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Instruments / Technical Area	Publication Details	Published
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	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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