Bulletin of the American Physical Society
APS March Meeting 2022
Volume 67, Number 3
Monday–Friday, March 14–18, 2022; Chicago
Session Z65: Topological Superconductivity IIIFocus Recordings Available
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Chair: Fan Zhang, UT Dallas Room: Hyatt Regency Hotel -Grant Park C |
Friday, March 18, 2022 11:30AM - 12:06PM |
Z65.00001: Majorana corner modes in transition metal dichalcogenides Invited Speaker: Yi-Ting Hsu The material search for topological superconductors is fueled by the prediction of Majorana zero modes (MZM), which are exotic boundary excitations that are promising candidates for topological qubits. Yet, materials hosting topological superconductivity with MZMs are still rare to date. In this talk, I will show that MZM can occur at the corners of two- or three-dimensional inversion-symmetric superconductors without utilizing proximity effect. Using monolayer WTe2 and bulk MoTe2 as case studies, I will present how to obtain comprehensive recipes for realizing such ‘higher-order’ topological superconductors from mathematically derived topological invariants. Then by combining ab initio band structure calculations and pairing symmetry analyses, I will show that WTe2 and MoTe2 are promising candidates that satisfy the recipes and could host Majorana corner modes. |
Friday, March 18, 2022 12:06PM - 12:18PM |
Z65.00002: Majorana corner modes in triangular superconductor islands Hua Chen, Aidan Winblad We study the possibility of obtaining robust Majorana modes at the corners of triangular islands of different superconductor models, with the goal of finding alternative structures that can serve as building blocks of topological quantum computation. By considering both spinless p-wave and ferromagnetic Rashba s-wave superconductor models on an equilateral triangle subject to inhomogeneous supercurrents, we found that Majorana corner modes can generally appear if the system being considered can be made equivalent to a triangular chain model, which can be understood by calculating the bulk Z2 topological invariant. We also discussed the robustness of the corner modes in possible experimental realizations of the triangular islands. |
Friday, March 18, 2022 12:18PM - 12:30PM |
Z65.00003: Thickness-dependent superconductivity in layered 2M-WS2 Piumi I Samarawickrama, Piumi I Samarawickrama, Joseph McBride, Zhuangen Fu, Wenyong Wang, John Ackerman, Brian Leonard, Jinke Tang, TeYu Chien, Jifa Tian Topological superconductivity has become an active research field in recent years because it not only opens up opportunities in quantum physics, but also paves a way to revolutionize the current computing technology. Since the discovery of superconductivity in a binary transition metal dichalcogenide (TMD), 2M-WS2, in 2019, study of the possible topological properties and the corresponding pairing symmetry has attracted extensive attention. Here, we will first introduce the preparation of high quality 2M-WS2 nanolayers and then discuss the thickness dependence of the superconductivity in 2M-WS2. We find that the superconductivity in 2M-WS2 can persistent down to few layers. The corresponding transition temperature decreases from ~ 8.7 K to 6.9 K as the thickness varies from 40 nm to 3 nm. In the end, the pairing symmetry will also be discussed. Our discovery may provide evidence of the existence of topological properties in 2M-WS2 at the atomic scale. |
Friday, March 18, 2022 12:30PM - 12:42PM |
Z65.00004: Chiral Topological Superconductivity in Prototypical 2D Transition-Metal Dichalcogenides Christopher A Lane, Jian-Xin Zhu With our entrance into the noisy intermediate-scale quantum (NISQ) era in just the last few years, greater focus has been placed on quantum error mitigation to enable sustainable quantum supremacy. This ever-growing issue stems from the short coherence times plaguing current qubit platforms, requiring evermore 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. So far, very few material realizations have been theoretically predicted, let alone experimentally verified. To address this challenge, we combine state-of-the-art DFT and MBPT techniques, to determine the superconducting pairing instability and associated symmetries in TMDCs MoS2, WS2, and MoTe2. Our calculations yield a variety of superconducting states including d, f, d±id, and p±ip wave pairing modes. This study serves as a baseline in the systematic theoretical investigation of the superconducting properties of the TMDCs and other correlated 2D materials. |
Friday, March 18, 2022 12:42PM - 12:54PM |
Z65.00005: U(1)-enriched higher-order topological superconductor in WTe$_2$ with controllable Majorana zero modes Ammar Jahin We study the superconducting instability of monolayer WTe2. Modeling the electron-electron interactions of the system with a Hubbard on-site repulsive interaction, we still find a superconducting instability due to the Kohn-Luttinger mechanism. We find the leading instability of the resulting superconductor to be p-wave and to have nontrivial higher-order topology. In addition, we show that the U(1) spin-rotational symmetry in one of the superconducting states enables spatial control of the corner Majorana zero mdoes, leading to possible applications for fault tolerant quantum computations. |
Friday, March 18, 2022 12:54PM - 1:06PM |
Z65.00006: Exploring the superconducting state of a Dirac material Chris J Lygouras, Juan Chamorro, Tanya Berry, Thomas J Halloran, Junyi Zhang, Kagetora Mikuri, Jun Gouchi, Yoshiya Uwatoko, Satoru Nakatsuji, Ken Livi, Maxime Siegler, Yi Li, Collin L Broholm, Tyrel M McQueen Superconductivity in a doped magnetic Weyl semimetal is predicted to be characterized by a topologically non-trivial nodal gap function. Realizing superconductivity in a centrosymmetric, non-magnetic Dirac material is a natural starting point in pursuit of such topological superconductivity. Previous theoretical and experimental work has demonstrated the existence of Dirac fermions in LaCuSb2. Here, we report on the properties of the superconducting state of LaCuSb2, including its sensitivity to magnetic fields and stoichiometry. The bulk superconducting phase transition is confirmed with specific heat, and the possible models of the gap function are discussed. |
Friday, March 18, 2022 1:06PM - 1:18PM |
Z65.00007: Twisted multilayer nodal superconductors Etienne Lantagne-Hurtubise, Tarun Tummuru, Marcel Franz Twisted bilayers of nodal superconductors were recently proposed as an attractive platform to host topological superconducting phases with spontaneously broken time reversal symmetry. Here we extend this analysis to twisted multilayers consisting of two high-symmetry stackings with (i) alternating or (ii) constant twist angles between successive layers. The alternating-twist stacking can be mapped, in analogy to alternating-twist multilayer graphene, to a collection of twisted bilayers with renormalized interlayer couplings, albeit with a remnant nodal d-wave order parameter when the number L of layers is odd. In contrast, the constant twist stacking exhibits physics beyond twisted bilayers, including the presence of band touchings with cubic dispersion when L = 3 (mod 4). Owing to their diverging density of states at the Fermi level, such cubic band touchings are susceptible to interaction-induced instabilities, which we explore using a self-consistent BCS mean-field theory. |
Friday, March 18, 2022 1:18PM - 1:30PM |
Z65.00008: Observation of intrinsic topological superconductivity in transition metal dichalcogenide Abhay K Nayak, Aviram Steinbok, Yotam Roet, Jahyun Koo, Gilad Margalit, Irena Feldman, Avior Almoalem, Amit Kanigel, Gregory A Fiete, Binghai Yan, Yuval Oreg, Nurit Avraham, Haim Beidenkopf Topological superconductors may harbor non-Abelian excitation on its boundaries such as Majorana zero mode – an essential ingredient for quantum information processing. Although, Majorana edge mode has been observed in several hybrid heterostructures and epitaxially grown materials, it is yet to be demonstrated in a layered stoichiometric material. This is essential to eliminate disorder induced artifacts and to design robust devices for quantum computing. Here, we used scanning tunneling microscopy to visualize the boundary states of intrinsic topological superconductivity in a transition metal dichalcogenide (TMD). We observed a zero-bias peak at the vortex core as well as an exponentially localized in-gap edge mode on the step edges along arbitrary direction and termination. The robust one-dimensional Majorana edge mode dispersing across the entire superconducting gap has slightly different localization length along the different directions of the step edges. We propose a simple model of nodal topological superconductivity pertinent to TMDs that captures the essential phenomenon of the one-dimensional edge mode along different directions and edge terminations. This is the first observation of Majorana edge mode in an intrinsically topologically superconducting material, marking a significant advance towards engineering two-dimensional devices for quantum computation. |
Friday, March 18, 2022 1:30PM - 1:42PM |
Z65.00009: Spin-polarized zero bias peak from a single magnetic impurity at a s-wave superconductor Kyungwha Park, Bendeguz Nyari, Andras Laszloffy, Laszlo Szunyogh, Balazs Ujfalussy Topological superconductivity has emerged a promising platform for fault-tolerant quantum computing using Majorana modes. Since intrinsic topological superconductors are rare, various heterostructures including ferromagnetic atomic chains on s-wave superconductors were proposed to realize topological superconductivity. So far, most theoretical studies in the heterostructures were done using effective models based on single orbitals. We investigate the Yu-Shiba-Rusinov (YSR) states of a single magnetic impurity (Fe, Co, Mn) at the surface of superconducting Pb using the fully relativistic first-principles simulations including band structure of Pb and 3d orbitals of the impurity in the superconducting state. For the single Fe and Co impurities, we observe strong effects of spin-orbit coupling on the YSR states as the impurity moment rotates. As the rotation angle varies, we show that two symmetric YSR peaks merge and form a zero-bias peak with large spin polarization, although effective models predict zero normalized spin polarization for such a zero-bias peak. Our results reveal importance of realistic band structure and multiple 3d orbitals of the impurity in the calculations, and they are relevant to longer atomic chains considering canting and noncollinear magnetism. |
Friday, March 18, 2022 1:42PM - 1:54PM |
Z65.00010: Mobility exceeding 100,000cm2/Vs in shallow InAs quantum wells Teng Zhang, Tyler Lindemann, Kasun Premasiri, Tailung Wu, Michael J Manfra The two-dimensional electron gas (2DEG) formed in shallow InAs quantum wells with InGaAs barriers grown on InP is a promising platform for achieving topological superconductivity. Strong spin-orbit coupling and induced superconductivity are key ingredients. In addition, it is desirable to optimize 2DEG mobility. In this study, we assess the impact of modulation doping on shallow InAs quantum well heterostructures; we report on the magnetotransport properties as a function of the doping density and the spacer thickness. We find mobility may be enhanced in the modulation-doped heterostructures in which the silicon dopants are placed below the quantum well. Our analysis suggests that the doping layer pulls the electron wavefunction away from the surface, reducing the effect of surface scattering, leading to higher mobility. Impact on Rashba spin-orbit coupling and induced superconductivity will also be discussed. |
Friday, March 18, 2022 1:54PM - 2:06PM |
Z65.00011: 1D supercurrent in a graphene heterostructure Julien Barrier, Alexey Berdyugin, Piranavan Kumaravadivel, Na Xin, Jonathan R Prance, Minsoo Kim, Rashid Jalil, Roshan Krishna Kumar, Saurav Islam, Takashi Taniguchi, Kenji Watanabe, A K Geim Topological insulators have been the subject of an intense research effort, as they could host excitations with non-trivial braiding statistics like Majorana fermions or non-abelian anyons, quasiparticles proposed as a basis for fault-tolerant quantum computers [1,2]. |
Friday, March 18, 2022 2:06PM - 2:18PM |
Z65.00012: Majorana zero modes in encapsulated bilayer graphene Pablo San-Jose, Fernando Penaranda, Ramon Aguado, Elsa Prada The search of solid state platforms exhibiting robust topological superconductivity and Majorana bound states is beginning to expand from semiconducting nanowires and related systems to two-dimensional crystals. Some previous proposals to generate Majoranas in graphene relied on interactions in the Quantum Hall regime. In this work we study the feasibility of an alternative approach that exploits graphene bilayers encapsulated in transition metal dichalcogenides, and requires potentially much smaller magnetic fields. The strong spin-orbit coupling induced on the graphene bilayer is known to open a gap in the bilayer with fragile helical edge states. We show that, when subject to an in-plane Zeeman field, armchair edge states can be proximitized into a p-wave one-dimensional topological superconductor by contacting them with a conventional superconductor. We demonstrate the emergence of Majorana bound states, both with crystallographically perfect side-contacted edges and also in more realistic vertically-contacted samples. The computed phase diagram generalizes that of the Oreg-Lutchyn model for Rashba nanowires, and suggests the existence of a topological phase within experimental reach. |
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