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Shou-Cheng Zhang

Publications and source records attributed to Shou-Cheng Zhang.

At least 91 records · Page 5Linked to original sources
arXiv2012

Strongly Correlated Topological Superconductors and Topological Phase Transitions via Green's Function

We propose several topological order parameters expressed in terms of Green's function at zero frequency for topological superconductors, which generalizes the previous work for interacting insulators. The coefficient in topological field theory is expressed in terms of zero frequency Green's function. We also study topological phase transition beyond noninteracting limit in this zero frequency Green's function approach.

cond-mat.str-el↗
arXiv2012

Topological insulators from the Perspective of first-principles calculations

Topological insulators are new quantum states with helical gapless edge or surface states inside the bulk band gap.These topological surface states are robust against the weak time-reversal invariant perturbations, such as lattice distortions and non-magnetic impurities. Recently a variety of topological insulators have been predicted by theories, and observed by experiments. First-principles calculations have been widely used to predict topological insulators with great success. In this review, we summarize the current progress in this field from the perspective of first-principles calculations. First of all, the basic concepts of topological insulators and the frequently-used techniques within first-principles calculations are briefly introduced. Secondly, we summarize general methodologies to search for new topological insulators. In the last part, based on the band inversion picture first introduced in the context of HgTe, we classify topological insulators into three types with s-p, p-p and d-f, and discuss some representative examples for each type.

cond-mat.mes-hall↗
arXiv2012

Equivalent topological invariants of topological insulators

A time-reversal invariant topological insulator can be generally defined by the effective topological field theory with a quantized θcoefficient, which can only take values of 0 or π. This theory is generally valid for an arbitrarily interacting system and the quantization of the θinvariant can be directly measured experimentally. Reduced to the case of a non-interacting system, the θinvariant can be expressed as an integral over the entire three dimensional Brillouin zone. Alternatively, non-interacting insulators can be classified by topological invariants defined over discrete time-reversal invariant momenta. In this paper, we show the complete equivalence between the integral and the discrete invariants of the topological insulator.

cond-mat.str-el↗
arXiv2012

Simplified topological invariants for interacting insulators

We propose general topological order parameters for interacting insulators in terms of the Green's function at zero frequency. They provide an unified description of various interacting topological insulators including the quantum anomalous Hall insulators and the time reversal invariant insulators in four, three and two dimensions. Since only Green's function at zero frequency is used, these topological order parameters can be evaluated efficiently by most numerical and analytical algorithms for strongly interacting systems.

cond-mat.str-el↗
arXiv2012

Interacting topological phases and modular invariance

We discuss a (2+1) dimensional topological superconductor with $N_f$ left- and right-moving Majorana edge modes and a $\mathbb{Z}_2\times \mathbb{Z}_2$ symmetry. In the absence of interactions, these phases are distinguished by an integral topological invariant $N_f$. With interactions, the edge state in the case $N_f=8$ is unstable against interactions, and a $\mathbb{Z}_2\times \mathbb{Z}_2$ invariant mass gap can be generated dynamically. We show that this phenomenon is closely related to the modular invariance of type II superstring theory. More generally, we show that the global gravitational anomaly of the non-chiral Majorana edge states is the physical manifestation of the bulk topological superconductors classified by $\mathbb{Z}_8$.

cond-mat.str-el↗
arXiv2012

Axion topological field theory of topological superconductors

Topological superconductors are gapped superconductors with gapless and topologically robust quasiparticles propagating on the boundary. In this paper, we present a topological field theory description of three-dimensional time-reversal invariant topological superconductors. In our theory the topological superconductor is characterized by a topological coupling between the electromagnetic field and the superconducting phase fluctuation, which has the same form as the coupling of "axions" with an Abelian gauge field. As a physical consequence of our theory, we predict the level crossing induced by the crossing of special "chiral" vortex lines, which can be realized by considering s-wave superconductors in proximity with the topological superconductor. Our theory can also be generalized to the coupling with a gravitational field.

cond-mat.supr-con↗
arXiv2012

Topological invariants for interacting topological insulators with inversion symmetry

For interacting Z_2 topological insulators with inversion symmetry, we propose a simple topological invariant expressed in terms of the parity eigenvalues of the interacting Green's function at time-reversal invariant momenta. We derive this result from our previous formula involving the integral over the frequency-momenta space. This formula greatly simplifies the explicit calculation of Z_2 topological invariants in inversion symmetric insulators with strong interactions.

cond-mat.str-el↗
arXiv2012

Stationary phase approximation approach to the quasiparticle interference on the surface of a strong topological insulator

Topological insulators have surface states with unique spin-orbit coupling. With impurities on the surface, the quasiparticle interference pattern is an effective way to reveal the topological nature of the surface states, which can be probed by the scanning tunneling microscopy. In this paper, we present a general analytic formulation of the local density of states using the stationary phase approximation. The power laws of Friedel oscillations are discussed for a constant energy contour with a generic shape. In particular, we predict unique signature of magnetic impurities in comparison with nonmagnetic impurities for a surface state trapped in a "magnetic wall".

cond-mat.mes-hall↗
arXiv2012

Topological p-n Junction

We consider a junction between surface $p$-type and surface $n$-type on an ideal topological insulator in which carrier type and density in two adjacent regions are locally controlled by composition graded doping or electrical gating. Such junction setting on topological insulators are fundamental for possible device application. A single gapless chiral edge state localized along the junction interface appears in the presence of an external magnetic field, and it can be probed by scanning tunneling microscopy and transport measurements. We propose to realize this topological \emph{p-n} junction in (Bi$_{1-x}$Sb$_x$)$_2$Te$_3$, which has insulating bulk properties and a tunable surface state across the Dirac cone.

cond-mat.mes-hall↗
arXiv2011

Actinide Topological Insulator Materials with Strong Interaction

Topological band insulators have recently been discovered in spin-orbit coupled two and three dimensional systems. In this work, we theoretically predict a class of topological Mott insulators where interaction effects play a dominant role. In actinide elements, simple rocksalt compounds formed by Pu and Am lie on the boundary of metal to insulator transition. We show that interaction drives a quantum phase transition to a topological Mott insulator phase with a single Dirac cone on the surface.

cond-mat.str-el↗
arXiv2011

Half quantum spin Hall effect on the surface of weak topological insulators

We investigate interaction effects in three dimensional weak topological insulators (TI) with an even number of Dirac cones on the surface. We find that the surface states can be gapped by a surface charge density wave (CDW) order without breaking the time-reversal symmetry. In this sense, time reversal symmetry alone can not robustly protect the weak TI state in the presence of interactions. If the translational symmetry is additionally imposed in the bulk, a topologically non-trivial weak TI state can be obtained with helical edge states on the CDW domain walls. In other words, a CDW domain wall on the surface is topologically equivalent to the edge of a two-dimensional quantum spin Hall insulator. Therefore, the surface state of a weak topological insulator with translation symmetry breaking on the surface has a "half quantum spin Hall effect", in the same way that the surface state of a strong topological insulator with time-reversal symmetry breaking on the surface has a "half quantum Hall effect". The on-site and nearest neighbor interactions are investigated in the mean field level and the phase diagram for the surface states of weak topological insulators is obtained.

cond-mat.mes-hall↗
arXiv2011

Half-metallic Surface States and Topological Superconductivity in NaCoO$_2$ from first principles

Based on first-principles calculations, we predict a half-metallic surface state in layered bulk insulator NaCoO2, with tunable surface hole concentration. The half-metallic surface has a single Fermi surface with a helical spin texture, similar to the surface state of topological insulators, but with the key difference of time-reversal symmetry breaking in the present case. We propose the realization of topological superconductivity and Majorana fermions when the half-metallic surface states are in proximity contact with a conventional superconductor.

cond-mat.mtrl-sci↗
arXiv2011

Quantum Anomalous Hall Effect in Magnetic Topological Insulator GdBiTe3

The quantum anomalous Hall (QAH) state is a two-dimensional bulk insulator with a non-zero Chern number in absence of external magnetic fields. Protected gapless chiral edge states enable dissipationless current transport in electronic devices. Doping topological insulators with random magnetic impurities could realize the QAH state, but magnetic order is difficult to establish experimentally in the bulk insulating limit. Here we predict that the single quintuple layer of GdBiTe3 film could be a stoichiometric QAH insulator based on ab-initio calculations, which explicitly demonstrate ferromagnetic order and chiral edge states inside the bulk gap. We further investigate the topological quantum phase transition by tuning the lattice constant and interactions. A simple low-energy effective model is presented to capture the salient physical feature of this topological material.

cond-mat.mes-hall↗
arXiv2011

Topological field theory and thermal responses of interacting topological superconductors

We investigate the three-dimensional, time-reversal invariant topological superconductors with generic interaction by their response to external fields. The first description is a gravitational topological field theory, which gives a $Z_2$ classification of topological superconductors, and predicts a half-quantized thermal Hall effect on the surface. The second description introduces an s-wave proximity pairing field on the surface, and the associated topological defects give an integer $Z$ classification of the topological superconductors.

cond-mat.str-el↗
arXiv2011

Spin polarization of the quantum spin Hall edge states

While the helical character of the edge channels responsible for charge transport in the quantum spin Hall regime of a two-dimensional topological insulator is by now well established, an experimental confirmation that the transport in the edge channels is spin-polarized is still outstanding. We report experiments on nanostructures fabricated from HgTe quantum wells with an inverted band structure, in which a split gate technique allows us to combine both quantum spin Hall and metallic spin Hall transport in a single device. In these devices, the quantum spin Hall effect can be used as a spin current injector and detector for the metallic spin Hall effect, and vice versa, allowing for an all-electrical detection of spin polarization.

cond-mat.mes-hall↗
arXiv2011

Mechanism for a Pairing State with Time-Reversal Symmetry Breaking in Iron-Based Superconductors

The multipocket Fermi surfaces of iron-based superconductors promote pairing states with both s_{+-}-wave and d_{x^2-y^2}-wave symmetry. We argue that the competition between these two order parameters could lead to a time-reversal-symmetry breaking state with s+id-pairing symmetry in the iron-based superconductors, and propose serveral scenarios in which this phase may be found. To understand the emergence of such a pairing state on a more rigorous footing, we start from a microscopic 5-orbital description representative for the pnictides. Using a combined approach of functional renormalization group and mean-field analysis, we identify the microscopic parameters of the s+id-pairing state. There, we find the most promising region for s+id-pairing in the electron doped regime with an enhanced pnictogen height.

cond-mat.supr-con↗
arXiv2011

Topological insulators in filled skutterudites

We propose new topological insulators in cerium filled skutterudite (FS) compounds based on ab initio calculations. We find that two compounds CeOs4As12 and CeOs4Sb12 are zero gap materials with band inversion between Os-d and Ce-f orbitals, which are thus parent compounds of two and three-dimensional topological insulators just like bulk HgTe. At low temperature, both compounds become topological Kondo insulators, which are Kondo insulators in the bulk, but have robust Dirac surface states on the boundary. This new family of topological insulators has two advantages compared to previous ones. First, they can have good proximity effect with other superconducting FS compounds to realize Majarona fermions. Second, the antiferromagnetism of CeOs4Sb12 at low temperature provides a way to realize the massive Dirac fermion with novel topological phenomena.

cond-mat.mtrl-sci↗