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Photoinduced topological phase transition in monolayer Ti2SiCO2

Physical Review B 2024年第7期109卷 075141-075141页

作者： Pu Liu Chaoxi Cui Zhi-Ming Yu Centre for Quantum Physics Key Laboratory of Advanced Optoelectronic Quantum Architecture and Measurement (MOE) School of Physics Beijing Institute of Technology Beijing 100081 China and Beijing Key Lab of Nanophotonics & Ultrafine Optoelectronic Systems School of Physics Beijing Institute of Technology Beijing 100081 China

The TiSiCO-family monolayer X2YCO2 (X=Ti, Zr, Hf; Y=Si, Ge) is a two-dimensional second-order topological insulator with unique valley-layer coupling in equilibrium condition. In this work, based on the four-band tight-binding (TB) model of monolayer Ti2SiCO2 (ML-TiSiCO) and the Floquet theory, we study the nonequilibrium properties of the ML-TiSiCO under a periodic field of laser and a gate-electric field. We find the interaction between the time-periodic polarized light and the electric field can lead to a variety of intriguing topological phase transitions. By driving the system with only circularly polarized light (CPL), a photoinduced topological phase transition occurs from a second-order topological insulator to a Chern insulator with a Chern number of C=±2, and the sign of the Chern number C is determined by the chirality of the incident light. Further adding a perpendicular electric field, we find that the ML-TiSiCO exhibits a rich phase diagram, consisting of Chern insulators with different Chern numbers and various topological semimetals. In contrast, since the linearly polarized light (LPL) does not break time-reversal symmetry, the Chern number of the system would not be changed under the irradiation of LPL. However, there still exist many topological phases, including second-order topological insulator, topological semi-Dirac, Dirac, and valley-polarized Dirac semimetals under the interaction between the LPL and the electric field. Our results not only enhance the understanding of the fundamental properties of ML-TiSiCO but also broaden the potential applications of such materials in optoelectronic devices.

关键词： Edge states Electrical properties Electronic structure Phase diagrams Phase transitions Photoinduced effect Topological materials Topological phase transition 2-dimensional systems Dirac semimetal Semimetals Bloch-Floquet theorem Tight-binding model

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Floquet Chern vector topological insulators in three dimensions

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Physical Review B 2025年第13期111卷 134301-134301页

作者： Fangyuan Ma Junrong Feng Feng Li Ying Wu Di Zhou Key Lab of Advanced Optoelectronic Quantum Architecture and Measurement (MOE) School of Physics Beijing Institute of Technology Beijing 100081 China School of Physics Nanjing University of Science and Technology Nanjing 210094 China

We theoretically and numerically investigate Chern vector insulators and topological surface states in a three-dimensional lattice, based on phase-delayed temporally periodic interactions within the tight-binding model. These Floquet interactions break time-reversal symmetry, effectively inducing a gauge field analogous to magnetic flux. This gauge field results in Chern numbers in all spatial dimensions, collectively forming the Chern vector. This vector characterizes the topological phases and signifies the emergence of robust surface states. Numerically, we observe these states propagating unidirectionally without backscattering on all open surfaces of the three-dimensional system. Our paper paves the way for breaking time-reversal symmetry and realizing three-dimensional Chern vector topological insulators using temporally periodic Floquet techniques.

关键词： Topological insulators

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Two-Dimensional Topological Ferroelectric Metal with Giant Shift Current

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Physical Review Letters 2024年第18期133卷 186801页

作者： Liu Yang Lei Li Zhi-Ming Yu Menghao Wu Yugui Yao Department of Physics Hubei Engineering Research Center of Weak Magnetic-field Detection Centre for Quantum Physics Key Laboratory of Advanced Optoelectronic Quantum Architecture and Measurement (MOE) School of Physics Beijing Key Lab of Nanophotonics and Ultrafine Optoelectronic Systems School of Physics International Center for Quantum Materials School of Physics

The pursuit for “ferroelectric metal,” which combines seemingly incompatible spontaneous electric polarization and metallicity, has been assiduously ongoing but remains elusive. Unlike traditional ferroelectrics with a wide band gap, ferroelectric (FE) metals can naturally incorporate nontrivial band topology near the Fermi level, endowing them with additional exotic properties. Here, we show first-principles evidence that the metallic PtBi2 monolayer is an intrinsic two-dimensional (2D) topological FE metal, characterized by out-of-plane polarization and a moderate switching barrier. Moreover, it exhibits a topologically nontrivial electronic structure with Z2 invariant equal to 1, leading to a significant FE bulk photovoltaic effect. A slight strain can further enhance this effect to a remarkable level, which far surpasses that of previously reported 2D and 3D FE materials. Our Letter provides an important step toward realizing intrinsic monolayer topological FE metals and paves a promising way for future nonlinear optical devices.

关键词： Ferroelectricity Shift current Topological materials Van der Waals systems Density functional theory First-principles calculations

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Tunable optical topological transitions of plasmon polaritons in WTe_(2) van der Waals films

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Light(Science & Applications) 2023年第9期12卷 1807-1817页

作者： Yuangang Xie Chong Wang Fucong Fei Yuqi Li Qiaoxia Xing Shenyang Huang Yuchen Lei Jiasheng Zhang Lei Mu Yaomin Dai Fengqi Song Hugen Yan State Key Laboratory of Surface Physics Key Laboratory of Micro and NanoPhotonic Structures(Ministry of Education)and Department of PhysicsFudan University200433 ShanghaiChina Centre for Quantum Physics Key Laboratory of Advanced Optoelectronic Quantum Architecture and Measurement(MOE)School of PhysicsBeijing Institute of Technology100081 BeijingChina Beijing Key Lab of Nanophotonics&Ultrafine Optoelectronic Systems School of PhysicsBeijing Institute of Technology100081 BeijingChina National Laboratory of Solid State Microstructures Collaborative Innovation Center of Advanced Microstructuresand College of PhysicsNanjing University210093 NanjingChina Atom Manufacturing Institute(AMI) 211805 NanjingChina Center for Superconducting Physics and Materials National Laboratory of Solid State Microstructures and Department of PhysicsNanjing University211805 NanjingChina

Naturally existing in-plane hyperbolic polaritons and the associated optical topological transitions,which avoid the nano-structuring to achieve hyperbolicity,can outperform their counterparts in artificial *** plasmon polaritons are rare,but experimentally revealed recently in WTe_(2)van der Waals thin *** from phonon polaritons,hyperbolic plasmon polaritons originate from the interplay of free carrier Drude response and interband transitions,which promise good intrinsic ***,tunable in-plane hyperbolic plasmon polariton and its optical topological transition of the isofrequency contours to the elliptic topology in a natural material have not been *** we demonstrate the tuning of the optical topological transition through Mo doping and *** optical topological transition energy is tuned over a wide range,with frequencies ranging from 429 cm^(−1)(23.3 microns)for pure WTe_(2)to 270 cm^(−1)(37.0 microns)at the 50%Mo-doping level at 10 ***,the temperature-induced blueshift of the optical topological transition energy is also revealed,enabling active and reversible ***,the localized surface plasmon resonance in skew ribbons shows unusual polarization dependence,accurately manifesting its topology,which renders a reliable means to track the topology with far-field *** results open an avenue for reconfigurable photonic devices capable of plasmon polariton steering,such as canaling,focusing,and routing,and pave the way for low-symmetry plasmonic nanophotonics based on anisotropic natural materials.

关键词： topological transitions polar

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Majorana corner modes and tunable patterns in an altermagnet heterostructure

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Physical Review B 2023年第20期108卷 205410-205410页

作者： Yu-Xuan Li Cheng-Cheng Liu Centre for Quantum Physics Key Laboratory of Advanced Optoelectronic Quantum Architecture and Measurement (MOE) School of Physics Beijing Institute of Technology Beijing 100081 China

The mutual competition and synergy of magnetism and superconductivity provide us with a very valuable opportunity to access topological superconductivity and Majorana fermions. Here, we devise a heterostructure consisting of an s-wave superconductor, a two-dimensional topological insulator, and an altermagnet, which is classified as the third magnet and featured by zero magnetization but spin polarization in both real and reciprocal spaces. We find that the altermagnet can induce mass terms at the edges that compete with electron pairing, and mass domains are formed at the corners of the sample, resulting in zero-energy Majorana corner modes (MCMs). The presence or absence of MCMs can be engineered by only changing the direction of the Néel vector. Moreover, uniaxial strain can effectively manipulate the patterns of the MCMs, such as moving and interchanging MCMs. Experimental realization, remarkable advantages of our proposal, and possible braiding are discussed.

关键词： Magnetic order Topological superconductors Magnetic systems Topological materials

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General electronic structure calculation method for twisted systems

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Physical Review B 2025年第7期111卷 075434-075434页

作者： Junxi Yu Shifeng Qian Cheng-Cheng Liu Centre for Quantum Physics Key Laboratory of Advanced Optoelectronic Quantum Architecture and Measurement (MOE) and Beijing Key Lab of Nanophotonics and Ultrafine Optoelectronic Systems School of Physics Beijing Institute of Technology Beijing 100081 China Anhui Province Key Laboratory for Control and Applications of Optoelectronic Information Materials Department of Physics Anhui Normal University Anhui Wuhu 241000 China

In recent years, two-dimensional twisted systems have gained increasing attention. However, calculating the electronic structures in twisted materials has remained a challenge. To address this, we have developed a general computational methodology that can generate twisted geometries starting from a monolayer structure and obtain a precisely relaxed twisted structure through a machine learning-based method. Then, the electronic structure properties of the twisted material are calculated using the tight-binding (TB) and continuum model methods; thus, the entire process requires minimal computational resources. We introduce the theoretical methods for generating twisted structures and computing their electronic properties, and further provide calculations and brief analyses of the electronic structure properties for several typical two-dimensional materials with different characteristics. This paper serves as a solid foundation for researchers interested in studying twisted systems.

关键词： Electronic properties

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Stable higher-order topological Dirac semimetals with Z2 monopole charge in alternating-twist multilayer graphene and beyond

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Physical Review B 2023年第24期108卷 L241406-L241406页

作者： Shifeng Qian Yongpan Li Cheng-Cheng Liu Centre for Quantum Physics Key Laboratory of Advanced Optoelectronic Quantum Architecture and Measurement (MOE) School of Physics Beijing Institute of Technology Beijing 100081 China

We demonstrate that a class of stable Z2 monopole charge Dirac point (Z2DP) phases can robustly exist in real materials, which is counterintuitive: that is, a Z2DP is unstable and generally considered to be only the critical point of a Z2 nodal line (Z2NL) characterized by a Z2 monopole charge (the second Stiefel-Whitney number w2) with space-time inversion symmetry but no spin-orbital coupling. We explicitly reveal the higher-order bulk-boundary correspondence in the stable Z2DP phase. We propose the alternating-twist multilayer graphene, which can be regarded as 3D twisted bilayer graphene (TBG), as the first example to realize such stable Z2DP phase and show that the Dirac points in the 3D TBG are essentially degenerate at high-symmetry points protected by crystal symmetries and carry a nontrivial Z2 monopole charge (w2=1), which results in higher-order hinge states along the entire Brillouin zone of the kz direction. By breaking some crystal symmetries or tailoring interlayer coupling we are able to access Z2NL phases or other Z2DP phases with hinge states of adjustable length. In addition, we present other 3D materials which host Z2DPs in the electronic band structures and phonon spectra. We construct a minimal eight-band tight-binding lattice model that captures these nontrivial topological characters and furthermore tabulate all possible space groups to allow the existence of the stable Z2DP phases, which will provide direct and strong guidance for the realization of the Z2 monopole semimetal phases in (among others) electronic materials, metamaterials, and electrical circuits.

关键词： Electronic structure Topological phases of matter Dirac semimetal Twisted bilayer graphene Density functional theory Tight-binding model Wannier function methods k dot p method

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Giant enhancement of exciton radiative lifetime by ferroelectric polarization: The case of monolayer TiOCl2

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Physical Review B 2023年第23期107卷 235407-235407页

作者： Hongwei Qu Yuanchang Li Key Lab of Advanced Optoelectronic Quantum Architecture and Measurement (MOE) and Advanced Research Institute of Multidisciplinary Science Beijing Institute of Technology Beijing 100081 People's Republic of China

Exciton binding energy and lifetime are the two most important parameters controlling exciton dynamics, and the general consensus is that the larger the former, the larger the latter. However, our first-principles study of monolayer ferroelectric TiOCl2 shows that this is not always the case. We find that ferroelectric polarization tends to weaken exciton binding but enhance exciton lifetime. This stems from the different effects of the induced built-in electric field and structural distortion by the spontaneous polarization: the former always destabilizes or even dissociates the exciton while the latter leads to a relaxation of the selection rule and activates excitons that are otherwise not optically active. Their combined effect leads to a halving of the exciton binding energy but a substantial increase in lifetime by 40 times. Our results deepen the understanding of the interaction of light with ferroelectric materials and provide new insights into the use of ferroelectricity to control exciton dynamics.

关键词： Excitons Ferroelectricity 2-dimensional systems First-principles calculations

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Predictable Gate-Field Control of Spin in Altermagnets with Spin-Layer Coupling

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Physical Review Letters 2024年第5期133卷 056401-056401页

作者： Run-Wu Zhang Chaoxi Cui Runze Li Jingyi Duan Lei Li Zhi-Ming Yu Yugui Yao <sup>1</sup>Key Lab of Advanced Optoelectronic Quantum Architecture and Measurement (MOE) Beijing Key Lab of Nanophotonics &amp Ultrafine Optoelectronic Systems and School of Physics <a href="https://***/01skt4w74">Beijing Institute of Technology</a> Beijing 100081 China. <sup>2</sup>International Center for Quantum Materials <a href="https://***/01skt4w74">Beijing Institute of Technology</a> Zhuhai 519000 China.

Spintronics, a technology harnessing electron spin for information transmission, offers a promising avenue to surpass the limitations of conventional electronic devices. While the spin directly interacts with the magnetic field, its control through the electric field is generally more practical, and has become a focal point in the field. Here, we propose a mechanism to realize static and almost uniform effective magnetic field by gate-electric field. Our method employs two-dimensional altermagnets with valley-mediated spin-layer coupling (SLC), in which electronic states display valley-contrasted spin and layer polarization. For the low-energy valley electrons, a uniform gate field is approximately identical to a uniform magnetic field, leading to predictable control of spin. Through symmetry analysis and ab initio calculations, we predict altermagnetic monolayer Ca(CoN)2 and its family materials as potential candidates hosting SLC. We show that an almost uniform magnetic field (Bz) indeed is generated by gate field (Ez) in Ca(CoN)2 with Bz∝Ez in a wide range, and Bz reaches as high as about 103 T when Ez=0.2 eV/Å. Furthermore, owing to the clean band structure and SLC, one can achieve perfect and switchable spin and valley currents and significant tunneling magnetoresistance in Ca(CoN)2 solely using the gate field. Our work provides new opportunities to generate predictable control of spin and design spintronic devices that can be controlled by purely electric means.

关键词： Electrical properties Electronic structure First-principles calculations Spintronics Valley degrees of freedom Valleytronics

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Cornertronics in Two-Dimensional Second-Order Topological Insulators

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Physical Review Letters 2024年第17期133卷 176602页

作者： Yilin Han Chaoxi Cui Xiao-Ping Li Ting-Ting Zhang Zeying Zhang Zhi-Ming Yu Yugui Yao Centre for Quantum Physics Key Laboratory of Advanced Optoelectronic Quantum Architecture and Measurement (MOE) School of Physics Beijing Key Lab of Nanophotonics and Ultrafine Optoelectronic Systems School of Physics School of Physical Science and Technology Beijing National Laboratory for Condensed Matter Physics Institute of Physics College of Mathematics and Physics

Traditional electronic devices rely on the electron’s intrinsic degrees of freedom (d.o.f.) to process information. However, additional d.o.f., like the valley, can emerge in the low-energy states of certain systems. Here, we show that the quantum dots constructed from two-dimensional second-order topological insulators posses a new kind of d.o.f., namely corner freedom, related to the topological corner states that reside at different corners of the systems. Since the corner states are well separated in real space, they can be individually and intuitively manipulated, giving rise to the concept of cornertronics. Via symmetry analysis and material search, we identify the TiSiCO-family monolayers as the first prototype of cornertronics materials, where the corner states can be controlled by both electric and optical fields due to novel corner-layer coupling effect and corner-contrasted linear dichroism. Furthermore, we find that the band gap of the TiSiCO nanodisk lies in the terahertz region and is robust to size reduction. These results indicate that the TiSiCO nanodisks can be used to design terahertz devices with ultrasmall size and electric-field tunable band gap. Besides, the TiSiCO nanodisks are simultaneously sensitive to both the strength and polarization of the terahertz waves. Our findings not only pave the way for cornertronics, but also open a new direction for research in two-dimensional second-order topological insulators, quantum dots, and terahertz electronics.

关键词： Electrical properties First-principles calculations Topological materials quantum dots

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