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Yong Xu

Publications and source records attributed to Yong Xu.

At least 469 records · Page 26Linked to original sources

Dirac semimetal phase in the hexagonal LiZnBi

Based on first-principles calculations, we find that LiZnBi, a metallic hexagonal $ABC$ compound, can be driven into a Dirac semimetal with a pair of Dirac points by strain. The nontrivial topological nature of the strained LiZnBi is directly demonstrated by calculating its $\mathbb{Z}_2$ index and the surface states, where the Fermi arcs are clearly observed. The low-energy states as well as topological properties are shown to be sensitive to the strain configurations. The finding of Dirac semimetal phase in LiZnBi may intrigue further researches on the topological properties of hexagonal $ABC$ materials and promote new practical applications.

cond-mat.mtrl-sci↗

A survey of sparse representation: algorithms and applications

Sparse representation has attracted much attention from researchers in fields of signal processing, image processing, computer vision and pattern recognition. Sparse representation also has a good reputation in both theoretical research and practical applications. Many different algorithms have been proposed for sparse representation. The main purpose of this article is to provide a comprehensive study and an updated review on sparse representation and to supply a guidance for researchers. The taxonomy of sparse representation methods can be studied from various viewpoints. For example, in terms of different norm minimizations used in sparsity constraints, the methods can be roughly categorized into five groups: sparse representation with $l_0$-norm minimization, sparse representation with $l_p$-norm (0$<$p$<$1) minimization, sparse representation with $l_1$-norm minimization and sparse representation with $l_{2,1}$-norm minimization. In this paper, a comprehensive overview of sparse representation is provided. The available sparse representation algorithms can also be empirically categorized into four groups: greedy strategy approximation, constrained optimization, proximity algorithm-based optimization, and homotopy algorithm-based sparse representation. The rationales of different algorithms in each category are analyzed and a wide range of sparse representation applications are summarized, which could sufficiently reveal the potential nature of the sparse representation theory. Specifically, an experimentally comparative study of these sparse representation algorithms was presented. The Matlab code used in this paper can be available at: http://www.yongxu.org/lunwen.html.

cs.CV↗

Interacting spin-orbit-coupled spin-1 Bose-Einstein condensates

The recent experimental realization of spin-orbit (SO) coupling for spin-1 ultracold atoms opens an interesting avenue for exploring SO-coupling-related physics in large-spin systems, which is generally unattainable in electronic materials. In this paper, we study the effects of interactions between atoms on the ground states and collective excitations of SO-coupled spin-1 Bose-Einstein condensates (BECs) in the presence of a spin-tensor potential. We find that ferromagnetic interaction between atoms can induce a stripe phase exhibiting in-phase or out-of-phase modulating patterns between spin-tensor and zero-spin-component density waves. We characterize the phase transitions between different phases using the spin-tensor density as well as the collective dipole motion of the BEC. We show that there exists a double maxon-roton structure in the Bogoliubov-excitation spectrum, attributed to the three band minima of the SO-coupled spin-1 BEC.

cond-mat.quant-gas↗

Dirac and Weyl Rings in Three Dimensional Cold Atom Optical Lattices

Recently three dimensional topological quantum materials with gapless energy spectra have attracted considerable interests in many branches of physics. Besides the celebrated example, Dirac and Weyl points which possess gapless point structures in the underlying energy dispersion, the topologically protected gapless spectrum can also occur along a ring, named Dirac and Weyl nodal rings. Ultra-cold atomic gases provide an ideal platform for exploring new topological materials with designed symmetries. However, whether Dirac and Weyl rings can exist in the single-particle spectrum of cold atoms remains elusive. Here we propose a realistic model for realizing Dirac and Weyl rings in the single-particle band dispersion of a cold atom optical lattice. Our scheme is based on previously experimentally already implemented Raman coupling setup for realizing spin-orbit coupling. Without the Zeeman field, the model preserves both pseudo-time-reversal and inversion symmetries, allowing Dirac rings. The Dirac rings split into Weyl rings with a Zeeman field that breaks the pseudo-time-reversal symmetry. We examine the superfluidity of attractive Fermi gases in this model and also find Dirac and Weyl rings in the quasiparticle spectrum.

cond-mat.quant-gas↗

Converting normal insulators into topological insulators via tuning orbital levels

Tuning the spin-orbit coupling strength via foreign element doping and/or modifying bonding strength via strain engineering are the major routes to convert normal insulators to topological insulators. We here propose an alternative strategy to realize topological phase transition by tuning the orbital level. Following this strategy, our first-principles calculations demonstrate that a topological phase transition in some cubic perovskite-type compounds CsGeBr$_3$ and CsSnBr$_3$ could be facilitated by carbon substitutional doping. Such unique topological phase transition predominantly results from the lower orbital energy of the carbon dopant, which can pull down the conduction bands and even induce band inversion. Beyond conventional approaches, our finding of tuning the orbital level may greatly expand the range of topologically nontrivial materials.

cond-mat.mtrl-sci↗

Large-Gap Quantum Spin Hall States in Stanene Grown on Substrate

Two-dimensional stanene is a promising candidate material for realizing room-temperature quantum spin Hall (QSH) effect. Monolayer stanene has recently been fabricated by molecular beam epitaxy, but shows metallic features on Bi$_2$Te$_3$(111) substrate, which motivates us to study the important influence of substrate. Based on first-principles calculations, we find that varying substrate conditions considerably tunes electronic properties of stanene. The supported stanene gives either trivial or QSH states, with significant Rashba splitting induced by inversion asymmetry. More importantly, large-gap (up to 0.3 eV) QSH states are realizable when growing stanene on various substrates, like the anion-terminated (111) surfaces of SrTe, PbTe, BaSe and BaTe. These findings provide significant guidance for future research of stanene and large-gap QSH states.

cond-mat.mtrl-sci↗

Epitaxial Growth of Two-Dimensional Stanene

Ultrathin semiconductors present various novel electronic properties. The first experimental realized two-dimensional (2D) material is graphene. Searching 2D materials with heavy elements bring the attention to Si, Ge and Sn. 2D buckled Si-based silicene was realized by molecular beam epitaxy (MBE) growth1,2. Ge-based germanene was realized by mechanical exfoliation3. Sn-based stanene has its unique properties. Stanene and its derivatives can be 2D topological insulators (TI) with a very large band gap as proposed by first-principles calculations4, or can support enhanced thermoelectric performance5, topological superconductivity6 and the near-room-temperature quantum anomalous Hall (QAH) effect7. For the first time, in this work, we report a successful fabrication of 2D stanene by MBE. The atomic and electronic structures were determined by scanning tunneling microscopy (STM) and angle-resolved photoemission spectroscopy (ARPES) in combination with first-principles calculations. This work will stimulate the experimental study and exploring the future application of stanene.

cond-mat.mtrl-sci↗

Effects of ferroelectric polarization on surface phase diagram: an evolutionary algorithm study of the BaTiO$_{3}$(001) surface

We have constructed the surface phase diagram of the BaTiO$_{3}$(001) surface by employing an evolutionary algorithm for surface structure prediction, where the ferroelectric polarization is included as a degree of freedom. Among over 1000 candidate structures explored, a surface reconstruction of (2$\times$1)-TiO is discovered to be thermodynamically stable and have the \emph{p2mm} plane group symmetry as observed experimentally. We find that the influence of ferroelectric polarization on the surface free energy can be either negligibly small or sizably large (over 1 eV per ($2 \times 1$) supercell), depending strongly on the surface structure and resulting in a significant distinction of surface phase diagram with varying ferroelectric polarization. It is therefore feasible to control the surface stability by applying an external electric field. Our results may have important implications in understanding the surface reconstruction of ferroelectric materials and tuning surface properties.

cond-mat.mtrl-sci↗

Bright solitons in a 2D spin-orbit-coupled dipolar Bose-Einstein condensate

We study a two-dimensional spin-orbit-coupled dipolar Bose-Einstein condensate with repulsive contact interactions by both the variational method and the imaginary time evolution of the Gross-Pitaevskii equation. The dipoles are completely polarized along one direction in the 2D plane so as to provide an effective attractive dipole-dipole interaction. We find two types of solitons as the ground states arising from such attractive interactions: a plane wave soliton with a spatially varying phase and a stripe soliton with a spatially oscillating density for each component. Both types of solitons possess smaller size and higher anisotropy than the soliton without spin-orbit coupling. Finally, we discuss the properties of moving solitons, which are nontrivial because of the violation of Galilean invariance.

cond-mat.quant-gas↗

Gap Solitons in Spin-Orbit-Coupled Bose-Einstein Condensates in Optical Lattices

While different ways to realize spin-orbit coupling in Bose-Einstein condensates exist, not all are currently experimentally implementable. Here we present a detailed study of gap solitons in a Bose- Einstein condensate with experimentally realizable spin-orbit coupling and discuss two cases relating to a spin-dependent parity symmetry. In the parity symmetric case, two families of fundamental gap solitons in second linear energy gap are demonstrated with opposite sign of the parity, with one family having single humped densities and the other double humped ones. In the case of broken parity symmetry, the fundamental modes manifest spin-polarization. Both families possess an opposite sign of the spin-polarization.

cond-mat.quant-gas↗

Berezinskii-Kosterlitz-Thouless Phase Transition in 2D Spin-Orbit Coupled Fulde-Ferrell Superfluids

The experimental observation of traditional Zeeman-field induced Fulde-Ferrell-Larkin-Ovchinnikov (FFLO) superfluids has been hindered by various challenges, in particular, the requirement of low dimensional systems. In 2D, finite temperature phase fluctuations lead to extremely small Berezinskii-Kosterlitz-Thouless (BKT) transition temperature for FFLO superfluids, raising serious concerns regarding their experimental observability. Recently, it was shown that FFLO superfluids can be realized using a Rashba spin-orbit coupled Fermi gas subject to Zeeman fields, which may also support topological excitations such as Majorana fermions in 2D. Here we address the finite temperature BKT transition issue in this system, which may exhibit gapped, gapless, topological, and gapless topological FF phases. We find a large BKT transition temperature due to large effective superfluid densities, making it possible to observe 2D FF superfluids at finite temperature. In addition, we show that gapless FF superfluids can be stable due to their positive superfluid densities. These findings pave the way for the experimental observation of 2D gapped and gapless FF superfluids and their associated topological excitations at finite temperature.

cond-mat.quant-gas↗

Disentangling the magnetoelectric and thermoelectric transport in topological insulator thin films

We report transport studies on (Bi,Sb)2Te3 topological insulator thin films with tunable electronic band structure. We find a doping and temperature regime in which the Hall coefficient is negative indicative of electron-type carriers, whereas the Seebeck coefficient is positive indicative of hole-type carriers. This sign anomaly is due to the distinct transport behaviors of the bulk and surface states: the surface Dirac fermions dominate magnetoelectric transport while the thermoelectric effect is mainly determined by the bulk states. These findings may inspire new ideas for designing topological insulator-based high efficiency thermoelectric devices.

cond-mat.mtrl-sci↗

Topological Fulde-Ferrell Superfluids of a Spin-Orbit Coupled Fermi Gas

Topological Fermi superfluids have played the central role in various fields of physics. However, all previous studies focus on the cases where Cooper pairs have zero center-of-mass momenta (i.e. normal superfluids). The topology of Fulde-Ferrell superfluids with nonzero momentum pairings have never been explored until recent findings that Fulde-Ferrell superfluids in a spin-orbit coupled Fermi gas can accommodate Majorana fermions in real space in low dimensions and Weyl fermions in momentum space in three dimension. In this review, we first discuss the mechanism of pairings in spin-orbit coupled Fermi gases in optical lattices subject to Zeeman fields, showing that spin-orbit coupling as well as Zeeman fields enhance Fulde-Ferrell states while suppress Larkin-Ovchinnikov states. We then present the low temperature phase diagram including both FF superfluids and topological FF superfluids phases in both two dimension and three dimension. In one dimension, Majorana fermions as well as phase dependent order parameter are visualized. In three dimension, we show the properties of Weyl fermions in momentum space such as anisotropic linear dispersion, Fermi arch, and gaplessness away from $k_{\perp}=0$. Finally, we discuss some possible methods to probe FF superfluids and topological FF superfluids in cold atom systems.

cond-mat.quant-gas↗

Dark Solitons with Majorana Fermions in Spin-Orbit-Coupled Fermi Gases

We show that a single dark soliton can exist in a spin-orbit-coupled Fermi gas with a high spin imbalance, where spin-orbit coupling favors uniform superfluids over non-uniform Fulde-Ferrell-Larkin-Ovchinnikov states, leading to dark soliton excitations in highly imbalanced gases. Above a critical spin imbalance, two topological Majorana fermions (MFs) without interactions can coexist inside a dark soliton, paving a way for manipulating MFs through controlling solitons. At the topological transition point, the atom density contrast across the soliton suddenly vanishes, suggesting a signature for identifying topological solitons.

cond-mat.quant-gas↗

Two-dimensional time-reversal-invariant topological superconductivity in a doped quantum spin Hall insulator

Time-reversal-invariant topological superconductors have a full paring gap in the bulk and gapless Majorana states at the edge or on the surface. Here, we theoretically propose topological superconductivity in a doped quantum spin Hall insulator. We study the pairing symmetry of doped Sn$X$ film within a two-orbital model, and show that it realizes a topological superconductor when the intraorbital attractive interaction is stronger than the interorbital interaction, which is generic for a doped quantum spin Hall insulator with strong spin-orbit coupling. Edge channels are studied in a tight-binding model numerically. Finally, we discuss the possible topological superconductivity in Sn$X$ film by comparing to $3d$ superconductivity in bulk Sn.

cond-mat.supr-con↗

Stable two-dimensional dumbbell stanene: a quantum spin Hall insulator

We predict from first-principles calculations a novel structure of stanene with dumbbell units (DB), and show that it is a two-dimensional topological insulator with inverted band gap which can be tuned by compressive strain. Furthermore, we propose that the boron nitride sheet and reconstructed ($2\times2$) InSb(111) surfaces are ideal substrates for the experimental realization of DB stanene, maintaining its non-trivial topology. Combined with standard semiconductor technologies, such as magnetic doping and electrical gating, the quantum anomalous Hall effect, Chern half metallicity and topological superconductivity can be realized in DB stanene on those substrates. These properties make the two-dimensional supported stanene a good platform for the study of new quantum spin Hall insulator as well as other exotic quantum states of matter.

cond-mat.mes-hall↗

Anisotropic Weyl Fermions from Quasiparticle Excitation Spectrum of a 3D Fulde-Ferrell Superfluid

Weyl fermions, first proposed for describing massless chiral Dirac fermions in particle physics, have not been observed yet in experiments. Recently, much effort has been devoted to explore Weyl fermions around band touching points of single particle energy dispersions in certain solid state materials (named \textit{Weyl semimetals}), similar as graphene for Dirac fermions. Here we show that such Weyl semimetals also exist in the quasiparticle excitation spectrum of a three-dimensional (3D) spin-orbit coupled Fulde-Ferrell (FF) superfluid. By varying Zeeman fields, the properties of Weyl fermions, such as their creation and annihilation, number and position, as well as anisotropic linear dispersions around band touching points, can be tuned. We study the manifestation of anisotropic Weyl fermions in sound speeds of FF fermionic superfluids, which are detectable in experiments.

cond-mat.quant-gas↗

Enhanced Thermoelectric Performance and Anomalous Seebeck Effects in Topological Insulators

Improving the thermoelectric figure of merit zT is one of the greatest challenges in material science. Recent discovery of topological insulators (TIs) offers new promise in this prospect. In this work, we demonstrate theoretically that zT is strongly size dependent in TI, and the size parameter can be tuned to enhance zT to be significantly greater than 1. Furthermore, we show that the life time of the edge states in TI is strongly energy dependent, leading to large and anomalous Seebeck effects with an opposite sign to the Hall effect. These striking properties make TIs the promising material for thermoelectrics science and technology.

cond-mat.mtrl-sci↗