Bulletin of the American Physical Society
APS March Meeting 2020
Volume 65, Number 1
Monday–Friday, March 2–6, 2020; Denver, Colorado
Session J55: Photoemission Studies of Topological Materials |
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Sponsoring Units: DCMP Chair: Michael Osofsky, United States Naval Research Laboratory Room: Mile High Ballroom 2B |
Tuesday, March 3, 2020 2:30PM - 2:42PM |
J55.00001: Evolution of the band structure of α-Sn(001) thin film on InSb(001) Ko-Hsuan Chen, Keng-Yung Lin, Sheng-Wen Huang, Chao-Kai Cheng, Hsiao-Yu Lin, Shang-Wei Lien, Tay-Rong Chang, Cheng-Maw Cheng, Minghwei Hong, Jueinai Kwo Determining the topological phase of a material is a popular research route in condensed matter physics. For elemental material of α-Sn, its topological phase has been under debate. α-Sn(001) with in-plane compressive strain is expect to be a topological Dirac semimetal in theory, but experimental evidence of 3D Dirac state is still lacking. In contrast, a spin-momentum locked topological surface state (TSS) was found for α-Sn(001) on InSb(001), suggesting a topological insulator (TI). In this work, molecular beam epitaxy grown α-Sn(001) on InSb(001) and the thickness dependent angle-resolved photoemission spectroscopy (ARPES) study from 3 bilayers (BLs) to 370 BLs are reported. Streaky (2×1) reflection high energy electron diffraction patterns and narrowθ-rocking curve (FWHM of 0.0163°) in x-ray diffraction indicated an excellent crystallinity. Sharp TSSs were attained by ARPES in films thicker than 6 BLs without resorting to extra doping or surface treatments. No TSSs were observed for film thinner than 5 BLs, indicating a crossover from 3D TI to trivial insulator. In an extremely thick α-Sn film of 370 BLs, no evident kz dependence on the Dirac state was found in photon energy range of 10-40 eV, suggesting α-Sn(001) on InSb(001) more likely to be in TI phase. |
Tuesday, March 3, 2020 2:42PM - 2:54PM |
J55.00002: Band structure study of elemental topological material α-Sn on InSb(111)B Keng-Yung Lin, Ko-Hsuan Chen, Sheng-Wen Huang, Chao-Kai Cheng, Cheng-Maw Cheng, Minghwei Hong, Jueinai Kwo Research efforts on studying the band structure of elemental topological α-Sn have increased substantially in recent years. Bulk α-Sn is a zero-gap material with an inherent band inversion due to spin-orbit coupling. Interestingly, theoretical calculation proposed strained-induced topological phase transitions from topological Dirac semimetal (TDS) to topological insulator (TI) in α-Sn thin films. Among them, compressively strained α-Sn thin films on InSb(111)B were experimentally claimed to be a 3D TDS for 30-bilayer (BL) film by angle-resolved photoemission spectroscopy (ARPES).1 In this work, α-Sn thin films were grown on InSb(111)B by molecular beam epitaxy (MBE) with epi-InSb as the starting surfaces prepared in the same MBE chamber. Single-crystal α-Sn thin films with excellent crystallinity and smooth surface morphology were characterized by X-ray diffraction and atomic force microscopy; their band structures were studied by ARPES. Much clearer band dispersions were observed for our 30-BL film compared to those in the literature, with virtually no kz dependence over a wide photon energy range from 18 eV to 52 eV for the claimed 3D Dirac state. Our results, therefore, do not support the 3D TDS phase observed in the previous work.1 |
Tuesday, March 3, 2020 2:54PM - 3:06PM |
J55.00003: Clarifying superconducting proximity effects in topological insulating (Bi1-xSbx)2Te3 films on niobium Joseph Hlevyack, Sahand Najafzadeh, Meng-Kai Lin, Takahiro Hashimoto, Akihiro Tsuzuki, Tsubaki Nagashima, Weilu Zhang, Akiko Fukushima, Yang Bai, Peng Chen, Ro-Ya Liu, Yao Li, David Floetotto, José Avila, James N Eckstein, Kozo Okazaki, Shik Shin, Tai-Chang Chiang Coupling a topological insulator (TI) with an s-wave superconductor (SC) can initiate p-wave-like pairing in the nontrivial surface states. However, this proximity-induced pairing is still not well understood and challenging to realize, possibly due to a short superconducting coherence length and/or Fermi surface/lattice mismatch at the TI/SC interface. Using a novel cleavage-based “flip-chip” method, we have successfully prepared single-crystalline (Bi1-xSbx)2Te3 thin films with a predetermined thickness (2-5 quintuple layers) on bulk Nb substrate. By carefully tuning the composition x, we confirm using angle-resolved photoemission spectroscopy (ARPES) that the Fermi level lies in the band gap for all (Bi1-xSbx)2Te3 films, thus yielding a lightly n-doped TI on a SC even in the thin-film limit. Proximity-induced superconducting gaps in these samples are measured by ultrahigh-resolution laser ARPES. The results will be compared with those from prior studies of heavily n-doped TI films on Nb. Our study helps to reveal the mechanisms of proximity-induced pairing in TIs and also underscores the limitations of using the proximity effect to realize topological superconductivity. |
Tuesday, March 3, 2020 3:06PM - 3:18PM |
J55.00004: Topological chiral crystals Tyler Cochran, Guoqing Chang, Ilya Belopolski, Kaustuv Manna, Daniel S Sanchez, Zijia Cheng, Xian Yang, Daniel Multer, Songtian Zhang, Nana Shumiya, Maksim Litskevich, Jiaxin Yin, Suyang Xu, Claudia Felser, Hsin Lin, Zahid Hasan Topological chiral crystals host guaranteed topological crossings in the bulk band structure and can possess Fermi arc surface states that span the entire surface Brillouin zone [1]. These materials can give rise to novel opitcal and magneto-transport response. In this presentation, we discuss recent photoemission experiments exploring the topology of chiral fermions [2]. By combining measurements with ultraviolet and soft x-ray incident photons, we determine the topological invariant directly. Our results shed new light on the relationship between structural chirality and topology. |
Tuesday, March 3, 2020 3:18PM - 3:30PM |
J55.00005: Observation of topological surface state in a superconducting material Gyanendra Dhakal, Md Mofazzel Hosen, Ayana Ghosh, Christopher Lane, Karolina Górnicka, Michal J. Winiarski, Klauss Dimitri, Firoza Kabir, Christopher Sims, Sabin Regmi, William Neff, Dariusz Kaczorowski, Jian-Xin Zhu, Tomasz Klimczuk, Madhab Neupane The discovery of the topological insulator phase has ignited massive research interests in novel quantum materials. Topological materials with superconductivity further invigorate the importance of materials providing the platform to study the interplay between these two unique states. However, the candidates of such materials are rare. Here, we report a systematic angle-resolved photoemission spectroscopy (ARPES) study of superconducting material CaBi2 [ Tc = 2 K], corroborated by the first-principles calculations. Our study reveals the presence of topological Dirac state at the corner-point of the Brillouin zone with topological protection. Furthermore, our transport measurements show the presence of large magnetoresistance in this compound. Our results provide a platform to study the superconductivity and topology in CaBi2. |
Tuesday, March 3, 2020 3:30PM - 3:42PM |
J55.00006: Electronic structure of PtPb4 studied by Angle-Resolved Photoemission Spectroscopy Kyungchan Lee, Daixiang Mou, Lin-Lin Wang, Na Hyun Jo, Yun Wu, Benjamin Schrunk, John Wilde, Manh Cuong Nguyen, Amelia Estry, Cai-Zhuang Wang, Andreas Kreyssig, Sergey L. Bud'ko, Kai-Ming Ho, Paul C Canfield, Adam Kaminski We investigate the electronic structure of PtPb4 by using ultrahigh resolution, laser based Angle Resolved Photoemission Spectroscopy(ARPES) and Density functional theory(DFT) calculations. This material is a compound related to PtSn4, which exhibits exotic topological properties such as Dirac node arcs. The Fermi surface(FS) of PtPb4 has at least two electron pockets at the center of the Brillouin zone(BZ) and several hole pockets around the zone boundaries. Although the crystal structure has 4-fold symmetry, the ARPES data reveals a FS with only 2-fold symmetry, showing a rectangular FS sheet around X points and two intersected circular FS sheet around Y points. X-ray scattering data points to the stacking order as a key factor for the breaking of the 4-fold symmetry observed in ARPES data. The experimental results are in a reasonable agreement with the DFT calculations. |
Tuesday, March 3, 2020 3:42PM - 3:54PM |
J55.00007: Gating monolayer WTe2 devices in nano-ARPES Paul Nguyen, Cheng Chen, Roland Koch, Aaron Bostwick, Natalie Teutsch, Viktor Kandyba, Alex Victorovich Barinov, Neil R Wilson, Yulin Chen, Eli Rotenberg, Xiaodong Xu, David Cobden We investigate the effects of electrostatic doping on a 2D topological semimetal, monolayer WTe2, employing nanometer-resolution angle-resolved photoemission spectroscopy (nano-ARPES). The exfoliated WTe2 monolayer flake rests on an hBN flake and is capped by a monolayer of hBN to prevent oxidation and allow cleaning. During the photoemission measurements a gate voltage can be applied between a thin graphite electrode beneath the hBN and a graphene contact that overlaps the WTe2 on top. The gate is observed to shift the Fermi energy by more than 100 meV, from valence to conduction band of monolayer WTe2. The temperature and doping dependence of the spectrum yield insights into the nature of the bulk conductivity. |
Tuesday, March 3, 2020 3:54PM - 4:06PM |
J55.00008: Electronic Structure of Topological Material CaMnSb2 Hongtao Rong, Junbao He, Yongqing Cai, Guodong Liu, Lin Zhao, Zuyan Xu, Xingjiang Zhou
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Tuesday, March 3, 2020 4:06PM - 4:18PM |
J55.00009: Coexistence of topological nontrivial phase and Rashba-type surface state in HoSb Md Mofazzel Hosen, Gyanendra Dhakal, Baokai Wang, Narayan Poudel, Bahadur Singh, Klauss Dimitri, Firoza Kabir, Christopher Sims, Sabin Regmi, William Neff, Daniel Murray, Franziska Weickert, Krzysztof Gofryk, Orest Pavlosiuk, Piotr Wisniewski, Dariusz Kaczorowski, Arun Bansil, Madhab Neupane Using the high resolution angle-resolved photoemission spectroscopy (ARPES), magnetotransport, and parallel first-principles modeling, we report the discovery of a topologically nontrivial state coexisting with a Rashba-type surface state in a rare earth monopnictide semimetal. Our theoretical analysis reveals the presence of bulk band inversions at the Γ and X points of the Brillouin zone (BZ). Interestingly, the upper part of the observed Rashba-type surface state coincides with the nontrivial surface state at the X point. Magnetotransport study indicates that HoSb can be characterized as a correlated nearly-complete electron-hole-compensated semimetal. Therefore, our findings provide HoSb as a new materials platform for exploring the interplay between coexisting topological and Rashba-type surface state and the non-saturating XMR effect in the rare earth monopnictide family. |
Tuesday, March 3, 2020 4:18PM - 4:30PM |
J55.00010: Observation of spin polarized surface states in polar Weyl semimetal MoTe2 Yuma Tanaka, Mohammad Saeed Bahramy, Hidefumi Takahashi, Kenta Kuroda, Koichiro Yaji, Ayumi Harasawa, Takeshi Kondo, Shik Shin, Shintaro Ishiwata, Kyoko Ishizaka Since the theoretical prediction of the low temperature phase as a type-II Weyl semimetal, the electronic structure of β-MoTe2 has been actively studied by angle-resolved photoelectron spectroscopy. Until now, some segment-like band features resembling Fermi arcs are experimentally observed, which are indicative of the topological surface states connecting the bulk Weyl nodes. However, the band dispersions near the Fermi level is complicated, and clear interpretation is not easy. Here we investigate the spin polarizations of the Fermi arc features for both polar terminations by utilizing the high-resolution spin and angle-resolved photoelectron spectroscopy (SARPES). For respective surface terminations, the different patterns and spin polarizations of Fermi arcs are clearly observed. In the presentation, we will compare the result with the first-principles calculation and discuss the details of the spin polarized surface states. |
Tuesday, March 3, 2020 4:30PM - 4:42PM |
J55.00011: Termination Dependent Topological Surface States in Nodal Loop Semimetal HfP2 Christopher Sims, M. Mofazzel Hosen, Hugo Aramberri, Cheng-Yi Huang, Gyanendra Dhakal, Klauss Dimitri, Firoza Kabir, Sabin Regmi, XIAOTING ZHOU, Tay-Rong Chang, Hsin Lin, Dariusz Kaczorowski, Nicholas Kioussis, Madhab Neupane Symmetry plays a major role in all disciplines of physics. Within the field of topological materials |
Tuesday, March 3, 2020 4:42PM - 4:54PM |
J55.00012: Topological hinge state in a 1D stacking material Ryo Noguchi, Masaru Kobayashi, Kenta Kuroda, Takanari Takahashi, Zhanzhi Jiang, Zifan Xu, Daehun Lee, Motoaki Hirayama, Masayuki Ochi, Tetsuro Shirasawa, Peng Zhang, Chun Lin, Cédric Bareille, Koichiro Yaji, Ayumi Harasawa, Viktor Kandyba, Alessio Giampietri, Alex Victorovich Barinov, Timur Kim, Cephise Cacho, Shik Shin, Ryotaro Arita, Keji Lai, Takao Sasagawa, Takeshi Kondo Due to a higher order bulk boundary correspondence, a higher-order topological insulator hosts one-dimensional helical edge states around the hinges of the crystal. However, spectroscopic evidence for topological hinge states has been so far limited to semimetallic materials [1], where the measured edge conductivity is blurred by bulk carries. In this contribution, we show evidence for topological hinge states in a semiconducting quasi-one-dimensional material. Importantly, the crystal has naturally cleavable top and side surfaces which are stacked via van-der-Waals forces, and therefore the electronic structure around the hinge can be investigated in a cleaved surface or in an exfoliated thin flake. Our experiments by high-resolution angle-resolved photoemission spectroscopy reveal quasi-1D states inside the bulk band gap. Moreover, the corresponding signals with high conductivity are detected at the edges of the crystal by microwave impedance microscopy. These observations, therefore, indicate the existence of topological hinge states in a 1D stacking material. |
Tuesday, March 3, 2020 4:54PM - 5:06PM |
J55.00013: Sparse Big Data Spectroscopy: Advanced techniques for maximally resolving microARPES data on topological insulators Erica Kotta, Lin Miao, Yishuai Xu, Stanley A Breitweiser, Chris Jozwiak, Aaron Bostwick, Eli Rotenberg, Wenhan Zhang, Weida Wu, Takehito Suzuki, Joseph G Checkelsky, L. Andrew Wray Topological insulators are bulk semiconductors that manifest in-gap surface states with massless Dirac-like dispersion due to the topological bulk-boundary correspondence principle. These surface states can be manipulated by the interface environment to display various emergent phenomena. Here, we use angle-resolved photoemission spectroscopy (ARPES) and scanning tunnelling microscopy (STM) to investigate the interplay of crystallographic inhomogeneity with the topologically ordered band structure in a model topological insulator. We develop quantitative analysis methods to obtain spectroscopic information in spite of a limited dwell time on each measured point. We find that the band energies vary on the scale of 50 meV across the sample surface, and this enables single-sample measurements that are analogous to a multi-sample doping series. By focusing separately on the bulk and surface electrons we reveal a hybridisation-like interplay between fluctuations in the surface and bulk state energetics. |
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J55.00014: Topological Lifshitz transitions and Fermi arc manipulation in Weyl semimetal NbAs Haifeng Yang, Lexian Yang, Zhongkai Liu, Yan Sun, Andrei Bernevig, Claudia Felser, Binghai Yan, Yulin Chen Surface Fermi arcs (SFAs), the unique open Fermi-surfaces (FSs) discovered recently in topological Weyl semimetals (TWSs), are unlike closed FSs in conventional materials and can give rise to many exotic phenomena, such as anomalous SFA-mediated quantum oscillations, chiral magnetic effects, three-dimensional quantum Hall effect, non-local voltage generation and anomalous electromagnetic wave transmission. Here, by using in-situ surface decoration, we demonstrate successful manipulation of the shape, size and even the connections of SFAs in a model TWS, NbAs, and observe their evolution that leads to an unusual topological Lifshitz transition not caused by the change of the carrier concentration. The phase transition teleports the SFAs between different parts of the surface Brillouin zone. Despite the dramatic surface evolution, the existence of SFAs is robust and each SFA remains tied to a pair of Weyl points of opposite chirality, as dictated by the bulk topology. |
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J55.00015: Observation of unconventional chiral fermions with long Fermi arcs in CoSi Zhicheng Rao In condensed system, Chiral fermions exist at nodes (that is, robust band-crossing points protected from being gapped by nontrivial band topology) that carry a nonzero Chern number. Recently, we reported two types of unconventional chiral fermions in CoSi, which are threefold-degenerate node at point and fourfold-degenerate node at R point. These nodes at point and R point carry nonzero Chern number and we observed two surface Fermi arcs on surface to connect their projection on (001) surface, which are related by a π rotation about Γ. Further, we show the two helical surface states encircle the projection of Γ point and R point, which can reveal the topological nature of bulk nodes in CoSi. |
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