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Dirac and Weyl Materials: Fundamental Aspects and Some Spintronics ApplicationsSep 21 2016Dirac and Weyl materials refer to a class of solid materials which host low-energy quasiparticle excitations that can be described by the Dirac and Weyl equations in relativistic quantum mechanics. Starting with the advent of graphene as the first prominent ... More

Dirac and Weyl Materials: Fundamental Aspects and Some Spintronics ApplicationsSep 21 2016Nov 03 2016Dirac and Weyl materials refer to a class of solid materials which host low-energy quasiparticle excitations that can be described by the Dirac and Weyl equations in relativistic quantum mechanics. Starting with the advent of graphene as the first prominent ... More

Chirality Hall Effect in Weyl SemimetalsApr 03 2015We generalize a semiclassical theory and use the argument of angular momentum conservation to examine the ballistic transport in lightly-doped Weyl semimetals, taking into account various phase-space Berry curvatures. We predict universal transverse shifts ... More

Edge states in Graphene: from gapped flat band to gapless chiral modesOct 12 2008Oct 14 2008We study edge-states in graphene systems where a bulk energy gap is opened by inversion symmetry breaking. We find that the edge-bands dispersion can be controlled by potentials applied on the boundary with unit cell length scale. Under certain boundary ... More

Buckled honeycomb lattice and unconventional magnetic responseSep 06 2014We study the magnetic response of buckled honeycomb-lattice materials. The buckling breaks the sublattice symmetry, enhances the spin-orbit coupling, and allows the tuning of a topological quantum phase transition. As a result, there are two doubly degenerate ... More

Goos-Hänchen-like shifts at metal/superconductor interfaceMar 26 2018At a normal-metal/superconductor interface, an incident electron from the normal-metal (N) side can be normally reflected as an electron or Andreev reflected as a hole. We show that pronounced lateral shifts along the interface between the incident and ... More

Predicted Unusual Magnetoresponse in Type-II Weyl SemimetalsApr 14 2016Aug 19 2016We show several distinct signatures in the magneto-response of type-II Weyl semimetals. The energy tilt tends to squeeze the Landau levels (LLs), and for a type-II Weyl node, there always exists a critical angle between the B-fileld and the tilt, at which ... More

Scattering universality classes of side jump in anomalous Hall effectNov 14 2010The anomalous Hall conductivity has an important extrinsic contribution known as side jump contribution, which is independent of both scattering strength and disorder density. Nevertheless, we discover that side jump has strong dependence on the spin ... More

Unconventional pairing induced anomalous transverse shift in Andreev reflectionAug 23 2017Sep 20 2018Superconductors with unconventional pairings have been a fascinating subject of research, for which a central issue is to explore effects that can be used to characterize the pairing. The process of Andreev reflection--the reflection of an electron as ... More

Transverse Shift in Andreev ReflectionJun 12 2017An incoming electron is reflected back as a hole at a normal-metal-superconductor interface, a process known as Andreev reflection. We predict that there exists a universal transverse shift in this process due to the effect of spin-orbit coupling in the ... More

Electron-Magnon Scattering in Anomalous Hall EffectNov 16 2009We study the role played by electron-magnon scattering in the anomalous Hall effect. We find that it has important contributions distinct from other scattering processes like impurities scattering and phonon scattering. As a demonstration, we calculate ... More

Electric control of topological phase transitions in Dirac semimetal thin filmsMar 27 2015We investigate the effect of a vertical electric field on a Dirac semimetal thin film. We show that through the interplay between the quantum confinement effect and the field-induced coupling between sub-bands, the sub-band gap can be tuned and inverted, ... More

Ferroelectricity and Antiferroelectricity in Elemental Group-V Monolayer MaterialsJun 18 2017Ferroelectricity is usually found in compound materials composed by different elements. Here, based on first-principles calculations, we reveal the first example of spontaneous electrical polarization and ferroelectricity in stable two-dimensional elemental ... More

Anomalous spatial shifts in interface electronic scatteringJan 02 2019The anomalous spatial shifts at interface scattering, first studied in geometric optics, recently found their counterparts in the electronic context. It was shown that both longitudinal and transverse shifts, analogous to the Goos-Hanchen and Imbert-Fedorov ... More

Hybrid Nodal Loop Metal: Unconventional Magnetoresponse and Material RealizationFeb 03 2018A nodal loop is formed by band crossing along a one-dimensional closed manifold, with each point on the loop a linear nodal point in the transverse dimensions and can be classified as type-I or type-II depending on the band dispersion. Here, we propose ... More

Borophene as an extremely high capacity electrode material for Li-ion and Na-ion batteriesAug 05 2016Two-dimensional (2D) materials as electrodes is believed to be the trend for future Li-ion and Na-ion batteries technologies. Here, by using first-principles methods, we predict that the recently reported borophene (2D born sheet) can serve as an ideal ... More

Coexistence of four-band nodal rings and triply-degenerate nodal points in centrosymmetric metal diboridesApr 12 2017Topological metals with protected band-crossing points have been attracting great interest. Here we report novel topological band features in a family of metal diboride materials. Using first- principles calculations, we show that these materials are ... More

Three-dimensional honeycomb carbon: Junction line distortion and novel emergent fermionsJun 28 2018Carbon enjoys a vast number of allotropic forms, each possessing unique properties determined by the lattice structures and bonding characters. Here, based on first-principles calculations, we propose a new three-dimensional carbon allotrope--hC28. We ... More

Two-dimensional Weyl Half Semimetal and Tunable Quantum Anomalous Hall Effect in Monolayer PtCl$_{3}$Mar 20 2019We propose a new topological quantum state of matter---the two-dimensional (2D) Weyl half semimetal (WHS), which features 2D Weyl points at Fermi level belonging to a single spin channel, such that the low-energy electrons are described by fully spin-polarized ... More

Low-Energy Effective Hamiltonian for Giant-Gap Quantum Spin Hall Insulators in Honeycomb X-Hydride/Halide (X=N-Bi) MonolayersFeb 24 2014Sep 22 2014Using the tight-binding method in combination with first-principles calculations, we systematically derive a low-energy effective Hilbert subspace and Hamiltonian with spin-orbit coupling for two-dimensional hydrogenated and halogenated group-V monolayers. ... More

Theory of I-V Characteristics of Magnetic Josephson JunctionsOct 15 2010We analyze the electrical characteristics of a circuit consisting of a free thin-film magnetic layer and source and drain electrodes that have opposite magnetization orientations along the free magnet's two hard directions. We find that when the circuit's ... More

Hourglass Dirac Chain Metal in Rhenium DioxideMay 03 2017Nonsymmorphic symmetries, which involve fractional lattice translations in crystalline materials, can generate exotic types of fermionic excitations that are robust against spin-orbit coupling. Here we report on a hourglass-type dispersion in the bulk ... More

$d$-Orbital Topological Insulator and Semimetal in Antifluorite Cu$_2$S Family: Contrasting Spin Helicities, Nodal Box, and Hybrid Surface StatesMar 27 2017We reveal a class of three-dimensional $d$-orbital topological materials in the antifluorite Cu$_2$S family. Derived from the unique properties of low-energy $t_{2g}$ states, their phases are solely determined by the sign of spin-orbit coupling (SOC): ... More

Weyl-loop half metal in Li$_3$(FeO$_3$)$_2$Nov 13 2018Nodal-line metals and semimetals, as interesting topological states of matter, have been mostly studied in nonmagnetic materials. Here, based on first-principles calculations and symmetry analysis, we predict that fully spin-polarized Weyl loops can be ... More

Quadratic and Cubic Nodal Lines Stabilized by Crystalline SymmetryJul 07 2018In electronic band structures, nodal lines may arise when two (or more) bands contact and form a one-dimensional manifold of degeneracy in the Brillouin zone. Around a nodal line, the dispersion for the energy difference between the bands is typically ... More

Anomalous tunneling characteristic of Weyl semimetals with tilted energy dispersionOct 20 2016Aug 22 2017Weyl semimetal is a recently discovered state of quantum matter, which generally possesses tilted energy dispersion. Here, we investigate the electron tunneling through a Weyl semimetal p-n-p junction. The angular dependence of electron tunneling exhibits ... More

Blue Phosphorene Oxide: Strain-tunable Quantum Phase Transitions and Novel 2D Emergent FermionsOct 18 2016Tunable quantum phase transitions and novel emergent fermions in solid state materials are fascinating subjects of research. Here, we propose a new stable two-dimensional (2D) material, the blue phosphorene oxide (BPO), which exhibits both. Based on first-principles ... More

Strain effects on electronic and optic properties of monolayer C$_2$N holey two-dimensional crystalsOct 04 2015A new two-dimensional material, the C$_2$N holey 2D (C$_2$N-$h$2D) crystal, has recently been synthesized. Here we investigate the strain effects on the properties of this new material by first-principles calculations. We show that the material is quite ... More

Blue Phosphorene Oxide: Strain-tunable Quantum Phase Transitions and Novel 2D Emergent FermionsOct 18 2016Oct 20 2016Tunable quantum phase transitions and novel emergent fermions in solid state materials are fascinating subjects of research. Here, we propose a new stable two-dimensional (2D) material, the blue phosphorene oxide (BPO), which exhibits both. Based on first-principles ... More

Almost ideal nodal-loop semimetal in monoclinic CuTeO$_3$ materialMar 12 2018Jun 28 2018Nodal-loop semimetals are materials in which the conduction and valence bands cross on a one-dimensional loop in the reciprocal space. For the nodal-loop character to manifest in physical properties, it is desired that the loop is close to the Fermi level, ... More

Engineering Topological Surface States and Giant Rashba Spin Splitting in BiTeI/Bi$_2$Te$_3$ HeterostructuresNov 19 2013The search for strongly inversion asymmetric topological insulators is an active research field because these materials possess distinct properties compared with the inversion symmetric ones. In particular, it is desirable to realize a large Rashba spin-splitting ... More

Magnetic control of the valley degree of freedom of massive Dirac fermions with application to transition metal dichalcogenidesSep 16 2013We study the valley-dependent magnetic and transport properties of massive Dirac fermions in multivalley systems such as the transition metal dichalcogenides. The asymmetry of the zeroth Landau level between valleys and the enhanced magnetic susceptibility ... More

Spin Polarized and Valley Helical Edge Modes in Graphene NanoribbonsMar 07 2011Inspired by recent progress in fabricating precisely zigzag-edged graphene nanoribbons and the observation of edge magnetism, we find that spin polarized edge modes with well-defined valley index can exist in a bulk energy gap opened by a staggered sublattice ... More

Spin-orbit-free Weyl-loop and Weyl-point semimetals in a stable three-dimensional carbon allotropeMay 09 2015Topological band theory has revolutionized our understanding of electronic structure of materials, in particular, a novel state - Weyl semimetal - has been predicted for systems with strong spin-orbit coupling (SOC). Here, a new class of Weyl semimetals, ... More

Multivariable Scaling for the Anomalous Hall EffectMar 11 2015We derive a general scaling relation for the anomalous Hall effect in ferromagnetic metals involving multiple competing scattering mechanisms, described by a quadratic hypersurface in the space spanned by the partial resistivities. We also present experimental ... More

Strain-induced Isostructural and Magnetic Phase Transitions in Monolayer MoN$_2$Aug 18 2016Feb 22 2017The change of bonding status, typically occurring only in chemical processes, could dramatically alter the material properties. Here, we show that a tunable breaking and forming of a diatomic bond can be achieved through physical means, i.e., by a moderate ... More

Three-dimensional Pentagon Carbon with a genesis of emergent fermionsJun 11 2017Carbon, the basic building block of our universe, enjoys a vast number of allotropic structures. Owing to its bonding characteristic, most carbon allotropes possess the motif of hexagonal rings. Here, with first-principles calculations, we discover a ... More

Valley-Layer Coupling: A New Design Principle for ValleytronicsApr 13 2019We introduce the concept of valley-layer coupling (VLC) in two-dimensional materials, where the low-energy electronic states in the emergent valleys have valley-contrasted layer polarization such that each state is spatially localized on the top or bottom ... More

Two-dimensional Spin-Orbit Dirac Point in Monolayer HfGeTeJun 27 2017Oct 12 2017Dirac points in two-dimensional (2D) materials have been a fascinating subject of research, with graphene as the most prominent example. However, the Dirac points in existing 2D materials, including graphene, are vulnerable against spin-orbit coupling ... More

Singular Effects of Spin-Flip Scattering on Gapped Dirac FermionsNov 17 2010Aug 27 2011We investigate the effects of spin-flip scattering on the Hall transport and spectral properties of gapped Dirac fermions. We find that in the weak scattering regime, the Berry curvature distribution is dramatically compressed in the electronic energy ... More

Theory of orbital magnetization in disordered systemsJan 17 2012We present a general formula of the orbital magnetization of disordered systems based on the Keldysh Green's function theory in the gauge-covariant Wigner space. In our approach, the gauge invariance of physical quantities is ensured from the very beginning, ... More

Two-dimensional honeycomb borophene oxide: A promising anode material offering super high capacity for Li/Na-ion batteriesApr 30 2019Rational design of novel two-dimensional (2D) electrode materials with high capacity is crucial for the further development of Li-ion and Na-ion batteries. Herein, based on first-principles calculations, we systemically investigate Li and Na storage behaviors ... More

Hourglass Weyl loops in two dimensions: Theory and material realization in monolayer GaTeI familyFeb 25 2019May 20 2019Nodal loops in two-dimensional (2D) systems are typically vulnerable against spin-orbit coupling (SOC). Here, we explore 2D systems with a type of doubly degenerate nodal loops that are robust under SOC and feature an hourglass type dispersion. We present ... More

Composite Dirac SemimetalApr 12 2019Weak topological insulators and Dirac semimetals are gapped and nodal phases with distinct topological properties, respectively. Here, we propose a novel topological phase that exhibits features of both and is dubbed composite Dirac semimetal (CDSM). ... More

Tunable ferroelectricity and anisotropic electric transport in monolayer $β$-GeSeDec 12 2017Apr 11 2018Low-dimensional ferroelectricity has attracted tremendous attention due to its huge potential in device applications. Here, based on first-principles calculations, we predict the existence of spontaneous in-plane electrical polarization and ferroelectricity ... More

Temperature dependence of side-jump spin Hall conductivityNov 08 2018Mar 15 2019In the conventional paradigm of the spin Hall effect, the side-jump contribution to the spin Hall conductivity is independent of the density of disorder, thus remains unchanged in clean samples when the phonon density varies with temperature. To the contrary, ... More

Composite Dirac SemimetalApr 12 2019Apr 16 2019Weak topological insulators and Dirac semimetals are gapped and nodal phases with distinct topological properties, respectively. Here, we propose a novel topological phase that exhibits features of both and is dubbed composite Dirac semimetal (CDSM). ... More

Circumventing the no-go theorem: A single Weyl point surrounded by nodal wallsMar 21 2019Despite of a rapidly expanding inventory of possible crystalline Weyl semimetals, all of them are constrained by the Nielsen-Ninomiya no-go theorem, namely, that left- and right-handed Weyl points appear in pairs. With time-reversal (T) symmetry, an even ... More

Strain-induced Isostructural and Magnetic Phase Transitions in Monolayer MoN$_2$Aug 18 2016The change of bonding status, typically occurring only in chemical processes, could dramatically alter the material properties. Here, we show that a tunable breaking and forming of a diatomic bond can be achieved through physical means, i.e., by a moderate ... More

Topological metallic states in spin-orbit coupled bilayer systemsFeb 26 2014We investigate the influence of different spin-orbit couplings on topological phase transitions in the bilayer Kane-Mele model. We find that the competition between intrinsic spin-orbit coupling and Rashba spin-orbit coupling can lead to two dimensional ... More

Topological bosonic states on ribbons of honeycomb latticeAug 09 2018The topological properties of hardcore bosons on ribbons of honeycomb lattice are studied using quantum Monte Carlo simulations. We map out a rich phase diagram with the superfluid and insulator phases at various fillings. Particularly, it is revealed ... More

Nonsymmorphic cubic Dirac point and crossed nodal rings across the ferroelectric phase transition in LiOsO$_3$Apr 25 2018Apr 30 2018Crystalline symmetries can generate exotic band-crossing features, which can lead to unconventional fermionic excitations with interesting physical properties. We show how a cubic Dirac point---a four-fold-degenerate band-crossing point with cubic dispersion ... More

Gate tunable current partition in graphene based topological zero linesJan 31 2017We demonstrate new mechanisms for gate tunable current partition at topological zero-line intersections in a graphene-based current splitter. Based on numerical calculations of the non-equilibrium Green's functions and Landauer-B\"{u}ttiker formula, we ... More

Temperature dependence of side-jump spin Hall conductivityNov 08 2018May 14 2019In the conventional paradigm of the spin Hall effect, the side-jump contribution to the spin Hall conductivity is independent of the density of disorder, thus remains unchanged in clean samples when the phonon density varies with temperature. To the contrary, ... More

Valley-polarized quantum anomalous Hall phase and disorder induced valley-filtered chiral edge channelsDec 25 2014We investigate the topological and transport properties of the recently discovered valley-polarized quantum anomalous Hall (VQAH) phase. In single layer, the phase is realized through the competition between two types of spin-orbit coupling, which breaks ... More

Transverse shift in crossed Andreev reflectionSep 07 2018Nov 30 2018Crossed Andreev reflection (CAR) is an intriguing effect that occurs in a normal-superconductor-normal junction. In CAR, an incoming electron from one terminal is coherently scattered as an outgoing hole into the other terminal. Here, we reveal that there ... More

Anomalous Magnetic Susceptibility and Hall Effect from Valley Degrees of FreedomMar 30 2011We study the magnetic and transport properties of epitaxial graphene films in this letter. We predict enhanced signal of magnetic susceptibility and relate it to the intrinsic valley magnetic moments. There is also an anomalous contribution to the ordinary ... More

Hybrid Structures and Strain-Tunable Electronic Properties of Carbon NanothreadsMar 13 2018The newly synthesized ultrathin carbon nanothreads have drawn great attention from the carbon community. Here, based on first-principles calculations, we investigate the electronic properties of carbon nanothreads under the influence of two important ... More

Theory of phonon side jump contribution in Anomalous Hall transportJun 12 2019The role of electron-phonon scattering in finite-temperature anomalous Hall effect is still poorly understood. In this work, we present a Boltzmann theory for the side-jump contribution from electron-phonon scattering, which is derived from the microscopic ... More

Anisotropic Quantum Confinement Effect and Electric Control of Surface States in Dirac Semimetal NanostructuresAug 13 2014Nov 20 2014The recent discovery of Dirac semimetals represents a new achievement in our fundamental understanding of topological states of matter. Due to their topological surface states, high mobility, and exotic properties associated with bulk Dirac points, these ... More

All-electric spin transistor based on a side-gate-modulated two-dimensional topological insulatorOct 23 2015We propose and investigate a spin transistor device consisting of two ferromagnetic leads connected by a two-dimensional topological insulator as the channel material. It exploits the unique features of the topological spin-helical edge states, such that ... More

Theoretical prediction of MoN2 monolayer as a high capacity electrode material for metal ion batteriesSep 20 2016Benefited from the advantages on environmental benign, easy purification, and high thermal stability, the recently synthesized two-dimensional (2D) material MoN2 shows great potential for clean and renewable energy applications. Here, through first-principles ... More

Interfacial Multiferroics of TiO2/PbTiO3 Heterostructure Driven by Ferroelectric Polarization DiscontinuityFeb 22 2017Novel phenomena appear when two different oxide materials are combined together to form an interface. For example, at the interface of LaAlO3/SrTiO3, two dimensional conductive states form to avoid the polar discontinuity and magnetic properties are found ... More

Hybrid Dirac Semimetal in CaAgBi Materials FamilyJun 13 2017Based on their formation mechanisms, Dirac points in three-dimensional systems can be classified as accidental or essential. The former can be further distinguished into type-I and type-II, depending on whether the Dirac cone spectrum is completely tipped ... More

Type-II nodal loops: theory and material realizationMay 05 2017Aug 22 2017Nodal loop appears when two bands, typically one electron-like and one hole-like, are crossing each other linearly along a one-dimensional manifold in the reciprocal space. Here we propose a new type of nodal loop which emerges from crossing between two ... More

Electrically tunable valley polarization in Weyl semimetals with tilted energy dispersionMay 19 2017Feb 09 2018Tunneling transport across the p-n-p junction of Weyl semimetal with tilted energy dispersion is investigated. We report that the electrons around different valleys experience opposite direction refractions at the barrier interface when the energy dispersion ... More

Quantum Anomalous Hall Effect in Graphene from Rashba and Exchange EffectsMay 10 2010Sep 10 2010We investigate the possibility of realizing quantum anomalous Hall effect in graphene. We show that a bulk energy gap can be opened in the presence of both Rashba spin-orbit coupling and an exchange field. We calculate the Berry curvature distribution ... More

Nonsymmorphic-symmetry-protected hourglass Dirac loop, nodal line, and Dirac point in bulk and monolayer $X_3$SiTe$_6$ ($X=$ Ta, Nb)Oct 23 2017Jan 17 2018Nonsymmorphic space group symmetries can generate exotic band-crossings in topological metals and semimetals. Here, based on symmetry analysis and first-principles calculations, we reveal rich band-crossing features in the existing layered compounds Ta$_3$SiTe$_6$ ... More

Monolayer Mg$_{2}$C: Negative Poisson's ratio and unconventional 2D emergent fermionsJun 21 2018Novel two-dimensional (2D) emergent fermions and negative Poisson's ratio in 2D materials are fascinating subjects of research. Here, based on first-principles calculations and theoretical analysis, we predict that the hexacoordinated Mg$_{2}$C monolayer ... More

Two-dimensional nodal-loop half metal in monolayer MnNMar 12 2019Two-dimensional (2D) materials with nodal-loop band crossing have been attracting great research interest. However, it remains a challenge to find 2D nodal loops that are robust against spin-orbit coupling (SOC) and realized in magnetic states. Here, ... More

Nodal surface semimetals: Theory and material realizationDec 28 2017Mar 13 2018We theoretically study the three-dimensional topological semimetals with nodal surfaces protected by crystalline symmetries. Different from the well-known nodal-point and nodal-line semimetals, in these materials, the conduction and valence bands cross ... More

Tunable Half-metallic Magnetism in Atom-thin Holey Two-dimensional C$_2$N MonolayerFeb 10 2017Aug 28 2017Exploring two-dimensional (2D) materials with magnetic ordering is a focus of current research. It remains a challenge to achieve tunable magnetism in a material of one-atom-thickness without introducing extrinsic magnetic atoms or defects. Here, based ... More

Two-dimensional second-order topological insulator in graphdiyneApr 22 2019A second-order topological insulator (SOTI) in $d$ spatial dimensions features topologically protected gapless states at its $(d-2)$-dimensional boundary at the intersection of two crystal faces, but is gapped otherwise. As a novel topological state, ... More

Anderson Localization from Berry-Curvature Interchange in Quantum Anomalous Hall SystemJan 27 2016We theoretically investigate the localization mechanism of the quantum anomalous Hall effect (QAHE) in the presence of spin-flip disorders. We show that the QAHE keeps quantized at weak disorders, then enters a Berry-curvature mediated metallic phase ... More

Hexagonal supertetrahedral boron: A topological metal with multiple spin-orbit-free emergent fermionsJan 24 2019We predict a new three-dimensional (3D) boron allotrope based on systematic first-principles electronic structure calculations. This allotrope can be derived by substituting each carbon atom in a hexagonal diamond lattice with a B$_{4}$ tetrahedron and ... More

Nodal Loop and Nodal Surface States in Ti3Al Family MaterialsMay 04 2018May 09 2018Topological metals and semimetals are new states of matter which attract great interest in current research. Here, based on first-principles calculations and symmetry analysis, we propose that the family of titanium-based compounds Ti3X (X=Al, Ga, Sn, ... More

Artificial gravity field, astrophysical analogues, and topological phase transitions in strained topological semimetalsSep 02 2016May 10 2017Effective gravity and gauge fields are emergent properties intrinsic for low-energy quasiparticles in topological semimetals. Here, taking two Dirac semimetals as examples, we demonstrate that applied lattice strain can generate warped spacetime, with ... More

Two-dimensional honeycomb borophene oxide: Strong anisotropy and nodal loop transformationOct 06 2018Jan 23 2019The search for topological semimetals is mainly focused on heavy-element compounds as following the footsteps of previous research on topological insulators, with less attention on light-element materials. However, the negligible spin orbit coupling with ... More

Three-dimensional quantum Hall effect and metal-insulator transition in ZrTe5Jul 07 2018Dec 28 2018Symmetry, dimensionality, and interaction are crucial ingredients for phase transitions and quantum states of matter. As a prominent example, the integer quantum Hall effect (QHE) represents a topological phase generally regarded as characteristic for ... More

Multiple Unpinned Dirac Points in Group-Va Single-layers with Phosphorene StructureSep 18 2015Apr 27 2016Emergent Dirac fermion states underlie many intriguing properties of graphene, and the search for them constitute one strong motivation to explore two-dimensional (2D) allotropes of other elements. Phosphorene, the ultrathin layers of black phosphorous, ... More

Quadratic contact point semimetal: Theory and material realizationJun 27 2018Sep 20 2018Most electronic properties of metals are determined solely by the low-energy states around the Fermi level, and for topological metals/semimetals, these low-energy states become distinct because of their unusual energy dispersion and emergent pseudospin ... More

Electric Field-Tuned Topological Phase Transition in Ultra-Thin Na3Bi - Towards a Topological TransistorMay 22 2018Feb 05 2019The electric field induced quantum phase transition from topological to conventional insulator has been proposed as the basis of a topological field effect transistor [1-4]. In this scheme an electric field can switch 'on' the ballistic flow of charge ... More

Intrinsic Magnetoconductivity of Non-magnetic MetalsSep 23 2016We present a comprehensive study of magnetoconductivity for general three-dimensional non-magnetic metals within the Berry-curvature-corrected semiclassical and Boltzmann framework. We find a new contribution, which is intrinsic in the sense that its ... More

New fermions on the line in topological symmorphic metalsMay 22 2016Topological metals and semimetals (TMs) have recently drawn significant interest. These materials give rise to condensed matter realizations of many important concepts in high-energy physics, leading to wide-ranging protected properties in transport and ... More

Field Induced Positional Shift of Bloch Electrons and its Dynamical ImplicationsFeb 11 2014We derive the field correction to the Berry curvature of Bloch electrons, which can be traced back to a positional shift due to the interband mixing induced by external electromagnetic fields. The resulting semiclassical dynamics is accurate to second ... More

Geometrical effects in orbital magnetic susceptibilityNov 02 2014Apr 06 2015Within the wave-packet semiclassical approach, the Bloch electron energy is derived to second order in the magnetic field and classified into gauge-invariant terms with clear physical meaning, yielding a fresh understanding of the complex behavior of ... More

Type-II Topological Dirac Semimetals: Theory and Materials Prediction (VAl3 family)Jun 24 2016The discoveries of Dirac and Weyl semimetal states in spin-orbit compounds led to the realizations of elementary particle analogs in table-top experiments. In this paper, we propose the concept of a three-dimensional type-II Dirac fermion and identify ... More

Theory of Electromotive Force Induced by Domain Wall MotionSep 07 2007Mar 12 2008We formulate a theory on the dynamics of conduction electrons in the presence of moving magnetic textures in ferromagnetic materials. We show that the variation of local magnetization in both space and time gives rise to topological fields, which induce ... More

Dirac and Weyl Superconductors in Three DimensionsFeb 27 2014We introduce the concept of 3D Dirac (Weyl) superconductors (SC), which have protected bulk four(two)-fold nodal points and surface Andreev arcs at zero energy. We provide a sufficient criterion for realizing them in centrosymmetric SCs with odd-parity ... More

Germagraphene as promising anode material for Lithium-ion batteries predicted from first-principles calculationsOct 05 2018Finding electrode materials with high capacity is a key challenge for developing Lithium-ion batteries (LIBs). Graphene was once expected to be a promising candidate, but it turns out to be too inert to interact with Li. Here, by using the first-principles ... More

Tunable Hyperbolic Dispersion and Negative Refraction in Natural Electride MaterialsFeb 18 2017Hyperbolic (or indefinite) materials have attracted significant attention due to their unique capabilities for engineering electromagnetic space and controlling light propagation. A current challenge is to find a hyperbolic material with wide working ... More

Perfect Valley Filter in Topological Domain WallDec 31 2014We propose a realization of perfect valley filters based on the chiral domain-wall channels between a quantum anomalous Hall insulator and a quantum valley Hall insulator. Uniquely, all these channels reside in the same valley and propagate unidirectionally, ... More

Lehmer Transform and its Theoretical PropertiesMay 13 2018We propose a new class of transforms that we call {\it Lehmer Transform} which is motivated by the {\it Lehmer mean function}. The proposed {\it Lehmer transform} decomposes a function of a sample into their constituting statistical moments. Theoretical ... More

Temperature dependence of side-jump spin Hall conductivityNov 08 2018Feb 13 2019In the conventional paradigm of the spin Hall effect, the side-jump contribution to the spin Hall conductivity is independent of the density of disorder, thus remains unchanged in clean samples when the phonon density varies with temperature. To the contrary, ... More

Valleytronics in merging Dirac cones: All-electric-controlled valley filter, valve and universal reversible logic gateNov 16 2017Nov 28 2017Despite much anticipation of valleytronics as a candidate to replace the ageing CMOS-based information processing, its progress is severely hindered by the lack of practical ways to manipulate valley polarization all-electrically in an electrostatic setting. ... More

Towards Three-Dimensional Weyl-SurfaceSemimetals in Graphene NetworksJan 06 2016Graphene as a two-dimensional (2D) topological Dirac semimetal has attracted much attention for its outstanding properties and potential applications. However, three-dimensional (3D) topological semimetals for carbon materials are still rare. Searching ... More

Electronic, Dielectric, and Plasmonic Properties of Two-Dimensional Electride Materials X$_2$N (X=Ca, Sr): A First-Principles StudyFeb 09 2015Based on first-principles calculations, we systematically study the electronic, dielectric, and plasmonic properties of two-dimensional (2D) electride materials X$_2$N (X=Ca, Sr). We show that both Ca$_2$N and Sr$_2$N are stable down to monolayer thickness. ... More

Hourglass Weyl loops in two dimensions: Theory and material realization in monolayer GaTeI familyFeb 25 2019Nodal loops in two-dimensional (2D) systems are typically vulnerable against spin-orbit coupling (SOC). Here, we explore 2D systems with a type of doubly degenerate nodal loops that are robust under SOC and feature an hourglass type dispersion. We present ... More

Electron Spectral Functions of Reconstructed Quantum Hall EdgesApr 30 2004Aug 03 2005During the reconstruction of the edge of a quantum Hall liquid, Coulomb interaction energy is lowered through the change in the structure of the edge. We use theory developed earlier by one of the authors [K. Yang, Phys. Rev. Lett. 91, 036802 (2003)] ... More

Bond-randomness-induced Neel order in weakly coupled antiferromagnetic spin chainsJul 25 2002May 21 2003Quasi-one-dimensional antiferromagnetic (AF) quantum spin systems show a wide range of interesting phenomena such as the spin-Peierls transition and disorder driven long range ordering. While there is no magnetic long range order in strictly one-dimensional ... More