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Design of Antiferromagnetic Second-order Band Topology with Rotation Topological Invariants in Two Dimensions

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arXiv 2023年

作者： Zhan, Fangyang Qin, Zheng Xu, Dong-Hui Zhou, Xiaoyuan Ma, Da-Shuai Wang, Rui Institute for Structure and Function Department of Physics Chongqing Key Laboratory for Strongly Coupled Physics Chongqing University Chongqing400044 China Center of Quantum materials and devices Chongqing University Chongqing400044 China

The existence of fractionally quantized topological corner states serves as a key indicator for two-dimensional second-order topological insulators (SOTIs), yet has not been experimentally observed in realistic materials. Here, based on effective model analysis and symmetry arguments, we propose a strategy for achieving SOTI phases with in-gap corner states in two dimensional systems with antiferromagnetic (AFM) order. We uncover by a minimum lattice model that the band topology originates from the interplay between intrinsic spin-orbital coupling and interlayer AFM exchange interactions. Using first principles calculations, we show that the 2D AFM SOTI phases can be realized in (MnBi2Te4)(Bi2Te3)m films. Moreover, we demonstrate that the nontrivial corner states are linked to rotation topological invariants under three-fold rotation symmetry C3, resulting in C3-symmetric SOTIs with corner charges fractionally quantized to 3n |e| (mod e). Due to the great recent achievements in (MnBi2Te4)(Bi2Te3)m systems, our results providing reliable material candidates for experimentally accessible AFM higher-order band topology would draw intense attentions. Copyright © 2023, The Authors. All rights reserved.

关键词： Topology

Topological Rashba-like edge states in large-gap quantum spin Hall insulators

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Physical Review Materials 2018年第11期2卷 114207-114207页

作者： Yuanjun Jin Li-Yong Gan Rui Wang Jinzhu Zhao Yueyue Shan Junfeng Liu Hu Xu Department of Physics and Institute for Quantum Science and Engineering Southern University of Science and Technology Shenzhen 518055 China School of Materials Science and Engineering Key Laboratory of Advanced Energy Storage Materials of Guangdong Province South China University of Technology 510641 Guangzhou China Institute for Structure and Function and Department of Physics Chongqing University 400030 Chongqing China

The search for large-band-gap quantum spin Hall (QSH) insulators, in which the dissipationless gapless edge states lie inside the bulk gap, is critical for their practical applications at room temperature. Based on first-principles calculations, we propose a sort of QSH insulator in a rectangular structure, i.e., AuSb, AuAs, and CuAs monolayers, which exhibits sizable bulk gaps up to 258 meV. These compounds host unexpected topological Rashba-like edge states, which exhibit strong dispersion with a nearly-free-electron feature. Such nontrivial edge states originate from spin-orbital coupling and an abrupt discontinuous potential across the surface. Furthermore, we argue that the AuSb monolayer can be epitaxially grown on a rutile TiO2(110) surface. Importantly, the thermodynamic stability of AuSb is enhanced while the topological features are perfectly preserved due to the weak van der Waals interaction at the interface. Our findings provide promising large-gap QSH candidates with exotic topological Rashba-like edge states that are in favor of experimental realization at room temperature.

关键词： Electronic structure First-principles calculations Rashba coupling Surface states Topological insulators

Chiral-Flux-Phase-Based Topological Superconductivity in Kagome Systems with Mixed Edge Chiralities

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Physical Review Letters 2023年第8期131卷 086601-086601页

作者： Junjie Zeng Qingming Li Xun Yang Dong-Hui Xu Rui Wang Institute for Structure and Function and Department of Physics and Chongqing Key Laboratory for Strongly Coupled Physics Chongqing University Chongqing 400044 People’s Republic of China Department of Physics Shijiazhuang University Shijiazhuang Hebei 050035 People’s Republic of China Center of Quantum materials and devices Chongqing University Chongqing 400044 People’s Republic of China

Recent studies have attracted intense attention on the quasi-2D kagome superconductors AV3Sb5 (A=K, Rb, and Cs) where the unexpected chiral flux phase (CFP) associates with the spontaneous time-reversal symmetry breaking in charge density wave states. Here, commencing from the 2-by-2 charge density wave phases, we bridge the gap between topological superconductivity and time-reversal asymmetric CFP in kagome systems. Several chiral topological superconductor (TSC) states featuring distinct Chern numbers emerge for an s-wave or a d-wave superconducting pairing symmetry. Importantly, these CFP-based TSC phases possess unique gapless edge modes with mixed chiralities (i.e., both positive and negative chiralities), but with the net chiralities consistent with the Bogoliubov–de Gennes Chern numbers. We further study the transport properties of a two-terminal junction, using Chern insulator or normal metal leads via atomic Green’s function method with Landauer-Büttiker formalism. In both cases, the normal electron tunneling and the crossed Andreev reflection oscillate as the chemical potential changes, but together contribute to plateau transmissions (1 and 32, respectively) that exhibit robustness against disorder. These behaviors can be regarded as the signature of a TSC hosting edge states with mixed chiralities.

关键词： Andreev reflection Charge density waves Edge states Spin-orbit coupling Spin-singlet pairing Superconductivity Topological phases of matter Topological superconductors 2-dimensional systems Bogoliubov-de Gennes equations Green's function methods Landauer formula Second quantization Tight-binding model

Learning Interacting Theories from Data

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Physical Review X 2023年第4期13卷 041033-041033页

作者： Claudia Merger Alexandre René Kirsten Fischer Peter Bouss Sandra Nestler David Dahmen Carsten Honerkamp Moritz Helias Institute of Neuroscience and Medicine (INM-6) and Institute for Advanced Simulation (IAS-6) and JARA-Institute Brain Structure-Function Relationships (INM-10) Jülich Research Centre Jülich Germany RWTH Aachen University Aachen Germany Department of Physics University of Ottawa Ottawa Canada

One challenge of physics is to explain how collective properties arise from microscopic interactions. Indeed, interactions form the building blocks of almost all physical theories and are described by polynomial terms in the action. The traditional approach is to derive these terms from elementary processes and then use the resulting model to make predictions for the entire system. But what if the underlying processes are unknown? Can we reverse the approach and learn the microscopic action by observing the entire system? We use invertible neural networks to first learn the observed data distribution. By the choice of a suitable nonlinearity for the neuronal activation function, we are then able to compute the action from the weights of the trained model; a diagrammatic language expresses the change of the action from layer to layer. This process uncovers how the network hierarchically constructs interactions via nonlinear transformations of pairwise relations. We test this approach on simulated datasets of interacting theories and on an established image dataset (MNIST). The network consistently reproduces a broad class of unimodal distributions; outside this class, it finds effective theories that approximate the data statistics up to the third cumulant. We explicitly show how network depth and data quantity jointly improve the agreement between the learned and the true model. This work shows how to leverage the power of machine learning to transparently extract microscopic models from data.

关键词： Complex systems Inverse problems Learning Network inference

Perspective: Floquet engineering topological states from effective models towards realistic materials

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arXiv 2024年

作者： Zhan, Fangyang Chen, Rui Ning, Zhen Ma, Da-Shuai Wang, Ziming Xu, Dong-Hui Wang, Rui Institute for Structure and Function Department of Physics Chongqing Key Laboratory for Strongly Coupled Physics Chongqing University Chongqing400044 China Center of Quantum Materials and Devices Chongqing University Chongqing400044 China Department of Physics Hubei University Wuhan430062 China

With significant advances in classifying and cataloguing topological matter, the focus of topological physics has shifted towards quantum control, particularly the creation and manipulation of topological phases of matter. Floquet engineering, the concept of tailoring a system by periodic fields, offers a powerful tool to manipulate electronic properties of condensed systems, and even to create exotic non-equilibrium topological states that are impossibly present in equilibrium scenarios. In this perspective, we give a brief review of recent progress in theoretical investigations of Floquet engineering topological states from effective models towards realistic materials. We show that light irradiation can realize various desired topological states through the introduction of symmetry breaking, such as first- and higher-order Weyl fermions, quadrupole topological insulator with periodic driving and disorder, quantum anomalous Hall effects with a tunable Chern number, as well as beyond. Moreover, based on first-principles calculations and Floquet theorem, we show several realistic material candidates proposed as potential hosts for promising Floquet topological states, facilitating their verification in experiments. We believe that our perspective on Floquet engineering of topological states will advance further studies of rich exotic light-induced phenomena in condensed matter physics. Copyright © 2024, The Authors. All rights reserved.

关键词： Electronic properties

Topological Weyl and Node-line semimetals in ferromagnetic vanadium-phosphorous-oxide β-V2OPO4 compound

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arXiv 2017年

作者： Jin, Y.J. Wang, R. Zhao, J.Z. Chen, Z.J. Zhao, Y.J. Xu, H. Department of Physics South University of Science and Technology of China Shenzhen518055 China Institute for Structure and Function Department of physics Chongqing University Chongqing400044 China Dalian Institute of Chemical Physics Chinese Academy of Sciences Dalian116023 China Department of Physics South China University of Technology Guangzhou510640 China

We propose that the topological semimetal features can co-exist with ferromagnetic ground state in vanadium-phosphorous-oxide β-V2OPO4 compound from first-principles calculations. In this magnetic system with inversion symmetry, the direction of magnetization is able to manipulate the symmetric protected band structures from a node-line type to a Weyl one in the presence of spin-orbital-coupling. The node-line semimetal phase is protected by the mirror symmetry with the reflection-invariant plane perpendicular to magnetic order. Within mirror symmetry breaking due to the magnetization along other directions, the gapless node-line loop will degenerate to only one pair of Weyl points protected by the rotational symmetry along the magnetic axis, which are largely separated in momentum space. Such Weyl semimetal phase provides a nice candidate with the minimum number of Weyl points in a condensed matter system. The results of surface band calculations confirm the non-trivial topology of this proposed compound. This findings provide a realistic candidate for the investigation of topological semimetals with time-reversal symmetry breaking, particularly towards the realization of quantum anomalous Hall effect in Weyl semimetals. Copyright © 2017, The Authors. All rights reserved.

关键词： Vanadium compounds

Ferromagnetic Type-II Weyl Semimetal in Pyrite Chromium Dioxide

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arXiv 2017年

作者： Wang, R. Jin, Y.J. Zhao, J.Z. Chen, Z.J. Zhao, Y.J. Xu, H. Department of Physics South University of Science and Technology of China Shenzhen518055 China Institute for Structure and Function Department of physics Chongqing University Chongqing400044 China Dalian Institute of Chemical Physics Chinese Academy of Sciences Dalian116023 China Department of Physics South China University of Technology Guangzhou510640 China

Magnetic topological materials have recently drawn significant importance and interest, due to their topologically nontrivial electronic structure within spontaneous magnetic moments and band inversion. Based on first-principles calculations, we propose that chromium dioxide, in its ferromagnetic pyrite structure, can realize one pair of type-II Weyl points between the Nth and (N + 1)th bands, where N is the total number of valence electrons per unit cell. Other Weyl points between the (N − 1)th and Nth bands also appear close to the Fermi level due to the complex topological electronic band structure. The symmetry analysis elucidates that the Weyl points arise from a triply-degenerate point splitting due to the mirror reflection symmetry broken in the presence of spin-orbital coupling, which is equivalent to an applied magnetic field along the direction of magnetization. The Weyl points located on the magnetic axis are protected by the three-fold rotational symmetry. The corresponding Fermi arcs projected on both (001) and (110) surfaces are calculated as well and observed clearly. This finding opens a wide range of possible experimental realizations of type-II Weyl fermions in a system with time-reversal breaking. Copyright © 2017, The Authors. All rights reserved.

关键词： Electronic structure

One-dimensional chiral anomaly and its disorder response

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Physical Review B 2023年第19期108卷 195103-195103页

作者： Zheng Qin Dong-Hui Xu Zhen Ning Rui Wang Institute for Structure and Function and Department of Physics Chongqing University Chongqing 400044 People's Republic of China Chongqing Key Laboratory for Strongly Coupled Physics Chongqing 400044 People's Republic of China Center of Quantum Materials and Devices Chongqing University Chongqing 400044 People's Republic of China

The condensed-matter realization of chiral anomaly has attracted tremendous interest in exploring unexpected phenomena of quantum field theory. Here, we show that one-dimensional (1D) chiral anomaly (i.e., 1D nonconservational chiral current under a background electromagnetic field) can be realized in a generalized Su-Schrieffer-Heeger model where a single gapless Dirac cone occurs. Based on the topological Thouless pump and anomalous dynamics of chiral displacement, we elucidate that such a system possesses the half-integer quantization of the winding number. Moreover, we investigate the evolution of 1D chiral anomaly with respect to two typical types of disorder, i.e., on-site disorder and bond disorder. The results show that the on-site disorder tends to smear the gapless Dirac cone. However, we propose a strategy to stabilize the half-integer quantization, facilitating its experimental detection. Furthermore, we demonstrate that the bond disorder causes a unique crossover with disorder-enhanced topological charge pumping, driving the system into a topological Anderson insulator phase.

关键词： Electronic structure Topological phase transition Topological phases of matter 1-dimensional systems Dirac semimetal Second quantization Tight-binding model k dot p method

Room-Temperature Ferromagnetic Topological Phase in CrO2: From Tripe Fermions to Weyl Fermions

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arXiv 2017年

作者： Wang, R. Jin, Y.J. Duan, L. Zhao, J.Z. Xu, H. Jin, C.Q. Department of Physics Southern University of Science and Technology Shenzhen518055 China Institute for Structure and Function Department of physics Chongqing University Chongqing400044 China Institute of Physics Chinese Academy of Sciences Beijing100190 China Institute for Quantum Science and Engineering Southern University of Science and Technology Shenzhen518055 China

Ferromagnetic topological semimetals due to their band topology co-existing with intrinsic magnetization exerted important influences on early study of topological fermions. However, they have not been observed in experiments up to now. In this work, we propose that rutile CrO2, a widely used half-metallic ferromagnetic material in magnetic recording taps, exhibits unexpected ferromagnetic topological features. Using first-principles calculations and symmetry analysis, we reveal that rutile CrO2 hosts the triple nodal points in the absence of spin orbital coupling (SOC). By taking into account of SOC, each triple nodal point splits into two Weyl points, which are located on the magnetic axis with four-fold rotational symmetry. Notably, the Fermi arcs and accompanying quasiparticle interference patterns are clearly visible, which facilitate experimental observations. In addition, we find that another experimentally synthesized CrO2 in CaCl2 structure, also hosts the topologically nontrivial ferromagnetic phase. Our findings substantially advance the experimental realization of ferromagnetic topological semimetals. More importantly, room-temperature time-reversal-breaking Weyl fermions in CrO2 may result in promising applications related to the topological phenomena in industry. Copyright © 2017, The Authors. All rights reserved.

关键词： Chromium compounds