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Instruments / Technical Area	Publication Details	Published
I05-ARPES	Evidence for a higher-order topological insulator in a three-dimensional material built from van der Waals stacking of bismuth-halide chains Ryo Noguchi , Masaru Kobayashi , Zhanzhi Jiang , Kenta Kuroda , Takanari Takahashi , Zifan Xu , Daehun Lee , Motoaki Hirayama , Masayuki Ochi , Tetsuroh Shirasawa , Peng Zhang , Chun Lin , Cédric Bareille , Shunsuke Sakuragi , Hiroaki Tanaka , So Kunisada , Kifu Kurokawa , Koichiro Yaji , Ayumi Harasawa , Viktor Kandyba , Alessio Giampietri , Alexei Barinov , Timur K. Kim , Cephise Cacho , Makoto Hashimoto , Donghui Lu , Shik Shin , Ryotaro Arita , Keji Lai , Takao Sasagawa , Takeshi Kondo Nature Materials 98 DOI: 10.1038/s41563-020-00871-7 Diamond Proposal Number(s): [20445] Show Abstract ▼ Abstract: Low-dimensional van der Waals materials have been extensively studied as a platform with which to generate quantum effects. Advancing this research, topological quantum materials with van der Waals structures are currently receiving a great deal of attention. Here, we use the concept of designing topological materials by the van der Waals stacking of quantum spin Hall insulators. Most interestingly, we find that a slight shift of inversion centre in the unit cell caused by a modification of stacking induces a transition from a trivial insulator to a higher-order topological insulator. Based on this, we present angle-resolved photoemission spectroscopy results showing that the real three-dimensional material Bi4Br4 is a higher-order topological insulator. Our demonstration that various topological states can be selected by stacking chains differently, combined with the advantages of van der Waals materials, offers a playground for engineering topologically non-trivial edge states towards future spintronics applications.	Jan 2021
I05-ARPES	Observation of small Fermi pockets protected by clean CuO2 sheets of a high-Tc superconductor So Kunisada , Shunsuke Isono , Yoshimitsu Kohama , Shiro Sakai , Cédric Bareille , Shunsuke Sakuragi , Ryo Noguchi , Kifu Kurokawa , Kenta Kuroda , Yukiaki Ishida , Shintaro Adachi , Ryotaro Sekine , Timur K. Kim , Cephise Cacho , Shik Shin , Takami Tohyama , Kazuyasu Tokiwa , Takeshi Kondo Science 369, 833-838 DOI: 10.1126/science.aay7311 Diamond Proposal Number(s): [20445, 20446] Show Abstract ▼ Abstract: In cuprate superconductors with high critical transition temperature (Tc), light hole-doping to the parent compound, which is an antiferromagnetic Mott insulator, has been predicted to lead to the formation of small Fermi pockets. These pockets, however, have not been observed. Here, we investigate the electronic structure of the five-layered Ba2Ca4Cu5O10(F,O)2, which has inner copper oxide (CuO2) planes with extremely low disorder, and find small Fermi pockets centered at (π/2, π/2) of the Brillouin zone by angle-resolved photoemission spectroscopy and quantum oscillation measurements. The d-wave superconducting gap opens along the pocket, revealing the coexistence between superconductivity and antiferromagnetic ordering in the same CuO2 sheet. These data further indicate that superconductivity can occur without contribution from the antinodal region around (π, 0), which is shared by other competing excitations.	Aug 2020
I05-ARPES	Three-dimensional electronic structure in ferromagnetic Fe3Sn2 with breathing kagome bilayers Hiroaki Tanaka , Yuita Fujisawa , Kenta Kuroda , Ryo Noguchi , Shunsuke Sakuragi , Cedric Bareille , Barnaby Smith , Cephise Cacho , Sung Won Jung , Takayuki Muro , Yoshinori Okada , Takeshi Kondo Physical Review B 101 DOI: 10.1103/PhysRevB.101.161114 Diamond Proposal Number(s): [24488] Show Abstract ▼ Abstract: A large anomalous Hall effect (AHE) has been observed in ferromagnetic Fe 3 Sn 2 with breathing kagome bilayers. To understand the underlying mechanism for this, we investigate the electronic structure of Fe 3 Sn 2 by angle-resolved photoemission spectroscopy (ARPES). In particular, we use both vacuum ultraviolet light (VUV) and soft x ray (SX), which allow surface-sensitive and relatively bulk-sensitive measurements, respectively, and distinguish bulk states from surface states, which should be unlikely related to the AHE. While VUV-ARPES observes two-dimensional bands mostly due to surface states, SX-ARPES reveals three-dimensional band dispersions with a periodicity of the rhombohedral unit cell in the bulk. Our data show a good consistency with a theoretical calculation based on density functional theory, suggesting a possibility that Fe 3 Sn 2 is a magnetic Weyl semimetal.	Apr 2020
I05-ARPES	A weak topological insulator state in quasi-one-dimensional bismuth iodide Ryo Noguchi , T. Takahashi , K. Kuroda , M. Ochi , T. Shirasawa , M. Sakano , C. Bareille , M. Nakayama , M. D. Watson , K. Yaji , A. Harasawa , H. Iwasawa , P. Dudin , T. K. Kim , M. Hoesch , V. Kandyba , A. Giampietri , A. Barinov , S. Shin , R. Arita , T. Sasagawa , Takeshi Kondo Nature 98 DOI: 10.1038/s41586-019-0927-7 Diamond Proposal Number(s): [15095, 16161] Show Abstract ▼ Abstract: The major breakthroughs in understanding of topological materials over the past decade were all triggered by the discovery of the Z2-type topological insulator—a type of material that is insulating in its interior but allows electron flow on its surface. In three dimensions, a topological insulator is classified as either ‘strong’ or ‘weak’ and experimental confirmations of the strong topological insulator rapidly followed theoretical predictions. By contrast, the weak topological insulator (WTI) has so far eluded experimental verification, because the topological surface states emerge only on particular side surfaces, which are typically undetectable in real three-dimensional crystals. Here we provide experimental evidence for the WTI state in a bismuth iodide, β-Bi4I4. Notably, the crystal has naturally cleavable top and side planes—stacked via van der Waals forces—which have long been desirable for the experimental realization of the WTI state. As a definitive signature of this state, we find a quasi-one-dimensional Dirac topological surface state at the side surface (the (100) plane), while the top surface (the (001) plane) is topologically dark with an absence of topological surface states. We also find that a crystal transition from the β-phase to the α-phase drives a topological phase transition from a nontrivial WTI to a normal insulator at roughly room temperature. The weak topological phase—viewed as quantum spin Hall insulators stacked three-dimensionally—will lay a foundation for technology that benefits from highly directional, dense spin currents that are protected against backscattering.	Feb 2019
I05-ARPES	Orbital-selective metal-insulator transition lifting the t 2 g band hybridization in the Hund metal Sr3(Ru 1−xMnx)2O7 M. Nakayama , T. Kondo , K. Kuroda , C. Bareille , M. D. Watson , S. Kunisada , R. Noguchi , T. K. Kim , M. Hoesch , Y. Yoshida , S. Shin Physical Review B 98 DOI: 10.1103/PhysRevB.98.161102 Diamond Proposal Number(s): [15095, 16161, 17816, 20445] Show Abstract ▼ Abstract: We reveal the orbital-selective metal-insulator transition in a 4d multiorbital system, Sr3(Ru1−xMnx)2O7(x=0−0.2), by means of angle-resolved photoemission spectroscopy. With a small substitution (x=0.05) of Mn with localized 3d orbitals, the spectra for the 4dxy band become incoherent with a large gap, while the 4dxz/yz bands stay metallic with sharp quasiparticle peaks. Intriguingly, this lifts band hybridization among 4d orbitals, and causes a two- to one-dimensional crossover in the remaining Fermi surface. Significant nesting in the remaining Fermi surface is implied to induce the reported magnetic ordering, and yields the insulating bulk state at higher Mn substitution, where all bands are gapped and totally incoherent. The availability of orbital-selective tuning of the pseudogap by a small impurity will prompt further theoretical studies of Hund's metals, which were recognized only recently in condensed matter physics.	Oct 2018
I05-ARPES	Experimental determination of the topological phase diagram in cerium monopnictides K. Kuroda , M. Ochi , H. s. Suzuki , M. Hirayama , M. Nakayama , R. Noguchi , C. Bareille , S. Akebi , S. Kunisada , T. Muro , M. D. Watson , H. Kitazawa , Y. Haga , T. K. Kim , M. Hoesch , S. Shin , R. Arita , T. Kondo Physical Review Letters 120 DOI: 10.1103/PhysRevLett.120.086402 Diamond Proposal Number(s): [16161] Show Abstract ▼ Abstract: Experimental determinations of bulk band topology in the solid states have been so far restricted to only indirect investigation through the probing of surface states predicted by electronic structure calculations. We here present an alternative approach to determine the band topology by means of bulk-sensitive soft x-ray angle-resolved photoemission spectroscopy. We investigate the bulk electronic structures of the series materials, Ce monopnictides (CeP, CeAs, CeSb, and CeBi). By performing a paradigmatic study of the band structures as a function of their spin-orbit coupling, we draw the topological phase diagram and unambiguously reveal the topological phase transition from a trivial to a nontrivial regime in going from CeP to CeBi induced by the band inversion. The underlying mechanism of the phase transition is elucidated in terms of spin-orbit coupling in concert with their semimetallic band structures. Our comprehensive observations provide a new insight into the band topology hidden in the bulk states.	Feb 2018
I05-ARPES	Fermi arc electronic structure and Chern numbers in the type-II Weyl semimetal candidate Mo x W 1 − x Te 2 Ilya Belopolski , Su-yang Xu , Yukiaki Ishida , Xingchen Pan , Peng Yu , Daniel S. Sanchez , Hao Zheng , Madhab Neupane , Nasser Alidoust , Guoqing Chang , Tay-rong Chang , Yun Wu , Guang Bian , Shin-ming Huang , Chi-cheng Lee , Daixiang Mou , Lunan Huang , You Song , Baigeng Wang , Guanghou Wang , Yao-wen Yeh , Nan Yao , Julien E. Rault , Patrick Le Fèvre , François Bertran , Horng-tay Jeng , Takeshi Kondo , Adam Kaminski , Hsin Lin , Zheng Liu , Fengqi Song , Shik Shin , M. Zahid Hasan Physical Review B 94 DOI: 10.1103/PhysRevB.94.085127 Diamond Proposal Number(s): [13653] Show Abstract ▼ Abstract: It has recently been proposed that electronic band structures in crystals can give rise to a previously overlooked type of Weyl fermion, which violates Lorentz invariance and, consequently, is forbidden in particle physics. It was further predicted that MoxW1−xTe2 may realize such a type-II Weyl fermion. Here, we first show theoretically that it is crucial to access the band structure above the Fermi level ɛF to show a Weyl semimetal in MoxW1−xTe2. Then, we study MoxW1−xTe2 by pump-probe ARPES and we directly access the band structure >0.2 eV above ɛF in experiment. By comparing our results with ab initio calculations, we conclude that we directly observe the surface state containing the topological Fermi arc. We propose that a future study of MoxW1−xTe2 by pump-probe ARPES may directly pinpoint the Fermi arc. Our work sets the stage for the experimental discovery of the first type-II Weyl semimetal in MoxW1−xTe2.	Aug 2016

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