声子晶体中的多重拓扑相

研究了圆环型波导依照蜂窝结构排列的声子晶体系统中的拓扑相变.利用晶格结构的点群对称性实现赝自旋,并在圆环中引入旋转气流来打破时间反演对称性.通过紧束缚近似模型计算的解析结果表明,没有引入气流时,调节几何参数,系统存在普通绝缘体和量子自旋霍尔效应绝缘体两个相;引入气流后,可以实现新的时间反演对称性破缺的量子自旋霍尔效应相,而增大气流强度,则可以实现量子反常霍尔效应相.这三个拓扑相可以通过自旋陈数来分类.通过有限元软件模拟了多个系统中边界态的传播,发现不同于量子自旋霍尔效应相,量子反常霍尔相系统的表面只支持一种自旋的边界态,并且它无需时间反演对称性保护.     

Topological Phase Transition with Larger Winding Number in the Non-Hermitian Dimerized Lattice

The topological phase transitions among normal insulator phase, two kinds of topological insulator phases, and topological semimetal phase are shown based on the non-Hermitian dimerized Su–Schrieffer–Heeger (SSH) model with the nonreciprocal intercell and long-range hopping. In contrast to the previous work, it is found that the topological insulator phase in the present SSH model can hold the larger non-Bloch winding number accompanied by exceptional winding of the generalized Brillouin zone around the gap-closing points. Compared with the usual topological insulator phase in non-Hermitian SSH model, the topological insulator with the larger winding number owns two pairs of zero energy modes with a distinct form of edge localization in the gap. The physical mechanism of the distinct edge localization for zero energy modes via a equivalent Hermitian version of the non-Hermitian SSH model is revealed. Additionally, the process of the phase transition is visualized among normal insulator phase, topological insulator phases, and topological semimetal phase in detail via the evolution of the gap-closing points on the plane of generalized Brillouin zone. This work further verifies the non-Bloch theory and enrich the investigation about the topologically nontrivial phase with the larger topological invariant in the non-Hermitian SSH model.     

Topological magnetic insulators with corundum structure

Topological insulators are new states of quantum matter in which surface states residing in the bulk insulating gap are protected by time-reversal symmetry. When a proper kind of antiferromagnetic long-range order is established in a topological insulator, the system supports axionic excitations. In this Letter, we study theoretically the electronic states in a transition metal oxide of corundum structure, in which both spin-orbit interaction and electron-electron interaction play crucial roles. A tight-binding model analysis predicts that materials with this structure can be strong topological insulators. Because of the electron correlation, an antiferromagnetic order may develop, giving rise to a topological magnetic insulator phase with axionic excitations.     

Topological phase transition in anisotropic square-octagon lattice with spin–orbit coupling and exchange field

We investigate the topological phase transitions in an anisotropic square-octagon lattice in the presence of spin–orbit coupling and exchange field. On the basis of the Chern number and spin Chern number, we find a number of topologically distinct phases with tuning the exchange field, including time-reversal-symmetry-broken quantum spin Hall phases, quantum anomalous Hall phases and a topologically trivial phase. Particularly, we observe a coexistent state of both the quantum spin Hall effect and quantum anomalous Hall effect. Besides, by adjusting the exchange filed, we find the phase transition from time-reversal-symmetry-broken quantum spin Hall phase to spin-imbalanced and spin-polarized quantum anomalous Hall phases, providing an opportunity for quantum spin manipulation. The bulk band gap closes when topological phase transitions occur between different topological phases. Furthermore, the energy and spin spectra of the edge states corresponding to different topological phases are consistent with the topological characterization based on the Chern and spin Chern numbers.     

Band gap tuning of defective silicon carbide nanotubes under external electric field: Density functional theory

We have theoretically investigated the effect of applying longitudinal and transverse electric field on silicon carbide nanotubes with different orientations of Stone Wales defects. We found that each type of Stone Wales defects maintained different formation energy. We have also successfully proved that the orientation of Stone Wales defects in silicon carbide nanotubes response quite differently upon applying external electric field, whereas, two important and interesting phenomena were observed. First, the semiconductor-metal phase transition occurred in silicon carbide nanotubes as well as the three types of Stone Wales defects. However, clear band gap variations were observed in all silicon carbide nanotubes under study. Second, the band gap variations in pristine silicon carbide nanotubes and nanotubes with different orientations of Stone Wales defects have the same trend, even though all silicon carbide nanotubes have clear band gap values under different strengths of the applied external electric field. However, band gap tuning under longitudinal electric field is less significant compared to band gap tuning under the transverse electric field.     

Superconducting quantum interference devices made of Sb-doped Bi2Se3 topological insulator nanoribbons

We report the fabrication and characterization of superconducting quantum interference devices (SQUIDs) made of Sb-doped Bi₂Se₃ topological insulator (TI) nanoribbon (NR) contacted with PbIn superconducting electrodes. When an external magnetic field was applied along the NR axis, the TI NR exhibited periodic magneto-conductance oscillations, the so-called Aharonov-Bohm oscillations, owing to one-dimensional subbands. Below the superconducting transition temperature of PbIn electrodes, we observed supercurrent flow through TI NR-based SQUID. The critical current periodically modulates with a magnetic field perpendicular to the SQUID loop, revealing that the periodicity corresponds to the superconducting flux quantum. Our experimental observations can be useful to explore Majorana bound states (MBS) in TI NR, promising for developing topological quantum information devices.     

Electronic properties of SnTe-class topological crystalline insulator materials

The rise of topological insulators in recent years has broken new ground both in the conceptual cognition of condensed matter physics and the promising revolution of the electronic devices.It also stimulates the explorations of more topological states of matter.Topological crystalline insulator is a new topological phase,which combines the electronic topology and crystal symmetry together.In this article,we review the recent progress in the studies of SnTe-class topological crystalline insulator materials.Starting from the topological identifications in the aspects of the bulk topology,surface states calculations,and experimental observations,we present the electronic properties of topological crystalline insulators under various perturbations,including native defect,chemical doping,strain,and thickness-dependent confinement effects,and then discuss their unique quantum transport properties,such as valley-selective filtering and helicity-resolved functionalities for Dirac fermions.The rich properties and high tunability make SnTe-class materials promising candidates for novel quantum devices.     

Topological quantum phase transition in 5d transition metal oxide Na2IrO3

We predict a quantum phase transition from normal to topological insulators in the 5d transition metal oxide Na2IrO3, where the transition can be driven by the change of the long-range hopping and trigonal crystal field terms. From the first-principles-derived tight-binding Hamiltonian, we determine the phase boundary through the parity analysis. In addition, our first-principles calculations for Na2IrO3 model structures show that the interlayer distance can be an important parameter for the existence of a three-dimensional strong topological insulator phase. Na2IrO3 is suggested to be a candidate material which can have both a nontrivial topology of bands and strong electron correlations.     

Bosonic topological insulator intermediate state in the superconductor-insulator transition

A low-temperature intervening metallic regime arising in the two-dimensional superconductor-insulator transition challenges our understanding of electronic fluids. Here we develop a gauge theory revealing that this emergent anomalous metal is a bosonic topological insulator where bulk transport is suppressed by mutual statistics interactions between out-of-condensate Cooper pairs and vortices and the longitudinal conductivity is mediated by symmetry-protected gapless edge modes. We explore the magnetic-field-driven superconductor-insulator transition in a niobium titanium nitride device and find marked signatures of a bosonic topological insulator behavior of the intervening regime with the saturating resistance. The observed superconductor-anomalous metal and insulator-anomalous metal dual phase transitions exhibit quantum Berezinskii-Kosterlitz-Thouless criticality in accord with the gauge theory.     

Cloaking magnetic field and generating electric field with topological insulator and high permeability material

Topological insulator is a new state of quantum matter. When applied magnetic field is applied on a topological insulator, not only the magnetic field is induced, but also the electric field is induced, vice versa. We designed bi-layer magnetic cloak with topological insulator and high permeability material (HPM), derived the electric field and magnetic field inside and outside the bi-layer topological insulator and HPM. Calculation and simulation results show that the applied magnetic field is cloaked by the bi-layer topological insulator and HPM, and the uniform electric field is induced in the cloaked region.     

Regular and irregular dynamics of spin-polarized wavepackets in a mesoscopic quantum dot at the edge of topological insulator

The dynamics of Dirac–Weyl spin-polarized wavepackets driven by a periodic electric field is considered for the electrons in a mesoscopic quantum dot formed at the edge of the two-dimensional HgTe/CdTe topological insulator with Dirac–Weyl massless energy spectra, where the motion of carriers is less sensitive to disorder and impurity potentials. It is observed that the interplay of strongly coupled spin and charge degrees of freedom creates the regimes of irregular dynamics in both coordinate and spin channels. The border between the regular and irregular regimes determined by the strength and frequency of the driving field is found analytically within the quasiclassical approach by means of the Ince–Strutt diagram for the Mathieu equation, and is supported by full quantum-mechanical simulations of the driven dynamics. The investigation of quasienergy spectrum by Floquet approach reveals the presence of non-Poissonian level statistics, which indicates the possibility of chaotic quantum dynamics and corresponds to the areas of parameters for irregular regimes within the quasiclassical approach. We find that the influence of weak disorder leads to partial suppression of the dynamical chaos. Our findings are of interest both for progress in the fundamental field of quantum chaotic dynamics and for further experimental and technological applications of spindependent phenomena in nanostructures based on topological insulators.     

Topological Anderson insulator in three dimensions

We show that disorder, when sufficiently strong, can transform an ordinary metal with strong spin-orbit coupling into a strong topological "Anderson" insulator, a new topological phase of quantum matter in three dimensions characterized by disordered insulating bulk and topologically protected conducting surface states.     

Electron transport near the Mott transition in n-GaAs and n-GaN

In this paper, we study the temperature dependence of the conductivity and the Hall coefficient near the metal–insulator phase transition. A theoretical investigation is performed within the effective mass approximation. The variational method is used to calculate the eigenvalues and eigenfunctions of the impurity states. Unlike previous studies, we have included nonlinear corrections to the screened impurity potential, because the Thomas–Fermi approximation is incorrect for the insulator phase. It is also shown that near the phase transition the exchange interaction is essential. The obtained temperature dependencies explain several experimental measurements in gallium arsenide (GaAs) and gallium nitride (GaN).     

单轴压力下Ge2X2Te5(X=Sb,Bi)薄膜拓扑相变的第一性原理研究

随着拓扑绝缘体的发现, 材料拓扑物性的研究成为凝聚态物理研究的热点领域. 本文基于第一性原理计算, 研究了化合物Ge₂X₂Te₅ (X=Sb, Bi) 的块体结构和二维单层和双层薄膜结构的拓扑物性, 以及单双层薄膜在垂直方向单轴压力下的拓扑量子相变. 研究发现, A型原子序列排列的这两种化合物都是拓扑绝缘体, 其单层薄膜都是普通金属, 而双层薄膜都是拓扑金属, 单层和双层薄膜在单轴加压过程中都没有发生拓扑量子相变; 这两种化合物的B型原子序列的晶体是普通绝缘体, 其所对应的薄膜, Ge₂Sb₂Te₅单层是普通金属, 双层薄膜和Ge₂Bi₂Te₅的单层和双层薄膜均为普通绝缘体, 但是在单轴加压过程中B 型原子序列所对应的单层和双层薄膜都转变为拓扑金属.     

Photoinduced Weyl semimetal phase and anomalous Hall effect in a three-dimensional topological insulator

Motivated by the fact that Weyl fermions can emerge in a three-dimensional topological insulator on breaking either time-reversal or inversion symmetries, we propose that a topological quantum phase transition to a Weyl semimetal phase occurs under the off-resonant circularly polarized light, in a three-dimensional topological insulator, when the intensity of the incident light exceeds a critical value. The circularly polarized light effectively generates a Zeeman exchange field and a renormalized Dirac mass, which are highly controllable. The phase transition can be exactly characterized by the first Chern number. A tunable anomalous Hall conductivity emerges, which is fully determined by the location of the Weyl nodes in momentum space, even in the doping regime. Our predictions are experimentally realizable through pump-probe angle-resolved photoemission spectroscopy and raise a new way for realizing Weyl semimetals and quantum anomalous Hall effects.     

Transport features of topological corner states in honeycomb lattice with multihollow structure

Higher-order topological phase in 2-dimensional (2D) systems is characterized by in-gap corner states, which are hard to detect and utilize. We numerically investigate transport properties of topological corner states in 2D honeycomb lattice, where the second-order topological phase is induced by an in-plane Zeeman field in the conventional Kane–Mele model. Through engineering multihollow structures with appropriate boundaries in honeycomb lattice, multiple corner states emerge, which greatly increases the probability to observe them. A typical two-probe setup is built to study the transport features of a diamond-shaped system with multihollow structures. Numerical results reveal the existence of global resonant states in bulk insulator, which corresponds to the resonant tunneling of multiple corner states and occupies the entire scattering region. Furthermore, based on the well separated energy levels of multiple corner states, a single-electron source is constructed.     

Exotic ferromagnetism in the two-dimensional quantum material C<Subscript>3</Subscript>N

The search for and study of exotic quantum states in novel low-dimensional quantum materials have triggered extensive research in recent years. Here, we systematically study the electronic and magnetic structures in the newly discovered two-dimensional quantum material C₃N within the framework of density functional theory. The calculations demonstrate that C₃N is an indirect-band semiconductor with an energy gap of 0.38 eV, which is in good agreement with experimental observations. Interestingly, we find van Hove singularities located at energies near the Fermi level, which is half that of graphene. Thus, the Fermi energy easily approaches that of the singularities, driving the system to ferromagnetism, under charge carrier injection, such as electric field gating or hydrogen doping. These findings not only demonstrate that the emergence of magnetism stems from the itinerant electron mechanism rather than the effects of local magnetic impurities, but also open a new avenue to designing field-effect transistor devices for possible realization of an insulator–ferromagnet transition by tuning an external electric field.     

Chemical memory erasure; a photochemical model

In this paper we propose an exactly solvable model of a topological insulator defined on a spin- \(\tfrac{1}{2}\) square decorated lattice. Itinerant fermions defined in the framework of the Haldane model interact via the Kitaev interaction with spin- \(\tfrac{1}{2}\) Kitaev sublattice. The presented model, whose ground state is a non-trivial topological phase, is solved exactly. We have found out that various phase transitions without gap closing at the topological phase transition point outline the separate states with different topological numbers. We provide a detailed analysis of the model’s ground-state phase diagram and demonstrate how quantum phase transitions between topological states arise. We have found that the states with both the same and different topological numbers are all separated by the quantum phase transition without gap closing. The transition between topological phases is accompanied by a rearrangement of the spin subsystem’s spectrum from band to flat-band states.     

Insulator–metal transition in Mn-doped NaNbO3 induced by chemical and thermal treatment

The transition between insulator and metal conductor states, induced by oxygen non-stoichiometry, was studied for NaNbO₃ : Mn crystals. Conditions for an optimal reduction were determined on the basis of TGA tests. The temperature dependencies of the resistance measured on the macroscale showed that the transition from thermally activated to metallic features depends on the level of oxygen deficiency. The LC-AFM measurement exhibited non-homogeneous electric resistance on the nanoscale. We ascribed the local insulator–metal transition to changeover in the electronic state of the Nb ions occurring in filaments. The Mn dopant stabilised the induced oxygen non-stoichiometry and the metallic conduction down to room temperature.