Bulletin of the American Physical Society
APS March Meeting 2020
Volume 65, Number 1
Monday–Friday, March 2–6, 2020; Denver, Colorado
Session U60: Topological Superconductivity: 3D TIs, Weyl Semimetals and TMDs 
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Sponsoring Units: DMP Chair: Vidya Madhavan, University of Illinois at UrbanaChampaign Room: Mile High Ballroom 4A 
Thursday, March 5, 2020 2:30PM  3:06PM 
U60.00001: Valley Hall transport and superconductivity signatures in twisted transition metal dichalcogenides Invited Speaker: Ning Wang Selected by Focus Topic Organizer (Peide Ye) 
Thursday, March 5, 2020 3:06PM  3:18PM 
U60.00002: Topological linkage for thermoelectricity and superconductivity in their nontrivial topological state XiaoJia Chen, Hao Yu, WeiJian Li, Liucheng Chen, ZiYu Cao, Di Li, Xiaoying Qin Most thermoelectric materials and topological insulators were made from heavy elements and have narrow band gaps and strong spinorbit couplings. The nontrivial topological insulating states are the essential base for the topological superconductivity. It remains unclear how the thermoelectric figure of merit performs in the topological state, and whether a thermoelectric material can evolve into a topological superconductor and what is the driving force if the latter happens. By choosing an ntype Bi_{2}Te_{2.7}Se_{0.3}, a highly efficient thermoelectric material and topological insulator, we perform measurements of the thermoelectric and superconducting properties at high pressures. We find that the figure of merit increases with pressure and exhibits a maximum at around 1 GPa where the topological phase transition takes place. After that, superconductivity appears and the critical temperature dramatically increases with increasing pressure in the phase with the nontrivial topological state. Its upper critical field exhibits different temperature dependence compared to the other superconducting phases without topological order. The topological phase transition is proposed to serve the linkage for the highperformance thermoelectricity and topological superconductivity. 
Thursday, March 5, 2020 3:18PM  3:30PM 
U60.00003: Theory of Giant InPlane Critical Field Enhancement in Superconducting Films on a Topological Insulator Surface Haoyun Deng, Pedro Schlottmann, Peng Xiong, Nicholas Evans Bonesteel Majorana zero modes (MZMs) are of interest due to potential applications in topological quantum computing. Topological insulators (TIs), with spinmomentum locked surface states, may enable emergence of MZMs when coupled to an swave superconductor (SC) via the proximity effect. We recently observed a giant enhancement of the inplane H_{c} in thin Al, a superconductor with minimal intrinsic spinorbit coupling (SOC), evaporated onto a 3dimensional strong TI.^{1} Here we present a theoretical analysis of this enhancement. Superconductivity at inplane fields well in excess of the ChanderasekharClogston (CC) limit is understood as a consequence of strong SOC induced in the thin Al films from the topological surface state through the inverse proximity effect. The CC model is combined with the Maki equation to calculate H_{c }at different temperatures. The calculated H_{c}(T) shows excellent quantitative agreement with the experimental result. A relatively large value of the induced spinorbit scattering rate (in comparison to the superconducting energy gap) is derived, indicating a strong proximity coupling in this TISC heterostructure which may be a promising platform for observing MZMs. 
Thursday, March 5, 2020 3:30PM  3:42PM 
U60.00004: Characterizing the Superconducting State of Cu_{x}Bi_{2}Se_{3} Through MuonSpin Relaxation/Rotation Emily Duden, Benjamin Frandsen, Markus Kriener, Yoichi Ando, Yasutomo J Uemura The discovery of topological superconductors has been a major objective in the field since they were first predicted theoretically. The doped topological insulator Cu_{x}Bi_{2}Se_{3} is a promising candidate for topological superconductivity, but there is much conflicting evidence on the subject. Identifying the superconducting symmetry of the material is paramount in determining whether it has a topologically nontrivial ground state. Muon spin relaxation/rotation (SR) experiments provide a heretofore unexplored method of probing the superconducting state of Cu_{x}Bi_{2}Se_{3}. Here, we present SR data collected on a singlecrystal sample of Cu_{0.3}Bi_{2}Se_{3}. Measurements conducted in zerofield conditions demonstrate that time reversal symmetry is preserved in the superconducting state, while transversefield measurements reveal superfluid density behavior that is consistent with pwave pairing. Such a scenario could support topological superconductivity in Cu_{x}Bi_{2}Se_{3}. However, a more conventional swave pairing scenario also provides a reasonable fit to the data, making an unambiguous conclusion difficult. Nevertheless, these results represent a valuable addition to the body of experimental data available for Cu_{x}Bi_{2}Se_{3}. 
Thursday, March 5, 2020 3:42PM  3:54PM 
U60.00005: Inplane Critical Field far beyond the Paramagnetic Limit for Thin Al Films on Topological Insulator Surface Zhu Lin, Zhilin Li, Haoyun Deng, Tianhan Liu, Gang Shi, Pedro Schlottmann, Nicholas Evans Bonesteel, Yongqing Li, Peng Xiong Heterostructures of an swave superconductor and a strong topological insulator are regarded as a viable platform for the creation of Majorana zero mode (MZM). However, the microscopic understanding of the proximity effect between the superconductor and topological surface state has yet to be fully elucidated. Here we report measurements of the inplane critical field (H_{C//}) of thin Al, a superconductor with minimal spinorbit interaction (SOI), on the surface of (Bi_{0.5}Sb_{0.5})_{2}TeSe_{2} (BSTS). H_{C//} of the Al films are directly compared with those of simultaneously grown control samples of Al films on SiO_{2}. Giant enhancement of H_{C//} beyond the paramagnetic limit was observed for the Al on BSTS. The enhancement diminishes with increasing Al thickness. The H_{C//}(T) of thin Al on SiO_{2} is consistent with the ChanderasekhaClogston theory, while the H_{C//}(T) of thin Al on BSTS can be welldescribed by a theory based the Maki equation invoking strong induced SOI in Al. The results provide direct evidence for an electrically transparent interface and strong proximity effect between the Al and BSTS. The Al/BSTS heterostructure may be a promising system for inducing topological superconductivity and controlled generation of MZMs. 
Thursday, March 5, 2020 3:54PM  4:06PM 
U60.00006: UniaxialStrain Control of Nematic Superconductivity in Sr_{x}Bi_{2}Se_{3} Ivan Kostylev, Shingo Yonezawa, Zhiwei Wang, Yoichi Ando, Yoshiteru Maeno Recently, nematic superconductivity, in which the superconducting gap spontaneously lifts the rotational symmetry of the lattice, has been discovered [1]. In nematic superconductivity, multiple superconducting domains with different nematic orientations can exist. These domains can be controlled by a conjugate external stimulus, such as the stain induced by a uniaxial stress with a piezoelectric device [2]. Here, we report for the first time control of the nematic superconductivity and their domains of Sr_{x}Bi_{2}Se_{3}, through externallyapplied uniaxial stress [3]. The suppression of subdomains indicates that it is the Δ_{4y} state that is most favored under compression along the basal BiBi bonds. These results provide an inevitable step towards microscopic understanding and future utilization of the unique topological nematic superconductivity. 
Thursday, March 5, 2020 4:06PM  4:18PM 
U60.00007: Doping and pressure effects on Weyl semimetal Mo_{1x}W_{x}Te_{2} Rabin Dahal, Liangzi Deng, Narayan Poudel, Melissa Gooch, Zheng Wu, HungCheng Wu, HungDuen Yang, ChungKai Chang, Paul C. W. Chu Systematic pressure and doping dependent study on Mo_{1x}W_{x}Te_{2 }was carried out to investigate the correlation among chemical doping, structural transition, and superconductivity. Doping dependent resistivity measurement at ambient pressure shows structural transition temperature (T_{s}) for MoTe_{2} to be 249 K, which continuously increases as doping level (x) increases and reaches a maximum value of 613 K for WTe_{2}. The structural transition of WTe_{2} at ambient pressure is also confirmed by conducting temperature dependent synchrotron Xray diffraction from 300 K to 673 K. Three samples with different doping (x = 0.10, 0.40, 0.75) were selected for high pressure measurement. The superconducting transition temperature  pressure (T_{c}  P) phase diagrams have been constructed and will be discussed. Our experimental results revealed a competition between the structural transition and superconductivity in Mo_{1x}W_{x}Te_{2}. Superconductivity emerges when the T_{s} is suppressed below 175 K and T_{c} increases as T_{s} is suppressed. 
Thursday, March 5, 2020 4:18PM  4:30PM 
U60.00008: Theory on superconducting chiral crystal and its applications ZheShen Gao, Wenyu He, Kam Tuen Law

Thursday, March 5, 2020 4:30PM  4:42PM 
U60.00009: Correlationdriven renormalization of Weyl nodes in the Ce113 series Vsevolod Ivanov, Sergey Savrasov We study the electronic properties of the inversionbroken CeTX3 (T = Co, Rh, Ir, X=Si,Ge) heavyfermion materials through a combination of LDA and LDA+Guzwiller methods. In this class of compounds the electronic mass is enhanced, causing narrowing the Ce felectron bands. Our LDA+Guzwiller procedure captures these renormalization effects, yielding a mass enhancement that is in good agreement with experimental specific heat measurements. We also track the renormalization of Weyl points by the correlations, which move them closer to the Fermi energy. We briefly comment on the interplay of Weyl physics and pressureinduced superconductivity in these compounds. 
Thursday, March 5, 2020 4:42PM  4:54PM 
U60.00010: Onedimensional states residing on edges and steps in fewlayer WTe_{2} Artem Kononov, Gulibusitan Abulizi, Kejian Qu, Jiaqiang Yan, David Mandrus, Kenji Watanabe, Takashi Taniguchi, Christian Schonenberger WTe_{2} is a layered material with rich topological properties. The bulk crystal is a typeII Weyl semimetal and the monolayer is a twodimensional topological insulator. Recently, it has been predicted that higher order topological insulator state can appear in WTe_{2}. An observation of 1D, highly conductive channels, known in this case as hinge states, is hindered by the bulk conductivity of WTe_{2}. Here, we employ the Josephson effect to disentangle the contribution of the hinge states from the bulk in electronic transport. We observe 1D current carrying states on edges and steps in fewlayer WTe_{2}. The width of the states is deduced to be below 100 nm. A supercurrent in them can be measured over distances up to 3 µm and in perpendicular magnetic field up to 2 T. Moreover, the dependence of the supercurrent on the magnetic field is compatible with the asymmetric Josephson effect predicted to occur in topological systems with broken inversion symmetry. 
Thursday, March 5, 2020 4:54PM  5:06PM 
U60.00011: Probing Proximity Effect in Topological Insulator/Superconductor Heterostructures Cequn Li, Brian Bersch, Natalie Briggs, Yifan Zhao, Chengye Dong, Timothy Bowen, Hemian Yi, Joshua Robinson, CuiZu Chang, Jun Zhu Topological insulator (TI)/superconductor heterostructures are predicted to host Majorana zero modes, which are building blocks of topological qubit. In this work, we synthesize highquality largearea TI/graphene (Gr)/gallium heterostructures combining confinement epitaxy and molecular beam epitaxy. Atomically thin Ga film superconducts at T_{c}~4K [B. Bersh, et. al, arXiv:1905.09938], and the growth of TI preserves its superconductivity. We probe the proximity effect in the TI film using fourterminal transport and tunneling spectroscopy. Hints of a proximity induced gap in the TI film are observed in transport measurements. We demonstrate the efficacy of a clean van der Waals tunnel junction in probing the superconducting gap of thin film NbSe_{2} and present tunneling spectroscopy measurements on TI/Gr/Ga heterostructures. 
Thursday, March 5, 2020 5:06PM  5:18PM 
U60.00012: Topological superconductivity in proximitized transition metal dichalcogenide Josephson junctions Joseph Pakizer, Alex Matos Abiague We theoretically investigate the emergence of topological superconductivity in Josephson junctions (JJ) built of a twodimensional transition metal dichalcogenide (TMD) layer in proximity to an swave superconductor and a magnetic insulator. We show that in a certain range of chemical potential and magnetic proximity strength, Majorana bound states form at the ends of the normal region of a TMDbased JJ. The evolution of the topological superconducting phase as the magnetization direction is varied is also investigated. The effect of the interplay between Ising spins, proximity exchange, and Rashba spinorbit coupling on the system transition to the topological superconducting phase, as well as the corresponding signatures on the ground state phase and critical current, are analyzed in detail for both phasefixed and phaseunbiased JJs. 
Thursday, March 5, 2020 5:18PM  5:30PM 
U60.00013: Superconductivity in Cd_{3}As_{2} thin films Alexey Suslov, Alexander Davydov, Leonid Oveshnikov, Leonid Morgun, Kliment Kugel, Vasiliy Zakhvalinskii, Evgeny Pilyuk, Alexey Kochura, Alexander Kuzmenko, Alexey Ril, Sergey Marenkin, Vladimir Pudalov, Boris Aronzon For the first time we observed superconductivity in Cd_{3}As_{2} thin films without applying external pressure. We studied thin (40 nm 100 nm) films of Cd_{3}As_{2} grown on various substrates by two various methods: thermal evaporation and magnetron sputtering from Cd_{3}As_{2} single crystals. Both Raman spectroscopy and Xray diffraction on the films show results typical for Cd_{3}As_{2} with tetragonal lattice. A zeroresistance state was registered by the fourprobe method on Hall bar shaped samples. Also a zeroresistance plateau was observed in dV/dI characteristics at low currents. The superconducting state was suppressed by applying a magnetic field or increasing the current above critical values H_{c} and I_{c}, respectively. H_{c} perpendicular to the film plane was smaller than the inplane critical field at the same temperature. Analysis of dependences of critical magnetic field H_{c} on critical temperature T_{c} reveals a clearly pronounced linear behavior within the intermediate temperature range, similar to that observed for superconducting bulk Cd_{3}As_{2} and Bi_{2}Se_{3} under pressure. 
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