Bulletin of the American Physical Society
APS March Meeting 2021
Volume 66, Number 1
Monday–Friday, March 15–19, 2021; Virtual; Time Zone: Central Daylight Time, USA
Session C52: Topological Superconductivity: 2D Materials and HeterostructuresFocus Session Live
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Sponsoring Units: DMP Chair: Anand Bhattacharya, Argonne National Laboratory |
Monday, March 15, 2021 3:00PM - 3:36PM Live |
C52.00001: Insulating and superconducting states of monolayer WTe2 Invited Speaker: David Cobden In monolayer WTe2, low symmetry, band inversion, heavy hole mass, electron-hole compensation, and strong spin-orbit coupling together provide a promising setting for nontrivial correlated and topological phenonema. I will summarize the experimental evidence for some of these phenomena. Undoped, this material behaves as a two-dimensional topological insulator, while at a surprisingly low level of electron doping it becomes superconducting below 1 K. Anisotropic magnetoresistance measurements of the edge conduction in the insulating state demonstrate that the edges states are indeed helical, and also reveal a tilted-Ising-like electron spin polarization in common with the bulk bands from which the superconducting state must be built. Some measurements suggest that the helical edge states of the topological insulator persist in the metallic and even in the superconducting regime, while being very weakly coupled to it. In addition, many pieces of evidence taken together indicate that the insulating behavior is driven by electron-hole pairing in what would otherwise be a semimetal, with the possible formation of zero-momentum excitonic insulator ordering near the neutral state that competes with the superconductivity at moderate doping. |
Monday, March 15, 2021 3:36PM - 3:48PM Live |
C52.00002: Supercurrent and band-inversion in bilayer graphene/WSe2 Josephson junctions Prasanna Rout, Nikos Papadopoulos, Kenji Watanabe, Takashi Taniguchi, Srijit Goswami In recent years 2D materials like WSe2 are utilised to induce spin-orbit interaction in bilayer graphene (BLG), which can even lead to topological band-inverted phase in a symmetrically WSe2 encapsulated BLG [6]. Here, we investigate the supercurrent in WSe2/BLG/ WSe2 Josephson junctions close to band-inverted regime by dual gate tuning of carrier density and displacement field (D). Along the charge neutrality line, we observe a suppression of supercurrent at D=0 indicative of band-inversion. In the absence of in-plane magnetic field, the current-phase relation (CPR) extracted from quantum interferometry measurement shows no phase shift as expected. However the application of field results in anomalous CPR with finite phase shift which is pronounced for p-doped regime with higher junction transparency. Our work opens novel possibilities for phase-controlled Josephson junctions as well as topological superconductivity in Van der Waals materials with spin-orbit coupling. |
Monday, March 15, 2021 3:48PM - 4:00PM Live |
C52.00003: Two dimensional 2M-WS2 nanolayers for superconductivity Piumi Samarawickrama, Rabindra Dulal, Zhuangen Fu, Uppalaiah Erugu, Wenyong Wang, John Ackerman, Brian Leonard, Jinke Tang, Te-Yu Chien, Jifa Tian A newly discovered VIB group transition metal dichalcogenide (TMD) material 2M-WS2 has attracted extensive attention due to its interesting physical properties such as topological superconductivity, nodeless superconductivity, and anisotropic Majorana bound states. However, the techniques to grow high-quality 2M-WS2 bulk crystals and the study of their physical properties at the nanometer scale are still limited. Here, we report a new route to grow high-quality 2M-WS2 single crystals and the observation of superconductivity in its thin layers. The crystal structure of the as-grown 2M-WS2 crystals was determined by X-ray diffraction (XRD) and scanning tunneling microscopy (STM). The chemical composition of the 2M-WS2 crystals was determined by energy dispersive x-ray spectroscopy (EDS) analysis. Our low temperature transport measurements demonstrate clear signatures of superconductivity of a 25 nm-thick 2M-WS2 flake with a critical temperature (TC) of ~8.5 K and an upper critical field of ~ 2.5 T at T = 1.5 K. Our work may pave new opportunities in studying the topological superconductivity at the atomic scale in simple 2D TMD materials. |
Monday, March 15, 2021 4:00PM - 4:12PM Live |
C52.00004: Link between superconductivity and a Lifshitz transition in intercalated BiSe Shay Sandik, Itai Silber, Avior Almoalem, Amit Ribak, Yuval Nitzav, Alexander Kuntsevich, Oleg Sobolevskiy, Yuriy Selivanov, Valerii Prudkoglyad, Yoram Dagan, Amit Kanigel In a topological superconductor the bulk of the sample is fully gapped while a topologically protected gapless state resides on the surface. Intercalation of Cu, Sr or Nb between the quintuple layers of the topological insulator Bi2Se3 increases the carrier concentration and leads to superconductivity. It was suggested that the superconducting state in these compounds is topological. Here we study the electronic structure of Sr-intercalated Bi2Se3 using angle resolved photoemission spectroscopy (ARPES) and Shubnikov-de Haas (SdH) oscillations. We show that despite the relatively low Hall number, the carrier density is large and the resulting Fermi surface is cylindrical-type. We suggest that superconductivity in intercalated Bi2Se3 is more likely in such quasi-2D open Fermi surface. |
Monday, March 15, 2021 4:12PM - 4:24PM Live |
C52.00005: Identifying Topological Superconductivity in the 2D Transition Metal Dichalcogenides Christopher Lane, Jian-Xin Zhu Significant advances have been made towards quantum supremacy in just the last few years. However, most platforms are still plagued by short coherence times and substantial overhead generated by error correction. To remedy this, Majorana fermion modes have been proposed as a class of topologically protected qubits that are immune to conventional decoherence sources. Topological superconductors are believed to host such exotic quasiparticles, but very few material realizations have been theoretically predicted, let alone experimentally verified. To address this challenge, we search for and identify topological superconductivity (TS) in the 2D transition metal dichalcogenides (TMD). By combining state-of-the-art DFT and MBPT techniques, we are able to determine the superconducting pairing instability and associated symmetries along with the band topology in material specific detail. A few candidate materials are analyzed, revealing possible signatures of TS. This study serves as an initial step in the systematic theoretical investigation of the superconducting properties of the TMD and other correlated 2D materials. |
Monday, March 15, 2021 4:24PM - 5:00PM Live |
C52.00006: Quanum Hall - superconductor hybrid devices Invited Speaker: Gleb Finkelstein This talk covers our two recent results: 1) realization of the quantum Hall-based SQUID in side-gated samples and 2) observation of the chiral Andreev edge states – hybrid electron/hole single particle modes propagating along the superconductor - quantum Hall interfaces. |
Monday, March 15, 2021 5:00PM - 5:12PM Live |
C52.00007: Superconductivity and Proximity Effect in Topological Insulator/Graphene/Gallium Heterostructures Cequn Li, Alexander Vera, Yifan Zhao, Timothy Bowen, Hemian Yi, Cui-Zu Chang, Joshua Robinson, Jun Zhu Proximity induced superconductivity in topological insulator (TI)/superconductor (SC) heterostructures is a potential platform to host Majorana zero modes. Through confinement heteroepitaxy (Briggset al. Nat. Mater. 19, 637–643 (2020)), we have achieved the intercalation of atomically thin Ga films at the interface of epitaxial graphene grown on SiC. In this work, we report on the superconducting properties of two-layer Ga using four-terminal transport and tunneling spectroscopy. We measure the temperature and magnetic field dependence of the superconducting Ga film (Tc~ 4 K) and evaluate its upper critical field as a function of temperature. The linear relationship between Hc2 and T suggests the 2D superconductivity of Ga. Transport tunneling spectroscopy on graphene/Ga films allows us to determine the superconducting gap of Ga. In finely controlled magnetic field sweeps, we observe evidence of single vortex motion and vortex trapping in the tunneling conductance. Subsequent molecular beam epitaxy growth of thin TI film (Bi0.2Sb0.8)2Te3 on graphene/Ga allows us to examine the superconducting proximity effect of the Dirac surface states. We present four-terminal transport and tunneling spectroscopy measurements on TI/graphene/Ga heterostructures. |
Monday, March 15, 2021 5:12PM - 5:24PM Live |
C52.00008: Magic angles and topology in twisted nodal superconductors Pavel Volkov, Jed Pixley, Justin Wilson We propose twisted bilayers of two-dimensional nodal superconductors as a new platform to realize topological and correlated superconducting phases. We show that the Fermi velocity of the Dirac excitations in the Bogoliubov-De Gennes quasiparticle dispersion is strongly renormalized by the interlayer hopping, vanishing at a "magic angle”, where a pair of Dirac points merge into a quadratic band touching that is unstable to the formation of correlation-induced phases. We demonstrate that magnetic field, electric gating, and current bias can be used for versatile control of the system. In particular, we show that current bias can open a topological gap, with the system being characterized by a non-zero Chern number that is equal to the number of nodes. This produces a quantized thermal Hall effect with gapless thermal currents on the boundary. |
Monday, March 15, 2021 5:24PM - 5:36PM Live |
C52.00009: Observation of topological electronic structure in quasi-1D superconductor TaSe3 Zhongkai Liu, Cheng Chen, Aiji Liang, Shuai Liu, Si-Min Nie, Junwei Huang, Meixiao Wang, Yiwei Li, Ding Pei, Haifeng Yang, Hui-Jun Zheng, yong zhang, Donghui Lu, Makoto Hashimoto, Alexei Barinov, Chris Jozwiak, Aaron Bostwick, Eli Rotenberg, Xufeng Kou, Lexian Yang, Yanfeng Guo, Zhijun Wang, Hongtao Yuan, Yulin Chen Topological superconductors (TSCs),with the capability to host Majorana bound states that can lead to non-Abelian statistics and application in quantum computation,have been one of the most intensively studied topics in condensed matter physics recently. Up to date, only a few compounds have been proposed as candidates of intrinsic TSCs, such as doped topological insulator CuxBi2Se3 and iron-based superconductor FeTe0.55Se0.45. Here, by carrying out synchrotron and laser based angle-resolved photoemission spectroscopy (ARPES), we systematically investigated the electronic structure of a quasi-1D superconductor TaSe3, and identified the nontrivial topological surface states. In addition, our scanning tunneling microscopy (STM) study revealed a clean cleaved surface with a persistent superconducting gap, proving it suitable for further investigation of potential Majorana modes. These results prove TaSe3 as a stoichiometric TSC candidate that is stable and exfoliable, therefore a great platform for the study of rich novel phenomena and application potentials. |
Monday, March 15, 2021 5:36PM - 5:48PM Live |
C52.00010: Dome-shaped superconducting behavior of Weyl semimetal Mo0.50W0.50Te2 under pressure Rabin Dahal, Liangzi Deng, Narayan Poudel, Melissa Gooch, Zheng Wu, Paul C. W. Chu Systematic high-pressure studies have been carried out on the type-II Weyl semimetal candidate Mo0.50W0.50Te2 using a diamond anvil cell in order to determine the effects of pressure on its magnetotransport properties. Superconductivity was successfully induced under 1.1 GPa with a superconducting transition temperature (Tc) of 2.6 K. Tc was found to continuously increase with increasing pressure, reaching a maximum value of 5.8 K under 14.3 GPa. With further increasing pressure above 14.3 GPa, the Tc remains constant and then begins to decrease at 17.5 GPa. The dome-shaped Tc - P phase diagram was mapped out for pressure up to 42 GPa. Under pressure, the magnetoresistance of Mo0.50W0.50Te2 is suppressed and its Hall coefficient changes sign. Details of the above results will be discussed. |
Monday, March 15, 2021 5:48PM - 6:00PM Live |
C52.00011: Chiral Electron-Hole Hybridized Edge Modes: Interference in Transport Along a Graphene-Superconductor Interface Alexey Bondarev, Harold U Baranger We study chiral transport in junctions coupling the integer quantum Hall state of graphene nanoribbons with an s-wave superconductor. On the lowest Landau plateau, Andreev processes lead to two particle-hole hybrid chiral edge modes. The electron-hole hybrid nature of the modes enables using them for charge interconversion. The probability to outcouple from the superconducting edge oscillates between electrons and holes whenever the momentum difference between the modes is tuned. We study the dispersion on armchair and zigzag cuts of the graphene edge numerically (tight-binding) as well as analytically in a continuum model. In the simplest Dirac-type graphene, an in-valley particle-hole symmetry constraints the mode momentum, thereby hindering oscillations to more slowly varying plateau-like transport. We show that breaking the symmetry via valley anisotropic terms gives distinct two-mode dispersion and recovers the Andreev oscillations. We extend valley-isospin selection rules on Andreev probability to the two mode case and provide more extensive examples of these transport oscillations as a function of chemical potential, magnetic field, or bias. |
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