arXiv subjects

Zhongqin Yang

Publications and source records attributed to Zhongqin Yang.

15 recordsLinked to original sources

arXiv2026

Phase-slip residual-order spin state in FeSe

In unconventional superconductors, the microscopic form of magnetic correlations is crucial for identifying the origin of spin fluctuations and the associated pairing interaction. FeSe superconducts without chemical doping and shows no static long-range magnetic order, yet inelastic neutron scattering reveals a strong stripe response, finite linewidths, and reproducible Neel-side spectral weight. Here we propose a phase-slip residual-order spin state (ROSS). Stripe, Neel, pair-checkerboard, and staggered trimer antiferromagnetic states can be unified as symmetric phase-slip derivatives of a stripe background, while more general asymmetric phase slips form lower-energy configurations and reconstruct the spin structure factor S(q) within a finite coherence length. The ROSS therefore reconciles the absence of static magnetic order with strong spin excitations, provides a microscopic picture for the origin of spin fluctuations in FeSe, and establishes a magnetic basis for understanding pairing in unconventional superconducting systems with similar magnetic fingerprints.

cond-mat.supr-con↗

arXiv2022

Two-dimensional half Chern-Weyl semimetal with multiple screw axes

Half topological states of matter and two-dimensional (2D) magnetism have gained much attention recently. In this paper, we propose a special topological semimetal phase called a 2D half Chern-Weyl semimetal (HCWS), which is a 2D magnetic Weyl semimetal bound to the half Chern insulator phase by symmetry, and the two phases can be converted to each other by manipulating the magnetization direction. We provide the symmetry conditions to realize this state in 2D systems with multiple screw axes. Tight-binding models with multiple basis and a predicted 2D material, monolayer TiTe, are shown as the concrete examples for HCWSs. The TiTe monolayer was shown to have a high ferremagnetic Curie temperature (~966 K) as well as a Coulomb correlation-enhanced spin-orbit coupling (SOC), and further demonstrates the effect of correlation-enhanced SOC on magnetocrystalline anisotropy energy and energy gap opening. Our work reveals a state with switchable and spin-resolved half body charge currents as well as half boundary charge currents, and will provide a platform for novel and high-performance topological spintronics devices.

cond-mat.mes-hall↗

arXiv2021

Quantum Spin-Valley Hall Kink States: From Concept to Materials Design

We propose a general and tunable platform to realize high-density arrays of quantum spin-valley Hall kink (QSVHK) states with spin-valley-momentum locking based on a two-dimensional hexagonal topological insulator. Through the analysis of Berry curvature and topological charge, the QSVHK states are found to be topologically protected by the valley-inversion and time-reversal symmetries. Remarkably, the conductance of QSVHK states remains quantized against either nonmagnetic or long-range magnetic disorder, verified by the Green function calculations. Based on first-principles results, we show that QSVHK states, protected with a gap up to 287 meV, can be realized in bismuthene by alloy engineering, surface functionalization, or electric field, supporting non-volatile applications of spin-valley filters, valves, and waveguides even at room temperature.

cond-mat.mes-hall↗

arXiv2020

Intrinsic topological phases in Mn$_2$Bi$_2$Te$_5$ tuned by the layer magnetization

The interplay between band topology and magnetic order could generate a variety of time-reversal-breaking gapped topological phases with exotic topological quantization phenomena, such as quantum anomalous Hall (QAH) insulators and axion insulators (AxI). Here by combining analytic models and first-principles calculations, we predict QAH and AxI phases can be realized in thin film of an intrinsic antiferromagnetic van der Waal material Mn$_2$Bi$_2$Te$_5$. The phase transition between QAH and AxI is tuned by the layer magnetization, which would provide a promising platform for chiral superconducting phases. We further present a simple and unified continuum model that captures the magnetic topological features, and is generic for Mn$_2$Bi$_2$Te$_5$ and MnBi$_2$Te$_4$ family materials.

cond-mat.mes-hall↗

arXiv2019

Novel Chern insulators with half-metallic edge states

The central target of spintronics research is to achieve flexible control of highly efficient and spin-polarized electronic currents. Based on first-principles calculations and k-p models, we demonstrate that Cu2S/MnSe heterostructures are a novel type of Chern insulators with half-metallic chiral edge states and a very high Fermi velocity (0.87 * 10^6 m/s). The full spin-polarization of the edge states is found to be robust against the tuning of the chemical potential. Unlike the mechanisms reported previously, this heterostructure has quadratic bands with a normal band order, that is, the p/d-like band is below the s-like band. Charge transfer between the Cu2S moiety and the substrate results in variation in the occupied bands, which together with spin-orbit coupling, triggers the appearance of the topological state in the system. These results imply that numerous ordinary semiconductors with normal band order may convert into Chern insulators with half-metallic chiral edge states through this mechanism, providing a strategy to find a rich variety of materials for dissipationless, 100% spin-polarized and high-speed spintronic devices.

cond-mat.mtrl-sci↗

arXiv2018

Engineering Topological quantum dot through planar magnetization in bismuthene

The discovery of quantum spin Hall materials with huge bulk gaps in experiment, such as bismuthene, provides a versatile platform for topological devices. We propose a topological quantum dot (QD) device in bismuthene ribbon in which two planar magnetization areas separate the sample into a QD and two leads. At zero temperature, peaks of differential conductance emerge, demonstrating the discrete energy levels from the confined topological edge states. The key parameters of the QD, the tunneling coupling strength with the leads and the discrete energy levels, can be controlled by the planar magnetization and the sample size. Specially, different from the conventional QD, we find that the angle between two planar magnetization orientations provides an effective way to manipulate the discrete energy levels. Combining the numerical calculation and the theoretical analysis, we identify that such manipulation originates from the unique quantum confinement effect of the topological edge states. Based on such mechanism, we find the spin transport properties of QDs can also be controlled.

cond-mat.mes-hall↗

arXiv2018

Origin of topologically trivial states and topological phase transitions in low-buckled plumbene

Combining tight-binding (TB) models with first-principles calculations, we investigate electronic and topological properties of plumbene. Different from the other two-dimensional (2D) topologically nontrivial insulators in group IVA (from graphene to stanene), low-buckled plumbene is a topologically trivial insulator. The plumbene without spin-orbit coupling exhibits simultaneously two kinds of degeneracies, i.e., quadratic non-Dirac and linear Dirac band dispersions around the Γand K/K' points, respectively. Our TB model calculations show that it is the coupling between the two topological states around the Γand K/K' points that triggers the global topologically trivial property of plumbene. Quantum anomalous Hall effects with Chern numbers of 2 or -2 can be, however, achieved after an exchange field is introduced. When the plumbene is functionalized with ethynyl (PbC2H), quantum spin Hall effects appear due to the breaking of the coupling effect of the local topological states.

cond-mat.mtrl-sci↗

arXiv2018

Giant Spin-valley Polarization and Multiple Hall Effect in Functionalized Bi Monolayers

Valleytronic materials, characterized by local extrema (valley) in their bands, and topological insulators have separately attracted great interest recently. However, the interplay between valleytronic and topological properties in one single system, likely to enable important unexplored phenomena and applications, has been largely overlooked so far. Here, by combining a tight-binding model with first-principles calculations, we find the large-band-gap quantum spin Hall effects (QSHEs) and valley Hall effects (VHEs) appear simultaneously in the Bi monolayers decorated with halogen elements, denoted as Bi2XY (X, Y = H, F, Cl, Br, or I). A staggered exchange field is introduced into the Bi2XY monolayers by transition metal atom (Cr, Mo, or W) doping or LaFeO3 magnetic substrates, which together with the strong SOC of Bi atoms generates a time-reversal-symmetry-broken QSHE and a huge valley splitting (up to 513 meV) in the system. With gate control, QSHE and anomalous charge, spin, valley Hall effects can be observed in the single system. These predicted multiple and exotic Hall effects, associated with various degrees of freedom of electrons, could enable applications of the functionalized Bi monolayers in electronics, spintronics, and valleytronics.

cond-mat.mes-hall↗

arXiv2018

Coupling effect of topological states and Chern insulators in two-dimensional triangular lattices

We investigate topological states of two-dimensional (2D) triangular lattices with multi-orbitals. Tight-binding model calculations of a 2D triangular lattice based on $\emph{p}_{x}$ and \emph{p}_{y} orbitals exhibit very interesting doubly degenerate energy points at different positions ($Γ$ and K/K$^{\prime}$) in momentum space, with quadratic non-Dirac and linear Dirac band dispersions, respectively. Counterintuitively, the system shows a global topologically trivial rather than nontrivial state with consideration of spin-orbit coupling due to the "destructive interference effect" between the topological states at the $Γ$ and K/K$^{\prime}$ points. The topologically nontrivial state can emerge by introducing another set of triangular lattices to the system (bitriangular lattices) due to the breakdown of the interference effect. With first-principles calculations, we predict an intrinsic Chern insulating behavior (quantum anomalous Hall effect) in a family of 2D triangular lattice metal-organic framework of Co(C$_{21}$N$_{3}$H$_{15}$) (TPyB-Co) from this scheme. Our results provide a different path and theoretical guidance for the search for and design of new 2D topological quantum materials.

cond-mat.mes-hall↗

arXiv2017

Strong magnetization and Chern insulators in compressed graphene/CrI$_{3}$ van der Waals heterostructures

Graphene-based heterostructures are a promising material system for designing the topologically nontrivial Chern insulating devices. Recently, a two-dimensional (2D) monolayer ferromagnetic insulator CrI$_{3}$ was successfully synthesized in experiments [Huang et al., Nature 546, 270 (2017)]. Here, these two interesting materials are proposed to build a heterostructure (Gr/CrI$_{3}$). Our first-principles calculations show that the system forms a van der Waals (vdW) heterostructure, relatively facilely fabricated in experiments. A Chern insulating state is acquired in the Gr/CrI$_{3}$ heterostructure if the vdW gap is compressed to certain extents by applying an external pressure. Amazingly, very strong magnetization (about 150 meV) is found in graphene, induced by the substrate CrI$_{3}$, despite the vdW interactions between them. A low-energy effective model is employed to understand the mechanism. The work functions, contact types, and band alignments of the Gr/CrI$_{3}$ heterostructure system are also studied. Our work demonstrates that the Gr/CrI$_{3}$ heterostructure is a promising system to observe the quantum anomalous Hall effect at high temperatures (up to 45 K) in experiments.

cond-mat.mtrl-sci↗

arXiv2017

Chern insulators without band inversion in MoS2 monolayers with 3d adatoms

Electronic and topological properties of MoS2 monolayers endowed with 3d transition metal (TM) adatoms (V-Fe) are explored by using ab initio methods and k.p models. Without the consideration of the Hubbard U interaction, the V, Cr, and Fe adatoms tend to locate on the top of the Mo atoms, while the most stable site for the Mn atom is at the hollow position of the Mo-S hexagon. After the Hubbard U is applied, the most stable sites of all the systems become the top of the Mo atoms. Chern insulators without band inversion are achieved in these systems. The V and Fe adsorption systems are the best candidates to produce the topological states. The k.p model calculations indicate that these topological states are determined by the TM magnetism, the C3v crystal field from the MoS2 substrate, and the TM atomic spin-orbit coupling (SOC). The special two-meron pseudospin texture is found to contribute to the topology. The apparent difference between the Berry curvatures for the V and Fe adsorption systems are also explored. Our results widen the understanding to the Chern insulators and are helpful for the applications of the MoS2 monolayers in the future electronics and spintronics.

cond-mat.mtrl-sci↗

arXiv2016

Quantum Spin-quantum Anomalous Hall Effect with Tunable Edge States in Sb Monolayer-based Heterostructures

A novel topological insulator with tunable edge states, called quantum spin-quantum anomalous Hall (QSQAH) insulator, is predicted in a heterostructure of a hydrogenated Sb (SbH) monolayer on a LaFeO3 substrate by using ab initio methods. The substrate induces a drastic staggered exchange field in the SbH film, which plays an important role to generate the QSQAH effect. A topologically nontrivial band gap (up to 35 meV) is opened by Rashba spin-orbit coupling, which can be enlarged by strain and electric field. To understand the underlying physical mechanism of the QSQAH effect, a tight-binding model based on px and py orbitals is constructed. With the model, the exotic behaviors of the edge states in the heterostructure are investigated. Dissipationless chiral charge edge states related to one valley are found to emerge along the both sides of the sample, while low-dissipation spin edge states related to the other valley flow only along one side of the sample. These edge states can be tuned flexibly by polarization-sensitive photoluminescence controls and/or chemical edge modifications. Such flexible manipulations of the charge, spin, and valley degrees of freedom provide a promising route towards applications in electronics, spintronics, and valleytronics.

cond-mat.mtrl-sci↗

arXiv2016

Enhanced Thermoelectric Properties of Dirac Semimetal Cd3As2

We report an investigation of temperature- and doping-dependent thermoelectric behaviors of topological semimetal Cd3As2. The electrical conductivity, thermal conductivity, Seebeck coefficient, and figure of merit (ZT) are calculated by using Boltzmann transport theory. The calculated thermoelectric properties of the pristine Cd3As2 match well the experimental results. The electron or hole doping, especially the latter, is found improving much the thermoelectric behaviors of the material. The optimum merit ZT of Cd3As2 with electron doping is found to be about 0.5 at T=700 K with n=1x1020 cm-3, much larger than the maximum experimental value obtained for the pristine Cd3As2 (~0.15). For the p-type Cd3As2, the maximal value of the Seebeck coefficient as a function of temperature increases apparently with the increase of the hole doping concentration and its position shifts drastically towards the lower temperature region compared to that of the n-type Cd3As2, leading to the optimum merit ZT of about 0.5 obtained at low temperature of 500K (p=1x1020 cm-3) in the p-type Cd3As2.

cond-mat.mtrl-sci↗

arXiv2006

Ab-initio electronic and magnetic structure in La_0.66Sr_0.33MnO_3: strain and correlation effects

The effects of tetragonal strain on electronic and magnetic properties of strontium-doped lanthanum manganite, La_{2/3}Sr_{1/3}MnO_3 (LSMO), are investigated by means of density-functional methods. As far as the structural properties are concerned, the comparison between theory and experiments for LSMO strained on the most commonly used substrates, shows an overall good agreement: the slight overestimate (at most of 1-1.5 %) for the equilibrium out-of-plane lattice constants points to possible defects in real samples. The inclusion of a Hubbard-like contribution on the Mn d states, according to the so-called "LSDA+U" approach, is rather ineffective from the structural point of view, but much more important from the electronic and magnetic point of view. In particular, full half-metallicity, which is missed within a bare density-functional approach, is recovered within LSDA+U, in agreement with experiments. Moreover, the half-metallic behavior, particularly relevant for spin-injection purposes, is independent on the chosen substrate and is achieved for all the considered in-plane lattice constants. More generally, strain effects are not seen to crucially affect the electronic structure: within the considered tetragonalization range, the minority gap is only slightly (i.e. by about 0.1-0.2 eV) affected by a tensile or compressive strain. Nevertheless, we show that the growth on a smaller in-plane lattice constant can stabilize the out-of-plane vs in-plane e_g orbital and significatively change their relative occupancy. Since e_g orbitals are key quantities for the double-exchange mechanism, strain effects are confirmed to be crucial for the resulting magnetic coupling.

cond-mat.mtrl-sci↗

arXiv2002

Nonlinear current-induced forces in Si atomic wires

We report first-principles calculations of current-induced forces in Si atomic wires as a function of bias and wire length. We find that these forces are strongly nonlinear as a function of bias due to the competition between the force originating from the scattering states and the force due to bound states. We also find that the average force in the wire is larger the shorter the wire, suggesting that atomic wires are more difficult to break under current flow with increasing length. The last finding is in agreement with recent experimental data.

cond-mat.mes-hall↗