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Realization of charge-four Weyl point in fermionic systems

Physical Review B 2023年第7期108卷 075130-075130页

作者： Xiaoliang Xiao Yuanjun Jin Da-Shuai Ma Weixiang Kong Jing Fan Rui Wang Xiaozhi Wu Institute for Structure and Function & Department of Physics Chongqing University Chongqing 400044 People's Republic of China Division of Physics and Applied Physics School of Physical and Mathematical Sciences Nanyang Technological University Singapore 637371 Singapore Center of Quantum materials and devices Chongqing University Chongqing 400044 People's Republic of China Center for Computational Science and Engineering Southern University of Science and Technology Shenzhen 518055 People's Republic of China Chongqing Key Laboratory for Strongly Coupled Physics Chongqing 401331 People's Republic of China

Unconventional quasiparticles of a twofold band degeneracy with |C|=4, named as charge-four Weyl point (CFWP), have been revealed in bosonic and artificial systems, while it is challenging in fermionic systems because of nonnegligible spin-orbit coupling. Herein, we propose a carbon allotrope, termed cP-C24, as an ideal platform to realize CFWP in the nonrelativistic limit. Besides one CFWP (C=+4), there are also eight type-I Weyl points (C=+1) and twelve type-II Weyl nodes (C=−1), making the topological charge in the whole Brillouin zone to be neutral. The characteristic quadruple helicoid surface states of CFWP are presented. Our work offering the avenue of CFWP in fermionic systems would absolutely advance the research on the chirality-dependent physical properties associated with unconventional topological quasiparticles.

关键词： Fermions Symmetry protected topological states 3-dimensional systems Crystal structures First-principles calculations Symmetries in condensed matter Wannier function methods k dot p method

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Ideal hourglass-type charge-three Weyl fermions in spinless systems

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Physical Review B 2024年第7期109卷 075160-075160页

作者： Xiaoliang Xiao Yuanjun Jin Da-Shuai Ma Weixiang Kong Jing Fan Rui Wang Xiaozhi Wu Institute for Structure and Function & Department of Physics Chongqing University Chongqing 400044 People's Republic of China Division of Physics and Applied Physics School of Physical and Mathematical Sciences Nanyang Technological University Singapore 637371 Singapore Center of Quantum Materials and Devices Chongqing University Chongqing 400044 People's Republic of China Center for Computational Science and Engineering Southern University of Science and Technology Shenzhen 518055 People's Republic of China Chongqing Key Laboratory for Strongly Coupled Physics Chongqing 400044 People's Republic of China

Hourglass-type Weyl fermions with the maximally topological charge of |C|=3, namely hourglass-type charge-three Weyl fermions (CTWFs), have seldom been reported due to the nonnegligible spin-orbit coupling effect and Pauli exclusion principles. Here, based on symmetry arguments and the low-energy effective k·p model, we confirm that two chiral space groups (SGs), i.e., SG P63 (No. 173) and SG P6322 (No. 182), can host ideal hourglass-type CTWFs in spinless systems. Moreover, we take Li2CO4 as a concrete example to show the ideal hourglass-type CTWFs, which possess ultralong sextuple- and triple-helicoid Fermi arcs, making them easier to be detected by experiments. Our work not only offers an avenue to search for ideal hourglass-type CTWFs but also provides a promising platform for experiments to verify this kind of unconventional quasiparticles with the ultralong sextuple- and triple-helicoid surface arcs.

关键词： Electronic structure Fermions First-principles calculations Symmetry protected topological states 3-dimensional systems Crystal structures Weyl semimetal High-throughput calculations Symmetries in condensed matter Wannier function methods k dot p method

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Symmetry-enforced nodal cage phonons in Th2BC2

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Physical Review B 2022年第17期105卷 174309-174309页

作者： Meng Wang Yu Wang Zhiyong Yang Jing Fan Baobing Zheng Rui Wang Xiaozhi Wu College of Physics and Optoelectronic Technology Baoji University of Arts and Sciences Baoji 721016 People's Republic of China Center for Computational Science and Engineering Southern University of Science and Technology Shenzhen 518055 People's Republic of China Institute for Structure and Function & Department of Physics Chongqing University Chongqing 400044 People's Republic of China Chongqing Key Laboratory for Strongly Coupled Physics Chongqing University Chongqing 400044 People's Republic of China Center of Quantum Materials and Devices Chongqing University Chongqing 400044 People's Republic of China

Exploring unique topological states in condensed-matter systems has attracted great interest especially for the topological phonons recently. Based on the unbiased structure prediction approach combined with first-principles calculations, the long-sought crystal structure of Th2BC2 is determined. Most importantly, we show by the symmetry analysis and the phonon tight-binding Hamiltonian that Th2BC2 hosts nodal surface phonons on the qz=±π plane, coexisting with nodal line phonons on the qy=0 and qy=±π planes, consequently, forming cagelike phonons. The nodal surface phonons are protected by the screw axis C̃2z, and the nodal line phonons are enforced by inversion and time-reversal symmetries, demonstrated by the codimension argument and the effective model analysis. In addition, we also investigate the phonon surface states and the isofrequency arc on the (100) surface, which benefit the confirmation of the nodal cage phonons in experiments. Our paper not only determines the long-sought crystal structure of Th2BC2, but also provides an ideal candidate to realize the exotic topological phonon excitations.

关键词： First-principles calculations Optical phonons Phonons Topological materials Crystal structures k dot p method

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Higher-order topology in twisted multilayer systems: a review

arXiv

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arXiv 2024年

作者： Hua, Chunbo Xu, Dong-Hui Key Laboratory of Artificial Micro- and Nanostructures of Ministry of Education School of Physics and Technology Wuhan University Wuhan430072 China School of Electronic and Information Engineering Hubei University of Science and Technology Xianning437100 China 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

In recent years, there has been a surge of interest in higher-order topological phases (HOTPs) across various disciplines within the field of physics. These unique phases are characterized by their ability to harbor topological protected boundary states at lower-dimensional boundaries, a distinguishing feature that sets them apart from conventional topological phases and is attributed to the higher-order bulk-boundary correspondence. Two-dimensional (2D) twisted systems offer an optimal platform for investigating HOTPs, owing to their strong controllability and experimental feasibility. Here, we provide a comprehensive overview of the latest research advancements on HOTPs in 2D twisted multilayer systems. We will mainly review the HOTPs in electronic, magnonic, acoustic, photonic and mechanical twisted systems, and finally provide a perspective of this topic. © 2024, CC BY.

关键词： Topology

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Floquet valley-polarized quantum anomalous Hall state in nonmagnetic heterobilayers

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Physical Review B 2022年第8期105卷 L081115-L081115页

作者： Fangyang Zhan Zhen Ning Li-Yong Gan Baobing Zheng Jing Fan Rui Wang Institute for Structure and Function & 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 for Computational Science and Engineering Southern University of Science and Technology Shenzhen 518055 People's Republic of China Center of Quantum Materials and Devices Chongqing University Chongqing 400044 People's Republic of China College of Physics and Optoelectronic Technology Baoji University of Arts and Sciences Baoji 721016 People's Republic of China

The valley-polarized quantum anomalous Hall (VQAH) state, which forwards a strategy for combining valleytronics and spintronics with nontrivial topology, attracts intensive interest in condensed-matter physics. So far, the exploration of VQAH states has still been limited to magnetic systems. Here, using the low-energy effective model and Floquet theorem, we propose an alternative mechanism to realize the Floquet VQAH state in nonmagnetic heterobilayers under light irradiation. We then realize this proposal via first-principles calculations in transition metal dichalcogenide heterobilayers, which initially possess the time-reversal invariant valley quantum spin Hall (VQSH) state. By irradiating circularly polarized light, the time-reversal invariant VQSH state can evolve into the VQAH state, behaving as an optically switchable topological spin-valley filter. These findings not only offer a rational scheme to realize the VQAH state in periodically driven nonmagnetic systems, but also pave a fascinating path for designing topological spintronic and valleytronic devices with high tunability. Our proposal can also be extended to design the general QAH effect without magnetic orders.

关键词： Chern insulators Edge states Photoinduced effect Spin filtering Topological materials Topological phase transition Topological phases of matter Valleytronics Transition metal dichalcogenides Bloch-Floquet theorem Density functional theory First-principles calculations Tight-binding model Wannier function methods k dot p method

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Symmetry-protected Dirac nodal lines and large spin Hall effect in V6Sb4 with kagome bilayer

arXiv

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arXiv 2022年

作者： Yang, Ye Wang, R. Shi, M.-Z. Wang, Z. Xiang, Z. Chen, X.-H. CAS Key Laboratory of Strongly-coupled Quantum Matter Physics Department of Physics University of Science and Technology of China Anhui Hefei230026 China Institute for Structure and Function Department of physics Chongqing University Chongqing400044 China Center of Quantum Materials and Devices Chongqing University Chongqing400044 China Chongqing Key Laboratory for Strongly Coupled Physics Chongqing University Chongqing400044 China Shanghai 200050 China CAS Center for Excellence in Quantum Information and Quantum Physics Anhui Hefei230026 China Collaborative Innovation Center of Advanced Microstructures Nanjing University Nanjing210093 China

Recently, a family of nonmagnetic kagome metals AV3Sb5 (A=K, Rb, and Cs) has attracted significant attention for realizing the intertwining of quantum order and nontrivial topology. However, these compounds have been identified to host complex band structures. Therefore, it is desirable to design and synthesize novel kagome materials with a simple band topology and good transport properties. In this study, using first-principles calculations, we present the electronic properties and the intrinsic spin Hall effect of V6Sb4, the latest experimentally synthesized vanadium-based compounds with a kagome bilayer. In the absence of spin-orbital coupling (SOC), this compound is a Dirac nodal line semimetal with symmetry-protected nodal rings near the Fermi level. Within the SOC, the spin-rotation symmetry breaks the gaps of the nodal rings with a small band gap. Furthermore, based on the Wannier tight-binding approach and the Kubo formula, we propose a large spin Hall effect in V6Sb4, which intrinsically originates from the spin Berry curvature. Our work further expands nonmagnetic kagome compounds for applications in spintronics accompanied by exotic quantum order. Copyright © 2022, The Authors. All rights reserved.

关键词： Energy gap

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Spin Signature of Majorana Fermions in Topological Nodal-Point Superconductors

arXiv

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arXiv 2024年

作者： Zeng, Junjie He, James Jun Ning, Zhen Xu, Dong-Hui Wang, Rui Institute for Structure and Function Department of Physics Chongqing Key Laboratory for Strongly Coupled Physics Chongqing University Chongqing400044 China Hefei National Research Center for Interdisciplinary Sciences at the Microscale University of Science and Technology of China Anhui Hefei230026 China Hefei National Laboratory Anhui Hefei230088 China Center of Quantum materials and devices Chongqing University Chongqing400044 China

In two-dimensional topological nodal superconductors, Majorana edge states have been conventionally believed to exhibit only spin-triplet pairing correlations. However, we reveal a substantial spin-singlet pairing component in Majorana edge states of antiferromagnetic topological nodal-point superconductors. This unexpected phenomenon emerges from the interplay between antiferromagnetic order and symmetry, resulting in Majorana edge states with a nearly flat band dispersion, deviating from the strictly flat band. Crucially, this phenomenon is detectable through spin-selective Andreev reflection, where the zero-bias conductance peaks are maximized when the spin of incident electrons is nearly antiparallel to that of Majorana edge excitations. This discovery unveils a unique spin signature for Andreev reflection resonances, advancing our fundamental understanding of spin-dependent mechanisms in topological superconductivity and representing a significant step towards the experimental detection of Majorana fermions. © 2024, CC BY-NC-ND.

关键词： Superconducting materials

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Hadron-quark phase transition in neutron star by combining the relativistic Brueckner-Hartree-Fock theory and Dyson-Schwinger equation approach

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Physical Review D 2023年第10期107卷 103009-103009页

作者： Pianpian Qin Zhan Bai Sibo Wang Chencan Wang Si-xue Qin Department of Physics and Chongqing Key Laboratory for Strongly Coupled Physics Chongqing University Chongqing 401331 China Institute of Theoretical Physics Chinese Academy of Sciences Beijing 100190 China School of Physics and Astronomy Sun Yat-Sen University Zhuhai 519082 China

Starting from the relativistic Brueckner-Hartree-Fock theory for nuclear matter and the Dyson-Schwinger equation approach for quark matter, the possible hadron-quark phase transition in the interior of a neutron star is explored. The first-order phase transition and crossover are studied by performing the Maxwell construction and three-window construction respectively. The mass-radius relation and the tidal deformability of the hybrid star are calculated and compared to the joint mass-radius observation of a neutron star and the constraints from gravitational wave detection. For the Maxwell construction, no stable quark core is found in the interior of a neutron star. For the three-window construction, the parameters of the smooth interpolation function are chosen in such a way to keep the thermodynamic stability and lead to a moderate crossover density region. To support a two-solar-mass neutron star under the three-window construction, the effective width of medium screening effects in quark matter should be around 0.35 GeV.

关键词： Nuclear matter in neutron stars QCD phase transitions

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Magnetic field induced valley-polarized quantum anomalous Hall effects in ferromagnetic van der Waals heterostructures

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Physical Review B 2022年第3期105卷 035131-035131页

作者： Fangyang Zhan Baobing Zheng Xiaoliang Xiao Jing Fan Xiaozhi Wu Rui Wang Institute for Structure and Function & 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 College of Physics and Optoelectronic Technology Baoji University of Arts and Sciences Baoji 721016 People's Republic of China Center for Computational Science and Engineering Southern University of Science and Technology Shenzhen 518055 People's Republic of China Center of Quantum Materials and Devices Chongqing University Chongqing 400044 People's Republic of China

The valley-polarized quantum anomalous Hall effect (VQAHE) attracts intensive interest since it uniquely combines valleytronics and spintronics with nontrivial band topology. Here, based on first-principles calculations and the Wannier-function-based tight-binding model, we reveal that valley-based Hall effects and especially the VQAHE induced by external magnetic fields can occur in two-dimensional (2D) ferromagnetic van der Waals heterostructures (vdWHs). The results show that valley splitting derived from the Zeeman exchange energy drives these vdWHs generating the valley anomalous Hall effect and then the VQAHE. The chiral edge states and quantized Hall conductance are utilized to confirm the presence of VQAHE. Besides, it is also found that applying external electric fields (or tuning interlayer distances) can facilitate the realization of VQAHE, and thus we present a phase diagram in a broad parameter regime of magnetic fields and electric fields (or interlayer distances). Our work not only offers a class of ferromagnetic vdWHs to realize various valley-based Hall phases, but also can guide advancements for designing topological devices with spin-valley filtering effects based on the VQAHE.

关键词： Chern insulators Electronic structure Magnetic order Spin accumulation Spin polarization Topological materials Topological phase transition Valleytronics Transition metal dichalcogenides First-principles calculations Wannier function methods

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Photoinduced quantum anomalous Hall states in the topological Anderson insulator

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Physical Review B 2022年第3期105卷 035103-035103页

作者： Zhen Ning Baobing Zheng Dong-Hui Xu Rui Wang Institute for Structure and Function & 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 College of Physics and Optoelectronic Technology & Advanced Titanium Alloys and Functional Coatings Cooperative Innovation Center Baoji University of Arts and Sciences Baoji 721016 People's Republic of China Department of Physics Hubei University Wuhan 430062 People's Republic of China Center for Quantum Materials and Devices Chongqing University Chongqing 400044 People's Republic of China

The realization of the quantum anomalous Hall (QAH) effect without magnetic doping attracts intensive interest since magnetically doped topological insulators usually possess inhomogeneity of ferromagnetic order. Here, we propose a different strategy to realize intriguing QAH states arising from the interplay of light and nonmagnetic disorder in two-dimensional topologically trivial systems. By combining the Born approximation and Floquet theory, we show that a time-reversal-invariant disorder-induced topological insulator, known as the topological Anderson insulator (TAI), would evolve into a time-reversal-broken TAI and then into a QAH insulator by shining circularly polarized light. We utilize spin and charge Hall conductivities, which can be measured in experiments directly, to distinguish these three different topological phases. This work not only offers an exciting opportunity to realize the high-temperature QAH effect without magnetic orders, but also is important for applications of topological states to spintronics.

关键词： Topological phases of matter Two-dimensional electron system Bloch-Floquet theorem Green's function methods k dot p method

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