arXiv subjects

Shou-Cheng Zhang

Publications and source records attributed to Shou-Cheng Zhang.

At least 73 records · Page 4Linked to original sources

arXiv2014

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↗

arXiv2014

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↗

arXiv2014

Topological States in Ferromagnetic CdO/EuO Superlattices and Quantum Wells

Based on ab initio calculations, we demonstrate that the ferromagnetic CdO/EuO superlattice is a simple Weyl semimetal with two linear Weyl nodes in the Brillouin zone, and the corresponding CdO/EuO quantum well realizes the stichometric quantum anomalous Hall state without random magnetic doping. In addition, a simple effective model is presented to describe the basic mechanism of spin polarized band inversion in this system.

cond-mat.mes-hall↗

arXiv2014

Generation and Electric Control of Spin-Coupled Valley Current in WSe2

The valley degree of freedom in layered transition-metal dichalcogenides (MX2) provides the opportunity to extend functionalities of novel spintronics and valleytronics devices. Due to spin splitting induced by spin-orbital coupling (SOC), the non-equilibrium charge carrier imbalance between two degenerate and inequivalent valleys to realize valley/spin polarization has been successfully demonstrated theoretically and supported by optical experiments. However, the generation of a valley/spin current by the valley polarization in MX2 remains elusive and a great challenge. Here, within an electric-double-layer transistor based on WSe2, we demonstrated a spin-coupled valley photocurrent whose direction and magnitude depend on the degree of circular polarization of the incident radiation and can be further greatly modulated with an external electric field. Such room temperature generation and electric control of valley/spin photocurrent provides a new property of electrons in MX2 systems, thereby enabling new degrees of control for quantum-confined spintronics devices.

cond-mat.mes-hall↗

arXiv2014

Quantum Spin Hall and Quantum Anomalous Hall States Realized in Junction Quantum Wells

Both quantum spin Hall and quantum anomalous Hall states are novel states of quantum matter with promising applications. We propose junction quantum wells comprising II-VI, III-V or IV semiconductors as a large class of new materials realizing the quantum spin Hall state. Especially, we find that the bulk band gap for the quantum spin Hall state can be as large as 0.1 eV. Further more, magnetic doping would induce the ferromagnetism in these junction quantum wells due to band edge singularities in the band-inversion regime and to realize the quantum anomalous Hall state.

cond-mat.mes-hall↗

arXiv2014

Universal Scaling of Quantum Anomalous Hall Plateau Transition

We study the critical properties of the quantum anomalous Hall (QAH) plateau transition in magnetic topological insulators. We introduce a microscopic model for the plateau transition in QAH effect at the coercive field and then map it to the network model of quantum percolation in the integer quantum Hall effect plateau transition. Generally, an intermediate plateau with zero Hall conductance could occur at the coercive field. $σ_{xx}$ would have double peaks at the coercivity while $ρ_{xx}$ only has single peak. Remarkably, this theoretical prediction is already borne out in experiment. Universal scaling of the transport coefficients $ρ_{xy}$ and $ρ_{xx}$ are predicted.

cond-mat.mes-hall↗

arXiv2014

Singlet Mott State Simulating the Bosonic Laughlin Wave Function

We study properties of a class of spin singlet Mott states for arbitrary spin S bosons on a lattice, with particle number per cite n=S/l+1, where l is a positive integer. We show that such a singlet Mott state can be mapped to a bosonic Laughlin wave function on the sphere with a finite number of particles at filling ν=1/2l. Spin, particle and hole excitations in the Mott state are discussed, among which the hole excitation can be mapped to the quasi-hole of the bosonic Laughlin wave function. We show that this singlet Mott state can be realized in a cold atom system on optical lattice, and can be identified using Bragg spectroscopy and Stern-Gerlach techniques. This class of singlet Mott states may be generalized to simulate bosonic Laughlin states with filling ν=q/2l.

cond-mat.quant-gas↗

arXiv2014

Topological Invariants and Ground-State Wave Functions of Topological Insulators on a Torus

We define topological invariants in terms of the ground states wave functions on a torus. This approach leads to precisely defined formulas for the Hall conductance in four dimensions and the topological magneto-electric $θ$ term in three dimensions, and their generalizations in higher dimensions. They are valid in the presence of arbitrary many-body interaction and disorder. These topological invariants systematically generalize the two-dimensional Niu-Thouless-Wu formula, and will be useful in numerical calculations of disordered topological insulators and strongly correlated topological insulators, especially fractional topological insulators.

cond-mat.str-el↗

arXiv2013

Quantum Anomalous Hall Effect in Magnetically Doped InAs/GaSb Quantum Wells

The quantum anomalous Hall effect has recently been observed experimentally in thin films of Cr doped (Bi,Sb)$_2$Te$_3$ at a low temperature ($\sim$ 30mK). In this work, we propose realizing the quantum anomalous Hall effect in more conventional diluted magnetic semiconductors with doped InAs/GaSb type II quantum wells. Based on a four band model, we find an enhancement of the Curie temperature of ferromagnetism due to band edge singularities in the inverted regime of InAs/GaSb quantum wells. Below the Curie temperature, the quantum anomalous Hall effect is confirmed by the direct calculation of Hall conductance. The parameter regime for the quantum anomalous Hall phase is identified based on the eight-band Kane model. The high sample quality and strong exchange coupling make magnetically doped InAs/GaSb quantum wells good candidates for realizing the quantum anomalous Hall insulator at a high temperature.

cond-mat.mes-hall↗

arXiv2013

Topological insulators with SU(2) Landau levels

We construct continuum models of 3D and 4D topological insulators by coupling spin-1/2 fermions to an SU(2) background gauge field, which is equivalent to a spatially dependent spin-orbit coupling. Higher dimensional generalizations of flat Landau levels are obtained in the Landau-like gauge. The 2D helical Dirac modes with opposite helicities and 3D Weyl modes with opposite chiralities are spatially separated along the third and fourth dimensions, respectively. Stable 2D helical Fermi surfaces and 3D chiral Fermi surfaces appear on open boundaries, respectively. The charge pumping in 4D Landau level systems shows quantized 4D quantum Hall effect.

cond-mat.str-el↗

arXiv2013

Large-gap quantum spin Hall insulators in tin films

The search of large-gap quantum spin Hall (QSH) insulators and effective approaches to tune QSH states is important for both fundamental and practical interests. Based on first-principles calculations we find two-dimensional tin films are QSH insulators with sizable bulk gaps of 0.3 eV, sufficiently large for practical applications at room temperature. These QSH states can be effectively tuned by chemical functionalization and by external strain. The mechanism for the QSH effect in this system is band inversion at the Γpoint, similar to the case of HgTe quantum well. With surface doping of magnetic elements, the quantum anomalous Hall effect could also be realized.

cond-mat.mtrl-sci↗

arXiv2013

Anomalous Edge Transport in the Quantum Anomalous Hall State

We predict by first-principles calculations that thin films of Cr-doped (Bi,Sb)$_2$Te$_3$ magnetic topological insulator have gapless non-chiral edge states coexisting with the chiral edge state. Such gapless non-chiral states are not immune to backscattering, which would explain dissipative transport in the quantum anomalous Hall (QAH) state observed in this system experimentally. Here we study the edge transport with both chiral and non-chiral states by Landaur-Büttiker formalism, and find that the longitudinal resistance is nonzero whereas Hall resistance is quantized to $h/e^2$. In particular, the longitudinal resistance can be greatly reduced by adding an extra floating probe even if it is not used, while the Hall resistance remains at the quantized value. We propose several transport experiments to detect the dissipative non-chiral edge channels. These results will facilitate the realization of pure dissipationless transport of QAH states in magnetic topological insulators.

cond-mat.mes-hall↗

arXiv2013

Quantum Anomalous Hall Effect with Higher Plateaus

Quantum anomalous Hall (QAH) effect in magnetic topological insulators is driven by the combination of spontaneous magnetic moments and spin-orbit coupling. Its recent experimental discovery raises the question if higher plateaus can also be realized. Here we present a general theory for QAH effect with higher Chern numbers, and show by first-principles calculations that thin film magnetic topological insulator of Cr-doped Bi$_2$(Se,Te)$_3$ is a candidate for the C=2 QAH insulator. Remarkably, whereas higher magnetic field leads to lower Hall conductance plateaus in the integer quantum Hall effect, higher magnetic moment leads to higher Hall conductance plateaus in the QAH effect.

cond-mat.mes-hall↗

arXiv2013

Chiral anomaly, Charge Density Waves, and Axion Strings from Weyl Semimetals

We study dynamical instability and chiral symmetry breaking in three dimensional Weyl semimetals, which turns Weyl semimetals into "axion insulators". Charge density waves (CDW) is found to be the natural consequence of the chiral symmetry breaking. The phase mode of this charge density wave state is identified as the axion, which couples to electromagnetic field in the topological $θ{\bf E}\cdot{\bf B}$ term. One of our main results is that the "axion strings" can be realized as the (screw or edge) dislocations in the charge density wave, which provides a simple physical picture for the elusive axion strings. These axion strings carry gapless chiral modes, therefore they have important implications for dissipationless transport properties of Weyl semimetals with broken symmetry.

cond-mat.str-el↗

arXiv2012

Models of three-dimensional fractional topological insulators

Time-reversal invariant three-dimensional topological insulators can be defined fundamentally by a topological field theory with a quantized axion angle theta of zero or pi. It was recently shown that fractional quantized values of theta are consistent with time-reversal invariance if deconfined, gapped, fractionally charged bulk excitations appear in the low-energy spectrum due to strong correlation effects, leading to the concept of a fractional topological insulator. These fractionally charged excitations are coupled to emergent gauge fields which ensure that the microscopic degrees of freedom, the original electrons, are gauge-invariant objects. A first step towards the construction of microscopic models of fractional topological insulators is to understand the nature of these emergent gauge theories and their corresponding phases. In this work, we show that low-energy effective gauge theories of both Abelian or non-Abelian type are consistent with a fractional quantized axion angle if they admit a Coulomb phase or a Higgs phase with gauge group broken down to a discrete subgroup. The Coulomb phases support gapless but electrically neutral bulk excitations while the Higgs phases are fully gapped. The Higgs and non-Abelian Coulomb phases exhibit multiple ground states on boundaryless spatial 3-manifolds with nontrivial first homology, while the Abelian Coulomb phase has a unique ground state. The ground state degeneracy receives an additional contribution on manifolds with boundary due to the induced boundary Chern-Simons term.

cond-mat.str-el↗

arXiv2012

Carrier-independent ferromagnetism and giant anomalous Hall effect in magnetic topological insulator

Breaking the time-reversal symmetry of a topological insulator (TI) by ferromagnetism can induce exotic magnetoelectric phenomena such as quantized anomalous Hall (QAH) effect. Experimental observation of QAH effect in a magnetically doped TI requires ferromagnetism not relying on the charge carriers. We have realized the ferromagnetism independent of both polarity and density of carriers in Cr-doped BixSb2-xTe3 thin films grown by molecular beam epitaxy. Meanwhile, the anomalous Hall effect is found significantly enhanced with decreasing carrier density, with the anomalous Hall angle reaching unusually large value 0.2 and the zero field Hall resistance reaching one quarter of the quantum resistance (h/e2), indicating the approaching of the QAH regime. The work paves the way to ultimately realize QAH effect and other unique magnetoelectric phenomena in TIs.

cond-mat.mtrl-sci↗

arXiv2012

Fluctuation-induced Topological Quantum Phase Transitions in Quantum Spin Hall and Quantum Anomalous Hall Insulators

We investigate the role of quantum fluctuations in topological quantum phase transitions of quantum spin Hall insulators and quantum anomalous Hall insulators. Employing the variational cluster approximation to obtain the single-particle Green's function of the interacting many-body system, we characterize different phases by direct calculation of the recently proposed topological order parameter for interacting systems. We pinpoint the influence of quantum fluctuations on the quantum spin Hall to Mott insulator transition in several models. Furthermore, we propose a general mechanism by which a topological quantum phase transition can be driven by the divergence of the self energy induced by interactions.

cond-mat.str-el↗

arXiv2012

Spin-orbital Texture in Topological Insulators

Relativistic spin-orbit coupling plays an essential role in the field of topological insulators and quantum spintronics. It gives rise to the topological non-trivial band structure and enables electric manipulation of the spin degree of freedom. Because of the spin-orbit coupling, rich spin-orbital coupled textures can exist both in momentum and in real space. For three dimensional topological insulators in the Bi$_2$Se$_3$ family, topological surface states with p$_z$ orbitals have a left-handed spin texture for the upper Dirac cone and a right-handed spin texture for the lower Dirac cone. In this work, we predict a new form of the spin-orbital texture associated with the p$_x$ and p$_y$ orbitals. For the upper Dirac cone, a left-handed (right-handed) spin texture is coupled to the "radial" ("tangential") orbital texture, whereas for the lower Dirac cone, the coupling of spin and orbital textures is the exact opposite. The "tangential" ("radial") orbital texture is dominant for the upper (lower) Dirac cone, leading to the right-handed spin texture for the in-plane orbitals of both the upper and lower Dirac cones. A spin-resovled and photon polarized angle-resolved photoemission spectroscopy experiment is proposed to observe this novel spin-orbital texture.

cond-mat.mes-hall↗