complexity for personalized treatments, as well as for enhancing survival predictions. Despite the progress made in integrating pathology and genomic data, most existing methods cannot mine the complex inter-modality relations thoroughly. Additionally, identifying explainable features from these models that govern preclinical discovery and clinical prediction is crucial for cancer diagnosis, prognosis, and therapeutic response studies. We propose PONET- a novel biological pathway-informed pathology-genomic deep model that integrates pathological images and genomic data not only to improve survival prediction but also to identify genes and pathways that cause different survival rates in patients. Empirical results on six of The Cancer Genome Atlas (TCGA) datasets show that our proposed method achieves superior predictive performance and reveals meaningful biological interpretations. The proposed method establishes insight into how to train biologically informed deep networks on multimodal biomedical data which will have general applicability for understanding diseases and predicting response and resistance to treatment.

preprint2023arXiv

Emergent Electronic Kagome Lattice in Correlated Charge-Density-Wave State of 1T-TaS$_2$

Quantum materials with tunable correlated and/or topological electronic states, such as the electronic Kagome lattice, provide an ideal platform to study the exotic quantum properties. However, the real-space investigations on the correlated electronic Kagome lattice have been rarely reported. Herein, we report on the electronic Kagome lattice emerging in the correlated charge-density-wave (CDW) state of 1T-TaS$_2$ at ~200 K via variable-temperature scanning tunneling microscopy (VT-STM). This emergent Kagome lattice can be considered a fractional electron-filling superstructure with reduced translational and rotational symmetries, confirmed by STM measurements and density functional theory simulations. The characteristic band structure and density of states of this electronic Kagome lattice are further explored based on theoretical calculations. Our results demonstrate a self-organized electronic Kagome lattice from the correlated CDW state via the effective tuning parameter of temperature and provide a platform to directly explore the interplay of correlated electrons and topological physics.

preprint2023arXiv

Improving Target Speaker Extraction with Sparse LDA-transformed Speaker Embeddings

As a practical alternative of speech separation, target speaker extraction (TSE) aims to extract the speech from the desired speaker using additional speaker cue extracted from the speaker. Its main challenge lies in how to properly extract and leverage the speaker cue to benefit the extracted speech quality. The cue extraction method adopted in majority existing TSE studies is to directly utilize discriminative speaker embedding, which is extracted from the pre-trained models for speaker verification. Although the high speaker discriminability is a most desirable property for speaker verification task, we argue that it may be too sophisticated for TSE. In this study, we propose that a simplified speaker cue with clear class separability might be preferred for TSE. To verify our proposal, we introduce several forms of speaker cues, including naive speaker embedding (such as, x-vector and xi-vector) and new speaker embeddings produced from sparse LDA-transform. Corresponding TSE models are built by integrating these speaker cues with SepFormer (one SOTA speech separation model). Performances of these TSE models are examined on the benchmark WSJ0-2mix dataset. Experimental results validate the effectiveness and generalizability of our proposal, showing up to 9.9% relative improvement in SI-SDRi. Moreover, with SI-SDRi of 19.4 dB and PESQ of 3.78, our best TSE system significantly outperforms the current SOTA systems and offers the top TSE results reported till date on the WSJ0-2mix.

preprint2023arXiv

Multi-Task Learning with Prior Information

Multi-task learning aims to boost the generalization performance of multiple related tasks simultaneously by leveraging information contained in those tasks. In this paper, we propose a multi-task learning framework, where we utilize prior knowledge about the relations between features. We also impose a penalty on the coefficients changing for each specific feature to ensure related tasks have similar coefficients on common features shared among them. In addition, we capture a common set of features via group sparsity. The objective is formulated as a non-smooth convex optimization problem, which can be solved with various methods, including gradient descent method with fixed stepsize, iterative shrinkage-thresholding algorithm (ISTA) with back-tracking, and its variation -- fast iterative shrinkage-thresholding algorithm (FISTA). In light of the sub-linear convergence rate of the methods aforementioned, we propose an asymptotically linear convergent algorithm with theoretical guarantee. Empirical experiments on both regression and classification tasks with real-world datasets demonstrate that our proposed algorithms are capable of improving the generalization performance of multiple related tasks.

preprint2023arXiv

Randomized Greedy Algorithms and Composable Coreset for k-Center Clustering with Outliers

In this paper, we study the problem of {\em $k$-center clustering with outliers}. The problem has many important applications in real world, but the presence of outliers can significantly increase the computational complexity. Though a number of methods have been developed in the past decades, it is still quite challenging to design quality guaranteed algorithm with low complexity for this problem. Our idea is inspired by the greedy method, Gonzalez's algorithm, that was developed for solving the ordinary $k$-center clustering problem. Based on some novel observations, we show that a simple randomized version of this greedy strategy actually can handle outliers efficiently. We further show that this randomized greedy approach also yields small coreset for the problem in doubling metrics (even if the doubling dimension is not given), which can greatly reduce the computational complexity. Moreover, together with the partial clustering framework proposed in arXiv:1703.01539 , we prove that our coreset method can be applied to distributed data with a low communication complexity. The experimental results suggest that our algorithms can achieve near optimal solutions and yield lower complexities comparing with the existing methods.

preprint2023arXiv

The Mars Orbiter Magnetometer of Tianwen-1: In-flight Performance and First Science Results

Mars Orbiter MAGnetometer (MOMAG) is a scientifc instrument onboard the orbiter of China's first mission for Mars -- Tianwen-1. It started to routinely measure the magnetic field from the solar wind to magnetic pile-up region surrounding Mars since November 13, 2021. Here we present its in-flight performance and first science results based on the first one and a half months' data. By comparing with the magnetic field data in the solar wind from the Mars Atmosphere and Volatile EvolutioN (MAVEN), the magnetic field by MOMAG is at the same level in magnitude, and the same magnetic structures with the similar variations in three components could be found in MOMAG data. In the first one and a half months, we recognize 158 clear bow shock (BS) crossings from MOMAG data, whose locations statistically match well with the modeled average BS. We also identify 5 pairs of simultaneous BS crossings of the Tianwen-1's orbiter and MAVEN. These BS crossings confirm the global shape of modeled BS as well as the south-north asymmetry of the Martian BS. Two presented cases in this paper suggest that the BS is probably more dynamic at flank than near the nose. So far, MOMAG performs well, and provides accurate magnetic field vectors. MOMAG is continuously scanning the magnetic field surrounding Mars. These measurements complemented by observations from MAVEN will undoubtedly advance our understanding of the plasma environment of Mars.

preprint2022arXiv

3D Interconnected Magnetic Nanowire Networks as Potential Integrated Multistate Memristors

Interconnected magnetic nanowire (NW) networks offer a promising platform for 3-dimensional (3D) information storage and integrated neuromorphic computing. Here we report discrete propagation of magnetic states in interconnected Co nanowire networks driven by magnetic field and current, manifested in distinct magnetoresistance (MR) features. In these networks, when only a few interconnected NWs were measured, multiple MR kinks and local minima were observed, including a significant minimum at a positive field during the descending field sweep. Micromagnetic simulations showed that this unusual feature was due to domain wall (DW) pinning at the NW intersections, which was confirmed by off-axis electron holography imaging. In a complex network with many intersections, sequential switching of nanowire sections separated by interconnects was observed, along with stochastic characteristics. The pinning/depinning of the DWs can be further controlled by the driving current density. These results illustrate the promise of such interconnected networks as integrated multistate memristors.

preprint2022arXiv

A review of knowledge graph application scenarios in cyber security

Facing the dynamic complex cyber environments, internal and external cyber threat intelligence, and the increasing risk of cyber-attack, knowledge graphs show great application potential in the cyber security area because of their capabilities in knowledge aggregation, representation, management, and reasoning. However, while most research has focused on how to develop a complete knowledge graph, it remains unclear how to apply the knowledge graph to solve industrial real challenges in cyber-attack and defense scenarios. In this review, we provide a brief overview of the basic concepts, schema, and construction approaches for the cyber security knowledge graph. To facilitate future research on cyber security knowledge graphs, we also present a curated collection of datasets and open-source libraries on the knowledge construction and information extraction task. In the major part of this article, we conduct a comparative review of the different works that elaborate on the recent progress in the application scenarios of the cyber security knowledge graph. Furthermore, a novel comprehensive classification framework is created to describe the connected works from nine primary categories and eighteen subcategories. Finally, we have a thorough outlook on several promising research directions based on the discussion of existing research flaws.

preprint2022arXiv

CohortVA: A Visual Analytic System for Interactive Exploration of Cohorts based on Historical Data

In history research, cohort analysis seeks to identify social structures and figure mobilities by studying the group-based behavior of historical figures. Prior works mainly employ automatic data mining approaches, lacking effective visual explanation. In this paper, we present CohortVA, an interactive visual analytic approach that enables historians to incorporate expertise and insight into the iterative exploration process. The kernel of CohortVA is a novel identification model that generates candidate cohorts and constructs cohort features by means of pre-built knowledge graphs constructed from large-scale history databases. We propose a set of coordinated views to illustrate identified cohorts and features coupled with historical events and figure profiles. Two case studies and interviews with historians demonstrate that CohortVA can greatly enhance the capabilities of cohort identifications, figure authentications, and hypothesis generation.

preprint2022arXiv

CoTe2: A quantum critical Dirac metal with strong spin fluctuations

Quantum critical points separating weak ferromagnetic and paramagnetic phases trigger many novel phenomena. Dynamical spin fluctuations not only suppress the long-range order, but can also lead to unusual transport and even superconductivity. Combining quantum criticality with topological electronic properties presents a rare and unique opportunity. Here, by means of ab initio calculations and magnetic, thermal, and transport measurements, we show that the orthorhombic CoTe$_2$ is close to ferromagnetism, which appears suppressed by spin fluctuations. Calculations and transport measurements reveal nodal Dirac lines, making it a rare combination of proximity to quantum criticality and Dirac topology.

preprint2022arXiv

Demonstration of room-temperature continuous-wave operation of InGaAs/AlGaAs quantum well lasers directly grown on on-axis silicon (001)

Room-temperature continuous-wave operation of InGaAs/AlGaAs quantum well lasers directly grown on on-axis silicon (001) has been demonstrated. A 420 nm thick GaAs epilayer completely free of antiphase domains was initially grown on the silicon substrate in a metal-organic chemical vapor deposition system and the other epilayers including four sets of five-period strained-layer superlattices and the laser-structural layers were successively grown in a molecular beam epitaxy system. The lasers were prepared as broad-stripe Fabry-Perot ones with a stripe width of 21.5 um and a cavity length of 1 mm. Typically, the threshold current and the corresponding threshold current density are 186.4 mA and 867 A/cm2, respectively. The lasing wavelength is around 980 nm and the slope efficiency is 0.097 W/A with a single-facet output power of 22.5 mW at an injection current of 400 mA. This advancement makes the silicon-based monolithic optoelectronic integration relevant to quantum well lasers more promising with an enhanced feasibility.

preprint2022arXiv

Dynamic Group Transformer: A General Vision Transformer Backbone with Dynamic Group Attention

Recently, Transformers have shown promising performance in various vision tasks. To reduce the quadratic computation complexity caused by each query attending to all keys/values, various methods have constrained the range of attention within local regions, where each query only attends to keys/values within a hand-crafted window. However, these hand-crafted window partition mechanisms are data-agnostic and ignore their input content, so it is likely that one query maybe attends to irrelevant keys/values. To address this issue, we propose a Dynamic Group Attention (DG-Attention), which dynamically divides all queries into multiple groups and selects the most relevant keys/values for each group. Our DG-Attention can flexibly model more relevant dependencies without any spatial constraint that is used in hand-crafted window based attention. Built on the DG-Attention, we develop a general vision transformer backbone named Dynamic Group Transformer (DGT). Extensive experiments show that our models can outperform the state-of-the-art methods on multiple common vision tasks, including image classification, semantic segmentation, object detection, and instance segmentation.

preprint2022arXiv

Emergent superconductivity in van der Waals Kagome material Pd3P2S8 under high pressure

Kagome lattice systems have been proposed to host rich physics, which provide an excellent platform to explore unusual quantum states. Here, we report on the discovery of superconductivity in van der Waals material Pd3P2S8 under pressure. The superconductivity is observed in Pd3P2S8 for those pressures where the temperature dependence of the resistivity changes from a semiconducting-like behavior to that of a normal metal. The superconducting transition temperature Tc increases with applied pressure and reaches ~ 6.83 K at 79.5 GPa. Combining high-pressure XRD, Raman spectroscopy and theoretical calculations, our results demonstrate that the observed superconductivity induced by high pressure in Pd3P2S8 is closely related to the formation of amorphous phase, which results from the structural instability due to the enhanced coupling between interlayer Pd and S atoms upon compression.

preprint2022arXiv

Enriched Robust Multi-View Kernel Subspace Clustering

Subspace clustering is to find underlying low-dimensional subspaces and cluster the data points correctly. In this paper, we propose a novel multi-view subspace clustering method. Most existing methods suffer from two critical issues. First, they usually adopt a two-stage framework and isolate the processes of affinity learning, multi-view information fusion and clustering. Second, they assume the data lies in a linear subspace which may fail in practice as most real-world datasets may have non-linearity structures. To address the above issues, in this paper we propose a novel Enriched Robust Multi-View Kernel Subspace Clustering framework where the consensus affinity matrix is learned from both multi-view data and spectral clustering. Due to the objective and constraints which is difficult to optimize, we propose an iterative optimization method which is easy to implement and can yield closed solution in each step. Extensive experiments have validated the superiority of our method over state-of-the-art clustering methods.

preprint2022arXiv

Exploring the impact of spatiotemporal granularity on the demand prediction of dynamic ride-hailing

Dynamic demand prediction is a key issue in ride-hailing dispatching. Many methods have been developed to improve the demand prediction accuracy of an increase in demand-responsive, ride-hailing transport services. However, the uncertainties in predicting ride-hailing demands due to multiscale spatiotemporal granularity, as well as the resulting statistical errors, are seldom explored. This paper attempts to fill this gap and to examine the spatiotemporal granularity effects on ride-hailing demand prediction accuracy by using empirical data for Chengdu, China. A convolutional, long short-term memory model combined with a hexagonal convolution operation (H-ConvLSTM) is proposed to explore the complex spatial and temporal relations. Experimental analysis results show that the proposed approach outperforms conventional methods in terms of prediction accuracy. A comparison of 36 spatiotemporal granularities with both departure demands and arrival demands shows that the combination of a hexagonal spatial partition with an 800 m side length and a 30 min time interval achieves the best comprehensive prediction accuracy. However, the departure demands and arrival demands reveal different variation trends in the prediction errors for various spatiotemporal granularities.

preprint2022arXiv

Hydrogenation induced magnetic and electronic transitions in monolayer electride Gd$_2$C: A first-principles study

The recently synthesized two-dimensional electride Gd$_2$C was proposed to be a ferromagnetic metal that possesses multiple pairs of Weyl points and may display a large anomalous Hall conductivity [Liu \textit{et al.}, Phys. Rev. Lett. \textbf{125}, 187203 (2020)]. In view of its layered structure, here we carry out first-principles studies on the magnetic and electronic properties of Gd$_2$C in the ultrathin monolayer limit. We find that monolayer Gd$_2$C remains ferromagnetic like the bulk form and the hydrogenation can effectively tune its magnetism and electronic structure. With one-sided coverage of hydrogen atoms, monolayer Gd$_2$C becomes a half-metal with one spin channel around the Fermi level. For two-sided hydrogenation, monolayer Gd$_2$C transforms to an antiferromagnetic insulator with a band gap of 0.8 eV. Our studies show that monolayer electride Gd$_2$C can perform multiple magnetic and electronic transitions with different levels of hydrogenation and may be also adopted to construct a planar heterojunction with selective area adsorption of hydrogen atoms, which has promising applications in future electronic and spintronic devices.

preprint2022arXiv

Improving Adversarial Waveform Generation based Singing Voice Conversion with Harmonic Signals

Adversarial waveform generation has been a popular approach as the backend of singing voice conversion (SVC) to generate high-quality singing audio. However, the instability of GAN also leads to other problems, such as pitch jitters and U/V errors. It affects the smoothness and continuity of harmonics, hence degrades the conversion quality seriously. This paper proposes to feed harmonic signals to the SVC model in advance to enhance audio generation. We extract the sine excitation from the pitch, and filter it with a linear time-varying (LTV) filter estimated by a neural network. Both these two harmonic signals are adopted as the inputs to generate the singing waveform. In our experiments, two mainstream models, MelGAN and ParallelWaveGAN, are investigated to validate the effectiveness of the proposed approach. We conduct a MOS test on clean and noisy test sets. The result shows that both signals significantly improve SVC in fidelity and timbre similarity. Besides, the case analysis further validates that this method enhances the smoothness and continuity of harmonics in the generated audio, and the filtered excitation better matches the target audio.

preprint2022arXiv

Joint CNN and Transformer Network via weakly supervised Learning for efficient crowd counting

Currently, for crowd counting, the fully supervised methods via density map estimation are the mainstream research directions. However, such methods need location-level annotation of persons in an image, which is time-consuming and laborious. Therefore, the weakly supervised method just relying upon the count-level annotation is urgently needed. Since CNN is not suitable for modeling the global context and the interactions between image patches, crowd counting with weakly supervised learning via CNN generally can not show good performance. The weakly supervised model via Transformer was sequentially proposed to model the global context and learn contrast features. However, the transformer directly partitions the crowd images into a series of tokens, which may not be a good choice due to each pedestrian being an independent individual, and the parameter number of the network is very large. Hence, we propose a Joint CNN and Transformer Network (JCTNet) via weakly supervised learning for crowd counting in this paper. JCTNet consists of three parts: CNN feature extraction module (CFM), Transformer feature extraction module (TFM), and counting regression module (CRM). In particular, the CFM extracts crowd semantic information features, then sends their patch partitions to TRM for modeling global context, and CRM is used to predict the number of people. Extensive experiments and visualizations demonstrate that JCTNet can effectively focus on the crowd regions and obtain superior weakly supervised counting performance on five mainstream datasets. The number of parameters of the model can be reduced by about 67%~73% compared with the pure Transformer works. We also tried to explain the phenomenon that a model constrained only by count-level annotations can still focus on the crowd regions. We believe our work can promote further research in this field.

preprint2022arXiv

Manipulation of Dirac band curvature and momentum-dependent g-factor in a kagome magnet YMn6Sn6

The Zeeman effect describes the energy change of an atomic quantum state in magnetic field. The magnitude and the direction of this change depend on the dimensionless Lande g-factor. In quantum solids, the response of the Bloch electron states to the magnetic field also exhibits the Zeeman effect with an effective g-factor that was theoretically predicted to be dependent on the momentum. While typically negligible in many ordinary solids, the momentum-dependent variation of the g-factor is theorized to be substantially enhanced in many topological and magnetic systems. However, the momentum-dependence of the g-factor is notoriously difficult to extract and it is yet to be directly experimentally measured. In this work, we report the experimental discovery of a strongly momentum-dependent g-factor in a kagome magnet YMn6Sn6. Using spectroscopic-imaging scanning tunneling microscopy, we map the evolution of a massive Dirac band in the vicinity of the Fermi level as a function of magnetic field. We find that electronic states at different lattice momenta exhibit markedly different Zeeman energy shifts, giving rise to an anomalous g-factor that peaks around the Dirac point. Our work provides the first momentum-resolved visualization of Dirac band curvature manipulation by magnetic field, which should in principle be highly relevant to other topological kagome magnets.

preprint2022arXiv

Maximum Correntropy Value Decomposition for Multi-agent Deep Reinforcemen Learning

We explore value decomposition solutions for multi-agent deep reinforcement learning in the popular paradigm of centralized training with decentralized execution(CTDE). As the recognized best solution to CTDE, Weighted QMIX is cutting-edge on StarCraft Multi-agent Challenge (SMAC), with a weighting scheme implemented on QMIX to place more emphasis on the optimal joint actions. However, the fixed weight requires manual tuning according to the application scenarios, which painfully prevents Weighted QMIX from being used in broader engineering applications. In this paper, we first demonstrate the flaw of Weighted QMIX using an ordinary One-Step Matrix Game (OMG), that no matter how the weight is chosen, Weighted QMIX struggles to deal with non-monotonic value decomposition problems with a large variance of reward distributions. Then we characterize the problem of value decomposition as an Underfitting One-edged Robust Regression problem and make the first attempt to give a solution to the value decomposition problem from the perspective of information-theoretical learning. We introduce the Maximum Correntropy Criterion (MCC) as a cost function to dynamically adapt the weight to eliminate the effects of minimum in reward distributions. We simplify the implementation and propose a new algorithm called MCVD. A preliminary experiment conducted on OMG shows that MCVD could deal with non-monotonic value decomposition problems with a large tolerance of kernel bandwidth selection. Further experiments are carried out on Cooperative-Navigation and multiple SMAC scenarios, where MCVD exhibits unprecedented ease of implementation, broad applicability, and stability.

preprint2022arXiv

MKQ-BERT: Quantized BERT with 4-bits Weights and Activations

Recently, pre-trained Transformer based language models, such as BERT, have shown great superiority over the traditional methods in many Natural Language Processing (NLP) tasks. However, the computational cost for deploying these models is prohibitive on resource-restricted devices. One method to alleviate this computation overhead is to quantize the original model into fewer bits representation, and previous work has proved that we can at most quantize both weights and activations of BERT into 8-bits, without degrading its performance. In this work, we propose MKQ-BERT, which further improves the compression level and uses 4-bits for quantization. In MKQ-BERT, we propose a novel way for computing the gradient of the quantization scale, combined with an advanced distillation strategy. On the one hand, we prove that MKQ-BERT outperforms the existing BERT quantization methods for achieving a higher accuracy under the same compression level. On the other hand, we are the first work that successfully deploys the 4-bits BERT and achieves an end-to-end speedup for inference. Our results suggest that we could achieve 5.3x of bits reduction without degrading the model accuracy, and the inference speed of one int4 layer is 15x faster than a float32 layer in Transformer based model.

preprint2022arXiv

Superconductivity in monolayer Ba$_2$N electride: a first-principles study

The exploration of superconductivity in low-dimensional materials has attracted intensive attention for decades. Based on first-principles electronic structure calculations, we have systematically investigated the electronic and superconducting properties of the two-dimensional electride Ba$_2$N in the monolayer limit. Our results show that monolayer Ba$_2$N has a low work function of 3.0 eV and a predicted superconducting transition temperature ($T_c$) of 3.4 K. The superconductivity can be further improved with the tensile strain, which results from the increase of density of states at the Fermi level as well as the enhanced coupling between inner-layer electrons and phonons. Remarkably, at the 4$\%$ tensile strain, the acoustic branches have noticeable softening at the K point of Brillouin zone and the superconducting $T_c$ can reach 10.8 K. The effect of lattice strain on the electron transfer from the superficial region to the inner-layer region of monolayer Ba$_2$N may also apply to other electride materials and influence their physical properties.

preprint2022arXiv

Time Resolution of the 4H-SiC PIN Detector

We address the determination of the time resolution for the $\rm 100~μm$ 4H-SiC PIN detectors fabricated by Nanjing University (NJU). The time response to $\rm β$ particles from a $\rm ^{90}$Sr source is investigated for the detection of the minimum ionizing particles (MIPs). We study the influence of different reverse voltages, which correspond to different carrier velocities and device sizes, and how this correlates with the detector capacitance. We determine a time resolution $\rm (94\pm1)~ps$ for $\rm 100~μm$ 4H-SiC PIN detector. A fast simulation software, termed RASER (RAdiation SEmiconductoR), is developed, and validated by comparing the waveform obtained from simulated and measured data. The simulated time resolution is $\rm (73\pm 1)~ps$ after considering the intrinsic leading contributions of the detector to time resolution.

preprint2022arXiv

Timing performance simulation for 3D 4H-SiC detector

To meet high radiation challenge for detectors in future high-energy physics, a novel 3D 4H-SiC detector was investigated. SiC detectors could potentially operate in radiation harsh and room temperature environment because of its high thermal conductivity and high atomic displacement threshold energy. 3D structure, which decouples thickness and distance between electrodes, further improves timing performance and radiation hardness of the detector. We developed a simulation software - RASER (RAdiation SEmiconductoR) to simulate the time resolution of planar and 3D 4H-SiC detectors with different parameters and structures, and the reliability of the software is verified by comparing time resolution results of simulation with data. The rough time resolution of 3D 4H-SiC detector was estimated, and the simulation parameters could be used as guideline to 3D 4H-SiC detector design and optimization.

preprint2022arXiv

Ultrasensitive Sub-monolayer Palladium Induced Chirality Switching and Topological Evolution of Skyrmions

Chiral spin textures are fundamentally interesting, with promise for device applications. Stabilizing chirality is conventionally achieved by introducing Dzyaloshinskii-Moriya interaction (DMI) in asymmetric multilayers where the thickness of each layer is at least a few monolayers. Here we report an ultrasensitive chirality switching in (Ni/Co)n multilayer induced by capping with only 0.22 monolayer of Pd. Using spin-polarized low-energy electron microscopy, we monitor the gradual evolution of domain walls from left-handed to right-handed Neel walls and quantify the DMI induced by the Pd capping layer. We also observe the chiral evolution of a skyrmion during the DMI switching, where no significant topological protection is found as the skyrmion winding number varies. This corresponds to a minimum energy cost of < 1 attojoule during the skyrmion chirality switching. Our results demonstrate the detailed chirality evolution within skyrmions during the DMI sign switching, which is relevant to practical applications of skyrmionic devices.

preprint2022arXiv

Weakly-supervised High-fidelity Ultrasound Video Synthesis with Feature Decoupling

Ultrasound (US) is widely used for its advantages of real-time imaging, radiation-free and portability. In clinical practice, analysis and diagnosis often rely on US sequences rather than a single image to obtain dynamic anatomical information. This is challenging for novices to learn because practicing with adequate videos from patients is clinically unpractical. In this paper, we propose a novel framework to synthesize high-fidelity US videos. Specifically, the synthesis videos are generated by animating source content images based on the motion of given driving videos. Our highlights are three-fold. First, leveraging the advantages of self- and fully-supervised learning, our proposed system is trained in weakly-supervised manner for keypoint detection. These keypoints then provide vital information for handling complex high dynamic motions in US videos. Second, we decouple content and texture learning using the dual decoders to effectively reduce the model learning difficulty. Last, we adopt the adversarial training strategy with GAN losses for further improving the sharpness of the generated videos, narrowing the gap between real and synthesis videos. We validate our method on a large in-house pelvic dataset with high dynamic motion. Extensive evaluation metrics and user study prove the effectiveness of our proposed method.

preprint2021arXiv

Intrinsic ferromagnetic and antiferromagnetic axion insulators in van der Waals materials Mn\emph{X}$_{2}$\emph{B}$_{2}$\emph{T}$_{6}$ family

The MnBi$_{2}$Te$_{4}$ family has attracted significant attention due to its rich topological states such as the quantum anomalous Hall (QAH) insulator state, the axion insulator state, and the magnetic Weyl semimetal state. Nevertheless, the intrinsic antiferromagnetic (AFM) interlayer coupling in MnBi$_{2}$Te$_{4}$ partly hinders the realization of "high-temperature" QAH effect. Here, by using first-principles electronic structure calculations, we design a new class of materials Mn\emph{X}$_{2}$\emph{B}$_{2}$\emph{T}$_{6}$ (\emph{X}=Ge, Sn, or Pb; \emph{B}=Sb or Bi; \emph{T}=Se or Te) based on the \emph{X}$_{2}$\emph{B}$_{2}$\emph{T}$_{5}$ structures rather than the Bi$_{2}$Te$_{3}$ family. We find that each septuple-layer Mn\emph{B}$_{2}$\emph{T}$_{4}$ is sandwiched by two [\emph{X}\emph{T}] layers, which may turn the AFM interlayer coupling into a ferromagnetic (FM) coupling. The calculations specifically demonstrate that \emph{MnGe}$_{2}$\emph{Sb}$_{2}$\emph{Te}$_{6}$, \emph{MnGe}$_{2}$\emph{Bi}$_{2}$\emph{Te}$_{6}$, and \emph{MnPb}$_{2}$\emph{Bi}$_{2}$\emph{Te}$_{6}$ are FM axion insulators, while MnGe$_{2}$Sb$_{2}$Se$_{6}$, MnGe$_{2}$Bi$_{2}$Se$_{6}$, MnSn$_{2}$Sb$_{2}$Te$_{6}$, and MnSn$_{2}$Bi$_{2}$Te$_{6}$ are A-type AFM axion insulators. These seven materials all have an out-of-plane easy axis of magnetization. The Mn\emph{X}$_{2}$\emph{B}$_{2}$\emph{T}$_{6}$ family thus offers a promising platform beyond the MnBi$_{2}$Te$_{4}$ family for the realization of quantized magnetoelectric effect and "high-temperature" QAH effect in future experiments.

preprint2021arXiv

Pressure induced superconductivity in WB2 and ReB2 through modifying the B layers

The recent discovery of superconductivity up to 32 K in the pressurized MoB2 reignites the interests in exploring high-Tc superconductors in transition-metal diborides. Inspired by that work, we turn our attention to the 5d transition-metal diborides. Here we systematically investigate the responses of both structural and physical properties of WB2 and ReB2 to external pressure, which possess different types of boron layers. Similar to MoB2, the pressure-induced superconductivity was also observed in WB2 above 60 GPa with a maximum Tc of 15 K at 100 GPa, while no superconductivity was detected in ReB2 in this pressure range. Interestingly, the structures at ambient pressure for both WB2 and ReB2 persist to high pressure without structural phase transitions. Theoretical calculations suggest that the ratio of flat boron layers in this class of transition-metal diborides may be crucial for the appearance of high Tc. The combined theoretical and experimental results highlight the effect of geometry of boron layers on superconductivity and shed light on the exploration of novel high-Tc superconductors in borides.

preprint2021arXiv

Two-dimensional Dirac nodal-line semimetal protected by symmetry

Dirac nodal line semimetals (DNLSs) host relativistic quasiparticles in their one-dimensional (1D) Dirac nodal line (DNL) bands that are protected by certain crystalline symmetries. Their novel low-energy fermion quasiparticle excitations and transport properties invite studies of relativistic physics in the solid state where their linearly dispersing Dirac bands cross at continuous lines with four-fold degeneracy. In materials studied up to now, the four-fold degeneracy, however, has been vulnerable to suppression by the ubiquitous spin-orbit coupling (SOC). Despite the current effort to discover 3D DNLSs that are robust to SOC by theory, positive experimental evidence is yet to emerge. In 2D DNLSs, because of the decreased total density of states as compared with their 3D counterparts, it is anticipated that their physical properties would be dominated by the electronic states defined by the DNL. It has been even more challenging, however, to discover robust 2D DNLSs against SOC because of their lowered symmetry; no such materials have yet been predicted by theory. By combining molecular beam epitaxy growth, STM, nc-AFM characterisation, with DFT calculations and space group theory analysis, here we reveal a novel class of 2D crystalline DNLSs that host the exact symmetry that protects them against SOC. The discovered quantum material is a brick phase 3-AL Bi(110), whose symmetry protection and thermal stability are imparted by the compressive vdW epitaxial growth on black phosphorus substrates. The BP substrate templates the growth of 3-AL Bi(110) nano-islands in a non-symmorphic space group structure. This crystalline symmetry protects the DNL electronic phase against SOC independent of any orbital or elemental factors. We theoretically establish that this intrinsic symmetry imparts a general, robust protection of DNL in a series of isostructural 2D quantum materials.

preprint2020arXiv

3D Nanomagnetism in Low Density Interconnected Nanowire Networks

Free-standing, interconnected metallic nanowire networks with density as low as 40 mg/cm^{3} have been achieved over cm-scale areas, using electrodeposition into polycarbonate membranes that have been ion-tracked at multiple angles. Networks of interconnected magnetic nanowires further provide an exciting platform to explore 3-dimensional nanomagnetism, where their structure, topology and frustration may be used as additional degrees of freedom to tailor the materials properties. New magnetization reversal mechanisms in cobalt networks are captured by the first-order reversal curve method, which demonstrate the evolution from strong demagnetizing dipolar interactions to intersections-mediated domain wall pinning and propagation, and eventually to shape-anisotropy dominated magnetization reversal. These findings open up new possibilities for 3-dimensional integrated magnetic devices for memory, complex computation, and neuromorphics.

preprint2020arXiv

Concept of the Solar Ring Mission: Overview

The concept of the Solar Ring mission was gradually formed from L5/L4 mission concept, and the proposal of its pre-phase study was funded by the National Natural Science Foundation of China in November 2018 and then by the Strategic Priority Program of Chinese Academy of Sciences in space sciences in May 2019. Solar Ring mission will be the first attempt to routinely monitor and study the Sun and inner heliosphere from a full 360-degree perspective in the ecliptic plane. The current preliminary design of the Solar Ring mission is to deploy six spacecraft, grouped in three pairs, on a sub-AU orbit around the Sun. The two spacecraft in each group are separated by about 30 degrees and every two groups by about 120 degrees. This configuration with necessary science payloads will allow us to establish three unprecedented capabilities: (1) determine the photospheric vector magnetic field with unambiguity, (2) provide 360-degree maps of the Sun and the inner heliosphere routinely, and (3) resolve the solar wind structures at multiple scales and multiple longitudes. With these capabilities, the Solar Ring mission aims to address the origin of solar cycle, the origin of solar eruptions, the origin of solar wind structures and the origin of severe space weather events. The successful accomplishment of the mission will advance our understanding of the star and the space environment that hold our life and enhance our capability of expanding the next new territory of human.

preprint2020arXiv

In-plane Néel wall chirality and orientation of interfacial Dzyaloshinskii-Moriya vector in magnetic films

The interfacial Dzyaloshinskii-Moriya interaction (DMI) is of great interest as it can stabilize chiral spin structures in thin films. Experiments verifying the orientation of the interfacial DMI vector remain rare, in part due to the difficulty of separating vector components of DMI. In this study, Fe/Ni bilayers and Co/Ni multilayers were deposited epitaxially onto Cu(001) and Pt(111) substrates, respectively. By tailoring the effective anisotropy, spin reorientation transitions (SRTs) are employed to probe the orientation of the DMI vector by measuring the spin structure of domain walls on both sides of the SRTs. The interfacial DMI is found to be sufficiently strong to stabilize chiral Néel walls in the out-of-plane magnetized regimes, while achiral Néel walls are observed in the in-plane magnetized regimes. These findings experimentally confirm that the out-of-plane component of the DMI vector is insignificant in these fcc(001) and fcc(111) oriented interfaces, even in the presence of atomic steps.

preprint2020arXiv

Nitrogen magneto-ionics

So far, magneto-ionics, understood as voltage-driven ion transport in magnetic materials, has largely relied on controlled migration of oxygen ion/vacancy and, to a lesser extent, lithium and hydrogen. Here, we demonstrate efficient, room-temperature, voltage-driven nitrogen transport (i.e., nitrogen magneto-ionics) by electrolyte-gating of a single CoN film (without an ion-reservoir layer). Nitrogen magneto-ionics in CoN is compared to oxygen magneto-ionics in Co3O4, both layers showing a nanocrystalline face-centered-cubic structure and reversible voltage-driven ON-OFF ferromagnetism. In contrast to oxygen, nitrogen transport occurs uniformly creating a plane-wave-like migration front, without assistance of diffusion channels. Nitrogen magneto-ionics requires lower threshold voltages and exhibits enhanced rates and cyclability. This is due to the lower activation energy for ion diffusion and the lower electronegativity of nitrogen compared to oxygen. These results are appealing for the use of magneto-ionics in nitride semiconductor devices, in applications requiring endurance and moderate speeds of operation, such as brain-inspired computing.

preprint2020arXiv

Synergistically creating sulfur vacancies in semimetal-supported amorphous MoS2 for efficient hydrogen evolution

The presence of elemental vacancies in materials is inevitable according to statistical thermodynamics, which will decide the chemical and physical properties of the investigated system. However, the controlled manipulation of vacancies for specific applications is a challenge. Here we report a facile method for creating large concentrations of S vacancies in the inert basal plane of MoS2 supported on semimetal CoMoP2. With a small applied potential, S atoms can be removed in the form of H2S due to the optimized free energy of formation. The existence of vacancies favors electron injection from the electrode to the active site by decreasing the contact resistance. As a consequence, the activity is increased by 221 % with the vacancy-rich MoS2 as electrocatalyst for hydrogen evolution reaction (HER). A small overpotential of 75 mV is needed to deliver a current density of 10 mA cm-2, which is considered among the best values achieved for MoS2. It is envisaged that this work may provide a new strategy for utilizing the semimetal phase for structuring MoS2 into a multi-functional material.

preprint2020arXiv

The ABC130 barrel module prototyping programme for the ATLAS strip tracker

For the Phase-II Upgrade of the ATLAS Detector, its Inner Detector, consisting of silicon pixel, silicon strip and transition radiation sub-detectors, will be replaced with an all new 100 % silicon tracker, composed of a pixel tracker at inner radii and a strip tracker at outer radii. The future ATLAS strip tracker will include 11,000 silicon sensor modules in the central region (barrel) and 7,000 modules in the forward region (end-caps), which are foreseen to be constructed over a period of 3.5 years. The construction of each module consists of a series of assembly and quality control steps, which were engineered to be identical for all production sites. In order to develop the tooling and procedures for assembly and testing of these modules, two series of major prototyping programs were conducted: an early program using readout chips designed using a 250 nm fabrication process (ABCN-25) and a subsequent program using a follow-up chip set made using 130 nm processing (ABC130 and HCC130 chips). This second generation of readout chips was used for an extensive prototyping program that produced around 100 barrel-type modules and contributed significantly to the development of the final module layout. This paper gives an overview of the components used in ABC130 barrel modules, their assembly procedure and findings resulting from their tests.

preprint2020arXiv

Vehicular Fog Computing Enabled Real-time Collision Warning via Trajectory Calibration

Vehicular fog computing (VFC) has been envisioned as a promising paradigm for enabling a variety of emerging intelligent transportation systems (ITS). However, due to inevitable as well as non-negligible issues in wireless communication, including transmission latency and packet loss, it is still challenging in implementing safety-critical applications, such as real-time collision warning in vehicular networks. In this paper, we present a vehicular fog computing architecture, aiming at supporting effective and real-time collision warning by offloading computation and communication overheads to distributed fog nodes. With the system architecture, we further propose a trajectory calibration based collision warning (TCCW) algorithm along with tailored communication protocols. Specifically, an application-layer vehicular-to-infrastructure (V2I) communication delay is fitted by the Stable distribution with real-world field testing data. Then, a packet loss detection mechanism is designed. Finally, TCCW calibrates real-time vehicle trajectories based on received vehicle status including GPS coordinates, velocity, acceleration, heading direction, as well as the estimation of communication delay and the detection of packet loss. For performance evaluation, we build the simulation model and implement conventional solutions including cloud-based warning and fog-based warning without calibration for comparison. Real-vehicle trajectories are extracted as the input, and the simulation results demonstrate that the effectiveness of TCCW in terms of the highest precision and recall in a wide range of scenarios.

preprint2019arXiv

Band splitting with vanishing spin polarizations in noncentrosymmetric crystals

The Dresselhaus and Rashba effects are well-known phenomena in solid-state physics, in which spin-orbit coupling (SOC) splits spin-up and spin-down energy bands of nonmagnetic non-centrosymmetric crystals. Here, we discover a new phenomenon, dubbed as band splitting with vanishing spin polarizations (BSVSP), in which, as usual, SOC splits the energy bands in nonmagnetic non-centrosymmetric systems; surprisingly, however, both split bands show no net spin polarization along certain high-symmetry lines in the Brillouin zone. In order to rationalize this phenomenon, we propose a new classification of point groups into pseudo-polar and non-pseudo-polar groups. By means of first-principles simulations, we demonstrate that BSVSP can take place in both symmorphic (e.g., bulk GaAs) and non-symmorphic systems (e.g., two dimensional ferroelectric SnTe). Furthermore, we propose a novel linear magnetoelectric coupling in reciprocal space, which could be employed to tune the spin polarization with an external electric field. The BSVSP effect and its manipulation could therefore pave a new way to novel spintronic devices.

preprint2019arXiv

Emergent superconductivity in single crystalline $\mathrm{MgTi}_2\mathrm{O}_4$ films via structural engineering

Spinel compounds have demonstrated rich functionalities but rarely shown superconductivity. Here, we report the emergence of superconductivity in the spinel $\mathrm{MgTi}_2\mathrm{O}_4$, known to be an insulator with a complicated order. The superconducting transition is achieved by engineering a superlattice of $\mathrm{MgTi}_2\mathrm{O}_4$ and $\mathrm{SrTiO}_3$. The onset transition temperature in the $\mathrm{MgTi}_2\mathrm{O}_4$ layer can be tuned from 0 to 5 K in such geometry, concurrently with a stretched $c$-axis (from 8.51 to 8.53 Å) compared to the bulk material. Such a positive correlation without saturation suggests ample room for the further enhancement. Intriguingly, the superlattice exhibits isotropic upper critical field $H_{\mathrm{c}2}$ that breaks the Pauli limit, distinct from the highly anisotropic feature of interface superconductivity. The origin of superconductivity in the $\mathrm{MgTi}_2\mathrm{O}_4$ layer is understood in combination with the electron energy loss spectra and the first-principles electronic structure calculations, which point to the birth of superconductivity in the $\mathrm{MgTi}_2\mathrm{O}_4$ layer by preventing the Ti-Ti dimerization. Our discovery not only provides a platform to explore the interplay between the superconductivity and other exotic states, but also opens a new window to realize superconductivity in the spinel compounds as well as other titanium oxides.

preprint2019arXiv

Interfacial-Redox-Induced Tuning of Superconductivity in YBa$_{2}$Cu$_{3}$O$_{7-δ}$

Solid state ionic approaches for modifying ion distributions in getter/oxide heterostructures offer exciting potentials to control material properties. Here we report a simple, scalable approach allowing for total control of the superconducting transition in optimally doped YBa$_{2}$Cu$_{3}$O$_{7-δ}$ (YBCO) films via a chemically-driven ionic migration mechanism. Using a thin Gd capping layer of up to 20 nm deposited onto 100 nm thick epitaxial YBCO films, oxygen is found to leach from deep within the YBCO. Progressive reduction of the superconducting transition is observed, with complete suppression possible for a sufficiently thick Gd layer. These effects arise from the combined impact of redox-driven electron doping and modification of the YBCO microstructure due to oxygen migration and depletion. This work demonstrates an effective ionic control of superconductivity in oxides, an interface induced effect that goes well into the quasi-bulk regime, opening up possibilities for electric field manipulation.

preprint2019arXiv

Interlayer quantum transport in Dirac semimetal BaGa$_2$

Quantum limit is quite easy to achieve once the band crossing exists exactly at the Fermi level ($E_F$) in topological semimetals. In multilayered Dirac fermion system, the density of Dirac fermions on the zeroth Landau levels (LLs) increases in proportion to the magnetic field, resulting in intriguing angle- and field-dependent interlayer tunneling conductivity near the quantum limit. BaGa$_2$ is an example of multilayered Dirac semimetal with anisotropic Dirac cone close to $E_F$, providing a good platform to study its interlayer transport properties. In this paper, we report the negative interlayer magnetoresistance (NIMR, I//c and B//c) induced by the tunneling of Dirac fermions on the zeroth LLs of neighbouring Ga layers in BaGa$_2$. When the field deviates from the c-axis, the interlayer resistivity $ρ_{zz}(θ)$ increases and finally results in a peak with the field perpendicular to the c-axis. These unusual interlayer transport properties (NIMR and resistivity peak with B$\perp$c) are observed together for the first time in Dirac semimetal under ambient pressure and are well explained by the model of tunneling between Dirac fermions in the quantum limit.

preprint2019arXiv

Nanoparticles manipulation in 3D nanotips excited with plasmonic vortex

Recent advances in nanotechnologies have prompted the need for tools to accurately and non invasively manipulate individual nanoobjects. Among the possible strategies, optical forces have been widely used to enable nano optical tweezers capable of trapping or moving a specimen with unprecedented accuracy. Here, we propose an architecture consisting of a nanotip excited with a plasmonic vortex enabling effective dynamical control of nanoparticles in three dimensions. The optical field generated by the structure can be used to manipulate single dielectric nanoparticles acting on the total angular momentum of light used to illuminate the structure. We demonstrate that it is possible to stably trap or force the beaming of the particle from specific points, thus enabling a new platform for nanoparticle manipulation and sorting.

preprint2019arXiv

Possible phonon-induced electronic bi-stability in VO$_2$ for ultrafast memory at room temperature

VO$_{2}$ is a model material system which exhibits a metal to insulator transition at 67$^\circ$C. This holds potential for future ultrafast switching in memory devices, but typically requires a purely electronic process to avoid the slow lattice response. The role of lattice vibrations is thus important, but it is not well understood and it has been a long-standing source of controversy. We use a combination of ultrafast spectroscopy and ab initio quantum calculations to unveil the mechanism responsible for the transition. We identify an atypical Peierls vibrational mode which acts as a trigger for the transition. This rules out the long standing paradigm of a purely electronic Mott transition in VO$_{2}$; however, we found a new electron-phonon pathway for a purely reversible electronic transition in a true bi-stable fashion under specific conditions. This transition is very atypical, as it involves purely charge-like excitations and requires only small nuclear displacement. Our findings will prompt the design of future ultrafast electro-resistive non-volatile memory devices.

preprint2019arXiv

Quantum spin Hall effect in monolayer and bilayer TaIrTe$_{4}$

Generally, stacking two quantum spin Hall insulators gives rise to a trivial insulator. Here, based on first-principles electronic structure calculations, we confirm that monolayer TaIrTe$_{4}$ is a quantum spin Hall insulator and remarkably find that bilayer TaIrTe$_{4}$ is still a quantum spin Hall insulator. Theoretical analysis indicates that the covalent-like interlayer interaction in combination with the small bandgap at time-reversal invariant $Γ$ point results in new band inversion in bilayer TaIrTe$_{4}$, namely, the emergence of quantum spin Hall phase. Meanwhile, a topological phase transition can be observed by increasing the interlayer distance in bilayer TaIrTe$_{4}$. Considering that bulk TaIrTe$_{4}$ is a type-II Weyl semimetal, layered TaIrTe$_{4}$ thus provides an ideal platform to realize different topological phases at different dimensions.

preprint2019arXiv

Uni-traveling-carrier photodetector with high-contrast grating focusing-reflection mirrors

A novel uni-traveling-carrier photodetector (UTC-PD) structure with an integrated focusing-reflection (FR) mirror realized by a non-periodic concentric circular high-contrast grating (NP-CC-HCG), referred to as FR-UTC-PD, is proposed to enhance responsivity in conventional UTC-PDs. The FR-UTC-PD allows improving the responsivity by 36.5% at a 1.55-um wavelength as compared to a UTC-PD without integrated an FR mirror with 84.59% reflectivity. For 40-um-diameter PDs, the obtained 3-dB bandwidths are unaltered with values of 18 GHz at -3.0 V bias voltage. The radio-frequency (RF) output power and photocurrent are -1.77 dBm and 17.56 mA, respectively, at 10 GHz and the -6.0 V bias voltage.

preprint2018arXiv

Optical control of magnetism in NiFe/VO2 heterostructures

Optical methods for magnetism manipulation have been considered as a promising strategy for ultralow-power and ultrahigh-speed spin switches, which becomes a hot spot in the field of spintronics. However, a widely applicable and efficient method to combine optical operation with magnetic modulation is still highly desired. Here, the strongly correlated electron material VO2 is introduced to realize phase-transition based optical control of the magnetism in NiFe. The NiFe/VO2 bilayer heterostructure features appreciable modulations in electrical conductivity (55%), coercivity (60%), and magnetic anisotropy (33.5%). Further analyses indicate that interfacial strain coupling plays a crucial role in this modulation. Utilizing this optically controlled magnetism modulation feature, programmable Boolean logic gates (AND, OR, NAND, NOR, XOR, NXOR and NOT) for high-speed and low-power data processing are demonstrated based on this engineered heterostructure. As a demonstration of phase-transition spintronics, this work may pave the way for next-generation electronics in the post-Moore era.

preprint2016arXiv

A certification scheme for the boson sampler

Boson sampling can provide strong evidence that the computational power of a quantum computer outperforms a classical one via currently feasible linear optics experiments. However, how to identify an actual boson sampling device against any classical computing imposters is an ambiguous problem due to the computational complexity class in which boson sampling lies. The certification protocol based on bosonic bunching fails to rule out the so-called mean-field sampling. We propose a certification scheme to distinguish the boson sampling from the mean-field sampling for any random scattering matrices chosen via the Harr-measure. We numerically analyze our scheme and the influence of imperfect input states caused by non-simultaneous arrival photons.

preprint2016arXiv

A Routing Mechanism Based on Social Networks and Betweenness Centrality in Delay-Tolerant Networks

With the growing popularity of mobile smart devices, the existing networks are unable to meet the requirement of many complex scenarios; current network architectures and protocols do not work well with the network with high latency and frequent disconnections. To improve the performance of these networks some scholars opened up a new research field, delay-tolerant networks, in which one of the important research subjects is the forwarding and routing mechanism of data packets. This paper presents a routing scheme based on social networks owing to the fact that nodes in computer networks and social networks have high behavioural similarity. To further improve efficiency this paper also suggests a mechanism, which is the improved version of an existing betweenness centrality based routing algorithm. The experiments showed that the proposed scheme has better performance than the existing friendship routing algorithms.

preprint2016arXiv

Beyond the Interface Limit: Structural and Magnetic Depth Profiles of Voltage-Controlled Magneto-Ionic Heterostructures

Electric-field control of magnetism provides a promising route towards ultralow power information storage and sensor technologies. The effects of magneto-ionic motion have so far been prominently featured in the direct modification of interface chemical and physical characteristics. Here we demonstrate magnetoelectric coupling moderated by voltage-driven oxygen migration beyond the interface limit in relatively thick AlOx/GdOx/Co (15 nm) films. Oxygen migration and its ramifications on the Co magnetization are quantitatively mapped with polarized neutron reflectometry under thermal and electro-thermal conditionings. The depth-resolved profiles uniquely identify interfacial and bulk behaviors and a semi-reversible suppression and recovery of the magnetization. Magnetometry measurements show that the conditioning changes the microstructure so as to disrupt long-range ferromagnetic ordering, resulting in an additional magnetically soft phase. X-ray spectroscopy confirms electric field induced changes in the Co oxidation state but not in the Gd, suggesting that the GdOx transmits oxygen but does not source or sink it. These results together provide crucial insight into controlling magnetic heterostructures via magneto-ionic motion, not only at the interface, but also throughout the bulk of the films.

preprint2016arXiv

Crystallographic dependent transport properties and oxygen issue in superconducting LiTi2O4 thin films

A systematic study is performed on the spinel oxide, i.e. LiTi2O4 thin films oriented along [111]-, [110]-, and [001]-directions, to reveal the crystallographic dependence of transport properties. With decreasing temperature, the LiTi2O4 displays roughly identical onset temperatures of twofold symmetry of in-plane angular dependent magnetoresistivity (AMR) (at ~ 100 K), crossover from negative- to positive- magnetoresistance (at ~ 50 K), and coherence length in the superconducting state. While, the twofold symmetry in AMR itself suggests anisotropic electron scattering. The superconducting transition temperature (Tc) seems insensitive to the lattice parameter. Moreover, the spherical aberration-corrected scanning transmission electron microscopy (Cs-STEM) discloses that oxygen vacancies exist in the LiTi2O4 films. These oxygen vacancies cause the change of lattice but show little influence on superconductivity, differing from high-Tc cuprates where subtle variation of oxygen way lead to a significant change in superconductivity.

preprint2016arXiv

Damped large amplitude oscillations in a solar prominence and a bundle of coronal loops

We investigate the evolutions of two prominences (P1,P2) and two bundles of coronal loops (L1,L2), observed with SDO/AIA near the east solar limb on 2012 September 22. It is found that there were large-amplitude oscillations in P1 and L1, but no detectable motions in P2 and L2. These transverse oscillations were triggered by a large-scale coronal wave, originating from a large flare in a remote active region behind the solar limb. By carefully comparing the locations and heights of these oscillating and non-oscillating structures, we conclude that the propagating height of the wave is between 50 Mm and 130 Mm. The wave energy deposited in the oscillating prominence and coronal loops is at least of the order of $10^{28}$ erg. Furthermore, local magnetic field strength and Alfvén speeds are derived from the oscillating periods and damping time scales, which are extracted from the time series of the oscillations. It is demonstrated that oscillations can be used in not only coronal seismology, but also revealing the properties of the wave.

preprint2016arXiv

Dynamics of a multicomponent vesicle in shear flow

We study the fully nonlinear, nonlocal dynamics of two-dimensional multicomponent vesicles in a shear flow with matched viscosity of the inner and outer fluids. Using a nonstiff, pseudo-spectral boundary integral method, we investigate dynamical patterns induced by inhomogeneous bending for a two phase system. Numerical results reveal that there exist novel phase-treading and tumbling mechanisms that cannot be observed for a homogeneous vesicle. In particular, unlike the well-known steady tank-treading dynamics characterized by a fixed inclination angle, here the phase-treading mechanism leads to unsteady periodic dynamics with an oscillatory inclination angle. When the average phase concentration is around 1/2, we observe tumbling dynamics even for very low shear rate, and the excess length required for tumbling is significantly smaller than the value for the single phase case. We summarize our results in phase diagrams in terms of the excess length, shear rate, and concentration of the soft phase. These findings go beyond the well known dynamical regimes of a homogeneous vesicle and highlight the level of complexity of vesicle dynamics in a fluid due to heterogeneous material properties.

preprint2016arXiv

Interplay between multiple charge-density waves and the relationship with superconductivity in Pd$_x$HoTe$_{3}$

HoTe$_{3}$, a member of the rare-earth tritelluride ($R$Te$_{3}$) family, and its Pd-intercalated compounds, Pd$_x$HoTe$_{3}$, where superconductivity (SC) sets in as the charge-density wave (CDW) transition is suppressed by the intercalation of a small amount of Pd, are investigated using angle-resolved photoemission spectroscopy (ARPES) and electrical resistivity. Two incommensurate CDWs with perpendicular nesting vectors are observed in HoTe$_{3}$ at low temperatures. With a slight Pd intercalation ($x$ = 0.01), the large CDW gap decreases and the small one increases. The momentum dependence of the gaps along the inner Fermi surface (FS) evolves from orthorhombicity to near tetragonality, manifesting the competition between two CDW orders. At $x$ = 0.02, both CDW gaps decreases with the emergence of SC. Further increasing the content of Pd for $x$ = 0.04 will completely suppress the CDW instabilities and give rise to the maximal SC order. The evolution of the electronic structures and electron-phonon couplings (EPCs) of the multiple CDWs upon Pd intercalation are carefully scrutinized. We discuss the interplay between multiple CDW orders, and the competition between CDW and SC in detail.

preprint2016arXiv

Investigating Energetic X-Shaped Flares on the Outskirts of A Solar Active Region

Typical solar flares display two quasi-parallel, bright ribbons on the chromosphere. In between is the polarity inversion line (PIL) separating concentrated magnetic fluxes of opposite polarity in active regions (ARs). Intriguingly a series of flares exhibiting X-shaped ribbons occurred at the similar location on the outskirts of NOAA AR 11967, where magnetic fluxes were scattered, yet three of them were alarmingly energetic. The X shape, whose center coincided with hard X-ray emission, was similar in UV/EUV, which cannot be accommodated in the standard flare model. Mapping out magnetic connectivities in potential fields, we found that the X morphology was dictated by the intersection of two quasi-separatrix layers, i.e., a hyperbolic flux tube (HFT), within which a separator connecting a double null was embedded. This topology was not purely local but regulated by fluxes and flows over the whole AR. The nonlinear force-free field model suggested the formation of a current layer at the HFT, where the current dissipation can be mapped to the X-shaped ribbons via field-aligned heat conduction. These results highlight the critical role of HFTs in 3D magnetic reconnection and have important implications for astrophysical and laboratory plasmas.

preprint2016arXiv

Large magnetoresistance in LaBi: origin of field-induced resistivity upturn and plateau in compensated semimetals

The discovery of non-magnetic extreme magnetoresistance (XMR) materials has induced great interests because the XMR phenomenon challenges our understanding of how a magnetic field can alter electron transport in semimetals. Among XMR materials, the LaSb shows XMR and field-induced exotic behaviors but it seems to lack the essentials for these properties. Here, we study the magnetotransport properties and electronic structure of LaBi, isostructural to LaSb. LaBi exhibits large MR as in LaSb, which can be ascribed to the nearly compensated electron and hole with rather high mobilities. More importantly, our analysis suggests that the XMR as well as field-induced resistivity upturn and plateau observed in LaSb and LaBi can be well explained by the two-band model with the compensation situation. We present the critical conditions leading to these field-induced properties. It will contribute to understanding the XMR phenomenon and explore novel XMR materials.

preprint2016arXiv

Magnetic fingerprint of interfacial coupling between CoFe and nanoscale ferroelectric domain walls

Magnetoelectric coupling in ferromagnet/multiferroic systems is often manifested in the exchange bias effect, which may have combined contributions from multiple sources, such as domain walls, chemical defects or strain. In this study we magnetically "fingerprint" the coupling behavior of CoFe grown on epitaxial BiFeO3 (BFO) thin films by magnetometry and first-order-reversal-curves (FORC). The contribution to exchange bias from 71°, 109° and charged ferroelectric domain walls (DWs) was elucidated by the FORC distribution. CoFe samples grown on BFO with 71° DWs only exhibit an enhancement of the coercivity, but little exchange bias. Samples grown on BFO with 109° DWs and mosaic DWs exhibit a much larger exchange bias, with the main enhancement attributed to 109° and charged DWs. Based on the Malozemoff random field model, a varying-anisotropy model is proposed to account for the exchange bias enhancement. This work sheds light on the relationship between the exchange bias effect of the CoFe/BFO heterointerface and the ferroelectric DWs, and provides a path for multiferroic device analysis and design.

preprint2016arXiv

Magnetic Yoking and Tunable Interactions in FePt-Based Hard/Soft Bilayers

Assessing and controlling magnetic interactions in magnetic nanostructures are critical to nanomagnetic and spintronic explorations, such as magnetic recording media, permanent magnets, magnetic memory and logic devices, etc. Here we demonstrate an extremely sensitive magnetic yoking effect and tunable interactions in FePt based hard/soft bilayers mediated by the soft layer. Below the exchange length, a thin soft layer strongly exchange couples to the perpendicular moments of the hard layer; above the exchange length, just a few nanometers thicker, the soft layer moments turn in-plane and act to yoke the dipolar fields from the adjacent hard layer perpendicular domains. The evolution from exchange to dipolar-dominated interactions is experimentally captured by first-order reversal curves, the delta-M method, and polarized neutron reflectometry, and confirmed by micromagnetic simulations. These findings demonstrate an effective yoking approach to design and control magnetic interactions in wide varieties of magnetic nanostructures and devices.

preprint2016arXiv

Perfect charge compensation in extremely large magnetoresistance materials LaSb and LaBi revealed by the first-principles calculations

By the first-principles electronic structure calculations, we have systematically studied the electronic structures of recently discovered extremely large magnetoresistance (XMR) materials LaSb and LaBi. We find that both LaSb and LaBi are semimetals with the electron and hole carriers in perfect balance. The calculated carrier densities in the order of $10^{20}$ cm$^{-3}$ are in good agreement with the experimental values, implying long mean free time of carriers and thus high carrier mobilities. With a semiclassical two-band model, the perfect charge compensation and high carrier mobilities naturally explain (i) the XMR observed in LaSb and LaBi; (ii) the non-saturating quadratic dependence of XMR on external magnetic field; and (iii) the resistivity plateau in the turn-on temperature behavior at very low temperatures. The explanation of these features without resorting to the topological effect indicates that they should be the common characteristics of all perfectly electron-hole compensated semimetals.

preprint2016arXiv

Raman scattering in transition metal dichalcogenides MTe2 (M = Mo, W)

We performed comparable polarized Raman scattering studies of MoTe2 and WTe2. By rotating crystals to tune the angle between the principal axis of the crystals and the polarization of the incident/scattered light, we obtained the angle dependence of the intensities for all the observed modes, which is perfectly consistent with careful symmetry analysis. Combining these results with first-principles calculations, we clearly identified the observed phonon modes in the different phases of both crystals. Fifteen Raman-active phonon modes (10Ag+5Bg) in the high-symmetry phase 1T'-MoTe2 (300 K) were well assigned, and all the symmetry-allowed Raman modes (11A1+6A2) in the low-symmetry phase Td-MoTe2 (10 K) and 12 Raman phonons (8A1+4A2) in Td-WTe2 were observed and identified. The present work provides basic information about the lattice dynamics in transition-metal dichalcogenides and may shed some light on the understanding of the extremely large magnetoresistance (MR) in this class of materials.

preprint2016arXiv

Stereoscopic Observation of Slipping Reconnection in A Double Candle-Flame-Shaped Solar Flare

The 2011 January 28 M1.4 flare exhibits two side-by-side candle-flame-shaped flare loop systems underneath a larger cusp-shaped structure during the decay phase, as observed at the northwestern solar limb by the Solar Dynamics Observatory (SDO). The northern loop system brightens following the initiation of the flare within the southern loop system, but all three cusp-shaped structures are characterized by ~ 10 MK temperatures, hotter than the arch-shaped loops underneath. The "Ahead" satellite of the Solar Terrestrial Relations Observatory (STEREO) provides a top view, in which the post-flare loops brighten sequentially, with one end fixed while the other apparently slipping eastward. By performing stereoscopic reconstruction of the post-flare loops in EUV and mapping out magnetic connectivities, we found that the footpoints of the post-flare loops are slipping along the footprint of a hyperbolic flux tube (HFT) separating the two loop systems, and that the reconstructed loops share similarity with the magnetic field lines that are traced starting from the same HFT footprint, where the field lines are relatively flexible. These results argue strongly in favor of slipping magnetic reconnection at the HFT. The slipping reconnection was likely triggered by the flare and manifested as propagative dimmings before the loop slippage is observed. It may contribute to the late-phase peak in Fe XVI 33.5 nm, which is even higher than its main-phase counterpart, and may also play a role in the density and temperature asymmetry observed in the northern loop system through heat conduction.

preprint2016arXiv

Sudden gap-closure across the topological phase transition in Bi$_{2-x}$In$_{x}$Se$_{3}$

The phase transition from a topological insulator to a trivial band insulator is studied by angle-resoled photoemission spectroscopy on Bi$_{2-x}$In$_{x}$Se$_{3}$ single crystals. We first report the complete evolution of the bulk band structures throughout the transition. The robust surface state and the bulk gap size ($\sim$ 0.50 eV) show no significant change upon doping for $x$ = 0.05, 0.10 and 0.175. At $x$ $\geq$ 0.225, the surface state completely disappears and the bulk gap size increases, suggesting a sudden gap-closure and topological phase transition around $x \sim$ 0.175$-$0.225. We discuss the underlying mechanism of the phase transition, proposing that it is governed by the combined effect of spin-orbit coupling and interactions upon band hybridization. Our study provides a new venue to investigate the mechanism of the topological phase transition induced by non-magnetic impurities.

preprint2016arXiv

Thermodynamic Spectrum of Solar Flares Based on SDO/EVE Observations: Techniques and Statistical Results

SDO/EVE provides rich information of the thermodynamic processes of solar activities, particularly of solar flares. Here, we develop a method to construct thermodynamic spectrum (TDS) charts based on the EVE spectral lines. This tool could be potentially useful to the EUV astronomy to learn the eruptive activities on the distant astronomical objects. Through several cases, we illustrate what we can learn from the TDS charts. Furthermore, we apply the TDS method to 74 flares equal to or greater than M5.0-class, and reach the following statistical results. First, EUV peaks are always behind the soft X-ray (SXR) peaks and stronger flares tend to have a faster cooling rate. There is a power law correlation between the peak delay times and the cooling rates, suggesting a coherent cooling process of flares from SXR to EUV emissions. Second, there are two distinct temperature drift patterns, called Type I and Type II. For Type I flares, the enhanced emission drifts from high to low temperature like a quadrilateral, whereas for Type II flares, the drift pattern looks like a triangle. Statistical analysis suggests that Type II flares are more impulsive than Type I flares. Third, for late-phase flares, the peak intensity ratio of the late phase to the main phase is roughly correlated with the flare class, and the flares with a strong late phase are all confined. We believe that the re-deposition of the energy carried by a flux rope, that unsuccessfully erupts out, into thermal emissions is responsible for the strong late phase found in a confined flare. Besides, with some cases we illustrate the signatures of the flare thermodynamic process in the chromosphere and transition region in TDS charts. These results provide new clues to advance our understanding of the thermodynamic processes of solar flares and associated solar eruptions, e.g., coronal mass ejections.

preprint2016arXiv

Tropical variants of some complex analysis results

Tropical Nevanlinna theory studies value distribution of continuous piecewise linear functions of a real variable. In this paper, we use the reasoning from tropical Nevanlinna theory to present tropical counterparts of some classical complex results related to Fermat type equations, Hayman conjecture and Bruck conjecture.

preprint2015arXiv

A Solar Coronal Jet Event Triggers A Coronal Mass Ejection

We present the multi-point and multi-wavelength observation and analysis on a solar coronal jet and coronal mass ejection (CME) event in this paper. Employing the GCS model, we obtained the real (three-dimensional) heliocentric distance and direction of the CME and found it propagate in a high speed over 1000 km/s . The jet erupted before and shared the same source region with the CME. The temporal and spacial relation- ship between them guide us the possibility that the jet triggered the CME and became its core. This scenario could promisingly enrich our understanding on the triggering mechanism of coronal mass ejections and their relations with coronal large-scale jets. On the other hand, the magnetic field configuration of the source region observed by the SDO/HMI instrument and the off- limb inverse Y-shaped configuration observed by SDO/AIA 171 A passband, together provide the first detailed observation on the three-dimensional reconnection process of large-scale jets as simulated in Pariat et al. 2009. The erupting process of the jet highlights that filament-like materials are important during the eruption not only of small-scale X-ray jets (Sterling et al. 2015) but also probably of large-scale EUV jets. Based on our observation and analysis, we propose a most possible mechanism for the whole event with a blob structure overlaying the three-dimensional structure of the jet to describe the interaction between the jet and the CME.

preprint2015arXiv

An M-Estimator for Reduced-Rank High-Dimensional Linear Dynamical System Identification

High-dimensional time-series data are becoming increasingly abundant across a wide variety of domains, spanning economics, neuroscience, particle physics, and cosmology. Fitting statistical models to such data, to enable parameter estimation and time-series prediction, is an important computational primitive. Existing methods, however, are unable to cope with the high-dimensional nature of these problems, due to both computational and statistical reasons. We mitigate both kinds of issues via proposing an M-estimator for Reduced-rank System IDentification (MR. SID). A combination of low-rank approximations, L-1 and L-2 penalties, and some numerical linear algebra tricks, yields an estimator that is computationally efficient and numerically stable. Simulations and real data examples demonstrate the utility of this approach in a variety of problems. In particular, we demonstrate that MR. SID can estimate spatial filters, connectivity graphs, and time-courses from native resolution functional magnetic resonance imaging data. Other applications and extensions are immediately available, as our approach is a generalization of the classical Kalman Filter-Smoother Expectation-Maximization algorithm.

preprint2015arXiv

Extremely Large EUV Late Phase of Solar Flares

The second peak in the Fe XVI 33.5 nm line irradiance observed during solar flares by Extreme ultraviolet Variability Experiment (EVE) is known as Extreme UltraViolet (EUV) late phase. Our previous paper (Liu et al. 2013) found that the main emissions in the late phase are originated from large-scale loop arcades that are closely connected to but different from the post flare loops (PFLs), and we also proposed that a long cooling process without additional heating could explain the late phase. In this paper, we define the extremely large late phase because it not only has a bigger peak in the warm 33.5 irradiance profile, but also releases more EUV radiative energy than the main phase. Through detailedly inspecting the EUV images from three point-of-view, it is found that, besides the later phase loop arcades, the more contribution of the extremely large late phase is from a hot structure that fails to erupt. This hot structure is identified as a flux rope, which is quickly energized by the flare reconnection and later on continuously produces the thermal energy during the gradual phase. Together with the late-phase loop arcades, the fail to erupt flux rope with the additional heating create the extremely large EUV late phase.

preprint2015arXiv

First-principles study of FeSe epitaxial films on SrTiO3

The discovery of high temperature superconductivity in FeSe films on SrTiO3 substrate has inspired great experimental and theoretical interests. First-principles density functional theory calculations, which have played an important role in the study of bulk iron-based superconductors, also participate in the investigation of interfacial superconductivity. In this article, we review the calculation results on the electronic and magnetic structures of FeSe epitaxial films, emphasizing on the interplay between different degrees of freedom, such as charge, spin, and lattice vibrations. Furthermore, the comparison between FeSe monolayer and bilayer films on SrTiO3 is discussed.

preprint2015arXiv

First-principles study of magnetic frustration in FeSe epitaxial films on SrTiO$_3$

The effects of electron doping and phonon vibrations on the magnetic properties of monolayer and bilayer FeSe epitaxial films on SrTiO$_3$ have been studied, respectively, using first-principles calculations with van der Waals correction. For monolayer FeSe epitaxial film, the combined effect of electron doping and phonon vibrations readily leads to magnetic frustration between the collinear antiferromagnetic state and the checkerboard antiferromagnetic Néel state. For bilayer FeSe epitaxial film, such magnetic frustration is much more easily induced by electron doping in its bottom layer than its top layer. The underlying physics is that the doped electrons are accumulated at the interface between the FeSe layers and the substrate. These results are consistent with existing experimental studies.

preprint2015arXiv

Nematic antiferromagnetic states in bulk FeSe

We revisit bulk FeSe through the systematic first-principles electronic structure calculations. We find that there are a series of staggered $n$-mer antiferromagnetic (AFM) states with corresponding energies below that of the collinear AFM state which is the ground state for the parent compounds of most iron-based superconductors. Here the staggered $n$-mer ($n$ any integer $>1$) means that a set of $n$ adjacent spins parallel on a line along $b$-axis with spins in antiparallel between $n$-mers and along $a$-axis. Among them, the lowest energy states are quasi-degenerate staggered dimer and staggered trimer AFM states as well as their any staggered combinations. Thus, to have the largest entropy to minimize the free energy at low temperature, the most favorable state is such a quasi-one-dimensional antiferromagnet in which along $b$-axis a variety of $n$-mers, mostly dimers and trimers, are randomly antiparallel aligned while along $a$-axis spins are antiparallel aligned, i.e. actually a nematic paramagnet. This finding accounts well for the absence of long-range magnetic order in bulk FeSe and meanwhile indicates the dominant stripe spin fluctuation and the nematicity as spin-driven.

preprint2015arXiv

New Reversal Mode in Exchange Coupled Antiferromagnetic/Ferromagnetic Disks: Distorted Viscous Vortex

Magnetic vortices have generated intense interest in recent years due to their unique reversal mechanisms, fascinating topological properties, and exciting potential applications. Additionally, the exchange coupling of magnetic vortices to antiferromagnets has also been shown to lead to a range of novel phenomena and functionalities. Here we report a new magnetization reversal mode of magnetic vortices in exchange coupled Ir20Mn80/Fe20Ni80 microdots: distorted viscous vortex reversal. Contrary to the previously known or proposed reversal modes, the vortex is distorted close to the interface and viscously dragged due to the uncompensated spins of a thin antiferromagnet, which leads to unexpected asymmetries in the annihilation and nucleation fields. These results provide a deeper understanding of the physics of exchange coupled vortices and may also have important implications for applications involving exchange coupled nanostructures.

preprint2015arXiv

On the origin of the Extreme-Ultraviolet late phase of solar flares

Solar flares typically have an impulsive phase that followed by a gradual phase as best seen in soft X-ray emissions. A recent discovery based on the EUV Variability Experiment (EVE) observations onboard the Solar Dynamics Observatory (SDO) reveals that some flares exhibit a second large peak separated from the first main phase peak by tens of minutes to hours, which is coined as the flare's EUV late phase. In this paper, we address the origin of the EUV late phase by analyzing in detail two late phase flares, an M2.9 flare on 2010 October 16 and an M1.4 flare on 2011 February 18, using multi-passband imaging observations from the Atmospheric Imaing Assembly (AIA) onboard SDO. We find that: (1) the late phase emission originates from a different magnetic loop system, which is much larger and higher than the main phase loop system. (2) The two loop systems have different thermal evolution. While the late phase loop arcade reaches its peak brightness progressively at a later time spanning for more than one hour from high to low temperatures, the main phase loop arcade reaches its peak brightness at almost the same time (within several minutes) in all temperatures. (3) Nevertheless, the two loop systems seem to be connected magnetically, forming an asymmetric magnetic quadruple configuration. (4) Further, the footpoint brightenings in UV wavelengths show a systematic delay of about one minute from the main flare region to the remote footpoint of the late phase arcade system. We argue that the EUV late phase is the result of a long-lasting cooling process in the larger magnetic arcade system.

preprint2015arXiv

Raman scattering in superconducting NdO1-xFxBiS2 crystals

The recently discovered layered BiS2-based superconductors have attracted a great deal of interest due to their structural similarity to cuprate and iron-pnictide superconductors. We have performed Raman scattering measurements on two superconducting crystals NdO0.5F0.5BiS2 (Tc = 4.5 K) and NdO0.7F0.3BiS2 (Tc = 4.8 K). The observed Raman phonon modes are assigned with the aid of first-principles calculations. The asymmetrical phonon mode around 118 cm-1 reveals a small electron-phonon (e-ph) coupling constant 0.16, which is insufficient to generate superconductivity at ~ 4.5 K. In the Raman spectra there exists a clear temperature-dependent hump around 100 cm-1, which can be well understood in term of inter-band vertical transitions around Fermi surface. The transitions get boosted when the particular rectangular-like Fermi surface meets band splitting caused by spin-orbit coupling. It enables a unique and quantitative insight into the band splitting.

preprint2015arXiv

Strong charge density wave fluctuation and sliding state in PdTeI with quasi-1D PdTe chains

In quasi-one-dimensional (quasi-1D) system, the charge density wave (CDW) transition temperature TCDW is usually lower than the mean-field-theory predicted TMF and a CDW fluctuation region exists between them. Here, we investigate the physical properties of PdTeI single crystal containing quasi-1D PdTe chains. Surprisingly, we find that the carrier concentration decreases gradually before the long-range CDW ordering state occurring at T1 ~ 110 K, reflecting the existence of strong CDW fluctuation with possible pseudogap state at T >> T1 because of dynamic charge separation of Pd ions (Pd3+ -> Pd2+ + Pd4+). Moreover, the sliding CDW state appears below T2 ~ 6 K. Combined such low T2 with the feature of multiple quasi-1D bands, PdTeI exhibits exotic crossover behavior from negative to huge positive magnetoresistance under magnetic field and field-induced localization. Thus, PdTeI provides a novel platform for studying the CDW fluctuation and the interplay between magnetic field and CDW state.

preprint2015arXiv

Study of $ψ(3686)\toπ^0 h_c, h_c\toγη_c$ via $η_c$ exclusive decays

The process $ψ(3686) \to π^0 h_c, h_c \to γη_c$ has been studied with a data sample of $106 \pm 4$ million $ψ(3686)$ events collected with the BESIII detector at the BEPCII storage ring. The mass and width of the $P$-wave charmonium spin-singlet state $h_c(^1P_1)$ are determined by simultaneously fitting distributions of the $π^0$ recoil mass for 16 exclusive $η_c$ decay modes. The results, $M(\hc) = 3525.31 \pm 0.11 {\rm (stat.)} \pm 0.14 {\rm (syst.)}$\,MeV/$c^2$ and $Γ(\hc) = 0.70 \pm 0.28 \pm 0.22$\,MeV, are consistent with and more precise than previous measurements. We also determine the branching ratios for the 16 exclusive $η_c$ decay modes, five of which have not been measured previously. New measurements of the $η_c$ line-shape parameters in the $E1$ transition $h_c\toγη_c$ are made by selecting candidates in the $h_c$ signal sample and simultaneously fitting the hadronic mass spectra for the 16 $η_c$ decay channels. The resulting $η_c$ mass and width values are $M(η_c) = 2984.49 \pm 1.16 \pm 0.52$\,MeV/$c^2$ and $Γ(η_c) = 36.4 \pm 3.2 \pm 1.7$\,MeV.

preprint2015arXiv

The dichotomy structure of Y chromosome Haplogroup N

Haplogroup N-M231 of human Y chromosome is a common clade from Eastern Asia to Northern Europe, being one of the most frequent haplogroups in Altaic and Uralic-speaking populations. Using newly discovered bi-allelic markers from high-throughput DNA sequencing, we largely improved the phylogeny of Haplogroup N, in which 16 subclades could be identified by 33 SNPs. More than 400 males belonging to Haplogroup N in 34 populations in China were successfully genotyped, and populations in Northern Asia and Eastern Europe were also compared together. We found that all the N samples were typed as inside either clade N1-F1206 (including former N1a-M128, N1b-P43 and N1c-M46 clades), most of which were found in Altaic, Uralic, Russian and Chinese-speaking populations, or N2-F2930, common in Tibeto-Burman and Chinese-speaking populations. Our detailed results suggest that Haplogroup N developed in the region of China since the final stage of late Paleolithic Era.

preprint2015arXiv

Type-I superconductivity in KBi2 single crystals

We report the detailed transport, magnetic, thermodynamic properties and theoretical calculation of KBi2 single crystals in superconducting and normal states. KBi2 shows metallic behavior at normal state and enters superconducting state below Tc = 3.573 K. Moreover, KBi2 exhibits low critical fields in all of measurements, field-induced crossover from second to first-order phase transition in specific heat measurement, typical magnetization isotherms of type-I superconductors, and small Ginzburg-Landau parameter kappaGL = 0.611. These results clearly indicate that KB2 is a type-I superconductor with thermodynamic critical field Hc = 234.3(3) Oe.

preprint2015arXiv

Ultralow-frequency collective compression mode and strong interlayer coupling in multilayer black phosphorus

The recent renaissance of black phosphorus (BP) as a two-dimensional 2D layered material has generated tremendous interest in its tunable electronic band gap and highly anisotropic transport properties that offer new opportunities for device applications. Many of these outstanding properties are attributed to its unique structural characters that still need elucidation. Here we show Raman measurements that reveal an ultralow-frequency collective compression mode (CCM), which is unprecedented among similar 2D layered materials. This novel CCM indicates an unusually strong interlayer coupling in BP, which is quantitatively supported by a phonon frequency analysis and first-principles calculations. Moreover, the CCM and another branch of low-frequency Raman modes shift sensitively with changing number of layers, allowing an accurate determination of the thickness up to tens of atomic layers, which is considerably higher than those previously achieved by using high-frequency Raman modes. These results offer fundamental insights and practical tools for exploring multilayer BP in new device applications.

preprint2014arXiv

A Prominence Eruption Driven by Flux Feeding from Chromospheric Fibrils

We present multi-wavelength observations of a prominence eruption originating from a quadrupolar field configuration, in which the prominence was embedded in a side-arcade. Within the two-day period prior to its eruption on 2012 October 22, the prominence was perturbed three times by chromospheric fibrils underneath, which rose upward, became brightened, and merged into the prominence, resulting in horizontal flows along the prominence axis, suggesting that the fluxes carried by the fibrils were incorporated into the magnetic field of the prominence. These perturbations caused the prominence to oscillate and to rise faster than before. The absence of intense heating within the first two hours after the onset of the prominence eruption, which followed an exponential increase in height, indicates that ideal instability played a crucial role. The eruption involved interactions with the other side-arcade, leading up to a twin coronal mass ejection, which was accompanied by transient surface brightenings in the central arcade, followed by transient dimmings and brightenings in the two side-arcades. We suggest that flux feeding from chromospheric fibrils might be an important mechanism to trigger coronal eruptions.

preprint2014arXiv

Accessing Different Spin-Disordered States using First Order Reversal Curves

Combined first order reversal curve (FORC) analyses of the magnetization (M-FORC) and magnetoresistance (MR-FORC) have been employed to provide a comprehensive study of the M-MR correlation in two canonical systems: a NiFe/Cu/FePt pseudo spin-valve (PSV) and a [Co/Cu]8 multilayer. In the PSV, due to the large difference in switching fields and minimal interactions between the NiFe and FePt layers, the M and MR show a simple one-to-one relationship during reversal. In the [Co/Cu]8 multilayer, the correlation between the magnetization reversal and MR evolution is more complex. This is primarily due to the similar switching fields of, and interactions between, the constituent Co layers. The FORC protocol accesses states with much higher spin disorders and larger MR than those found along the conventional major loop field-cycle. Unlike the M-FORC measurements, which only probe changes in the macroscopic magnetization, the MR-FORCs are more sensitive to the microscopic domain configurations, as those are most important in determining the resultant MR effect size. This approach is generally applicable to spintronic systems to realize the maximum spin-disorder and the largest MR.

preprint2014arXiv

An Unorthodox X-Class Long-Duration Confined Flare

We report the observation of an X-class long-duration flare which is clearly confined. It appears as a compact-loop flare in the traditional EUV passbands (171 and 195 Å), but in the passbands sensitive to flare plasmas (94 and 131 Å), it exhibits a cusp-shaped structure above an arcade of loops like other long-duration events. Inspecting images in a running difference approach, we find that the seemingly diffuse, quasi-static cusp-shaped structure consists of multiple nested loops that repeatedly rise upward and disappear approaching the cusp edge. Over the gradual phase, we detect numerous episodes of loop rising, each lasting minutes. A differential emission measure analysis reveals that the temperature is highest at the top of the arcade and becomes cooler at higher altitudes within the cusp-shaped structure, contrary to typical long-duration flares. With a nonlinear force-free model, our analysis shows that the event mainly involves two adjacent sheared arcades separated by a T-type hyperbolic flux tube (HFT). One of the arcades harbors a magnetic flux rope, which is identified with a filament that survives the flare owing to the strong confining field. We conclude that a new emergence of magnetic flux in the other arcade triggers the flare, while the preexisting HFT and flux rope dictate the structure and dynamics of the flare loops and ribbons during the long-lasting decay phase, and that a quasi-separatrix layer high above the HFT could account for the cusp-shaped structure.

preprint2014arXiv

Deposition Order Dependent Magnetization Reversal in Pressure Graded Co/Pd films

Magnetization reversal mechanisms and depth-dependent magnetic profile have been investigated in Co/Pd thin films magnetron-sputtered under continuously varying pressure with opposite deposition orders. For samples grown under increasing pressure, magnetization reversal is dominated by domain nucleation, propagation and annihilation; an anisotropy gradient is effectively established, along with a pronounced depth-dependent magnetization profile. However, in films grown under decreasing pressure, disorders propagate vertically from the bottom high-pressure region into the top low-pressure region, impeding domain wall motion and forcing magnetization reversal via rotation; depth-dependent magnetization varies in an inverted order, but the spread is much suppressed.

preprint2014arXiv

Effective anisotropy gradient in pressure graded [Co/Pd] multilayers

A vertically graded anisotropy profile has been proposed as an optimized balance of low coercivity and thermal stability for multilayers used in magnetic media. Deposition pressure is known to have a profound effect on the magnetic reversal properties of Co/Pd multilayers, making it an attractive control parameter for achieving an anisotropy gradient. We have used polarized neutron reflectometry to study the depth-dependent reversal behavior of "pressure-graded" Co/Pd, and observed pronounced gradients in the saturation magnetization and in the rate at which magnetization changes with field (the effective anisotropy). While the anisotropy gradient likely arises from a combination of factors intrinsic to deposition pressure, micromagnetic simulations indicate that the observed saturation magnetization gradient alone has a major effect on the resulting coercivity.

preprint2014arXiv

Elastic Properties of Chemical-Vapor-Deposited Monolayer MoS2, WS2, and Their Bilayer Heterostructures

Elastic properties of materials are an important factor in their integration in applications. Chemical vapor deposited (CVD) monolayer semiconductors are proposed as key components in industrial-scale flexible devices and building blocks of 2D van der Waals heterostructures. However, their mechanical and elastic properties have not been fully characterized. Here we report high 2D elastic moduli of CVD monolayer MoS2 and WS2 (~ 170 N/m), which is very close to the value of exfoliated MoS2 monolayers and almost half the value of the strongest material, graphene. The 2D moduli of their bilayer heterostructures are lower than the sum of 2D modulus of each layer, but comparable to the corresponding bilayer homostructure, implying similar interactions between the hetero monolayers as between homo monolayers. These results not only provide deep insight to understanding interlayer interactions in 2D van der Waals structures, but also potentially allow engineering of their elastic properties as desired.

preprint2014arXiv

Existence of Invariant Measures of Stochastic Systems with Delay in the Highest Order Partial Derivatives

In this note, we shall consider the existence of invariant measures for a class of infinite dimensional stochastic functional differential equations with delay whose driving semigroup is eventually norm continuous. The results obtained are applied to stochastic heat equations with distributed delays which appear in such terms having the highest order partial derivatives. In the systems, the associated driving semigroups are generally non eventually compact.

preprint2014arXiv

Probing the A1 to L10 Transformation in FeCuPt Using the First Order Reversal Curve Method

The A1- L10 phase transformation has been investigated in (001) FeCuPt thin films prepared by atomic-scale multilayer sputtering and rapid thermal annealing (RTA). Traditional x-ray diffraction is not always applicable in generating a true order parameter, due to non-ideal crystallinity of the A1 phase. Using the first-order reversal curve (FORC) method, the A1 and L10 phases are deconvoluted into two distinct features in the FORC distribution, whose relative intensities change with the RTA temperature. The L10 ordering takes place via a nucleation-and-growth mode. A magnetization-based phase fraction is extracted, providing a quantitative measure of the L10 phase homogeneity.

preprint2014arXiv

Quantitative Decoding of Interactions in Tunable Nanomagnet Arrays Using First Order Reversal Curves

To develop a full understanding of interactions in nanomagnet arrays is a persistent challenge, critically impacting their technological acceptance. This paper reports the experimental, numerical and analytical investigation of interactions in arrays of Co nanoellipses using the first-order reversal curve (FORC) technique. A mean-field analysis has revealed the physical mechanisms giving rise to all of the observed features: a shift of the non-interacting FORC-ridge at the low-H$_c$ end off the local coercivity H$_c$ axis; a stretch of the FORC-ridge at the high-H$_c$ end without shifting it off the H$_c$ axis; and a formation of a tilted edge connected to the ridge at the low-H$_c$ end. Changing from flat to Gaussian coercivity distribution produces a negative feature, bends the ridge, and broadens the edge. Finally, nearest neighbor interactions segment the FORC-ridge. These results demonstrate that the FORC approach provides a comprehensive framework to qualitatively and quantitatively decode interactions in nanomagnet arrays.

preprint2014arXiv

Stationarity of Stochastic Linear Equations with Additive Noise and Delays in the Unbounded Drift Terms

This paper continues the study of [11, 13] for stationary solutions of stochastic linear retarded functional differential equations with the emphasis on delays which appear in those terms including spatial partial derivatives. As a consequence, the associated stochastic equations have unbounded operators acting on the discrete or distributed delayed terms, while the operator acting on the instantaneous term generates a strongly continuous semigroup. We present conditions on the delay systems to obtain a unique stationary solution by combining spectrum analysis of unbounded operators and stochastic calculus. A few instructive cases are analyzed in detail to clarify the underlying complexity in the study of systems with unbounded delayed operators.

preprint2014arXiv

Tuning Perpendicular Anisotropy Gradient in Co/Pd Multilayers by Ion Irradiation

The tunability of Ar+ ion irradiation of Co/Pd multilayers has been employed to create depth-dependent perpendicular anisotropy gradients. By adjusting the Ar+ kinetic energy and fluence, the depth and lateral density of the local structural modification are controlled. First-order reversal curve analysis through X-ray magnetic circular dichroism and conventional magnetometry studies show that the local structural damage weakens the perpendicular anisotropy near the surface, leading to a magnetization tilting towards the in-plane direction. The ion irradiation method is complementary to, and may be used in conjunction with, other synthesis approaches to maximize the anisotropy gradient.

preprint2014arXiv

When and how does a prominence-like jet gain kinetic energy?

A jet is a considerable amount of plasma being ejected from the chromosphere or lower corona into the higher corona and is a common phenomenon. Usually, a jet is triggered by a brightening or a flare, which provides the first driving force to push plasma upward. In this process, magnetic reconnection is thought to be the mechanism to convert magnetic energy into thermal, nonthermal, and kinetic energies. However, most jets could reach an unusual high altitude and end much later than the end of its associated flare. This fact implies that there is another way to continuously transfer magnetic energy into kinetic energy even after the reconnection. The picture described above is well known in the community, but how and how much magnetic energy is released through a way other than reconnection is still unclear. By studying a prominence-like jet observed by SDO/AIA and STEREO-A/EUVI, we find that the continuous relaxation of the post-reconnection magnetic field structure is an important process for a jet to climb up higher than it could through only reconnection. The kinetic energy of the jet gained through the relaxation is 1.6 times that gained from the reconnection. The resultant energy flux is hundreds of times larger than the flux required for the local coronal heating, suggesting that such jets are a possible source to keep the corona hot. Furthermore, rotational motions appear all the time during the jet. Our analysis suggests that torsional Alfvén waves induced during reconnection could not be the only mechanism to release magnetic energy and drive jets.

preprint2013arXiv

Evidence for $η_{c} \rightarrow γγ$ and Measurement of $J/ψ\rightarrow 3γ$

The decay of $J/ψ$ to three photons is studied using $ψ(3686)\toπ^+π^-J/ψ$ in a sample of $1.0641\times10^8$ $ψ(3686)$ events collected with the BESIII detector. Evidence of the direct decay of $η_c$ to two photons, $η_c\toγγ$, is reported, and the product branching fraction is determined to be $\mathcal{B}(J/ψ\toγη_c,η_c\to γγ)=(4.5\pm1.2\pm0.6)\times10^{-6}$, where the first error is statistical and the second is systematic. The branching fraction for $J/ψ\to3γ$ is measured to be $(11.3\pm1.8\pm2.0)\times 10^{-6}$ with improved precision.

preprint2013arXiv

Evidence for η_{c}(2S) in ψ(3686) \to γK_{S}^{0}K^{\pm}π^{\mp}π^{+}π^{-}

We search for the M1 radiative transition ψ(3686) \to γη_{c}(2S) by reconstructing the exclusive η_{c}(2S) \to K_{S}^{0}K^{\pm}π^{\mp}π^{+}π^{-} decay using 1.06 \times 10^8 ψ(3686) events collected with the BESIII detector. The signal is observed with a statistical significance of greater than 4 standard deviations. The measured mass of the η_{c}(2S) is 3646.9 \pm 1.6(stat) \pm 3.6(syst) MeV/c^2, and the width is 9.9 \pm 4.8(stat) \pm 2.9(syst) MeV/c^2. The product branching fraction is measured to be B(ψ(3686) \to γη_{c}(2S)) \times B(η_{c}(2S) \to K_{S}^{0}K^{\pm}π^{\mp}π^{+}π^{-}) = (7.03 \pm 2.10(stat) \pm 0.70(syst)) \times 10^{-6}. This measurement complements a previous BESIII measurement of ψ(3686) \to γη_{c}(2S) with η_{c}(2S) \to K_{S}^{0} K^{\pm}π^{\mp} and K^{+}K^{-}π^{0}.

preprint2013arXiv

Investigation of atomic and electronic structures of MgOFeSe studied by the first-principles calculations

In order to assist the search of new superconductors in iron selenide materials by intercalation, we calculate the crystal and electronic structures of MgOFeSe using the first-principles density functional theory. MgOFeSe is isotructural to the parent compound of iron pnictide superconductor LaOFeAs. In LaOFeAs, the anion O$^{2-}$ is located at the center of each LaO tetrahedra. But for MgOFeSe, we find that the crystal structure with the cation Mg$^{2+}$ as the tetrahedral center in the MgO layer is energetically more stable. The low energy band structures around the Fermi surfaces of MgOFeSe are contributed mainly by Fe 3$d$ orbitals. The ground state of MgOFeSe is collinearly antiferromagnetically ordered. The height of Se atoms above the Fe-Fe layer is about 1.38 Å, which is close to the height of As from the Fe-Fe layer in the iron pnictide superconductors with optimal superconducting transition temperatures.

preprint2013arXiv

Ion irradiation of Fe-Fe oxide core-shell nanocluster films: Effect of interface on stability of magnetic properties

A cluster deposition method was used to produce films of loosely aggregated nanoclusters (NC) of Fe core-Fe3O4 shell or fully oxidized Fe3O4. Films of these NC on Si(100) or MgO(100)/Fe3O4(100) were irradiated to 10^16 Si2+/cm2 near room temperature using an ion accelerator. Ion irradiation creates structural change in the NC film with corresponding chemical and magnetic changes which depend on the initial oxidation state of the cluster. Films were characterized using magnetometry (hysteresis, first order reversal curves), microscopy (transmission electron, helium ion), and x-ray diffraction. In all cases, the particle sizes increased due to ion irradiation, and when a core of Fe is present, irradiation reduces the oxide shells to lower valent Fe species. These results show that ion irradiated behavior of the nanocluster films depends strongly on the initial nanostructure and chemistry, but in general saturation magnetization decreases slightly.

preprint2013arXiv

Low-Frequency Raman Modes and Electronic Excitations In Atomically Thin MoS2 Crystals

Atomically thin MoS$_{2}$ crystals have been recognized as a quasi-2D semiconductor with remarkable physics properties. This letter reports our Raman scattering measurements on multilayer and monolayer MoS$_{2}$, especially in the low-frequency range ($<$50 cm$^{-1}$). We find two low-frequency Raman modes with contrasting thickness dependence. With increasing the number of MoS$_{2}$ layers, one shows a significant increase in frequency while the other decreases following a 1/N (N denotes layer-number) trend. With the aid of first-principle calculations we assign the former as the shear mode $E_{2g}^{2}$ and the latter as the compression vibrational mode. The opposite evolution of the two modes with thickness demonstrates novel vibrational modes in atomically thin crystal as well as a new and more precise way to characterize thickness of atomically thin MoS$_{2}$ films. In addition, we observe a broad feature around 38 cm$^{-1}$ (~5 meV) which is visible only under near-resonance excitation and pinned at the fixed energy independent of thickness. We interpret the feature as an electronic Raman scattering associated with the spin-orbit coupling induced splitting in conduction band at K points in their Brillouin zone.

preprint2013arXiv

Measurement of $η^\prime\rightarrow π^+π^- e^+e^-$ and $η^\prime\rightarrow π^+π^-μ^+μ^-$

Based on a sample of 225.3 million J/ψevents accumulated with the BESIII detector at the BEPCII, the decays of η' to pi+pi-l+l- are studied via J/ψto γη'. A clear η' signal is observed in the pi+pi-e+e- mass spectrum, and the branching fraction is measured to be \BR(η' to pi+pi-e+e-) = (2.11\pm0.12 (stat.)\pm0.15 (syst.))\times10^{-3}, which is in good agreement with theoretical predictions and the previous measurement, but is determined with much higher precision. No η' signal is found in the pi+ pi- mu+ mu- mass spectrum, and the upper limit is determined to be \BR(η' to pi+ pi- mu+ mu-)<2.9\times10^{-5} at the 90% confidence level.

preprint2013arXiv

Measurements of Baryon Pair Decays of $χ_{cJ}$ Mesons

Using 106 $\times 10^{6}$ $ψ^{\prime}$ decays collected with the BESIII detector at the BEPCII, three decays of $χ_{cJ}$ ($J=0,1,2$) with baryon pairs ($\llb$, $\ssb$, $\SSB$) in the final state have been studied. The branching fractions are measured to be $\cal{B}$$(χ_{c0,1,2}\rightarrowΛ\barΛ) =(33.3 \pm 2.0 \pm 2.6)\times 10^{-5}$, $(12.2 \pm 1.1 \pm 1.1)\times 10^{-5}$, $(20.8 \pm 1.6 \pm 2.3)\times 10^{-5}$; $\cal{B}$$(χ_{c0,1,2}\rightarrowΣ^{0}\barΣ^{0})$ = $(47.8 \pm 3.4 \pm 3.9)\times 10^{-5}$, $(3.8 \pm 1.0 \pm 0.5)\times 10^{-5}$, $(4.0 \pm 1.1 \pm 0.5) \times 10^{-5}$; and $\cal{B}$$(χ_{c0,1,2}\rightarrowΣ^{+}\barΣ^{-})$ = $(45.4 \pm 4.2 \pm 3.0)\times 10^{-5}$, $(5.4 \pm 1.5 \pm 0.5)\times 10^{-5}$, $(4.9 \pm 1.9 \pm 0.7)\times 10^{-5}$, where the first error is statistical and the second is systematic. Upper limits on the branching fractions for the decays of $χ_{c1,2}\rightarrowΣ^{0}\barΣ^{0}$, $Σ^{+}\barΣ^{-}$, are estimated to be $\cal{B}$$(χ_{c1}\rightarrowΣ^{0}\barΣ^{0}) < 6.2\times 10^{-5}$, $\cal{B}$$(χ_{c2}\rightarrowΣ^{0}\barΣ^{0}) < 6.5\times 10^{-5}$, $\cal{B}$$(χ_{c1}\rightarrowΣ^{+}\barΣ^{-}) < 8.7\times 10^{-5}$ and $\cal{B}$$(χ_{c2}\rightarrowΣ^{+}\barΣ^{-}) < 8.8\times 10^{-5}$ at the 90% confidence level.

preprint2013arXiv

Mechanically Modulated Tunneling Resistance in Monolayer MoS2

We report on the modulation of tunneling resistance in MoS2 monolayers by nano-indentation using an atomic force microscope (AFM). The resistance between the conductive AFM tip and the bottom electrode separated by a monolayer MoS2 is reversibly reduced by up to 4 orders of magnitude, which is attributed to enhanced quantum tunneling when the monolayer is compressed by the tip force. Under the WKB approximation, the experimental data is quantitatively explained by using the metal-insulator-metal tunneling diode model. As an ideal tunneling medium, the defect-free, nanometer-thick MoS2 monolayer can serve as the active layer for non-impacting nano-electro-mechanical switches.

preprint2013arXiv

Multi-Wavelength Observations of the Spatio-Temporal Evolution of Solar Flares with AIA/SDO: I. Universal Scaling Laws of Space and Time Parameters

We extend a previous statistical solar flare study of 155 GOES M- and X-class flares observed with AIA/SDO (Aschwanden 2012) to all 7 coronal wavelengths (94, 131, 171, 193, 211, 304, 335 \ang) to test the wavelength-dependence of scaling laws and statistical distributions. Except for the 171 and 193 \ang\ wavelengths, which are affected by EUV dimming caused by coronal mass ejections (CMEs), we find near-identical size distributions of geometric (lengths $L$, flare areas $A$, volumes $V$, fractal dimension $D_2$), temporal (flare durations $T$), and spatio-temporal parameters (diffusion coefficient $κ$, spreading exponent $β$, and maximum expansion velocities $v_{max}$) in different wavelengths, which are consistent with the universal predictions of the fractal-diffusive avalanche model of a slowly-driven self-organized criticality (FD-SOC) system, i.e., $N(L) \propto L^{-3}$, $N(A) \propto A^{-2}$, $N(V) \propto V^{-5/3}$, $N(T) \propto T^{-2}$, $D_2=3/2$, for a Euclidean dimension $d=3$. Empirically we find also a new strong correlation $κ\propto L^{0.94\pm0.01}$ and the 3-parameter scaling law $L \propto κ T^{0.1}$, which is more consistent with the logistic-growth model than with classical diffusion. The findings suggest long-range correlation lengths in the FD-SOC system that operate in the vicinity of a critical state, which could be used for predictions of individual extreme events. We find also that eruptive flares (with accompanying CMEs), have larger volumes $V$, longer flare durations $T$, higher EUV and soft X-ray fluxes, and somewhat larger diffusion coefficients $κ$ than confined flares (without CMEs).

preprint2013arXiv

Observation of a charged charmoniumlike structure in e+e- to pi+pi-J/psi at \sqrt{s}=4.26 GeV

We study the process e+e- to pi+pi-J/psi at a center-of-mass energy of 4.260 GeV using a 525 pb^{-1} data sample collected with the BESIII detector operating at the Beijing Electron Positron Collider. The Born cross section is measured to be (62.9\pm 1.9\pm 3.7) pb, consistent with the production of the Y(4260). We observe a structure at around 3.9 GeV/c^2 in the π^\pm J/psi mass spectrum, which we refer to as the Z_c(3900). If interpreted as a new particle, it is unusual in that it carries an electric charge and couples to charmonium. A fit to the π^\pm J/psi invariant mass spectrum, neglecting interference, results in a mass of (3899.0\pm 3.6\pm 4.9) MeV/c^2 and a width of (46\pm 10\pm 20) MeV. Its production ratio is measured to be R=\frac{σ(e+e- to π^\pm Z_c(3900)^\mp to pi+pi-J/psi))} {σ(e+e- to pi+pi-J/psi)}=(21.5\pm 3.3\pm 7.5)%. In all measurements the first errors are statistical and the second are systematic.

preprint2013arXiv

Partial wave analysis of $J/ψ\to γηη$

Based on a sample of $2.25\times 10^{8}$ $J/ψ$ events collected with the BESIII detector at BEPCII, a full partial wave analysis on $J/ψ\toγηη$ was performed using the relativistic covariant tensor amplitude method. The results show that the dominant $0^{++}$ and $2^{++}$ components are from the $f_0(1710)$, $f_0(2100)$, $f_0(1500)$, $f_2'(1525)$, $f_2(1810)$ and $f_2(2340)$. The resonance parameters and branching fractions are also presented.

preprint2013arXiv

Partial wave analysis of $ψ(2S) \to p \bar{p}η$

Using a sample of $1.06 \times 10^{8}$ $ψ(2S)$ events collected with the BESIII detector at BEPCII, the decay $ψ(2S) \to p \bar{p}η$ is studied. A partial wave analysis determines that the intermediate state N(1535) with a mass of $1524\pm5^{+10}_{-4}$ MeV/$c^2$ and a width of $130^{+27+57}_{-24-10}$ MeV/$c^2$ is dominant in the decay; the product branching fraction is determined to be $B(ψ(2S) \to N(1535)\bar{p})\times B(N(1535)\to pη)+c.c. = (5.2\pm0.3^{+3.2}_{-1.2})\times 10^{-5}$. Furthermore, the branching fraction of $ψ(2S) \to ηp \bar{p}$ is measured to be $(6.4\pm0.2\pm0.6)\times 10^{-5}$.

preprint2013arXiv

Reversal mode instability and magnetoresistance in perpendicular (Co/Pd)/Cu/(Co/Ni) pseudo-spin-valves

We have observed distinct temperature-dependent magnetization reversal modes in a perpendicular (Co/Pd)4/Co/Cu/(Co/Ni)4/Co pseudo-spin-valve, which are correlated with spin-transport properties. At 300 K, magnetization reversal occurs by vertically correlated domains. Below 200 K the hysteresis loop becomes bifurcated due to laterally correlated reversal of the individual stacks. The magnetic configuration change also leads to higher spin disorders and a significant increase in the giant magnetoresistance effect. First order reversal curve measurements reveal that the coupled state can be re-established through field cycling, and allow direct determination of the interlayer coupling strength as a function of temperature.

preprint2013arXiv

Search for $η$ and $η'$ Invisible Decays in $J/ψ\toϕη$ and $ϕη'$

Using a sample of $(225.3\pm 2.8)\times 10^{6}$ $J/ψ$ decays collected with the BESIII detector at BEPCII, searches for invisible decays of $η$ and $η^\prime$ in $J/ψ\toϕη$ and $ϕη^\prime$ are performed. Decays of $ϕ\to K^{+}K^{-}$ are used to tag the $η$ and $η^\prime$ decays. No signals above background are found for the invisible decays, and upper limits at the 90% confidence level are determined to be $2.58\times10^{-4}$ for the ratio $\frac{\mathcal{B}(η\to\rm{invisible})}{\mathcal{B}(η\toγγ)}$ and $2.39\times10^{-2}$ for $\frac{\mathcal{B}(η^\prime\to\rm{invisible})}{\mathcal{B}(η^\prime \toγγ)}$.

preprint2013arXiv

Search for $η$ and $η^\prime\to π^+ e^- \barν_e +c.c.$ decays in $\jpsi \to ϕη$ and $ϕη^\prime$

Using a sample of 225.3 million $\jpsi$ events collected with the BESIII detector at the BEPCII $e^+e^-$ collider in 2009, searches for the decays of $η$ and $η^\prime\toπ^+ e^- \barν_e +c.c.$ in $\jpsi \to ϕη$ and $ϕη^\prime$ are performed. The $ϕ$ signals, which are reconstructed in $K^+K^-$ final states, are used to tag $η$ and $η^\prime$ semileptonic decays. No signals are observed for either $η$ or $η^\prime$, and upper limits at the 90% confidence level are determined to be $7.3\times 10^{-4}$ and $5.0\times 10^{-4}$ for the ratios $\frac{{\mathcal B}(η\to π^+ e^- \barν_e +c.c.)}{{\mathcal B}(η\to \pip\pim\piz)}$ and $\frac{{\mathcal B}(η^\prime\to π^+ e^-\barν_e +c.c.)}{{\mathcal B}(η^\prime \to \pip\pimη)}$, respectively. These are the first upper limit values determined for $η$ and $η^\prime$ semileptonic weak decays.

preprint2013arXiv

Search for Baryonic Decays of ψ(3770) and ψ(4040)

By analyzing data samples of 2.9 fb^{-1} collected at \sqrt s=3.773 GeV, 482 pb^{-1} collected at \sqrt s=4.009 GeV and 67 pb^{-1} collected at \sqrt s=3.542, 3.554, 3.561, 3.600 and 3.650 GeV with the BESIII detector at the BEPCII storage ring, we search for ψ(3770) and ψ(4040) decay to baryonic final states, including Λ\barΛπ^+π^-, Λ\barΛπ^0, Λ\barΛη, Σ^+ \barΣ^-, Σ^0 \barΣ^0, Ξ^-\barΞ^+ and Ξ^0\barΞ^0 decays. None are observed, and upper limits are set at the 90% confidence level.

preprint2013arXiv

Search for the Lepton Flavor Violation Process $J/ψ\to eμ$ at BESIII

We search for the lepton-flavor-violating decay of the $J/ψ$ into an electron and a muon using $(225.3\pm2.8)\times 10^{6}$ $J/ψ$ events collected with the BESIII detector at the BEPCII collider. Four candidate events are found in the signal region, consistent with background expectations. An upper limit on the branching fraction of $\mathcal{B}(J/ψ\to eμ)< 1.5 \times 10^{-7}$ (90% C.L.) is obtained.

preprint2013arXiv

Size-dependent magnetization switching characteristics and spin wave modes of FePt nanostructures

We present a comprehensive investigation of the size-dependent switching characteristics and spin wave modes of FePt nanoelements. Curved nanomagnets ("caps") are compared to flat disks of identical diameter and volume over a size range of 100 to 300nm. Quasi-static magnetization reversal analysis using first-order reversal curves (FORC) shows that spherical caps have lower vortex nucleation and annihilation fields than the flat disks. As the element diameter decreases, the reversal mechanism in the caps crosses over sooner to coherent rotation than in the disks. The magnetization dynamics are studied using optically induced small angle precession and reveal a strong size dependence that differs for the two shapes. Flat disks exhibit well-known center and edge modes at all sizes, but as the diameter of the caps increases from 100 to 300 nm, additional oscillation modes appear in agreement with dynamic micromagnetic simulations. In addition, we show that the three-dimensional curvature of the cap causes a much greater sensitivity to the applied field angle which provides an additional way for controlling the ultrafast response of nanomagnetic elements.

preprint2013arXiv

Study of $J/ψ\rightarrow ωp \bar{p}$ at BESIII

The decay $J/ψ\rightarrow ωp \bar{p}$ has been studied, using $225.3\times 10^{6}$ $J/ψ$ events accumulated at BESIII. No significant enhancement near the $p\bar{p}$ invariant-mass threshold (denoted as $X(p\bar{p})$) is observed. The upper limit of the branching fraction $\mathcal{B}(J/ψ\rightarrow ωX(p\bar{p}) \rightarrow ωp \bar{p})$ is determined to be $3.9\times10^{-6}$ at the 95% confidence level. The branching fraction of $J/ψ\rightarrow ωp \bar{p}$ is measured to be $\mathcal{B}(J/ψ\rightarrow ωp \bar{p}) =(9.0 \pm 0.2\ (\text{stat.})\pm 0.9\ (\text{syst.})) \times 10^{-4}$.

preprint2013arXiv

The influence of spin-phonon coupling on antiferromagnetic spin fluctuations in FeSe under pressure: the First-principles calculations with van der Waals corrections

The electronic structures, lattice dynamics, and magnetic properties of crystal β-FeSe under hydrostatic pressure have been studied by using the first-principles electronic structure calculations with van der Waals (vdW) corrections. With applied pressures, the energy bands around the Fermi energy level consisting mainly of Fe-3d orbitals show obvious energy shifts and occupation variations, and meanwhile the frequencies of all optical phonon modes increase. Among these phonon modes, the A1g mode, which relates to the Se height from the Fe-Fe plane, shows a clear frequency jump in the range between 5 and 6 GPa. This is also the pressure range within which the highest superconducting transition temperature Tc of FeSe is reached in experiments. In comparison with the other phonon modes, the zero-point vibration of the A1g mode induces the strongest fluctuation of local magnetic moment on Fe under a pressure from 0 to 9 GPa, and the induced fluctuation reaches the maximum around 5 GPa. These results suggest that the effect of phonon via spin-phonon coupling need to be considered when exploring the superconducting mechanism in iron-based superconductors.

preprint2013arXiv

The influence of structural disorder on magnetic domain formation in perpendicular anisotropy thin films

Using a combination of resonant soft x-ray scattering, magnetometry, x-ray reflectivity and microscopy techniques we have investigated the magnetic properties and microstructure of a series of perpendicular anisotropy Co/Pt multilayer films with respect to structural disorder tuned by varying the sputtering deposition pressure. The observed magnetic changes in domain size, shape and correlation length originate from structural and chemical variations in the samples, such as chemical segregation and grain formation as well as roughness at the surface and interfaces, which are all impacted by the deposition pressure. For low pressure samples we find evidence of a random "gas-like" distribution of magnetic domains, while in the higher pressure samples the domain structure exhibits only short range "liquid-like" positional ordering. The structural and chemical disorder induced by the higher deposition pressure first leads to an increase in the number of magnetic point defects that limit free domain wall propagation. Then, as the sputtering pressure is further increased, the domain wall energy density is lowered due to the formation of local regions with reduced magnetic moment, and finally magnetically void regions appear that confine the magnetic domains and clusters, similar to segregated granular magnetic recording media.

preprint2013arXiv

Tuning magnetic anisotropy in (001) oriented L10 (Fe1-xCux)55Pt45 films

We have achieved (001) oriented L10 (Fe1-xCux)55Pt45 thin films, with magnetic anisotropy up to 3.6x10^7 erg/cm^3, using atomic-scale multilayer sputtering and post annealing at 400 °C for 10 seconds. By fixing the Pt concentration, structure and magnetic properties are systematically tuned by the Cu addition. Increasing Cu content results in an increase in the tetragonal distortion of the L10 phase, significant changes to the film microstructure, and lowering of the saturation magnetization and anisotropy. The relatively convenient synthesis conditions, along with the tunable magnetic properties, make such materials highly desirable for future magnetic recording technologies.

preprint2012arXiv

A GPS Pseudorange Based Cooperative Vehicular Distance Measurement Technique

Accurate vehicular localization is important for various cooperative vehicle safety (CVS) applications such as collision avoidance, turning assistant, etc. In this paper, we propose a cooperative vehicular distance measurement technique based on the sharing of GPS pseudorange measurements and a weighted least squares method. The classic double difference pseudorange solution, which was originally designed for high-end survey level GPS systems, is adapted to low-end navigation level GPS receivers for its wide availability in ground vehicles. The Carrier to Noise Ratio (CNR) of raw pseudorange measurements are taken into account for noise mitigation. We present a Dedicated Short Range Communications (DSRC) based mechanism to implement the exchange of pseudorange information among neighboring vehicles. As demonstrated in field tests, our proposed technique increases the accuracy of the distance measurement significantly compared with the distance obtained from the GPS fixes.

preprint2012arXiv

Atomic and electronic structures of FeSe monolayer and bilayer thin films on SrTiO$_3$ (001): a first-principles study

By the first-principles electronic structure calculations, we have studied electronic structures of FeSe monolayer and bilayer thin films on SrTiO$_3$ (001) with SrO-termination or TiO$_2$ termination. We find that both FeSe monolayer and bilayer on either termination behave like a slightly doped semiconductor and a collinear antiferromagnetic order on Fe ions. There is no substantial charge transfer between the FeSe layers and the substrate. FeSe is adhered to the SrTiO$_3$ surface by a dipole-dipole interaction. The Fermi surface is mainly the contribution of Fe-3d orbitals. A valence band contributed mainly by the O-$2p$ orbitals in the TiO$_2$ layer is located slightly below the Fermi level, which can become conducting upon a small doping of holes.

preprint2012arXiv

Deconvoluting Reversal Modes in Exchange Biased Nanodots

Ensemble-averaged exchange bias in arrays of Fe/FeF2 nanodots has been deconvoluted into local, microscopic, bias separately experienced by nanodots going through different reversal modes. The relative fraction of dots in each mode can be modified by exchange bias. Single domain dots exhibit a simple loop shift, while vortex state dots have asymmetric shifts in the vortex nucleation and annihilation fields, manifesting local incomplete domain walls in these nanodots as magnetic vortices with tilted cores.

preprint2012arXiv

Evidence for the Direct Two-Photon Transition from $ψ(3686)$ to $J/ψ$

The two-photon transition $ψ(3686)\toγγJ/ψ$ is studied in a sample of 106 million $ψ(3686)$ decays collected by the BESIII detector. The branching fraction is measured to be $(3.1\pm0.6(\unit{stat})^{+0.8}_{-1.0}(\unit{syst})) \times10^{-4}$ using $J/ψ\to e^+e^-$ and $J/ψ\toμ^+μ^-$ decays, and its upper limit is estimated to be $4.5\times10^{-4}$ at the 90% conference level. This work represents the first measurement of a two-photon transition among charmonium states. The orientation of the $ψ(3686)$ decay plane and the $J/ψ$ polarization in this decay are also studied. In addition, the product branching fractions of sequential $E1$ transitions $ψ(3686)\toγχ_{cJ}, χ_{cJ}\toγJ/ψ(J=0,1,2)$ are reported.

preprint2012arXiv

Experimental study of psi' decays to K+K- pi^0 and K+K- eta

Using $(106\pm4)\times 10^6$ $\psip$ events accumulated with the BESIII detector at the BEPCII $e^+e^-$ collider, we present measurements of the branching fractions for psi' decays to $K^{+}K^{-}π^{0}$ and $K^{+}K^{-}η$. In these final states, the decay $ψ'\to K_2^{*}(1430)^+K^-+c.c.$ is observed for the first time, and its branching fraction is measured to be $(7.12\pm{0.62}{\rm (stat.)}^{+1.13}_{-0.61}{\rm (syst.)})\times 10^{-5}$, which indicates a violation of the helicity selection rule in $\psip$ decays. The branching fractions of $ψ'\to K^*(892)^+K^-+c.c., ϕη, ϕπ^0$ are also measured. The measurements are used to test the QCD predictions on charmonium decays.

preprint2012arXiv

First observation of the isospin violating decay $J/ψ\rightarrow Λ\barΣ^{0}+c.c.$

Using a sample of $(225.2\pm 2.8)\times 10^6$ $J/ψ$ events collected with the BESIII detector, we present results of a study of $J/ψ\rightarrow γΛ\barΛ$ and report the first observation of the isospin violating decay $J/ψ\rightarrowΛ\barΣ^{0}+c.c.$, in which $\barΣ^{0}$ decays to $γ\barΛ$. The measured branching fractions are $\mathcal{B}(J/ψ\rightarrow\barΛΣ^{0}$) = $(1.46\pm0.11\pm0.12) \times10^{-5}$ and $\mathcal{B}(J/ψ\rightarrowΛ\bar{Σ^{0}}$) = $(1.37\pm0.12\pm0.11) \times10^{-5}$. We search for $Λ(1520) \rightarrow γΛ$ decay, and find no evident signal, and an upper limit for the product branching fraction $\mathcal{B}(J/ψ\rightarrowΛ\barΛ(1520)+c.c.)\times \mathcal{B}(Λ(1520)\rightarrowγΛ)<4.1 \times10^{-6}$ is set at the 90% confidence level. We also report the observation of $η_{c}\rightarrowΛ\barΛ$ in $J/ψ\rightarrow γη_{c}$, $η_{c}\rightarrowΛ\barΛ$ and measure the branching fraction $\mathcal{B}(η_{c}\rightarrowΛ\barΛ) =(1.16\pm0.12 (stat)\pm0.19(syst)\pm0.28(PDG))\times10^{-3}$.

preprint2012arXiv

First observation of the M1 transition $ψ(3686)\to γη_c(2S)$

Using a sample of 106 million ψ(3686) events collected with the BESIII detector at the BEPCII storage ring, we have made the first measurement of the M1 transition between the radially excited charmonium S-wave spin-triplet and the radially excited S-wave spin-singlet states: ψ(3686)\toγη_c(2S). Analyses of the processes ψ(2S)\to γη_c(2S) with η_c(2S)\to \K_S^0 Kπand K^+K^-π^0 gave an η_c(2S) signal with a statistical significance of greater than 10 standard deviations under a wide range of assumptions about the signal and background properties. The data are used to obtain measurements of the η_c(2S) mass (M(η_c(2S))=3637.6\pm 2.9_\mathrm{stat}\pm 1.6_\mathrm{sys} MeV/c^2), width (Γ(η_c(2S))=16.9\pm 6.4_\mathrm{stat}\pm 4.8_\mathrm{sys} MeV), and the product branching fraction (\BR(ψ(3686)\to γη_c(2S))\times \BR(η_c(2S)\to K\bar Kπ) = (1.30\pm 0.20_\mathrm{stat}\pm 0.30_\mathrm{sys})\times 10^{-5}). Combining our result with a BaBar measurement of \BR(η_c(2S)\to K\bar K π), we find the branching fraction of the M1 transition to be \BR(ψ(3686)\toγη_c(2S)) = (6.8\pm 1.1_\mathrm{stat}\pm 4.5_\mathrm{sys})\times 10^{-4}.

preprint2012arXiv

Measurement of $χ_{cJ}$ decaying into $p\bar{n}π^{-}$ and $p\bar{n}π^{-}π^{0}$

Using a data sample of $1.06 \times 10^{8}$ $\psip$ events collected with the BESIII detector in 2009, the branching fractions of $χ_{cJ}\to p\bar{n}π^{-}$ and $χ_{cJ}\to p\bar{n}π^{-}π^{0}$ ($J$=0,1,2) are measured{Throughout the text, inclusion of charge conjugate modes is implied if not stated otherwise.}. The results for $χ_{c0}\to p\bar{n}π^{-}$ and $χ_{c2}\to p\bar{n}π^{-}$ are consistent with, but much more precise than those of previous measurements. The decays of $χ_{c1}\to p\bar{n}π^{-}$ and $χ_{cJ}\to p\bar{n}π^{-}π^{0}$ are observed for the first time.

preprint2012arXiv

Observation of $e^{+}e^{-} \to ηJ/ψ$ at center-of-mass energy $sqrt{s}=4.009$ GeV

Using a 478 pb$^{-1}$ data sample collected with the BESIII detector operating at the BEPCII storage ring at a center-of-mass energy of $\sqrt{s}=4.009$ GeV, the production of $e^{+}e^{-}\to ηJψ$ is observed for the first time with a statistical significance of greater than $10σ$. The Born cross section is measured to be $(32.1\pm 2.8 \pm 1.3)$ pb, where the first error is statistical and the second systematic. Assuming the $ηJψ$ signal is from a hadronic transition of the $ψ(4040)$, the fractional transition rate is determined to be ${\cal B}(ψ(4040)\to ηJψ)=(5.2\pm 0.5\pm 0.2\pm 0.5)\times 10^{-3}$, where the first, second, and third errors are statistical, systematic, and the uncertainty from the $ψ(4040)$ resonant parameters, respectively. The production of $e^{+}e^{-} \to π^0 Jψ$ is searched for, but no significant signal is observed, and ${\cal B}(ψ(4040)\to π^0 Jψ) < 2.8 \times 10^{-4}$ is obtained at the 90% confidence level.

preprint2012arXiv

Observation of $η_{c}$ decay into $Σ^{+}\barΣ^{-}$ and $Ξ^{-}\barΞ^{+}$ final states

Using a data sample of $2.25\times10^{8}$ $J/ψ$ events collected with the BESIII detector, we present the first observation of the decays of $η_{c}$ mesons to $Σ^{+}\barΣ^{-}$ and $Ξ^{-}\barΞ^{+}$. The branching fractions are measured to be $(2.11\pm0.28_{\rm stat.}\pm0.18_{\rm syst.}\pm0.50_{\rm PDG})\times10^{-3}$ and $(0.89\pm0.16_{\rm stat.}\pm0.08_{\rm syst.}\pm0.21_{\rm PDG})\times10^{-3}$ for $η_{c} \to Σ^{+}\barΣ^{-}$ and $Ξ^{-}\barΞ^{+}$, respectively. These branching fractions provide important information on the helicity selection rule in charmonium-decay processes.

preprint2012arXiv

Observation of chi_{cJ} Decays to ΛΛbarπ^{+}π^{-}

Decays of the chi_{cJ} states (J=0,\ 1,\ 2) to ΛΛbar\ pi^{+} π^{-}, including processes with intermediate Σ(1385), are studied through the E1 transition psi'-->γchi_{cJ} using 106 million psi' events collected with the BESIII detector at BEPCII. This is the first observation of chi_{cJ} decays to the final state ΛΛbar π^{+}π^{-}. The branching ratio of the intermediate process chi_{cJ}-->Σ(1385)^{+/-} \barΣ(1385)^{-/+} is also measured for the first time, and the results agree with the theoretical predictions based on the color-octet effect.

preprint2012arXiv

Positioning Accuracy Improvement via Distributed Location Estimate in Cooperative Vehicular Networks

The development of cooperative vehicle safety (CVS) applications, such as collision warnings, turning assistants, and speed advisories, etc., has received great attention in the past few years. Accurate vehicular localization is essential to enable these applications. In this study, motivated by the proliferation of the Global Positioning System (GPS) devices, and the increasing sophistication of wireless communication technologies in vehicular networks, we propose a distributed location estimate algorithm to improve the positioning accuracy via cooperative inter-vehicle distance measurement. In particular, we compute the inter-vehicle distance based on raw GPS pseudorange measurements, instead of depending on traditional radio-based ranging techniques, which usually either suffer from high hardware cost or have inadequate positioning accuracy. In addition, we improve the estimation of the vehicles' locations only based on the inaccurate GPS fixes, without using any anchors with known exact locations. The algorithm is decentralized, which enhances its practicability in highly dynamic vehicular networks. We have developed a simulation model to evaluate the performance of the proposed algorithm, and the results demonstrate that the algorithm can significantly improve the positioning accuracy.

preprint2012arXiv

Precision measurements of branching fractions for $ ψ'\toπ^0 Jψ$ and $ηJψ$

We present a precision study of the $\psip\toπ^0 J/ψ$ and $ηJ/ψ$ decay modes. The measurements are obtained using $106\times10^6$ $ψ'$ events accumulated with the BESIII detector at the BEPCII $\ee$ collider operating at a center-of-mass energy corresponding to the $\psip$ mass. We obtain $\mathcal{B}(\psip\toπ^0 J/ψ)=(1.26\pm0.02{\rm (stat.)}\pm0.03{\rm (syst.)})\times 10^{-3}$ and $\mathcal{B}(\psip\toηJ/ψ)=(33.75\pm0.17{\rm (stat.)}\pm0.86{\rm (syst.)})\times 10^{-3}$. The branching fraction ratio $R=\frac{\mathcal{B}(\psip\toπ^0 J/ψ)}{\mathcal{B}(\psip\toηJ/ψ)}$ is determined to be $(3.74\pm0.06 {\rm(stat.)}\pm0.04 {\rm(syst.)})\times 10^{-2}$. The precision of these measurements of $\mathcal{B}(\psip\toπ^{0} J/ψ)$ and $R$ represent a significant improvement over previously published values.

preprint2012arXiv

Rapid Microwave-Assisted Synthesis of Dextran-Coated Iron Oxide Nanoparticles for Magnetic Resonance Imaging

Currently, magnetic iron oxide nanoparticles are the only nano-sized magnetic resonance imaging (MRI) contrast agents approved for clinical use, yet commercial manufacturing of these agents has been limited or discontinued. Though there is still widespread demand for these particles both for clinical use and research, they are difficult to obtain commercially, and complicated syntheses make in-house preparation infeasible for most biological research labs or clinics. To make commercial production viable and increase accessibility of these products, it is crucial to develop simple, rapid, and reproducible preparations of biocompatible iron oxide nanoparticles. Here, we report a rapid, straightforward microwave-assisted synthesis of superparamagnetic dextran-coated iron oxide nanoparticles. The nanoparticles were produced in two hydrodynamic sizes with differing core morphologies by varying the synthetic method as either a two-step or single step process. A striking benefit of these methods is the ability to obtain swift and consistent results without the necessity for air, pH, or temperature sensitive techniques; therefore, reaction times and complex manufacturing processes are greatly reduced as compared to conventional synthetic methods. This is a great benefit for cost-effective translation to commercial production. The nanoparticles are found to be superparamagnetic and exhibit properties consistent for use in MRI. In addition, the dextran coating imparts the water-solubility and biocompatibility necessary for in vivo utilization.

preprint2012arXiv

Rapid Size-Controlled Synthesis of Dextran-Coated, 64Cu-Doped Iron Oxide Nanoparticles

Research into developing dual modality probes enabled for magnetic resonance imaging (MRI) and positron emission tomography (PET) has been on the rise recently due to the potential to combine the high resolution of MRI and the high sensitivity of PET. Current synthesis techniques for developing multimodal probes is largely hindered in part by prolonged reaction times during radioisotope incorporation - leading to a weakening of the radioactivity. Along with a time-efficient synthesis, the resulting products must fit within a critical size range (between 20-100nm) to increase blood retention time. In this work, we describe a novel, rapid, microwave-based synthesis technique to grow dextran-coated iron oxide nanoparticles doped with copper (DIO/Cu). Traditional methods for coprecipitation of dextran-coated iron oxide nanoparticles require refluxing for 2 hours and result in approximately 50 nm diameter particles. We demonstrate that microwave synthesis can produce 50 nm nanoparticles with 5 minutes of heating. We discuss the various parameters used in the microwave synthesis protocol to vary the size distribution of DIO/Cu, and demonstrate the successful incorporation of 64Cu into these particles with the aim of future use for dual-mode MR/PET imaging.

preprint2012arXiv

Search for hadronic transition $χ_{cJ}\toη_{c}π^{+}π^{-}$ and observation of $χ_{cJ}\to K\bar{K}πππ$

Hadronic transitions of $χ_{cJ}\to η_{c}π^{+}π^{-}$ (J=0, 1, 2) are searched for using a sample of $1.06\times 10^{8}$ $ψ(3686)$ events collected with the BESIII detector at the BEPCII storage ring. The $\etac$ is reconstructed with $K_{S}^{0}K^{+}π^{-}+c.c.$ and $K^{+}K^{-}π^{0}$ final states. No signals are observed in any of the three $χ_{cJ}$ states in either $\etac$ decay mode. At the 90% confidence level, the upper limits are determined to be $\BR(χ_{c0}\to η_{c}π^{+}π^{-})<0.07%$, $\BR(χ_{c1}\to η_{c}π^{+}π^{-})<0.32%$, and $\BR(χ_{c2}\to η_{c}π^{+}π^{-})<0.54%$. The upper limit of $\BR(χ_{c1}\to η_{c}π^{+}π^{-}$ is lower than the existing theoretical prediction by almost an order of magnitude. The branching fractions of $χ_{cJ}\to K_{S}^{0}K^{+}π^{-}π^{+}π^{-}+c.c.$, $K^{+}K^{-}π^{+}π^{-}π^{0}$, $ωK^{+}K^{-}$ and $ϕπ^{+}π^{-}π^{0}$ (J=0, 1, 2) are measured for the first time.

preprint2012arXiv

Study of $J/ψ\to p\bar{p}$ and $J/ψ\to n\bar{n}$

The decays $J/ψ\to p\bar{p}$ and $J/ψ\to n\bar{n}$ have been investigated with a sample of 225.2 million $J/ψ$ events collected with the BESIII detector at the BEPCII $e^+e^-$ collider. The branching fractions are determined to be $\mathcal{B}(J/ψ\to p\bar{p})=(2.112\pm0.004\pm0.031)\times10^{-3}$ and $\mathcal{B}(J/ψ\to n\bar{n})=(2.07\pm0.01\pm0.17)\times10^{-3}$. Distributions of the angle $θ$ between the proton or anti-neutron and the beam direction are well described by the form $1+α\cos^2θ$, and we find $α=0.595\pm0.012\pm0.015$ for $J/ψ\to p\bar{p}$ and $α=0.50\pm0.04\pm0.21$ for $J/ψ\to n\bar{n}$. Our branching-fraction results suggest a large phase angle between the strong and electromagnetic amplitudes describing the $J/ψ\to N\bar{N}$ decay.

preprint2012arXiv

Study of the near-threshold $ωϕ$ mass enhancement in doubly OZI suppressed $J/ψ\to γωϕ$ decays

A 2.25$\times10^8$ $\jpsi$ event sample accumulated with the BESIII detector is used to study the doubly OZI suppressed decay modes $\jpsi\toγ\of$, $ω\to\ppp$, $ϕ\to\kk$. A strong deviation ($>$ 30$σ$) from three-body $\jpsi\toγωϕ$ phase space is observed near the $ωϕ$ mass threshold that is consistent with a previous observation reported by the BESII experiment. A partial wave analysis with a tensor covariant amplitude that assumes that the enhancement is due to the presence of a resonance, the X(1810), is performed, and confirms that the spin-parity of the X(1810) is $0^{++}$. The mass and width of the X(1810) are determined to be $M=1795\pm7$(stat)$^{+13}_{-5}$(syst)$\pm$19(mod) MeV/$c^2$ and $Γ=95\pm10$(stat)$^{+21}_{-34}$(syst)$\pm$75(mod) MeV/$c^2$, respectively, and the product branching fraction is measured to be ${\cal B}(\jpsi\toγX(1810))\times{\cal B}(X(1810)\to\of)=(2.00\pm0.08$(stat)$^{+0.45}_{-1.00}$(syst)$\pm$1.30(mod))$\times10^{-4}$. %where the first error is the statistical error and the second the systematical error. These results are consistent within errors with those of the BESII experiment.

preprint2012arXiv

Two-photon widths of the $χ_{c0, 2}$ states and helicity analysis for $χ_{c2}\arγγ$}

Based on a data sample of 106 M $ψ^{\prime}$ events collected with the BESIII detector, the decays $ψ^{\prime}\arγχ_{c0, 2}$,$χ_{c0, 2}\arγγ$ are studied to determine the two-photon widths of the $χ_{c0, 2}$ states. The two-photon decay branching fractions are determined to be ${\cal B}(χ_{c0}\arγγ) = (2.24\pm 0.19\pm 0.12\pm 0.08)\times 10^{-4}$ and ${\cal B}(χ_{c2}\arγγ) = (3.21\pm 0.18\pm 0.17\pm 0.13)\times 10^{-4}$. From these, the two-photon widths are determined to be $Γ_{γγ}(χ_{c0}) = (2.33\pm0.20\pm0.13\pm0.17)$ keV, $Γ_{γγ}(χ_{c2}) = (0.63\pm0.04\pm0.04\pm0.04)$ keV, and $\cal R$ $=Γ_{γγ}(χ_{c2})/Γ_{γγ}(χ_{c0})=0.271\pm 0.029\pm 0.013\pm 0.027$, where the uncertainties are statistical, systematic, and those from the PDG ${\cal B}(ψ^{\prime}\arγχ_{c0,2})$ and $Γ(χ_{c0,2})$ errors, respectively. The ratio of the two-photon widths for helicity $λ=0$ and helicity $λ=2$ components in the decay $χ_{c2}\arγγ$ is measured for the first time to be $f_{0/2} =Γ^{λ=0}_{γγ}(χ_{c2})/Γ^{λ=2}_{γγ}(χ_{c2}) = 0.00\pm0.02\pm0.02$.

preprint2011arXiv

Chirality control via double vortices in asymmetric Co dots

Reproducible control of the magnetic vortex state in nanomagnets is of critical importance. We report on chirality control by manipulating the size and/or thickness of asymmetric Co dots. Below a critical diameter and/or thickness, chirality control is achieved by the nucleation of single vortex. Interestingly, above these critical dimensions chirality control is realized by the nucleation and subsequent coalescence of two vortices, resulting in a single vortex with the opposite chirality as found in smaller dots. Micromagnetic simulations and magnetic force microscopy highlight the role of edge-bound halfvortices in facilitating the coalescence process.

preprint2011arXiv

Critical Height for the Destabilization of Solar Prominences: Statistical Results from STEREO Observations

At which height will a prominence inclined to be unstable, or where is the most probable critical height for the prominence destabilization? This question is statistically studied based on 362 solar limb prominences well-recognized by SLIPCAT from 2007 April to the end of 2009. We found that there are about 71% disrupted prominences (DPs), among which about 42% of them did not erupt successfully and about 89% of them experienced a sudden destabilization (SD) process. After a comprehensive analysis of the DPs, the following findings are discovered. (1) Most DPs become unstable at the height of 0.06 -- 0.14 R$_\odot$ from the solar surface, and there are two most probable critical heights, at which a prominence is much likely to get unstable; the primary one is 0.13 R$_\odot$ and the secondary one is 0.19 R$_\odot$. (2) There exists upper limit for the erupting velocity of eruptive prominences (EPs), which decreases following a power law with increasing height and mass; the kinetic energy of EPs accordingly has an upper limit too, which decreases as critical height increases. (3) Stable prominences (SPs) are generally longer and heavier than DPs, and not higher than 0.4 R$_\odot$. (4) About 62% of EPs were associated with CMEs; but there is no difference in apparent properties between EPs associated with and without CMEs.

preprint2011arXiv

Field-effect modulation of conductance in VO2 nanobeam transistors with HfO2 as the gate dielectric

We study field-effect transistors realized from VO2 nanobeams with HfO2 as the gate dielectric. When heated up from low to high temperatures, VO2 undergoes an insulator-to-metal transition. We observe a change in conductance (~ 6 percent) of our devices induced by gate voltage when the system is in the insulating phase. The response is reversible and hysteretic, and the area of hysteresis loop becomes larger as the rate of gate sweep is slowed down. A phase lag exists between the response of the conductance and the gate voltage. This indicates the existence of a memory of the system and we discuss its possible origins.

preprint2011arXiv

On Regularity Property of Retarded Ornstein-Uhlenbeck Processes in Hilbert Spaces

In this work, some regularity properties of mild solutions for a class of stochastic linear functional differential equations driven by infinite dimensional Wiener processes are considered. In terms of retarded fundamental solutions, we introduce a class of stochastic convolutions which naturally arise in the solutions and investigate their Yosida approximants. By means of the retarded fundamental solutions, we find conditions under which each mild solution permits a continuous modification. With the aid of Yosida approximation, we study two kinds of regularity properties, temporal and spatial ones, for the retarded solution processes. By employing a factorization method, we establish a retarded version of Burkholder-Davis-Gundy's inequality for stochastic convolutions.

preprint2011arXiv

Shot noise in magnetic tunnel junctions from first principles

We compute the shot noise in ballistic and disordered Fe/MgO/Fe tunnel junctions by a wave function-matching method. For tunnel barriers with no more than 5 atomic layers we find a suppression of the Fano factor as a function of the magnetic configuration. For thicker MgO barriers the shot noise is suppressed up to a threshold bias indicating the onset of resonant tunneling. We find excellent agreement with recent experiments when interface disorder is taken into account

preprint2011arXiv

Some results on zeros distributions and uniqueness of derivatives of difference polynomials

We consider the zeros distributions on the derivatives of difference polynomials of meromorphic functions, and present some results which can be seen as the discrete analogues of Hayman conjecture \cite{hayman1}, also partly answer the question given in \cite[P448]{luolin}. We also investigate the uniqueness problems of difference-differential polynomials of entire functions sharing one common value. These theorems improve the results of Luo and Lin\cite{luolin} and some results of present authors \cite{liuliucao}.

preprint2011arXiv

Spin transfer torque on magnetic insulators

Recent experimental and theoretical studies focus on spin-mediated heat currents at interfaces between normal metals and magnetic insulators. We resolve conflicting estimates for the order of magnitude of the spin transfer torque by first-principles calculations. The spin mixing conductance G^\uparrow\downarrow of the interface between silver and the insulating ferrimagnet Yttrium Iron Garnet (YIG) is dominated by its real part and of the order of 10^14 Ω^-1m^-2, i.e. close to the value for intermetallic interface, which can be explained by a local spin model.

preprint2010arXiv

Cosmological inflation driven by holonomy corrections of loop quantum cosmology

At the level of heuristic effective dynamics, we investigate the cosmological inflation with holonomy corrections of loop quantum cosmology (LQC) in the $k=0$ Friedmann-Robertson-Walker model with a single inflaton field subject to a simple potential. In the symmetric bouncing scenario of LQC, the condition for occurrence of the quantum bounce naturally and uniquely fixes the initial conditions at the bouncing epoch. Around the quantum bounce, the universe undergoes a short super-inflationary phase, which drives the inflaton field to its potential hill and thus sets the proper initial conditions for the standard slow-roll inflation. Between the super-inflation and the standard inflation, there is a non-inflationary phase, which violates the slow-roll condition. The violation of slow roll is expected to give some suppression on the low angular power spectrum of the cosmic microwave background and different orders of holonomy corrections shall yield different suppressions.

preprint2010arXiv

Tuning carrier type and density in Bi2Se3 by Ca-doping

The carrier type and density in Bi2Se3 single crystals are systematically tuned by introducing a calcium (Ca) dopant. A carrier density of ~1x1017 cm-3 which corresponds to ~25 meV in the Fermi energy is obtained in both n- and p-type materials. Electrical transport properties show that the insulating behavior is achieved in low carrier density crystals. In addition, both the band gap and reduced effective mass of carriers are determined.

preprint2009arXiv

Origin of large moments in Mn$_x$Si$_{1-x}$ at small x

Recently, the magnetic moment/Mn, $M$, in Mn$_x$Si$_{1-x}$ was measured to be 5.0 $μ_B$/Mn, at $x$ =0.1%. To understand this observed $M$, we investigate several Mn$_x$Si$_{1-x}$ models of alloys using first-principles density functional methods. The only model giving $M = 5.0$ was a 513-atom cell having the Mn at a substitutional site, and Si at a second-neighbor interstitial site. The observed large moment is a consequence of the weakened d-p hybridization between the Mn and one of its nearest neighbor Si atoms, resulting from the introduction of the second-neighbor interstitial Si. Our result suggests a way to tune the magnetic moments of transition metal doped semiconductors.

preprint2009arXiv

Vertically Graded Anisotropy in Co/Pd Multilayers

Depth-grading of magnetic anisotropy in perpendicular magnetic media has been predicted to reduce the field required to write data without sacrificing thermal stability. To study this prediction, we have produced Co/Pd multilayers with depth-dependent Co layer thickness. Polarized neutron reflectometry shows that the thickness grading results in a corresponding magnetic anisotropy gradient. Magnetometry reveals that the anisotropy gradient promotes domain nucleation upon magnetization reversal - a clear experimental demonstration of the effectiveness of graded anisotropy for reducing write-field.

Kai Liu

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144 published item(s)

Toward Better Geometric Representations for Molecule Generative Models

An eigenvalue problem for self-similar patterns in Hele-Shaw flows

Fourier neural operator based fluid-structure interaction for predicting the vesicle dynamics

Interference of Two-Dimensional Bose-Einstein Condensates in Micro-Gravity

Deep Biological Pathway Informed Pathology-Genomic Multimodal Survival Prediction

Emergent Electronic Kagome Lattice in Correlated Charge-Density-Wave State of 1T-TaS$_2$

Improving Target Speaker Extraction with Sparse LDA-transformed Speaker Embeddings

Multi-Task Learning with Prior Information

Randomized Greedy Algorithms and Composable Coreset for k-Center Clustering with Outliers

The Mars Orbiter Magnetometer of Tianwen-1: In-flight Performance and First Science Results

3D Interconnected Magnetic Nanowire Networks as Potential Integrated Multistate Memristors

A review of knowledge graph application scenarios in cyber security

CohortVA: A Visual Analytic System for Interactive Exploration of Cohorts based on Historical Data

CoTe2: A quantum critical Dirac metal with strong spin fluctuations

Demonstration of room-temperature continuous-wave operation of InGaAs/AlGaAs quantum well lasers directly grown on on-axis silicon (001)

Dynamic Group Transformer: A General Vision Transformer Backbone with Dynamic Group Attention

Emergent superconductivity in van der Waals Kagome material Pd3P2S8 under high pressure

Enriched Robust Multi-View Kernel Subspace Clustering

Exploring the impact of spatiotemporal granularity on the demand prediction of dynamic ride-hailing

Hydrogenation induced magnetic and electronic transitions in monolayer electride Gd$_2$C: A first-principles study

Improving Adversarial Waveform Generation based Singing Voice Conversion with Harmonic Signals

Joint CNN and Transformer Network via weakly supervised Learning for efficient crowd counting

Manipulation of Dirac band curvature and momentum-dependent g-factor in a kagome magnet YMn6Sn6

Maximum Correntropy Value Decomposition for Multi-agent Deep Reinforcemen Learning

MKQ-BERT: Quantized BERT with 4-bits Weights and Activations

Superconductivity in monolayer Ba$_2$N electride: a first-principles study

Time Resolution of the 4H-SiC PIN Detector

Timing performance simulation for 3D 4H-SiC detector

Ultrasensitive Sub-monolayer Palladium Induced Chirality Switching and Topological Evolution of Skyrmions

Weakly-supervised High-fidelity Ultrasound Video Synthesis with Feature Decoupling

Intrinsic ferromagnetic and antiferromagnetic axion insulators in van der Waals materials Mn\emph{X}$_{2}$\emph{B}$_{2}$\emph{T}$_{6}$ family

Pressure induced superconductivity in WB2 and ReB2 through modifying the B layers

Two-dimensional Dirac nodal-line semimetal protected by symmetry

3D Nanomagnetism in Low Density Interconnected Nanowire Networks

Concept of the Solar Ring Mission: Overview

In-plane Néel wall chirality and orientation of interfacial Dzyaloshinskii-Moriya vector in magnetic films

Nitrogen magneto-ionics

Synergistically creating sulfur vacancies in semimetal-supported amorphous MoS2 for efficient hydrogen evolution

The ABC130 barrel module prototyping programme for the ATLAS strip tracker

Vehicular Fog Computing Enabled Real-time Collision Warning via Trajectory Calibration

Band splitting with vanishing spin polarizations in noncentrosymmetric crystals

Emergent superconductivity in single crystalline $\mathrm{MgTi}_2\mathrm{O}_4$ films via structural engineering

Interfacial-Redox-Induced Tuning of Superconductivity in YBa$_{2}$Cu$_{3}$O$_{7-δ}$

Interlayer quantum transport in Dirac semimetal BaGa$_2$

Nanoparticles manipulation in 3D nanotips excited with plasmonic vortex

Possible phonon-induced electronic bi-stability in VO$_2$ for ultrafast memory at room temperature

Quantum spin Hall effect in monolayer and bilayer TaIrTe$_{4}$

Uni-traveling-carrier photodetector with high-contrast grating focusing-reflection mirrors

Optical control of magnetism in NiFe/VO2 heterostructures

A certification scheme for the boson sampler

A Routing Mechanism Based on Social Networks and Betweenness Centrality in Delay-Tolerant Networks

Beyond the Interface Limit: Structural and Magnetic Depth Profiles of Voltage-Controlled Magneto-Ionic Heterostructures

Crystallographic dependent transport properties and oxygen issue in superconducting LiTi2O4 thin films

Damped large amplitude oscillations in a solar prominence and a bundle of coronal loops

Dynamics of a multicomponent vesicle in shear flow

Interplay between multiple charge-density waves and the relationship with superconductivity in Pd$_x$HoTe$_{3}$

Investigating Energetic X-Shaped Flares on the Outskirts of A Solar Active Region

Large magnetoresistance in LaBi: origin of field-induced resistivity upturn and plateau in compensated semimetals

Magnetic fingerprint of interfacial coupling between CoFe and nanoscale ferroelectric domain walls

Magnetic Yoking and Tunable Interactions in FePt-Based Hard/Soft Bilayers

Perfect charge compensation in extremely large magnetoresistance materials LaSb and LaBi revealed by the first-principles calculations

Raman scattering in transition metal dichalcogenides MTe2 (M = Mo, W)

Stereoscopic Observation of Slipping Reconnection in A Double Candle-Flame-Shaped Solar Flare

Sudden gap-closure across the topological phase transition in Bi$_{2-x}$In$_{x}$Se$_{3}$

Thermodynamic Spectrum of Solar Flares Based on SDO/EVE Observations: Techniques and Statistical Results

Tropical variants of some complex analysis results

A Solar Coronal Jet Event Triggers A Coronal Mass Ejection

An M-Estimator for Reduced-Rank High-Dimensional Linear Dynamical System Identification

Extremely Large EUV Late Phase of Solar Flares

First-principles study of FeSe epitaxial films on SrTiO3

First-principles study of magnetic frustration in FeSe epitaxial films on SrTiO$_3$

Nematic antiferromagnetic states in bulk FeSe

New Reversal Mode in Exchange Coupled Antiferromagnetic/Ferromagnetic Disks: Distorted Viscous Vortex

On the origin of the Extreme-Ultraviolet late phase of solar flares