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Topological states and competing magnetic fluctuations in iron germanides

Physical Review B 2023年第11期107卷 115116-115116页

作者： Huican Mao Guangwei Wang Xiaobo Ma Rui Liu Pengyu Zheng Zhiping Yin Department of Physics Center for Advanced Quantum Studies Beijing Normal University Beijing 100875 China

The correlated multiorbital multiband electronic structures of iron-based superconductors harbor an abundance of exotic electronic states, such as spin density waves, superconductivity, topological surface states, and Majorana zero modes. In this paper, we carry out density functional theory combined with dynamical mean-field theory calculations of the electronic structures, spin dynamics, and topological properties in YFe2Ge2 and MgFeGe and compare them with BaFe2As2 and LiFeAs. We find a coexistence of ferromagnetic and antiferromagnetic spin fluctuations in YFe2Ge2 and MgFeGe, which are attributed to the weakly dispersive bands just below the Fermi level. Moreover, we demonstrate that MgFeGe is in a strong topological phase and has two sets of Dirac-cone-like topological surface states on the (001) surface. The coexistence of superconductivity and nontrivial band topology in these two compounds demonstrates that it is interesting to explore topological superconductivity and Majorana zero modes in iron germanides.

关键词： Electronic structure Spin dynamics Surface states Iron-based superconductors Strongly correlated systems Topological materials Topological superconductors Density functional theory Dynamical mean field theory wannier function methods

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Magnetic skyrmion crystal at a topological insulator surface

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Physical Review B 2022年第3期105卷 035156-035156页

作者： Stefan Divic Henry Ling T. Pereg-Barnea Arun Paramekanti Department of Physics University of California Berkeley California 94720 USA 208-5800 Cooney Road Richmond British Columbia V6X3A8 Canada Department of Physics and the Centre for the Physics of Materials McGill University Montréal Québec H3A 2T8 Canada Department of Physics University of Toronto Toronto Ontario M5S 1A7 Canada

We consider a magnetic skyrmion crystal formed at the surface of a topological insulator. Incorporating the exchange interaction between the helical Dirac surface states and the periodic Néel or Bloch skyrmion texture, we obtain the resulting electronic band structure and discuss the constraints that symmetries impose on the energies and Berry curvature. We find substantive qualitative differences between the Néel and Bloch cases, with the latter generically permitting a multiband low energy tight-binding representation whose parameters are tightly constrained by symmetries. We explicitly compute the associated wannier orbitals, which resemble the ringlike chiral bound states of helical Dirac fermions coupled to a single skyrmion in a ferromagnetic background. We construct a two-band tight-binding model with real nearest-neighbor hoppings which captures the salient topological features of the low-energy bands. Our results are relevant to magnetic topological insulators (TIs), as well as to TI-magnetic thin film heterostructures, in which skyrmion crystals may be stabilized.

关键词： Chern insulators Dirac fermions Skyrmions Spin-orbit coupling Topological insulators Bloch wave theory Tight-binding model wannier function methods

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Spontaneous symmetry lowering of Si (001) towards two-dimensional ferro/antiferroelectric behavior

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Physical Review Materials 2019年第4期3卷 044410-044410页

作者： Chengcheng Xiao Xinwei Wang Xiaodong Pi Shengyuan A. Yang Yuanping Feng Yunhao Lu Shengbai Zhang State Key Laboratory of Silicon Materials School of Materials Science and Engineering Zhejiang University Hangzhou 310027 China Research Laboratory for Quantum Materials Singapore University of Technology and Design Singapore 487372 Singapore Department of Physics National University of Singapore Singapore 117542 Singapore Department of Physics Zhejiang University Hangzhou 310027 China Department of Physics Applied Physics and Astronomy Rensselaer Polytechnic Institute Troy New York 12180 USA

Electronic polarization, manifested in numerous physical phenomena such as ferroelectricity, piezoelectricity, and pyroelectricity, is of great importance to many technological applications. The presence of ferroelectricity in bulk elemental materials is extremely rare due to the high symmetry of the underlying lattices. However, the presence of a surface lowers the symmetry, which may provide a future avenue for achieving robust ferroelectric polarization. Using first-principles calculations, we show that a spontaneous electrical polarization can be realized on the silicon reconstructed (001) surface. The magnitude of this polarization is 0.45×10−10C/m, comparable to those in benchmarking two-dimensional ferroelectric compounds such as SnTe and SnSe. The symmetry lowering is originated from a Peierls instability that removes the degeneracy of surface dangling bonds, whereby opening a band gap. Although flip-flop of the surface dimers may destroy the ordered phase at elevated temperatures, we find that a modest compressive strain, which increases the local stress and interdimer coupling, can drastically increase the robustness of the ferroelectric state towards room-temperature operations. The technology for processing Si has been well developed and the Si (001) surface has also been widely used in various electronic devices. Our discovery here thus holds high promise in a multitude of fields, ranging from nanoscale random access memory to low power consumption energy storage devices.

关键词： Antiferroelectrics Structural phase transition Elemental semiconductors Surfaces Density functional theory Metropolis algorithm Nonequilibrium Green's function wannier function methods

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Fe(001) angle-resolved photoemission and intrinsic anomalous Hall conductivity in Fe seen by different ab initio approaches: LDA and GGA versus GW

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Physical Review B 2022年第11期105卷 115135-115135页

作者： E. Młyńczak I. Aguilera P. Gospodarič T. Heider M. Jugovac G. Zamborlini J.-P. Hanke C. Friedrich Y. Mokrousov C. Tusche S. Suga V. Feyer S. Blügel L. Plucinski C. M. Schneider Peter Grünberg Institut PGI Forschungszentrum Jülich and JARA- Fundamentals of Future Information Technologies 52425 Jülich Germany Jerzy Haber Institute of Catalysis and Surface Chemistry Polish Academy of Sciences Niezapominajek 8 30-239 Krakow Poland Institute of Physics Johannes Gutenberg University Mainz 55099 Mainz Germany Fakultät für Physik Universität Duisburg-Essen 47057 Duisburg Germany SANKEN Osaka University Ibaraki Osaka 567-0047 Japan

Many material properties such as the electronic transport characteristics depend on the details of the electronic band structure in the vicinity of the Fermi level. For an accurate ab initio description of the material properties, the electronic band structure must be known and theoretically reproduced with high fidelity. Here, we ask a question which of the ab initio methods compare the best to the experimental photoemission intensities from bcc Fe. We confront the photoemission data from Fe(001) thin film grown on Au(001) to the photoemission simulations based on different ab initio initial band structures: density functional theory (DFT) in the local density approximation (LDA) and the generalized gradient approximation (GGA) and GGA corrected with many-body perturbation theory in the GW approximation. We find the best comparison for the GW results. As a second step, we discuss how the calculated intrinsic anomalous Hall conductivity (AHC) in bcc Fe depends on the choice of the method that describes the electronic band structure and Fermi level position. We find very large differences in AHC between the three theoretical approaches and show that the AHC found for the experimental Fermi level location within the GW band structure is the closest to the literature results of transport experiments. This finding improves our understanding of not only the anomalous Hall effect itself, but also other related phenomena, such as the anomalous Nernst effect.

关键词： Anomalous Hall effect Electronic structure Elemental metals Ferromagnets Angle-resolved photoemission spectroscopy Free-electron model GGA GW method LDA wannier function methods

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Spin Weyl topological insulators

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Physical Review B 2024年第4期109卷 045126-045126页

作者： Rafael González-Hernández Bernardo Uribe Departamento de Física y Geociencias Universidad Del Norte Km. 5 Vía Antigua Puerto Colombia Barranquilla 080020 Colombia Departamento de Matemáticas y Estadística Universidad Del Norte Km. 5 Vía Antigua Puerto Colombia Barranquilla 080020 Colombia

The quantum nature of electron spin is crucial for establishing topological invariants in real materials. Since the spin does not in general commute with the Hamiltonian, some of the topological features of the material can be extracted from its study. In insulating materials, the spin operator induces a projected operator on valence states called the spin valence operator. Its spectrum contains information with regard to the different phases of the spin Chern class. If the spin valence spectrum is gapped, the spin Chern numbers are constant along parallel planes thus defining spin Chern insulating materials. If the spin valence spectrum is not gapped, the changes in the spin Chern numbers occur whenever this spectrum is zero. Materials whose spin valence spectrum is gapless will be denoted spin Weyl topological insulators and their definition together with some of their properties will be presented in this work. The classification of materials from the properties of the spin valence operator provides a characterization which complements the existing list of topological invariants.

关键词： Quantum spin Hall effect Topological insulators Topological order Topological materials Spin Density functional theory Topology wannier function methods

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Ab initio study of quantum oscillations in altermagnetic and nonmagnetic phases of RuO2

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Physical Review B 2024年第14期110卷 144410页

作者： Yingliang Huang Junwen Lai Jie Zhan Tianye Yu Rong Chen Peitao Liu Xing-Qiu Chen Yan Sun School of Materials Science and Engineering Shenyang National Laboratory for Materials Science

The altermagnet (AM) is a magnetic state with collinear antiferromagnetic ground state but it presents some transport properties that were only believed to exist in ferromagnets or noncollinear antiferromagnets. To have a comprehensive understanding of the transport properties of AMs, especially from the experimental point of view, a promising altermagnetic metal is crucial. In all the proposed altermagnetic metals, RuO2 has a special position, since it is the AM with the largest spin splitting and several important altermagnetism featured experiments were performed based on it. However, a very recent report based on sensitive muon-spin measurements suggests a supersmall local magnetization from Ru, i.e., a nonmagnetic ground state in RuO2. Therefore, a determination of the existence of the altermagnetic ground state is the basic starting point for all the previously altermagnetic transport properties in RuO2. In this work, we propose to identify its magnetic ground state from the Fermi surface (FS) via the electronic transport property of quantum oscillation (QO). We systematically analyzed the FSs of RuO2 in both nonmagnetic and altermagnetic states via first principles calculations. Our work should be helpful for future experiments on QO measurements to confirm its ground state by the interplay between transport measurements and computations.

关键词： Altermagnetism Fermi surface Shubnikov-de Haas effect Symmetry protected topological states Topological materials Density functional theory wannier function methods

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Two-dimensional Thouless pumping in time-space crystalline structures

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Physical Review B 2022年第14期106卷 144301-144301页

作者： Y. Braver C.-h. Fan G. Žlabys E. Anisimovas K. Sacha Institute of Theoretical Physics and Astronomy Vilnius University Saulėtekio 3 LT-10257 Vilnius Lithuania Instytut Fizyki Teoretycznej Uniwersytet Jagielloński ulica Profesora Stanisława Łojasiewicza 11 PL-30-348 Kraków Poland School of Physics and Optoelectronics Xiangtan University Hunan 411105 China Quantum Systems Unit Okinawa Institute of Science and Technology Graduate University Onna Okinawa 904-0495 Japan

Dynamics of a particle in a resonantly driven quantum well can be interpreted as that of a particle in a crystal-like structure, with the time playing the role of the coordinate. By introducing an adiabatically varied phase in the driving protocol, we demonstrate a realization of the Thouless pumping in such a time crystalline structure. Next, we extend the analysis beyond a single quantum well by considering a driven one-dimensional optical lattice, thereby engineering a 2D time-space crystalline structure. Such a setup allows us to explore adiabatic pumping in the spatial and the temporal dimensions separately, as well as to simulate simultaneous time-space pumping.

关键词： Cold gases in optical lattices Crystal phenomena Bose-Einstein condensates Ultracold gases wannier function methods

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Classification of crystalline insulators without symmetry indicators: Atomic and fragile topological phases in twofold rotation symmetric systems

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Physical Review B 2019年第11期100卷 115160-115160页

作者： Sander H. Kooi Guido van Miert Carmine Ortix Institute for Theoretical Physics Center for Extreme Matter and Emergent Phenomena Utrecht University Princetonplein 5 3584 CC Utrecht The Netherlands Dipartimento di Fisica “E. R. Caianiello” Università di Salerno I-84084 Fisciano (Salerno) Italy

Topological crystalline phases in electronic structures can be generally classified using the spatial symmetry characters of the valence bands and mapping them onto appropriate symmetry indicators. These mappings have been recently applied to identify thousands of topological electronic materials. There can exist, however, topological crystalline nontrivial phases that go beyond this paradigm: They cannot be identified using spatial symmetry labels and consequently lack any classification. In this work, we achieve the first of such classifications showcasing the paradigmatic example of two-dimensional crystals with twofold rotation symmetry. We classify the gapped phases in time-reversal invariant systems with strong spin-orbit coupling identifying a set of three Z2 topological invariants, which correspond to nested quantized partial Berry phases. By further isolating the set of atomic insulators representable in terms of exponentially localized symmetric wannier functions, we infer the existence of topological crystalline phases of the fragile type that would be diagnosed as topologically trivial using symmetry indicators and construct a number of microscopic models exhibiting this phase. Our work is expected to have important consequences given the central role fragile topological phases are expected to play in novel two-dimensional materials such as twisted bilayer graphene.

关键词： Geometric & topological phases Topological phases of matter 2-dimensional systems Bloch wave theory Tight-binding model wannier function methods

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Understanding the Activity of Single-Atom Catalysis from Frontier Orbitals

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Physical Review Letters 2020年第15期125卷 156001-156001页

作者： Zhaoming Fu Bowen Yang Ruqian Wu School of Physics Henan Normal University Xinxiang Henan 453007 China Department of Physics and Astronomy University of California Irvine California 92697-4575 USA

The d-band center and charge states are often used to analyze the catalytic activity of noble or transition metal surfaces and clusters, but their applicability for single-atom catalysts (SACs) is unsure. This work suggests that the spatial structure and orientation of frontier orbitals which are closest to the Fermi level of SACs play a vital role. Taking adsorption of several molecules and CO oxidization on C3N-supported single-atom Au as examples, we demonstrate that adsorption and catalytic activities are well correlated with the characteristics of frontier orbitals. This work provides an effective guidance for understanding the performance of single-atom catalysts.

关键词： Adsorption Chemical kinetics, dynamics & catalysis Surfaces Electronic structure First-principles calculations wannier function methods

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Prediction of coexistence of anomalous valley Hall and quantum anomalous Hall effects in breathing kagome-honeycomb lattices

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Physical Review B 2021年第24期104卷 245414-245414页

作者： Lei Liu Bao Zhao Jiayong Zhang Hairui Bao Hao Huan Yang Xue Yue Li Zhongqin Yang State Key Laboratory of Surface Physics and Key Laboratory of Computational Physical Sciences (MOE) & Department of Physics Fudan University Shanghai 200433 China School of Physics Science and Information Technology Shandong Key Laboratory of Optical Communication Science and Technology Liaocheng University Liaocheng 252059 China Jiangsu Key Laboratory of Micro and Nano Heat Fluid Flow Technology and Energy Application School of Physical Science and Technology Suzhou University of Science and Technology Suzhou 215009 China School of Science East China University of Science and Technology Shanghai 200237 China Shanghai Qi Zhi Institute Shanghai 200030 China

To explore the behaviors of valley degrees of freedom, we built a breathing kagome-honeycomb (BKH) lattice composed of a breathing kagome sublattice (BKL) and a breathing honeycomb sublattice (BHL). Many metal-organic frameworks (MOFs) without space-inversion symmetry (SIS) can be modeled as BKH lattices. The electronic states and the evolution of Dirac cones of the constructed BKH lattice under magnetic exchange and spin-orbit coupling interactions were investigated based on a tight-binding model and first-principles calculations. Coexistence states of anomalous valley Hall (AVH) and quantum anomalous Hall (QAH) effects, namely, the multiple Hall effects, were achieved in the BKH lattice with pz orbitals located at the BKL and px, py (and also dxy, dx2−y2) orbitals located at the BHL. The properties of the coexisting state, including the valley polarized status (at K or K′) and the chirality of the QAH effect, were determined by the overlapping styles of the bands from the two sublattices. An MOF of C6N3H3Au with a BKH lattice is proposed. Given the coexistence of the AVH and QAH effects in this built MOF, the metallic edge state protected by the topology of the MOF is 100% valley polarized, different from the ordinary QAH effect. Our results provide insights into the electronic states of the MOFs without SIS and a material platform with potential applications in not only valleytronics but also electronics and spintronics.

关键词： Topological phases of matter Valleytronics Honeycomb lattice Kagome lattice First-principles calculations Tight-binding model wannier function methods

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