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Xiaolin Wang

Xiaolin Wang appears in the imported research catalog. Authorship, coauthor and topic links are available while profile ownership is still unclaimed.

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cond-mat.mtrl-scicond-mat.mes-hallphysics.opticscond-mat.supr-conhep-exnlin.CDphysics.flu-dynArtificial IntelligenceBiological PhysicsComputation and LanguageComputational Engineering, Finance, and Sciencecond-mat.othercond-mat.softcond-mat.str-elDistributed, Parallel, and Cluster Computinghep-phMachine Learningmath.NAnucl-exphysics.chem-phquant-ph

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Published work

39 published item(s)

preprint2026arXiv

Language-free Experience at Expo 2025 Osaka

In line with the Global Communication Plan 2025, we have pursued the development of multilingual translation technologies to realize a language-barrier-free experience at Expo 2025 Osaka. Our work includes the advancement of simultaneous interpretation systems emphasizing high translation quality and low latency. Key achievements include chunk-based input segmentation, context-aware translation, and multi-engine machine translation technologies. Through demonstration deployments and collaboration with private companies, our technologies have led to real-world applications, with several services and systems showcased at Expo 2025 Osaka.

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preprint2022arXiv

Deep Learning Workload Scheduling in GPU Datacenters: Taxonomy, Challenges and Vision

Deep learning (DL) shows its prosperity in a wide variety of fields. The development of a DL model is a time-consuming and resource-intensive procedure. Hence, dedicated GPU accelerators have been collectively constructed into a GPU datacenter. An efficient scheduler design for such GPU datacenter is crucially important to reduce the operational cost and improve resource utilization. However, traditional approaches designed for big data or high performance computing workloads can not support DL workloads to fully utilize the GPU resources. Recently, substantial schedulers are proposed to tailor for DL workloads in GPU datacenters. This paper surveys existing research efforts for both training and inference workloads. We primarily present how existing schedulers facilitate the respective workloads from the scheduling objectives and resource consumption features. Finally, we prospect several promising future research directions. More detailed summary with the surveyed paper and code links can be found at our project website: https://github.com/S-Lab-System-Group/Awesome-DL-Scheduling-Papers

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preprint2022arXiv

Experimental Confirmation of the Universal Law for the Vibrational Density of States of Liquids

An analytical model describing the vibrational phonon density of states (VDOS) of liquids has long been elusive, mainly due to the difficulty in dealing with the imaginary modes dominant in the low-energy region, as described by the instantaneous normal mode (INM) approach. Nevertheless, Zaccone and Baggioli have recently developed such a model based on overdamped Langevin liquid dynamics. The model was proposed to be the universal law for the vibrational density of states of liquids. Distinct from the Debye law, g(ω) ~ ω2, for solids, the universal law for liquids reveals a linear relationship, g(ω) ~ ω, in the low-energy region. The universal law has been successfully verified with computer simulated VDOS for Lennard-Jones liquids. We further confirm this universal law with experimental VDOS measured by inelastic neutron scattering on real liquid systems including water, liquid metal, and polymer liquids. We have applied this model and extracted the effective relaxation rate for the short time dynamics for each liquid. The model has been further evaluated in the predication of the specific heat. The results have been compared with the existing experimental data as well as with values obtained by different approaches.

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preprint2022arXiv

Gate-tunable exchange bias effect in FePS3-Fe5GeTe2 van der Waals heterostructures

Electrical gate-manipulated exchange bias (EB) effect is a long-term goal for spintronics applications. Meanwhile, the emergence of van der Waals (vdW) magnetic heterostructures provides ideal platforms for the study of interlayer magnetic coupling. However, to date, the electrical gate-controlled EB effect has yet to be realized in vdW heterostructures. Here, for the first time, we realized electrically-controllable EB effects in a vdW antiferromagnetic (AFM)-ferromagnetic (FM) heterostructure, FePS3-Fe5GeTe2. For pristine FePS3-Fe5GeTe2 heterostructures, sizable EB effects can be generated due to the strong interface coupling, which also depend on the thickness of the ferromagnetic layers. By applying a solid protonic gate, the EB effects can be electrically tuned largely by proton intercalations and deintercalations. The EB field reaches up to 23% of the coercive field and the blocking temperature exceeds 50 K at Vg= -3.15 V. The proton intercalations not only tune the average magnetic exchange coupling, but also change the AFM configurations and transform the heterointerface between an uncompensated AFM-FM interface and a compensated AFM-FM interface. These alterations result in a dramatic modulation of the total interface exchange coupling and the resultant EB effects. The study is a significant step towards vdW heterostructure-based magnetic logic for future low-energy electronics.

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preprint2022arXiv

Optimizing Topological Switching in Confined 2D-Xene Nanoribbons via Finite-Size Effects

In a blueprint for topological electronics, edge state transport in a topological insulator material can be controlled by employing a gate-induced topological quantum phase transition. Here, by studying the width dependence of electronic properties, it is inferred that zigzag-Xene nanoribbons are promising materials for topological electronics with a display of unique physical characteristics associated with the intrinsic band topology and the finite-size effects on gate-induced topological switching. First, due to intertwining with intrinsic band topology-driven energy-zero modes in the pristine case, spin-filtered chiral edge states in zigzag-Xene nanoribbons remain gapless and protected against backward scattering even with finite inter-edge overlapping in ultra-narrow ribbons, i.e., a 2D quantum spin Hall material turns into a 1D topological metal. Second, mainly due to width- and momentum-dependent tunability of the gate-induced inter-edge coupling, the threshold-voltage required for switching between gapless and gapped edge states reduces as the width decreases, without any fundamental lower bound. Third, when the width of zigzag-Xene nanoribbons is smaller than a critical limit, topological switching between edge states can be attained without bulk bandgap closing and reopening. This is primarily due to the quantum confinement effect on the bulk band spectrum which increases the nontrivial bulk bandgap with decrease in width. The existence of such protected gapless edge states and reduction in threshold-voltage accompanied by enhancement in the bulk bandgap overturns the general wisdom of utilizing narrow-gap and wide channel materials for reducing the threshold-voltage in a standard field effect transistor analysis and paves the way toward low-voltage topological devices.

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preprint2022arXiv

Quarkonia production in ultra-peripheral PbPb collisions at LHCb

Measurements of coherent charmonium production cross sections together with their ratio in ultra-peripheral PbPb collisions are studied at a nucleon-nucleon centre-of-mass energy of $5.02\,\mathrm{TeV}$, the differential cross-sections are measured as a function of rapidity and transverse momentum, separately. The photo-production of \jpsi mesons at low transverse momentum is studied in peripheral PbPb collisions, which confirms coherent \jpsi production in hadronic collisions. These latest results significantly improve previous measurements and are compared with some theoretical predictions.

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preprint2021arXiv

An incompressible Eulerian method for fluid-structure interaction with mixed soft and rigid solids

We present a general simulation approach for incompressible fluid--structure interactions in a fully Eulerian framework using the reference map technique (RMT). The approach is suitable for modeling one or more rigid or finitely-deformable objects or soft objects with rigid components interacting with the fluid and with each other. It is also extended to control the kinematics of structures in fluids. The model is based on our previous Eulerian fluid--soft solver, and generalized to rigid structures by constraining the deformation-rate tensor in a projection framework. Several numerical examples are presented to illustrate the capability of the method.

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preprint2020arXiv

Overcoming Boltzmann's Tyranny in a Transistor via the Topological Quantum Field Effect

The sub-threshold swing is the fundamental critical parameter determining the operation of a transistor in low-power applications such as switches. It determines the fraction of dissipation due to the gate capacitance used for turning the device on and off, and in a conventional transistor it is limited by Boltzmann's tyranny to kTln(10)/q, or 60 mV per decade. Here, we demonstrate that the sub-threshold swing of a topological transistor, in which conduction is enabled by a topological phase transition via electric field switching, can be sizably reduced in a non-interacting system by modulating the Rashba spin-orbit interaction via a top-gate electric field. We refer to this as the Topological Quantum Field Effect and to the transistor as a Topological Quantum Field Effect transistor (TQFET). By developing a general theoretical framework for quantum spin Hall materials with honeycomb lattices we explicitly show that the Rashba interaction can reduce the sub-threshold swing by more than 25% compared to Boltzmann's limit in currently available materials, but without any fundamental lower bound, a discovery that can guide future materials design and steer the engineering of topological quantum devices.

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preprint2020arXiv

Quantum Anomalous Hall Effect in Magnetic Doped Topological Insulators and Ferromagnetic Spin-Gapless Semiconductors -- A Perspective Review

Quantum anomalous Hall effect, with a trademark of dissipationless chiral edge states for electronics/spintronics transport applications, can be realized in materials with large spin-orbit coupling and strong intrinsic magnetization. After Haldane seminal proposal, several models have been presented to control/enhance the spin-orbit coupling and intrinsic magnetic exchange interaction. After brief introduction of Haldane model for spineless fermions, following three fundamental quantum anomalous Hall models are discussed in this perspective review: (i) low-energy effective four band model for magnetic-doped topological insulator (Bi,Sb)2Te3 thin films, (ii) four band tight-binding model for graphene with magnetic adatoms, and (iii) two (three) band spinfull tight-binding model for ferromagnetic spin-gapless semiconductors with honeycomb (kagome) lattice where ground state is intrinsically ferromagnetic. These models cover two-dimensional Dirac materials hosting spinless, spinful and spin-degenerate Dirac points where various mass terms open a band gap and lead to quantum anomalous Hall effect. With emphasize on the topological phase transition driven by ferromagnetic exchange interaction and its interplay with spin-orbit-coupling, we discuss various symmetry constraints on the nature of mass term and the materialization of these models. We hope this study will shed light on the fundamental theoretical perspectives of quantum anomalous Hall materials.

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preprint2020arXiv

Spin gapless semiconductors

Spin gapless semiconductors (SGSs) are a new class of zero gap materials which have a fully spin polarised electrons and holes. They bridge zero gap materials and half-metals. The band structures of the SGSs can have two types of energy dispersions: Dirac linear dispersion and parabolic dispersion. The Dirac type SGSs exhibit fully spin polarized Dirac cones, and offer a platform for massless and fully spin polarized spintronics as well as dissipationless edge state via quantum anomalous Hall effect. Due to its fascinating spin and charge states, they hold great potential application in spintronics. There have been tremendous efforts worldwide on searching for suitable candidates of SGSs. In particularly, there is an increasing interest in searching for Dirac type SGSs. In the past decade, a large number of Dirac or parabolic type SGSs have been predicted by density functional theory and some of parabolic SGSs have been experimentally demonstrated. The SGSs hold great potential for high speed and low-energy consumption spintronics, electronics and optoelectronics. Here, we review both Dirac and parabolic types of SGSs in different materials systems and outline the concepts of SGSs, novel spin and charge states, and potential applications of SGSs in next generation spintronic devices.

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preprint2020arXiv

Understanding the mechanism of oxygen evolution reaction (OER) with the consideration of spin

Oxygen evolution reaction (OER) with intractable high overpotential is the rate-limiting step for rechargeable metal-air battery, water electrolysis systems, and solar fuels devices. There exists a spin state transition from spin singlet OH-/H2O reactant to spin triplet O2 product, which has not received enough attention yet. In this perspective, we attempt to retrospect electron behaviours during the whole OER process, with the consideration of spin attribute. Regardless of the adopted mechanisms by different electrocatalysts, for example, adsorbate evolution mechanism (AEM) or lattice oxygen mechanism (LOM), the underlying rationale is that active sites have to extract three in four electrons with the same spin direction before the formation of O=O. This spin-sensitive nature of OER superimposes additional high requirements on the electrocatalysts, especially on the spin structure, to compliment the fast electron transfer in the interface with spin selection and smoothly delivery afterwards. When optimizing the geometric and electronic structures catering for the spin-sensitive OER, awareness of the couplings between spin, charge, orbital and lattice is necessary. Some spin-correlated physical properties, such as (1) crystal field, (2) coordination, (3) oxidation, (4) bonding, (5) eg electron number, (6) conductivity and (7) magnetism, are also discussed briefly. It is hoped that our perspective could shed lights on the underlying physics of the slow kinetics of OER, providing a rational guidance for more effective energy conversion electrocatalysts designs.

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preprint2020arXiv

Weak localization and anti-localization in rare earth doped topological insulators

We study magneto-transport phenomena in two rare-earth doped topological insulators, SmxFexSb2-2xTe3 and SmxBi2-xTe2Se single crystals. The magneto-transport behaviours in both compounds exhibit a systematic crossover between weak anti-localization (positive magnetoresistance) and weak localization (negative magnetoresistance) with changes in temperatures and magnetic fields. The weak localization is caused by rare-earth-doping induced magnetization, and the weak anti-localization originates from topologically protected surface states. The transition between weak localization and weak anti-localization demonstrates a gap opening at the Dirac point of surface states in the quantum diffusive regime. This work demonstrates an effective way to manipulate the magneto-transport properties of the topological insulators by rare-earth element doping. Magnetometry measurements indicate that the Sm-dopant alone is paramagnetic, whereas the co-doped Fe-Sm state has short-range antiferromagnetic order. Our results hold potential for the realization of exotic topological effects in gapped topological insulator surface states.

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preprint2016arXiv

Study of nonleptonic $B_{q}^{\ast}$ ${\to}$ $D_{q}V$ and $P_{q} D^*$ weak decays

Motivated by the powerful capability of measurement for the $b$-flavored hadron rare decays at LHC and SuperKEKB/Belle-II, the nonleptonic $\bar{B}^{\ast}$ ${\to}$ $D\bar{D}^{\ast}$, $D{ρ^-}$, $DK^{\ast-}$, $πD^{\ast}$ and $KD^{\ast}$ weak decays are studied in detail. With the amplitudes calculated with factorization approach and the form factors of $B^{\ast}$ transition into pseudoscalar meson evaluated with the BSW model, branching fractions and polarization fractions are firstly presented. Numerically, the CKM-favored $\bar{B}_{q}^{\ast}$ ${\to}$ $D_{q}D_{s}^{{\ast}-}$ and $D_{q}ρ^{-}$ decays have large branching fractions, $\sim$ $10^{-8}$, which should be sought for with priority and firstly observed by LHC and Belle-II experiments. The $\bar{B}^{\ast}_q$ ${\to}$ $D_qK^{\ast}$ and $D_qρ$ decays are dominated by the longitudinal polarization states. While, the parallel polarization fractions of $\bar{B}^{\ast}_q$ ${\to}$ $D_q\bar{D}^{\ast}$ decays are comparable with the longitudinal ones, numerically, $f_{\parallel}$ $+$ $f_{L}$ ${\simeq}$ 95\% and $f_{L}:f_{\parallel}$ $\simeq$ $5:4$. Some comparisons between $\bar{B}^{*0}_q$ $\to$ $D_q V$ and their corresponding $\bar{B}^{0}_q$ $\to$ $D^*_q V$ decays are performed, and the relation $ f_{L,\parallel}(\bar{B}^{\ast 0}\to D V)\simeq f_{L,\parallel}(\bar{B}^0\to D^{\ast +} V^-) $ is presented. Besides, with the implication of $SU(3)$ flavor symmetry, some useful ratios $ R_{\rm du}$ and $ R_{\rm ds}$ are discussed in detail, and suggested to be verified experimentally.

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preprint2016arXiv

Study on the power scaling and mode instability property in an all-fiber narrow-linewidth tapered fiber amplifier

We develop a high-power narrow-linewidth fiber amplifier based on a tapered Yb-doped fiber. In the experiment, the narrow end of the tapered fiber is used as the input port of both pump source and signal light to form a robust all-fiber configuration. The stimulated Brillouin scattering (SBS) is effectively suppressed owing to its gradually increased mode area as well as continuously changed Brillouin frequency shift dependent on the core diameter. As a result, an output power of 260 W without SBS and mode instability (MI) is obtained, with a slope efficiency of 71.3 % and a narrow linewidth of ~2GHz. However, the MI rather than SBS is observed when output power reaches 260 W. The features of MI are experimentally studied in detail. It is pointed out that, the MI seems the primary and durative limitation factor of the tapered fiber amplifier for high-power narrow-linewidth output, although it has advantage to reduce other nonlinear effect. To the best of our knowledge, this is the first detailed experimental study on MI in a tapered fiber. At last, some worthwhile discussion about the optimization of the tapered fiber and the whole system for both SBS and MI suppression is conducted based on the present and previous results.

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preprint2016arXiv

Thermoelectric signals of state transition in polycrystalline SmB6

Topological Kondo insulator SmB6 has attracted quite a lot of attentions from condensed matter physics community. A number of unique electronic properties, including low- temperature resistivity anomaly, 1D electronic transport and 2D Fermi surfaces have been observed in SmB6. Here, we report on thermoelectric transport properties of polycrystalline SmB6 over a broad temperature from 300 K to 2 K. An anomalous transition in the temperature-dependent Seebeck coefficient S from S(T) ~ T-1 to S(T) ~ T was observed around 12 K. Such a transition demonstrates a transition of conductivity from 3D metallic bulk states to 2D metallic surface states with insulating bulk states. Our results suggest that the thermotransport measurements could be used for the characterization of state transition in topological insulators.

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preprint2015arXiv

Crossover of Magnetoresistance from Fourfold to Twofold Symmetry in SmB6 Single Crystal, a topological Kondo insulator

Topological Kondo insulators have been attracting great attention from the condensed-matter physics community due to their fascinating topological and strongly correlated properties. Here, we report angle-dependent c-axis magnetoresistance (MR) oscillations in a Kondo insulator, SmB6 single crystal, in a magnetic field of up to 13 T rotated in the ab-plane. Four-fold symmetric MR oscillations are first observed above 8 K, which result from the four-fold (C4) degeneracy of the bulk Fermi surface of SmB6. With decreasing temperature down to 2.3 K, the C4 symmetry of the MR oscillations gradually weakens and C2 symmetry appears. This demonstrates a crossover from three-dimensional bulk states to two-dimensional surface states and implies the possible emergence of topological nematic states. Our experimental observations shed new light on the metallic surface states and nematic states in the Kondo insulator SmB6.

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preprint2015arXiv

Giant enhancement in critical current density, up to a hundredfold, in superconducting NaFe0.97Co0.03As single crystals under hydrostatic pressure

Tremendous efforts towards improvement in the critical current density (Jc) of iron based superconductors (FeSCs), especially at relatively low temperatures and magnetic fields, have been made so far through different methods, resulting in real progress. Jc at high temperatures in high fields still needs to be further improved, however, in order to meet the requirements of practical applications. Here, we demonstrate a simple approach to achieve this. Hydrostatic pressure can significantly enhance Jc in NaFe0.97Co0.03As single crystals by at least tenfold at low field and more than a hundredfold at high fields. Significant enhancement in the in-field performance of NaFe0.97Co0.03As single crystal in terms of pinning force density (Fp) is found at high pressures. At high fields, the Fp is over 20 and 80 times higher than under ambient pressure at12K and 14K, respectively, at P=1GPa. We believe that the Co-doped NaFeAs compounds are very exciting and deserve to be more intensively investigated. Finally, it is worthwhile to say that by using hydrostatic pressure, we can achieve more milestones in terms of high Jc values in different superconductors.

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preprint2015arXiv

Influence of core NA on Thermal-Induced Mode Instabilities in High Power Fiber Amplifiers

We report on the influence of core NA on thermal-induced mode instabilities (MI) in high power fiber amplifiers. Influence of core NA and V-parameter on MI has been investigated numerically. It shows that core NA has larger influence on MI for fibers with smaller core-cladding-ratio, and the influence of core NA on threshold is more obvious when the amplifiers are pumped at 915nm. The dependence of threshold on V-parameter revealed that the threshold increases linearly as V-parameter decreases when V-parameter is larger than 3.5, and the threshold shows exponentially increase as V-parameter decreases when V-parameter is less than 3.5. We also discussed the effect of linewidth on MI, which indicates that the influence of linewidth can be neglected for linewidth smaller than 1nm when the fiber core NA is smaller than 0.07 and fiber length is shorter than 20m. Fiber amplifiers with different core NA were experimentally analyzed, which agreed with the theoretical predictions.

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preprint2015arXiv

Investigation of Electron-Phonon Coupling in Epitaxial Silicene by In-situ Raman Spectroscopy

In this letter, we report that the special coupling between Dirac fermion and lattice vibrations, in other words, electron-phonon coupling (EPC), in silicene layers on Ag(111) surface was probed by an in-situ Raman spectroscopy. We find the EPC is significantly modulated due to tensile strain, which results from the lattice mismatch between silicene and the substrate, and the charge doping from the substrate. The special phonon modes corresponding to two-dimensional electron gas scattering at edge sites in the silicene were identified. Detecting relationship between EPC and Dirac fermion through the Raman scattering will provide a direct route to investigate the exotic property in buckled two-dimensional honeycomb materials.

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preprint2015arXiv

Mg coating induced superconductivity in the FeSe ultrathin film

The transition from insulator to superconductor was achieved in the non-superconducting FeSe ultrathin film via simple Mg coating technique in present work. It was found that in non-superconducting FeSe ultrathin film without Mg coating, insulating \b{eta}-Fe1-xSe phase with iron-vacancy disorders is the main phase and more likely to be the parent phase of FeSe superconducting system. Proper Mg coating on the surface of FeSe films can lead to Mg entering FeSe crystal lattice to fill up these Fe vacancies rather than replace Fe. Simultaneously, additional electron doping is introduced and the electron carrier concentration in this parent phase can be tuned, which is responsible for the SIT and evolution of superconductivity in this system. However, abnormal decrease of electron concentration was found in FeSe film with excessive Mg coating, which brings about the severe degradation in superconducting performance.

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preprint2015arXiv

Observation of van Hove Singularities in Twisted Silicene Multilayers

Interlayer interactions perturb the electronic structure of two-dimensional materials and lead to new physical phenomena, such as van Hove singularities and Hofstadter's butterfly pattern. Silicene, the recently discovered two-dimensional form of silicon, is quite unique, in that silicon atoms adopt competing <i>sp</i><sup>2</sup> and <i>sp</i><sup>3</sup> hybridization states leading to a low-buckled structure promising relatively strong interlayer interaction. In multilayer silicene, the stacking order provides an important yet rarely explored degree of freedom for tuning its electronic structures through manipulating interlayer coupling. Here, we report the emergence of van Hove singularities in the multilayer silicene created by an interlayer rotation. We demonstrate that even a large-angle rotation (> 20<sup>o</sup>) between stacked silicene layers can generate a Moire pattern and van Hove singularities due to the strong interlayer coupling in multilayer silicene. Our study suggests an intriguing method for expanding the tunability of the electronic structure for electronic applications in this two-dimensional material.

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preprint2015arXiv

Polarization Effects on Thermal-Induced Mode Instabilities in High Power Fiber Lasers

We present detailed studies of the effect of polarization on thermal-induced mode instability (MI) in ytterbium-doped fiber amplifiers. Based on a steady-state theoretical model, which takes both electric fields along the two principal axes into consideration, the effect of polarization effects on the gain of Stokes wave was analyzed, which shows that the polarization characteristics of the fiber laser have no impact on the threshold of MI. Experimental validation of the theoretical analysis is presented with experimental results agreeing well with the theoretical results, in which polarization-maintained and non-polarization-maintained fiber lasers with core/inner cladding diameter of 30/250um and core NA of 0.07 were employed. The MI threshold power is measured to be about 367~386W.

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preprint2015arXiv

Quantum digital signature scheme

Digital signatures are the building blocks of modern communication to prevent masquerading by any party other than recipients, repudiation by signatory and forgery by any individual recipient. Digital signature scheme is said to be standard if the signature (a) is a pattern depending upon the message to be signed, (b) is built upon some information publically known and unique to the signatory, (c) can be stored by all the recipients. While classical methods provide computational security only, quantum mechanics guarantees information-theoretically secure and standard digital signature schemes. However, standard quantum digital signature schemes are based on quantum one-way functions and hence require long term quantum memory for storing quantum signatures, which is not practically feasible yet. We demonstrate here a standard quantum digital signature scheme by replacing quantum one-way functions with multiparty controlled EPR channels. It allows signatory to generate non-locally correlated quantum signatures, instead of multiple copies of a unique quantum state, and assures security against any individual since others have non-locally correlated information.

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preprint2015arXiv

Stability and scalability of piezoelectric flags

We investigate the effect of piezoelectric (PZT) material on the flutter speed, vibration mode and frequency, and energy harvesting power and efficiency of a flexible flag in various fluids. We develop a fully coupled fluid-solid-electric model by combining the inviscid vortex sheet model with a linear electro-mechanical coupling model. A resistance only circuit and a resonant resistance-inductance (RL) circuit are considered. For a purely resistive circuit, an increased electro-mechanical coupling factor results in an increased flutter speed, vibration frequency, averaged electric power and efficiency. A consistent optimal resistance is found that maximizes the flutter speed and the energy harvesting power. For a resonant RL circuit, by tuning the inductance to match the circuit frequency to the flag's vibration frequency, the flutter speed can be greatly decreased, and a larger averaged power and efficiency are obtained. We also consider a model scale set-up with several commonly used commercial materials for operating in air and water. Typical ranges of dimensionless parameters are obtained for four types of material that span a wide range of solid density and rigidity values. We find that the resistance only circuit is more effective when the flag is placed in a lighter fluid (e.g. air), while the RL circuit is able to reduce the flutter speed when the flag is placed in a heavier fluid (e.g. water).

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preprint2015arXiv

Study of dopant concentrations on thermal induced mode instability in high power fiber amplifiers

Dependence of mode instabilities (MI) on ytterbium dopant concentrations in high power fiber amplifiers has been investigated. It is theoretically shown that, by only varying the fiber length to maintain the same total small-signal pump absorption, the MI threshold is independent of dopant concentration. MI thresholds of gain fibers with ytterbium dopant concentration of 5.93X10^25/m3 and 1.02X10^26/m3 have been measured, which exhibit similar thresholds and agree with theoretical results. The result indicates that heavy doping of active fiber can be adopted to suppress nonlinear effects without decreasing MI threshold, which provides a method of maximizing the power output of fiber laser, taking into account the stimulated Brillouin scattering, stimulated Raman Scattering, and MI thresholds simultaneously.

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preprint2015arXiv

Study of wavelength dependence of mode instability based on a semi-analytical model

We present theoretical study of wavelength dependence of mode instability (MI) in high power fiber lasers, which employs an improved semi-analytical theoretical model. The influence of pump / seed wavelength and photodarkening on threshold has been studied. The results indicate promising MI suppression through pumping or seeding at an appropriate wavelength. Small amounts of photodarkening can lead to significant impact on MI.

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preprint2015arXiv

Theoretical study of transport properties of B40 and its endohedral borospherenes in single-molecule junctions

C60 fullerene has been studied extensively, as it is considered to be a good candidate for building single-molecule junctions. Here, we theoretically demonstrate that the conductance of single-molecule junctions based on a newly discovered molecule, borospherene (B40), is comparable to that for the C60-based junction with its more delocalized π electrons. The charge injection efficiency in the B40-based junction is improved, as up to 7 atoms in direct contact with the electrode are possible in the Au-B40-Au junction. Interestingly, a higher number of atoms in direct contact with the electrode does not result in a higher number of conduction channels because of the unique chemical bonding in the B40 molecule, without two-center two- electron bonds. The transport properties of Au-B40-Au junctions can be proved by doping. With a Ca, Sr, or Y atom encapsulated into the B40 cage, the conductance at zero bias increases significantly. Moreover, our calculations show that the lowest unoccupied molecular orbital dominates the low-bias transport, as the thermopower in these junctions is negative. Our study indicates that B40 is an attractive new platform for designing highly conductive single-molecule junctions for future molecular circuits.

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preprint2014arXiv

1.3kW monolithic linearly-polarized single-mode MOPA and strategies for mitigating mode instabilities

We report on the high power amplification of 1064nm linearly-polarized laser in all-fiber polarization-maintained MOPA, which can operate at output power level of 1.3kW. The main amplifier was pumped with six 915nm laser diodes, and the slope efficiency is 65.3%. The beam quality (M2) was measured to be <1.2 at full power operation. The polarization extinction rate of the fiber amplifier was measured to be above 94% before mode instabilities (MI) sets in, which reduced to about 90% after the onset of MI. Power scaling capability of strategies for suppressing MI is analyzed based on a novel semi-analytical model, the theoretical results of which agree with the experimental results. It shows that mitigating MI by coiling the gain fiber is an effective and practical way in standard double-cladding large mode area fiber, and, by tight coiling of the gain fiber to the radius of 5.5cm, the MI threshold can be increased to 3 times higher than that without coiling or loose coiling. Experimental study has been carried out to verify the idea, which has proved that MI was suppressed successfully in the amplifier by tight coiling.

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preprint2014arXiv

Effects of Oxygen Adsorption on the Surface State of Epitaxial Silicene on Ag(111)

Epitaxial silicene, which is one single layer of silicon atoms packed in a honeycomb structure, demonstrates a strong interaction with the substrate that dramatically affects its electronic structure. The role of electronic coupling in the chemical reactivity between the silicene and the substrate is still unclear so far, which is of great importance for functionalization of silicene layers. Here, we report the reconstructions and hybridized electronic structures of epitaxial 4x4 silicene on Ag(111), which are revealed by scanning tunneling microscopy and angle-resolved photoemission spectroscopy. The hybridization between Si and Ag results in a metallic surface state, which can gradually decay due to oxygen adsorption. X-ray photoemission spectroscopy confirms the decoupling of Si-Ag bonds after oxygen treatment as well as the relatively oxygen resistance of Ag(111) surface, in contrast to 4x4 silicene [with respect to Ag(111)]. First-principles calculations have confirmed the evolution of the electronic structure of silicene during oxidation. It has been verified experimentally and theoretically that the high chemical activity of 4x4 silicene is attributable to the Si pz state, while the Ag(111) substrate exhibits relatively inert chemical behavior.

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preprint2014arXiv

Hydrostatic pressure induced transition from δTc to δl pinning mechanism in MgB2

The impact of hydrostatic pressure up to 1.2 GPa on the critical current density (Jc) and the nature of the pinning mechanism in MgB2 have been investigated within the framework of the collective theory. We found that the hydrostatic pressure can induce a transition from the regime where pinning is controlled by spatial variation in the critical transition temperature (δT_c) to the regime controlled by spatial variation in the mean free path (δl). Furthermore, Tc and low field Jc are slightly reduced, although the Jc drops more quickly at high fields than at ambient pressure. We found that the pressure raises the anisotropy and reduces the coherence length, resulting in weak interaction of the vortex cores with the pinning centres. Moreover, the hydrostatic pressure can reduce the density of states [Ns(E)], which, in turn, leads to a reduction in the critical temperature from 39.7 K at P = 0 GPa to 37.7 K at P = 1.2 GPa.

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preprint2014arXiv

Hydrostatic pressure: A very effective approach to significantly enhance critical current density in granular Sr4V2O6Fe2As2 superconductor

Pressure is well known to significantly raise the superconducting transition temperature, Tc, in both iron pnictides and cuprate based superconductors. Little work has been done, however, on how pressure can affect the flux pinning and critical current density in the Fe-based superconductors. Here, we propose to use hydrostatic pressure to significantly enhance flux pinning and Tc in polycrystalline pnictide bulks. We have chosen Sr4V2O6Fe2As2 polycrystalline samples as a case study. We demonstrate that the hydrostatic pressure up to 1.2 GPa can not only significantly increase Tc from 15 K (underdoped) to 22 K, but also significantly enhance the irreversibility field, Hirr, by a factor of 4 at 7 K, as well as the critical current density, Jc, by up to 30 times at both low and high fields. It was found that pressure can induce more point defects, which are mainly responsible for the Jc enhancement. In addition, we found that the transformation from surface pinning to point pinning induced by pressure was accompanied by a reduction of anisotropy at high temperatures. Our findings provide an effective method to significantly enhance Tc, Jc, Hirr, and the upper critical field, Hc2, for other families of Fe-based superconductors in the forms of wires/tapes, films, and single crystal and polycrystalline bulks.

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preprint2014arXiv

Mitigating of Modal Instabilities in Linearly-Polarized Fiber Amplifiers by Shifting Pump Wavelength

We investigated the effct of pump wavelength on the modal instabilities (MI) in high power linearly-polarized Yb-doped fiber amplifiers. We built a novel semi-analytical model to determine the frequency coupling characteristics and power threshold of MI, which indicates promising MI suppression through pumping at an appropriate wavelength. By pumping at 915nm, the threshold can be enhanced by a factor of 2.36 as compared to that pumped at 976nm. Based on a high power linearly-polarized fiber amplifier platform, we studied the influence of pump wavelength experimentally. The threshold has been increased by a factor of 2 at 915nm, which agrees with the theoretical calculation and verified our theoretical model. Furthermore, we show that MI suppression by detuning the pump wavelength is weakened for fiber with large core-to-cladding ratio.

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preprint2014arXiv

Tuning the Band Gap in Silicene by Oxidation

Silicene monolayers grown on Ag(111) surfaces demonstrate a band gap that is tunable by oxygen adatoms from semimetallic to semiconducting type. By using low-temperature scanning tunneling microscopy, it is found that the adsorption configurations and amounts of oxygen adatoms on the silicene surface are critical for band-gap engineering, which is dominated by different buckled structures in R13xR13, 4x4, and 2R3x2R3 silicene layers. The Si-O-Si bonds are the most energy-favored species formed on R13xR13, 4x4, and 2R3x2R3 structures under oxidation, which is verified by in-situ Raman spectroscopy as well as first-principles calculations. The silicene monolayers retain their structures when fully covered by oxygen adatoms. Our work demonstrates the feasibility of tuning the band gap of silicene with oxygen adatoms, which, in turn, expands the base of available two-dimensional electronic materials for devices with properties that is hardly achieved with graphene oxide.

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preprint2014arXiv

Tuning the conductance of H$_{2}$O@C$_{60}$ by position of the encapsulated H$_{2}$O

The change of conductance of single molecule junctions in response to various external stimuli is the fundamental mechanism for single-molecule electronic devices with multiple functionalities. We propose a concept that the conductance of molecule systems can be tuned from its inside. The conductance is varied in C$_{60}$ with encapsulated H$_{2}$O, H$_{2}$O@C$_{60}$. The transport properties of the H$_{2}$O@C$_{60}$-based nanostructure sandwiched between electrodes are studied using first-principles calculations based on the non-equilibrium Green's function formalism. Our results show that the conductance of the H$_{2}$O@C$_{60}$ is sensitive to the position of the H$_{2}$O and its dipole direction inside the cage with changes in conductance up to 20%. Our study paves a way for the H$_{2}$O@C$_{60}$ molecule to be a new platform for novel molecule based electronics and sensors.

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preprint2014arXiv

Unabridged phase diagram for single-phased FeSexTe1-x thin films

A complete phase diagram and its corresponding physical properties are essential prerequisites to understand the underlying mechanism of iron based superconductivity. For the structurally simplest 11 (FeSeTe) system, earlier attempts using bulk samples have not been able to do so due to the fabrication difficulties. Here, thin FeSexTe1-x films with the Se content covering the full range were fabricated by using pulsed laser deposition method. Crystal structure analysis shows that all films retain the tetragonal structure in room temperature. Significantly, the highest superconducting transition temperature (TC = 20 K) occurs in the newly discovered domain, 0.6 - 0.8. The single-phased superconducting dome for the full Se doping range is the first of its kind in iron chalcogenide superconductors. Our results present a new avenue to explore novel physics as well as to optimize superconductors.

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preprint2013arXiv

Application of particle swarm optimization for enhanced cyclic steam stimulation in a offshore heavy oil reservoir

Three different variations of PSO algorithms, i.e. Canonical, Gaussian Bare-bone and Lévy Bare-bone PSO, are tested to optimize the ultimate oil recovery of a large heavy oil reservoir. The performance of these algorithms was compared in terms of convergence behaviour and the final optimization results. It is found that, in general, all three types of PSO methods are able to improve the objective function. The best objective function is found by using the Canonical PSO, while the other two methods give similar results. The Gaussian Bare-bone PSO may picks positions that are far away from the optimal solution. The Lévy Bare-bone PSO has similar convergence behaviour as the Canonical PSO. For the specific optimization problem investigated in this study, it is found that the temperature of the injection steam, CO2 composition in the injection gas, and the gas injection rates have bigger impact on the objective function, while steam injection rate and the liquid production rate have less impact on the objective function.

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preprint2013arXiv

Optimizing snake locomotion on an inclined plane

We develop a model to study the locomotion of snakes on an inclined plane. We determine numerically which snake motions are optimal for two retrograde traveling-wave body shapes---triangular and sinusoidal waves---across a wide range of frictional parameters and incline angles. In the regime of large transverse friction coefficient, we find power-law scalings for the optimal wave amplitudes and corresponding costs of locomotion. We give an asymptotic analysis to show that the optimal snake motions are traveling-wave motions with amplitudes given by the same scaling laws found in the numerics.

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preprint2012arXiv

Intermittent Josephson effect with feedback voltage and temperature oscillations in graphite-coated nanocapsules with superconducting TaC core

An intermittent Josephson effect in the form of voltage and temperature oscillations in the voltage - current curves near 2 K is observed in pellets consisting of superconducting TaC nanocapsules coated with graphite. This phenomenon is attributed to non-equilibrium conditions, when Cooper pairs across a junction, which stimulate the emission of photons and the feedback temperature change of the junction. It occurs in a three-dimensional granular framework composed of TaC/carbon/TaC tunneling junctions with a Mott metal-insulator transition, below the critical temperature Tc of non-ideal type-II superconductor TaC.

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preprint2011arXiv

s-d Electronic interactions induced H2 dissociation on the γ-U(100) surface and influences of niobium doping

The dissociation of hydrogen molecules on the γ-U(100) surface is systematically studied with the density functional theory method. Through potential energy surface calculations, we find that hydrogen molecules can dissociate without any barriers on the clean γ-U(100) surface. After careful electronic analysis, it is found that charge transfer between the hydrogen s and uranium d electronic states causes the dissociation, which is quite different from the dissociation of hydrogen molecules on other actinide metal surfaces. Considering that doping of 3d transition metal atoms can stabilize the αphase of U, we also study the influences of Nb-doping on the hydrogen dissociation process. We find that the 3d electronic states of Nb also take part in the hybridization with hydrogen s electronic states, which leads to the result that hydrogen molecules also dissociate without any energy barriers on the doped U surface. In addition, the free electronic energy lowers down more quickly for a hydrogen molecule approaching the doped U surface.

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