arXiv subjects

Tong Zhou

Publications and source records attributed to Tong Zhou.

At least 127 records · Page 7Linked to original sources

arXiv2020

Localized Excitons in NbSe$_2$-MoSe$_2$ Heterostructures

Neutral and charged excitons (trions) in atomically-thin materials offer important capabilities for photonics, from ultrafast photodetectors to highly-efficient light-emitting diodes and lasers. Recent studies of van der Waals (vdW) heterostructures comprised of dissimilar monolayer materials have uncovered a wealth of optical phenomena that are predominantly governed by interlayer interactions. Here, we examine the optical properties in NbSe$_2$ - MoSe$_2$ vdW heterostructures, which provide an important model system to study metal-semiconductor interfaces, a common element in optoelectronics. Through low-temperature photoluminescence (PL) microscopy we discover a sharp emission feature, L1, that is localized at the NbSe$_2$-capped regions of MoSe$_2$. L1 is observed at energies below the commonly-studied MoSe$_2$ excitons and trions, and exhibits temperature- and power-dependent PL consistent with exciton localization in a confining potential. Remarkably, L1 is very robust not just in different samples, but also under a variety of fabrication processes. Using first-principles calculations we reveal that the confinement potential required for exciton localization naturally arises from the in-plane band bending due to the changes in the electron affinity between pristine MoSe$_2$ and NbSe$_2$ - MoSe$_2$ heterostructure. We discuss the implications of our studies for atomically-thin optoelectronics devices with atomically-sharp interfaces and tunable electronic structures.

cond-mat.mes-hall↗

arXiv2020

Learning Oracle Attention for High-fidelity Face Completion

High-fidelity face completion is a challenging task due to the rich and subtle facial textures involved. What makes it more complicated is the correlations between different facial components, for example, the symmetry in texture and structure between both eyes. While recent works adopted the attention mechanism to learn the contextual relations among elements of the face, they have largely overlooked the disastrous impacts of inaccurate attention scores; in addition, they fail to pay sufficient attention to key facial components, the completion results of which largely determine the authenticity of a face image. Accordingly, in this paper, we design a comprehensive framework for face completion based on the U-Net structure. Specifically, we propose a dual spatial attention module to efficiently learn the correlations between facial textures at multiple scales; moreover, we provide an oracle supervision signal to the attention module to ensure that the obtained attention scores are reasonable. Furthermore, we take the location of the facial components as prior knowledge and impose a multi-discriminator on these regions, with which the fidelity of facial components is significantly promoted. Extensive experiments on two high-resolution face datasets including CelebA-HQ and Flickr-Faces-HQ demonstrate that the proposed approach outperforms state-of-the-art methods by large margins.

cs.CV↗

arXiv2020

Magnetic Properties and Electronic Structure of Magnetic Topological Insulator MnBi$_2$Se$_4$

The intrinsic magnetic topological insulators MnBi$_2$X$_4$ (X = Se, Te) are promising candidates in realizing various novel topological states related to symmetry breaking by magnetic order. Although much progress had been made in MnBi$_2$Te$_4$, the study of MnBi$_2$Se$_4$ has been lacking due to the difficulty of material synthesis of the desired trigonal phase. Here, we report the synthesis of multilayer trigonal MnBi$_2$Se$_4$ with alternating-layer molecular beam epitaxy. Atomic-resolution scanning transmission electron microscopy (STEM) and scanning tunneling microscopy (STM) identify a well-ordered multilayer van der Waals (vdW) crystal with septuple-layer base units in agreement with the trigonal structure. Systematic thickness-dependent magnetometry studies illustrate the layered antiferromagnetic ordering as predicted by theory. Angle-resolved photoemission spectroscopy (ARPES) reveals the gapless Dirac-like surface state of MnBi$_2$Se$_4$, which demonstrates that MnBi$_2$Se$_4$ is a topological insulator above the magnetic ordering temperature. These systematic studies show that MnBi$_2$Se$_4$ is a promising candidate for exploring the rich topological phases of layered antiferromagnetic topological insulators.

cond-mat.mtrl-sci↗

arXiv2020

Phase Control of Majorana Bound States in a Topological X Junction

Topological superconductivity supports exotic Majorana bound states (MBS) which are chargeless zero-energy emergent quasiparticles. With their non-Abelian exchange statistics and fractionalization of a single electron stored nonlocally as a spatially separated MBS, they are particularly suitable for implementing fault-tolerant topological quantum computing. While the main efforts to realize MBS have focused on one-dimensional systems, the onset of topological superconductivity requires delicate parameter tuning and geometric constraints pose significant challenges for their control and demonstration of non-Abelian statistics. To overcome these challenges, building on recent experimental advances in planar Josephson junctions (JJs), we propose a MBS platform of X-shaped JJs. This versatile implementation reveals how external flux control of the superconducting phase difference can generate and manipulate multiple MBS pairs to probe non-Abelian statistics. The underlying topological superconductivity exists over a large parameter space, consistent with materials used in our fabrication of such X junctions, as an important step towards scalable topological quantum computing.

cond-mat.mes-hall↗

arXiv2019

Electrical control of Majorana Bound States Using Magnetic Stripes

A hybrid semiconductor-superconductor nanowire on the top of a magnetic film in the stripe phase experiences a magnetic texture from the underlying fringing fields. The Zeeman interaction with the highly inhomogeneous magnetic textures generates a large synthetic spin-orbit coupling. We show that this platform can support the formation of Majorana bound states (MBS) localized at the ends of the nanowire. The transition to the topological superconducting phase not only depends on the nanowire parameters and stripe size but also on the relative orientation of the stripes with respect to the nanowire axis. Topological phase transitions with the corresponding emergence or destruction of MBS can be induced by reorienting the stripes or shifting their position, which can be achieved by passing a charge current through the magnetic film or by applying electrically-controlled strain to it. The proposed platform removes the need for external magnetic fields and offers a non-invasive electrical tuning of MBS with the perturbation (current or strain) acting only on thee magnetic film.

cond-mat.supr-con↗

arXiv2019

Experimental Evidence of the Topological Surface States in Mg3Bi2 Films Grown by Molecular Beam Epitaxy

Type-II nodal line semimetal (NLS) is a new quantum state hosting one-dimensional closed loops formed by the crossing of two bands which have the same sign in their slopes along the radial direction of the loop. According to the theoretical prediction, Mg3Bi2 is an ideal candidate for studying the type-II NLS by tuning its spin-orbit coupling (SOC). In this paper, high quality Mg3Bi2 films are grown by molecular beam epitaxy (MBE). By in-situ angle resolved photoemission spectroscopy (ARPES), a pair of surface resonance bands (SRBs) around Gamma point is clearly seen. It shows that Mg3Bi2 films grown by MBE is Mg(1)-terminated by comparing the ARPES data with the first principles calculations results. And, the temperature dependent weak anti-localization (WAL) effect in Mg3Bi2 films is observed under low magnetic field, which shows a clear two dimensional (2D) e-e scattering characteristics by fitting with the Hikami-Larkin-Nagaoka (HLN) model. Combining with ARPES, magneto-transport measurements and the first principles calculations, this work proves that Mg3Bi2 is a semimetal with topological surface states TSSs, which paves the way for Mg3Bi2 as an ideal materials platform for studying the exotic features of type-II nodal line semimetals (NLSs) and the topological phase transition by tuning its SOC.

cond-mat.mtrl-sci↗

arXiv2019

Dimensional crossover and topological nature of the thin films of a three-dimensional topological insulator by band gap engineering

Identification and control of topological phases in topological thin films offer great opportunity for fundamental research and the fabrication of topology-based devices. Here, combining molecular beam epitaxy, angle-resolved photoemission spectroscopy and ab-initio calculations, we investigate the electronic structure evolution in (Bi1-xInx)2Se3 films with thickness from 2 to 13 quintuple layers. We identify several phases with their characteristic topological nature and evolution between them, i.e., dimensional crossover from a three-dimensional topological insulator with gapless surface state to its two-dimensional counterpart with gapped surface state, and topological phase transition from topological insulator to a normal semiconductor with increasing In concentration x. Furthermore, by introducing In alloying as an external knob of band gap engineering, we experimentally demonstrated the trivial topological nature of Bi2Se3 thin films (below 6 quintuple layers) as two-dimensional gapped systems, in consistent with our theoretical calculations. Our results provide not only a comprehensive phase diagram of (Bi1-xInx)2Se3 and a route to control its phase evolution, but also a practical way to experimentally determine the topological properties of a gapped compound by topological phase transition and band gap engineering.

cond-mat.mtrl-sci↗

arXiv2019

Tunable magnetic textures in spin valves: From spintronics to Majorana bound states

Spin-valve structures in which a change of magnetic configuration is responsible for magnetoresistance have enabled impressive advances in spintronics, focusing on magnetically storing and sensing information. However, this mature technology also offers versatile control of the underlying fringing fields and entirely different applications by realizing topologically-nontrivial states. Together with proximity-induced superconductivity in a two-dimensional electron gas, these fringing fields realized in commercially-available spin valves could control Majorana bound states (MBS). Detailed support for the existence and control of MBS is obtained by combining accurate micromagnetic simulation of fringing fields used as an input in Bogoliubov de Gennes equation to calculate low-energy spectrum, wavefunction localization, and local charge neutrality. A generalized condition for quantum phase transition in these structures provides valuable guidance for the MBS evolution and implementing reconfigurable effective topological wires.

cond-mat.supr-con↗

arXiv2019

Structural Controllability of a Networked Dynamic System with LFT Parameterized Subsystems

This paper studies structural controllability for a networked dynamic system (NDS), in which each subsystem may have different dynamics, and unknown parameters may exist both in subsystem dynamics and in subsystem interconnections. In addition, subsystem parameters are parameterized by a linear fractional transformation (LFT). It is proven that controllability keeps to be a generic property for this kind of NDSs. Some necessary and sufficient conditions are then established respectively for them to be structurally controllable, to have a fixed uncontrollable mode, and to have a parameter dependent uncontrollable mode, under the condition that each subsystem interconnection link can take a weight independently. These conditions are scalable, and in their verifications, all arithmetic calculations are performed separately on each subsystem. In addition, these conditions also reveal influences on NDS controllability from subsystem input-output relations, subsystem uncontrollable modes and subsystem interconnection topology. Based on these observations, the problem of selecting the minimal number of subsystem interconnection links is studied under the requirement of constructing a structurally controllable NDS. A heuristic method is derived with some provable approximation bounds and a low computational complexity.

math.OC↗

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

Input Matrix Construction and Approximation Using a Graphic Approach

Given a state transition matrix (STM), we reinvestigate the problem of constructing the sparest input matrix with a fixed number of inputs to guarantee controllability. We give a new and simple graph theoretic characterization for the sparsity pattern of input matrices to guarantee controllability for a general STM admitting multiple eigenvalues, and provide a deterministic procedure with polynomial time complexity to construct real valued input matrices with arbi- trarily prescribed sparsity pattern satisfying controllability. Based on this criterion, some novel results on sparsely controlling a system are obtained. It is proven that the minimal number of inputs to guarantee controllability equals to the maximum geometric multiplicity of the STM under the constraint that some states are actuated-forbidden, extending the results of [28]. The minimal sparsity of input matrices with a fixed number of inputs is not necessarily equal to the minimal number of actuated states to ensure controllability. Furthermore, a graphic sub- modular function is built, leading to a greedy algorithm to efficiently approximate the minimal actuated states to assure controllability for general STMs. For the problem of approximating the sparsest input matrices with a fixed number of inputs, we propose a simple greedy algo- rithm (non-submodular) and a two-stage algorithm, and demonstrate that the latter algorithm, inspired from techniques in dynamic coloring, has a provable approximation guarantee. Finally, we present numerical results to show the efficiency and effectiveness of our approaches.

math.OC↗

arXiv2018

Minimal Structural Perturbations for Network Controllability: Complexity Analysis

Link (edge) addition/deletion or sensor/actuator failures are common structural perturbations for real network systems. This paper is related to the computation complexity of minimal (cost) link insertion, deletion and vertex deletion with respect to structural controllability of networks. Formally, given a structured system, we prove that: i) it is NP-hard to add the minimal cost of links (including links between state variables and from inputs to state variables) from a given set of links to make the system structurally controllable, even with identical link costs or a prescribed input topology; ii) it is NP-hard to determine the minimal cost of links whose deletion deteriorates structural controllability of the system, even with identical link costs or when the removable links are restricted in input links. It is also proven that determining the minimal cost of inputs whose deletion causes structural uncontrollability is NP-hard in the strong sense. The reductions in their proofs are technically independent. These results may serve an answer to the general hardness of optimally designing (modifying) a structurally controllable network topology and of measuring controllability robustness against link/actuator failures. Some fundamental approximation results for these related problems are also provided.

math.OC↗

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

Minimal Inputs/Outputs for Subsystems in a Networked System

Minimal input/output selection is investigated in this paper for each subsystem of a networked system. Some novel sufficient conditions are derived respectively for the controllability and observability of a networked system, as well as some necessary conditions. These conditions only depend separately on parameters of each subsystem and its in/out-degrees. It is proven that in order to be able to construct a controllable/observable networked system, it is necessary and sufficient that each subsystem is controllable/observable. In addition, both sparse and dense subsystem connections are helpful in making the whole system controllable/observable. An explicit formula is given for the smallest number of inputs/outputs for each subsystem required to guarantee controllability/observability of the whole system.

math.OC↗

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

Minimal Inputs/Outputs for a Networked System

This paper investigates the minimal number of inputs/outputs required to guarantees the controllability/observability of a system, under the condition that its state transition matrix (STM) is prescribed. It has been proved that this minimal number is equal to the maximum geometric multiplicity of the system STM. The obtained conclusions are in sharp contrast to those established for the problems of finding the sparest input/output matrix under the restriction of system controllability/observabilty, which have been proved to be NP-hard, and even impossible to be approximated within a multiplicative factor. Moreover, a complete parametrization is also provided for the input/output matrix of a system with this minimal input/output number.

eess.SY↗