Bulletin of the American Physical Society
APS March Meeting 2019
Volume 64, Number 2
Monday–Friday, March 4–8, 2019; Boston, Massachusetts
Session L03: Spectroscopies of Topological SystemsFocus

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Sponsoring Units: DCMP Chair: Erica Kotta, New York University Room: BCEC 107B 
Wednesday, March 6, 2019 11:15AM  11:27AM 
L03.00001: Topological Fermiarcs and Weyl node connectivity in the ferromagnetic Weyl semimetal Co_{3}Sn_{2}S_{2} Noam Morali, Rajib Batabyal, Pranab Kumar Nag, Enke Liu, Qiunan Xu, Yan Sun, Binghai Yan, Claudia Felser, Nurit Avraham, Haim Beidenkopf Topological “Fermiarc” surface states are guaranteed to exist on certain surfaces of Weyl semimetals. They connect the surface projection of the bulk Weyl nodes. However, in the presence of multiple pairs of surface projected Weyl nodes the actual connectivity among them is ambiguous. We use scanning tunneling spectroscopy to verify the classification of ferromagnetic Co_{3}Sn_{2}S_{2} as a Weyl semimetal as well as its time reversal broken origin. By studying three different surface terminations we show that Fermi arc connectivity and Fermiarc contour in Co_{3}Sn_{2}S_{2 }changes with the specific surface potential. While on the Sn surface we find intraBrillouin zone connectivity, on the Co surface the Fermi arcs connect Weyl nodes across the Brillouin zone edge. On the S termination the Fermi–arcs hybridise with nontopological bulk and surface states thus obscuring their connectivity. 
Wednesday, March 6, 2019 11:27AM  11:39AM 
L03.00002: A bidirectional photovoltaic shift on the surface of topological insulators Tomoki Yoshikawa, Kazuki Sumida, Yukiaki Ishida, Jiahua Chen, Munisa Nurmamat, Konstantin A. Kokh, Oleg E. Tereshchenko, Shik Shin, Akio Kimura Topological insulators (TIs) possess spinpolarized Dirac states on the surface as a result of the nontrivial topology of the bulk band structure. The surface of TIs is considered as a promising spinelectronic application. Recently, it was suggested that a generation of spinpolarized photocurrent would be possible with bulkinsulating TIs by utilizing surface photovoltage (SPV) effect. To date, SPV effect was observed for the bulkinsulating TI, Bi_{2}Te_{2}Se. However, due to the limited controllability of bulk carrier, only a unidirectional shift of the surface potential has been demonstrated. A bidirectional SPV is important for taking control of the ambipolar surface current by light. 
Wednesday, March 6, 2019 11:39AM  11:51AM 
L03.00003: Spectromicroscopic Analysis of Surface and Bulk Band Structure at a Disordered Topological Insulator Surface Erica Kotta, Lin Miao, Yishuai Xu, Aaron Bostwick, Chris Jozwiak, Eli Rotenberg, Takehito Suzuki, Joseph Checkelsky, Lewis A Wray The technique of angle resolved photoemission spectroscopy (ARPES) is widely applied to measure electronic band structures and has become central to our understanding of quantum material electronic structures over the last few decades. Recent improvements in coherent VUV light sources have driven rapid developments in the focused beam spot for ARPES, down to the d<10um diameter scale with little sacrifice, and the d~100nm scale with reduced flux. For this talk I will present ARPES spectromiscroscopy data which was used to map the surface band structure of a beamsensitive topological insulator sample (disorderenriched Bi2Se3). New data analysis methods are developed to obtain quantitative electronic structure information in spite of the flux sensitivity of the surface, revealing a complex structural environment in which fluctuations in the surface and bulk state energetics are found to be mostly decoupled. 
Wednesday, March 6, 2019 11:51AM  12:03PM 
L03.00004: Topological nodalline semimetals in ferromagnetic rareearthmetal monohalides SiMin Nie, Hongming Weng, Fritz Prinz Topological semimetals, extending the topological classification from insulators to metals, have greatly enriched our understanding of topological states in condensed matter. Here we identify layered materials as promising candidates for hosting TNLSs. Based on firstprinciples calculations and effective model analysis, we propose that layered ferromagnetic rareearthmetal monohalides LnX (Ln=La, Gd; X=Cl, Br) exhibit long pursued topological phases. Specifically, singlelayer LaX and singlelayer GdX are ideal twodimensional (2D) Weyl semimetals and largegap 2D quantum anomalous Hall insulators (QAHIs), with band gaps up to 61 meV, respectively. In addition, 3D LaX and 3D GdX are TNLSs with a pair of mirrorsymmetry protected nodal lines and 3D QAHIs, respectively. The nodal lines in 3D LaX extending through the whole Brillouin zone (BZ) are fairly robust against strong spinorbit coupling (SOC) and located close to the Fermi level, providing a novel platform toward exploring the exotic properties in nodalline fermions as well as related device designs. 
Wednesday, March 6, 2019 12:03PM  12:15PM 
L03.00005: Weak topological insulator state in quasionedimensional bismuth iodide observed by surfaceselective nanoARPES Ryo Noguchi, Takanari Takahashi, Kenta Kuroda, Masayuki Ochi, Tetsuro Shirasawa, Cédric Bareille, Masato Sakano, Mitsuhiro Nakayama, Matthew Watson, Koichiro Yaji, Ayumi Harasawa, Hideaki Iwasawa, Pavel Dudin, Timur Kim, Moritz Hoesch, Viktor Kandyba, Alessio Giampietri, Alexei Victorovich Barinov, Shik Shin, Ryotaro Arita, Takao Sasagawa, Takeshi Kondo Recently, various materials have been confirmed to be topologically nontrivial by angleresolved photoemission spectroscopy (ARPES) [1]. Despite intensive research on topological materials, a weak topological insulator (WTI) has so far remained hypothetical, since topological surface states (TSS) emerge only on the side surfaces, which are usually undetectable by conventional ARPES. We have overcome this difficulty by employing newly developed nanoARPES. Its high spatial resolution allows us to perform surface selective measurements, which is necessary for the discovery of a WTI state in real 3D crystals. 
Wednesday, March 6, 2019 12:15PM  12:27PM 
L03.00006: Large magnetic gap at the Dirac point in a Mninduced Bi_{2}Te_{3} heterostructure Oliver Rader, Emile Rienks, Partha M. Mandal, Ondrej Caha, Jan Ruzicka, Andreas Ney, Hubert Steiner, Valentyn V. Volobuev, Heiko H. Groiss, Saleem Khan, Jan Minár, Mihaela Albu, Hubert Ebert, Günther Bauer, Andrei Varykhalov, Jaime SánchezBarriga, Stefan Wimmer, Gunther Springholz A magnetic gap at the Dirac point of topological insulators has so far never been observed directly. Here, we use low temperature ARPES to reveal the magnetic gap of Mndoped Bi_{2}Te_{3} films which is present only below T_{C} of 1012 K. Surprisingly, the gap is 5 times larger than predicted by density functional theory. We show that this enhancement is due to a remarkable structure modification induced by Mn doping. Instead of a disordered impurity system, it forms an alternating sequence of septuple and quintuple layer blocks, with Mn in the center of the septuple layers. Mndoped Bi_{2}Se_{3} forms a similar heterostructure, however, only a large, albeit nonmagnetic gap is formed. We explain both differences based on the higher spinorbit interaction in Bi_{2}Te_{3}. The present findings provide crucial insights for pushing the lossless transport properties of topological insulators towards room temperature. 
Wednesday, March 6, 2019 12:27PM  12:39PM 
L03.00007: Directly photoexcited Dirac and Weyl fermions in ZrSiS and NbAs Jaehun Kim, Christopher P Weber, Leslie Schoop, Stuart S P Parkin, Robert Newby, Alexandr Nateprov, Bettina Lotsch, M. Bala Murali Krishna, Keshav M Dani, Hans Bechtel, Ernest Arushanov, Mazhar Ali We report ultrafast optical measurements of the Dirac linenode semimetal ZrSiS and the Weyl semimetal NbAs, using midinfrared pump photons from 86 meV to 500 meV to directly excite Dirac and Weyl fermions within the linearlydispersing bands. In NbAs the photoexcited Weyl fermions initially form a nonthermal distribution, signified by a brief spike in the differential reflectivity whose sign is controlled by the relative energy of the pump and probe photons. In ZrSiS electronelectron scattering rapidly thermalizes the electrons, and the spike is not observed. Subsequently hot carriers in both materials cool within a few picoseconds. This cooling, as seen in the two materials’ differential reflectivity, differs in sign, shape, and timescale. Nonetheless, we find that it may be described in a simple model of thermal electrons, without free parameters. The electronic cooling in ZrSiS is particularly fast, which may make the material useful for optoelectronic applications. 
Wednesday, March 6, 2019 12:39PM  12:51PM 
L03.00008: Symmetry protected band crossings of spinorbit coupled surface states of NbGeSb Igor Marković, Oliver J Clark, Federico Mazzola, Matthew Watson, Philip Murgatroyd, Matthew Dyer, Christopher Andrew Hooley, Jonathan Alaria, Philip King The electronic structure of materials in the ZrSiS family of nonsymmorphic semimetals hosts, alongside their bulk Dirac line nodes, peculiar surface states created by the breaking of nonsymmorphic symmetry at the surface layer [13]. We study the surface electronic structure of NbGeSb (isostructural to ZrSiS) using spin and angleresolved photoemission spectroscopy (ARPES) and densityfunctional theory (DFT). We observe two pairs of surface states, hosting large Rashbalike spinsplittings, split off from the bulk manifold. We find how these two branches intersect each other, forming a fourfold crossing structure along the Brillouin zone boundary. Surprisingly, we find a counterintuitive hybridisation structure around these crossings, with three remaining protected and just a single hybridised anticrossing. We demonstrate how a mirror symmetry line provides protection along the Brillouin zone face, opening new routes to realising complex intertwined spin textures of spinorbit coupled surface states. 
Wednesday, March 6, 2019 12:51PM  1:03PM 
L03.00009: NonHermitian HopfLink Exceptional Line Semimetals Yang Zhesen, Jiangping Hu We study a new class of nonHermitian topological phases in three dimension, where the topological robust band degeneracies are Hopflink exceptional lines. As a concrete example, we investigate the nonHermitian band structure of nodal line semimetals under nonHermitian perturbations, where the Fermi surfaces can transit from 1d nodal lines to 2d twisting surfaces with Hopflink boundaries. The topological invariants of these linked exceptional line phases are also proposed. Due to nonHermitian skin effect, We also find a huge difference between the band structure of periodic and open boundary conditions. After complexification of the momentum in the Bloch Hamiltonian, we find the correct bulkboundary correspondence in nonHermitian system. The possible experimental realizations are also discussed. 
(Author Not Attending)

L03.00010: Fragility of Fermi arcs in Dirac semimetals Yun Wu, Na Hyun Jo, LinLin Wang, Connor Schmidt, Kathryn Neilson, Benjamin Schrunk, Przemyslaw Swatek, Andrew Eaton, Sergey L. Bud'ko, Paul Canfield, Adam Kaminski We use tunable, vacuum ultraviolet laserbased angleresolved photoemission spectroscopy and density functional theory calculations to study the electronic properties of Dirac semimetal candidate PtBi_{2}. In addition to bulk electronic states we also find surface states as expected from theory and band calculations. The topological surface states form Fermi pockets rather than double Fermi arcs that were observed in Na_{3}Bi. The surface bands forming the Fermi pockets merge with bulk bands in proximity of the Dirac points projections, as expected. Our data confirms existence of Dirac states in PtBi_{2} and reveals the fragility of the Fermi arcs in Dirac semimetals. Because the Fermi arcs are not topologically protected in general, they can be deformed into Fermi pockets, as proposed by Kargarian et al.. Our results demonstrate validity of this theory in PtBi_{2} 
Wednesday, March 6, 2019 1:15PM  1:27PM 
L03.00011: WITHDRAWN ABSTRACT

Wednesday, March 6, 2019 1:27PM  1:39PM 
L03.00012: ARPES study on the electronic structure of VTe_{2} with double zigzag chains Mitsuishi Natsuki, Yusuke Sugita, Saeed Bahramy, Manabu Kamitani, Tatsuya Sonobe, Masato Sakano, Takahiro Shimojima, Koji Horiba, Hiroshi Kumigashira, Kazuaki Taguchi, Kouji Miyamoto, Taichi Okuda, Shintaro Ishiwata, Yukitoshi Motome, Kyoko Ishizaka Layered transition metal dichalcogenide (TMDC) has been a wellknown system to host a variety of charge density waves (CDW) reflecting its quasitwodimensionality. More recently, there has been increasing interest in spinorbit coupling effect and topological aspects of TMDC. VTe_{2} has a trigonal CdI_{2}type structure (socalled 1T phase) at high temperature, consisting of trigonal layers formed by edgesharing VTe_{6} octahedra. Below ~485 K, it undergoes a peculiar structural phase transition to the monoclinic 1T” phase. In this phase, VTe_{2} exhibits the 3×1×3 superstructure characterized by the formation of double zigzag chains of vanadium [1], which has been discussed in terms of CDW. In this talk, we report on the peculiar modification of the electronic structures via the 1T1T” phase transition clarified by utilizing angleresolved photoemission spectroscopy (ARPES) and firstprinciples calculations. 
Wednesday, March 6, 2019 1:39PM  1:51PM 
L03.00013: New class of Chiral topological nodes in transition metal silicide CoSi Zhicheng Rao Exotic chiral topological nodes with nonzero chern number and multiple degeneracy, distinct from conventional Dirac and Weyl nodes, could exist in crystalline semimetals with certain space group symmetries. The projections of these chiral topological nodes with opposite chern number are connected by topologically protected surface fermi arcs. Several other types of chiral topological nodes have been predicted by theorists, but the direct experiment evident is still lacked. Here, we will report that two types of new chiral nodes are revealed by using angelresolved photoemission spectroscopy in transition metal silicide CoSi. 
Wednesday, March 6, 2019 1:51PM  2:03PM 
L03.00014: Cyclotron antiresonance in the topological insulator Bi_{2}Te_{3} studied in Voigt geometry Sasa Dordevic, Hechang Lei, Cedomir Petrovic, Seongphill Moon, Dmitry Smirnov We have previously observed cyclotron antiresonance in a canonical 3D topological insulator Bi_{2}Te_{3} in Faraday geometry. We now 
Wednesday, March 6, 2019 2:03PM  2:15PM 
L03.00015: Observation of Quasionedimensional Edge State in ZrTe_{5} by Angleresolved Photoemission Spectroscopy Dong Qian, Fengfeng Zhu, Wenxiang Jiang Singlelayer ZrTe_{5} was known as a twodimensional topological insulator, while ZrTe_{5} bulk crystal consisting of many ZrTe_{5} layers has been argued as either a threedimensional strong topological insulator or a weak topological insulator. Using highresolution photonenergy dependent angleresolved photoemission spectroscopy, we succeeded in observing a quasionedimensional and 
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