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Kang CJ, Kim K, Min BI. Band theoretical approaches to topological physics in strongly-correlated f-electron Kondo systems. JOURNAL OF PHYSICS. CONDENSED MATTER : AN INSTITUTE OF PHYSICS JOURNAL 2022; 34:271501. [PMID: 35073534 DOI: 10.1088/1361-648x/ac4e47] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Subscribe] [Scholar Register] [Received: 10/07/2021] [Accepted: 01/24/2022] [Indexed: 06/14/2023]
Abstract
First-principles band structure theory on the basis of the density functional theory (DFT) plays an essential role in the investigation of topological properties of weakly-correlated systems. DFT band structures show clear bulk band crossings for Weyl and Dirac semimetals, and surface band crossings for topological insulators and topological-crystalline insulators. In contrast, for strongly-correlatedf-electron systems, their topological properties are relatively less explored because the simple DFT does not work properly in describing the electronic structures of strongly-correlatedfelectrons. In this perspective, we examine the band theoretical approaches to topological properties of strongly-correlatedf-electron Kondo systems. We recapitulate current status of understanding of electronic structures and topological properties of strongly-correlated 4f-electron systems, such as Ce, SmB6, and g-SmS, and also a 5f-electron system PuB4, the electronic structures of which were investigated by the DFT combined with the dynamical mean-field theory (DFT + DMFT). Finally, we provide future directions and perspectives of improving theoretical band approaches to search for new topologicalf-electron systems, as an outlook.
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Affiliation(s)
- Chang-Jong Kang
- Department of Physics, Chungnam National University, Daejeon 34134, Republic of Korea
| | - Kyoo Kim
- Korea Atomic Energy Research Institute (KAERI), 111 Daedeok-daero, Daejeon 34057, Republic of Korea
| | - B I Min
- Department of Physics, Pohang University of Science and Technology, Pohang 37673, Republic of Korea
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Ohkubo I, Aizawa T, Nakamura K, Mori T. Control of Competing Thermodynamics and Kinetics in Vapor Phase Thin-Film Growth of Nitrides and Borides. Front Chem 2021; 9:642388. [PMID: 34386477 PMCID: PMC8353106 DOI: 10.3389/fchem.2021.642388] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 12/16/2020] [Accepted: 06/22/2021] [Indexed: 11/13/2022] Open
Abstract
Thin-film growth is a platform technique that allows the preparation of various undeveloped materials and enables the development of novel energy generation devices. Preferred phase formation, control of crystalline orientation and quality, defect concentration, and stoichiometry in thin films are important for obtaining thin films exhibiting desired physical and chemical properties. In particular, the control of crystalline phase formation by utilizing thin-film technology favors the preparation of undeveloped materials. In this study, thin-film growth of transition metal nitride and rare-earth metal boride was performed using remote plasma-assisted molecular beam epitaxy and hybrid physical-chemical vapor deposition techniques, and was successfully achieved by tuning the competition between thermodynamics and kinetics during vapor-phase thin-film growth. Growth conditions of high crystalline quality titanium nitride thin films and high phase purity ytterbium boride thin films were not thermodynamically favorable. Appropriate control of the contribution degree of thermodynamics and kinetics during vapor-phase thin-film growth is crucial for fabricating high phase purity and high crystalline quality thin films.
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Affiliation(s)
- Isao Ohkubo
- International Center for Materials Nanoarchitectonics (WPI-MANA), National Institute for Materials Science (NIMS), Tsukuba, Japan
| | - Takashi Aizawa
- International Center for Materials Nanoarchitectonics (WPI-MANA), National Institute for Materials Science (NIMS), Tsukuba, Japan
| | - Katsumitsu Nakamura
- Department of Chemistry, College of Humanities and Sciences, Nihon University, Tokyo, Japan
| | - Takao Mori
- International Center for Materials Nanoarchitectonics (WPI-MANA), National Institute for Materials Science (NIMS), Tsukuba, Japan
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Anisimov MA, Samarin NA, Zhurkin VS, Bogach AV, Demishev SV, Voronov VV, Shitsevalova NY, Levchenko AV, Filipov VB, Glushkov VV. Evolution of thermoelectric properties in Eu xYb 1-xB 6family. JOURNAL OF PHYSICS. CONDENSED MATTER : AN INSTITUTE OF PHYSICS JOURNAL 2020; 32:465601. [PMID: 32698166 DOI: 10.1088/1361-648x/aba864] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Subscribe] [Scholar Register] [Received: 07/07/2020] [Accepted: 07/22/2020] [Indexed: 06/11/2023]
Abstract
Seebeck effect in the crystalline samples of EuxYb1-xB6(x= 0, 0.082, 0.127, 0.9, 1) was investigated at temperatures 2-300 K. For all the compounds thermopower is shown to be well described by the sum of diffusion (Sd=AT) and phonon drag components. The latter contribution is induced by quasilocal (Einstein) modes of ytterbium and europium ions with characteristic temperatures ΘE(YbB6) ≈ 91 K and ΘE(EuB6) ≈ 122 K. The estimation of effective massm*of the charge carriers proves that increasing of Eu content induces crossover from 'heavy' holes withmh*(x⩽ 0.127) ≈ 0.3-0.36m0to 'light' electrons withme*(x⩾ 0.9) ≈ 0.12-0.13m0(m0-free electron mass). For the Eu-rich compounds we propose the existence of additional point on the phase diagram, which corresponds to short-range magnetic order with enhanced spin fluctuations preceding the stabilization of magnetic polarons.
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Affiliation(s)
- M A Anisimov
- Prokhorov General Physics Institute of the Russian Academy of Sciences, Moscow 119991, Russia
| | - N A Samarin
- Prokhorov General Physics Institute of the Russian Academy of Sciences, Moscow 119991, Russia
| | - V S Zhurkin
- Moscow Institute of Physics and Technology, Dolgoprudny 141700, Russia
| | - A V Bogach
- Prokhorov General Physics Institute of the Russian Academy of Sciences, Moscow 119991, Russia
- National University of Science and Technology "MISIS", Moscow 119049, Russia
| | - S V Demishev
- Prokhorov General Physics Institute of the Russian Academy of Sciences, Moscow 119991, Russia
- Moscow Institute of Physics and Technology, Dolgoprudny 141700, Russia
| | - V V Voronov
- Prokhorov General Physics Institute of the Russian Academy of Sciences, Moscow 119991, Russia
| | - N Yu Shitsevalova
- Frantsevich Institute for Problems of Materials Science, National Academy of Sciences of Ukraine, Kiev 03680, Ukraine
| | - A V Levchenko
- Frantsevich Institute for Problems of Materials Science, National Academy of Sciences of Ukraine, Kiev 03680, Ukraine
| | - V B Filipov
- Frantsevich Institute for Problems of Materials Science, National Academy of Sciences of Ukraine, Kiev 03680, Ukraine
| | - V V Glushkov
- Prokhorov General Physics Institute of the Russian Academy of Sciences, Moscow 119991, Russia
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Sheets D, Flynn V, Kim J, Upton M, Casa D, Gog T, Fisk Z, Dzero M, Rosa PFS, Mazzone DG, Jarrige I, Zhu JX, Hancock J. Exploring itinerant states in divalent hexaborides using rare-earth L edge resonant inelastic x-ray scattering. JOURNAL OF PHYSICS. CONDENSED MATTER : AN INSTITUTE OF PHYSICS JOURNAL 2020; 32:135601. [PMID: 31791029 DOI: 10.1088/1361-648x/ab5e0f] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 06/10/2023]
Abstract
We present a study of resonant inelastic x-ray scattering (RIXS) spectra collected at the rare-earth L edges of divalent hexaborides YbB6 and EuB6. In both systems, RIXS-active features are observed at two distinct resonances separated by [Formula: see text] eV in incident energy, with angle-dependence suggestive of distinct photon scattering processes. RIXS spectra collected at the divalent absorption peak resemble the unoccupied 5d density of states calculated using density functional theory. We discuss possible origins of this correspondence including a scenario which changes the 4f valence. In addition, anomalous resonant scattering is observed at higher incident energy, where no corresponding absorption feature is present. Our results demonstrate the potential for L-edge RIXS to assess the itinerant-state properties of f -electron materials.
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Affiliation(s)
- Donal Sheets
- Department of Physics, University of Connecticut, Storrs, CT 06269, United States of America. Institute of Material Science, University of Connecticut, Storrs, CT 06269, United States of America
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Abstract
SmB6 is predicted to be the first member of the intersection of topological insulators and Kondo insulators, strongly correlated materials in which the Fermi level lies in the gap of a many-body resonance that forms by hybridization between localized and itinerant states. While robust, surface-only conductivity at low temperature and the observation of surface states at the expected high symmetry points appear to confirm this prediction, we find both surface states at the (100) surface to be topologically trivial. We find the \documentclass[12pt]{minimal}
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\begin{document}$${\bar{\varGamma }}$$\end{document}Γ¯ state to appear Rashba split and explain the prominent \documentclass[12pt]{minimal}
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\begin{document}$$\bar X$$\end{document}X¯ state by a surface shift of the many-body resonance. We propose that the latter mechanism, which applies to several crystal terminations, can explain the unusual surface conductivity. While additional, as yet unobserved topological surface states cannot be excluded, our results show that a firm connection between the two material classes is still outstanding. Samarium hexahoride is argued to be a topological Kondo insulator, but this claim remains under debate. Here, Hlawenka et al. provide a topologically trivial explanation for the conducting states at the (100) surface of samarium hexaboride; an explanation based on Rashba splitting and a surface shift of the Kondo resonance.
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Gan LY, Wang R, Jin YJ, Ling DB, Zhao JZ, Xu WP, Liu JF, Xu H. Emergence of topological nodal loops in alkaline-earth hexaborides XB6(X = Ca, Sr, and Ba) under pressure. Phys Chem Chem Phys 2017; 19:8210-8215. [DOI: 10.1039/c6cp08421d] [Citation(s) in RCA: 18] [Impact Index Per Article: 2.6] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/21/2022]
Abstract
Based on first-principles calculations, we report that external pressure can induce a topological phase transition in alkaline-earth hexaborides, XB6(X = Ca, Sr, and Ba).
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Affiliation(s)
- L.-Y. Gan
- Department of Physics
- South University of Science and Technology of China
- Shenzhen 518055
- China
- Key Laboratory of Advanced Technology of Materials (Ministry of Education)
| | - R. Wang
- Department of Physics
- South University of Science and Technology of China
- Shenzhen 518055
- China
- Institute for Structure and Function & Department of Physics
| | - Y. J. Jin
- Department of Physics
- South University of Science and Technology of China
- Shenzhen 518055
- China
| | - D. B. Ling
- Department of Physics
- Anhui University
- Hefei 230601
- China
| | - J. Z. Zhao
- Department of Physics
- South University of Science and Technology of China
- Shenzhen 518055
- China
| | - W. P. Xu
- Department of Physics
- South University of Science and Technology of China
- Shenzhen 518055
- China
| | - J. F. Liu
- Department of Physics
- South University of Science and Technology of China
- Shenzhen 518055
- China
| | - H. Xu
- Department of Physics
- South University of Science and Technology of China
- Shenzhen 518055
- China
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Jiao L, Rößler S, Kim DJ, Tjeng LH, Fisk Z, Steglich F, Wirth S. Additional energy scale in SmB 6 at low-temperature. Nat Commun 2016; 7:13762. [PMID: 27941948 PMCID: PMC5159841 DOI: 10.1038/ncomms13762] [Citation(s) in RCA: 46] [Impact Index Per Article: 5.8] [Reference Citation Analysis] [Abstract] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 04/12/2016] [Accepted: 10/31/2016] [Indexed: 11/09/2022] Open
Abstract
Topological insulators give rise to exquisite electronic properties because of their spin-momentum locked Dirac-cone-like band structure. Recently, it has been suggested that the required opposite parities between valence and conduction band along with strong spin-orbit coupling can be realized in correlated materials. Particularly, SmB6 has been proposed as candidate material for a topological Kondo insulator. Here we observe, by utilizing scanning tunnelling microscopy and spectroscopy down to 0.35 K, several states within the hybridization gap of about ±20 meV on well characterized (001) surfaces of SmB6. The spectroscopic response to impurities and magnetic fields allows to distinguish between dominating bulk and surface contributions to these states. The surface contributions develop particularly strongly below about 7 K, which can be understood in terms of a suppressed Kondo effect at the surface. Our high-resolution data provide insight into the electronic structure of SmB6, which reconciles many current discrepancies on this compound.
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Affiliation(s)
- L. Jiao
- Max-Planck-Institute for Chemical Physics of Solids, Nöthnitzer Str. 40, 01187 Dresden, Germany
| | - S. Rößler
- Max-Planck-Institute for Chemical Physics of Solids, Nöthnitzer Str. 40, 01187 Dresden, Germany
| | - D. J. Kim
- Department of Physics and Astronomy, University of California, Irvine, CA 92697, USA
| | - L. H. Tjeng
- Max-Planck-Institute for Chemical Physics of Solids, Nöthnitzer Str. 40, 01187 Dresden, Germany
| | - Z. Fisk
- Department of Physics and Astronomy, University of California, Irvine, CA 92697, USA
| | - F. Steglich
- Max-Planck-Institute for Chemical Physics of Solids, Nöthnitzer Str. 40, 01187 Dresden, Germany
- Center for Correlated Matter, Zhejiang University, Hangzhou 310058, China
- Institute of Physics, Chinese Academy of Sciences, Beijing 100190, China
| | - S. Wirth
- Max-Planck-Institute for Chemical Physics of Solids, Nöthnitzer Str. 40, 01187 Dresden, Germany
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Sun L, Wu Q. Pressure-induced exotic states in rare earth hexaborides. REPORTS ON PROGRESS IN PHYSICS. PHYSICAL SOCIETY (GREAT BRITAIN) 2016; 79:084503. [PMID: 27376406 DOI: 10.1088/0034-4885/79/8/084503] [Citation(s) in RCA: 5] [Impact Index Per Article: 0.6] [Reference Citation Analysis] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 06/06/2023]
Abstract
Finding the exotic phenomena in strongly correlated electron systems (SCESs) and understanding the corresponding microphysics have long been the research frontiers of condensed matter physics. The remarkable examples for the intriguing phenomena discovered in past years include unconventional superconductivity, heavy Fermion behaviors, giant magneto-resistance and so on. A fascinating type of rare earth hexaboride RB6 (R = Sm, Yb, Eu and Ce) belongs to a strongly correlated electron system (SCES), but shows unusual ambient-pressure and high-pressure behaviors beyond the phenomena mentioned above. Particularly, the recent discovery of the coexistence of an unusual metallic surface state and an insulating bulk state in SmB6, known to be a Kondo insulator decades ago, by theoretical calculations and many experimental measurements creates new interest for the investigation of the RB6. This significant progress encourages people to revisit the RB6 with an attempt to establish a new physics that links the SCES and the unusual metallic surface state which is a common feature of a topological insulator (TI). It is well known that pressure has the capability of tuning the electronic structure and modifying the ground state of solids, or even inducing a quantum phase transition which is one of the kernel issues in studies of SCESs. In this brief review, we will describe the progress in high pressure studies on the RB6 based on our knowledge and research interests, mainly focusing on the pressure-induced phenomena in YbB6 and SmB6, especially on the quantum phase transitions and their connections with the valence state of the rare earth ions. Moreover, some related high-pressure results obtained from CeB6 and EuB6 are also included. Finally, a summary is given in the conclusions and perspectives section.
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Affiliation(s)
- Liling Sun
- Institute of Physics and Beijing National Laboratory for Condensed Matter Physics, Chinese Academy of Sciences, Beijing 100190, People's Republic of China. Collaborative Innovation Center of Quantum Matter, Beijing 100190, People's Republic of China
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