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 A52: Topological Superconductivity: Quantum Wires and 2DEGFocus Live
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Sponsoring Units: DMP Chair: Alex Matos Abiague, Wayne State Univ |
Monday, March 15, 2021 8:00AM - 8:36AM Live |
A52.00001: Topological superconductivity in superconductor–semiconductor heterostructures Invited Speaker: Roman Lutchyn Superconductor–semiconductor heterostructures represent a versatile platform for realizing Majorana zero-energy modes. In this talk, I will explain how to engineer topological superconductivity at the interface of a conventional (s-wave) superconductor and a semiconductor with spin-orbit interaction. I will discuss state-of-the-art numerical approaches for modeling realistic devices which take into account proximity-induced superconductivity, orbital and Zeeman effect of an applied magnetic field, spin-orbit coupling as well as the electrostatic environment on equal footing. Finally, I will review recent materials science progress in growing superconductor–semiconductor heterostructures and discuss promising new directions. |
Monday, March 15, 2021 8:36AM - 8:48AM Live |
A52.00002: Non-Local Conductance in Three-Terminal Majorana Nanowires, Part 1 — Deterministic Control over Crossed Andreev Reflection Using Quantum Dots Guanzhong Wang, Tom Dvir, Nick van Loo, Grzegorz Mazur, Ghada Badawy, Sasa Gazibegovic, Erik P. A. M. Bakkers, Leo Kouwenhoven, Gijs De Lange The hallmark of proximity-induced superconductivity in a semiconducting system is the energy gap, resulting from the electron-hole correlations formed in the semiconductor through Andreev reflections. These correlations are also present above the gap, where the quasiparticle excitations have both an electron and a hole character. While conventional tunnel spectroscopy is only sensitive to the local density of states, non-local measurements are sensitive to the bulk properties of the proximitized system. We study the non-local conductance of an InSb nanowire coupled to superconducting Al, where the conductance is carried out through a quantum dot. By tuning the dot's potential and the applied bias, we deterministically control the non-local transport mechanism — crossed Andreev reflection or direct transport — and thus measure separately the electron and hole components of the quasiparticle wavefunction. We show that these two components are of similar magnitude, in full agreement with the BCS prediction. |
Monday, March 15, 2021 8:48AM - 9:00AM Live |
A52.00003: Non-local conductance in three-terminal Majorana nanowires part 2 - spin-polarization of the bulk bands Tom Dvir, Guanzhong Wang, Nick van Loo, Grzegorz Mazur, Ghada Badawy, Sasa Gazibegovic, Erik P. A. M. Bakkers, Leo Kouwenhoven, Gijs De Lange High spin-orbit semiconducting nanowires coupled to superconductors are predicted to undergo a topological phase transition at a finite magnetic field, accompanied by the appearance of Majorana zero modes. At the phase transition, the superconducting gap at the low momentum region of the Brillouin zone is closed and is re-opened due to the Zeeman term. The low energy excitations of the bulk nanowire bands are thus expected to be spin-polarized, and this polarization is expected to flip sign across the phase transition. Using non-local conductance measurements that include spin-polarized quantum dots, we measure the spin polarization of the bulk bands. With the application of small magnetic fields, we observe trivial spin polarization signatures that disappear following the closing of the induced gap. We discuss the implication of these results to the attempts of achieving topological superconductivity in porximitized nanowires. |
Monday, March 15, 2021 9:00AM - 9:12AM Live |
A52.00004: Crossed Andreev reflection in an InSb 2DEG in the quantum Hall state Ivan Kulesh, Candice Thomas, Sara Metti, Charles Guinn, Raymond Kallaher, Geoffrey C. Gardner, Michael Manfra, Srijit Goswami We embark on a novel approach to engineer a superconducting topological state of matter utilizing integer quantum hall (QH) edge states, coupled via crossed Andreev reflection (CAR) through a thin superconducting strip. This proposal has advantages in terms of control and manipulation of Majorona modes - building blocks for protected quantum information manipulation. Challenges arise due to conflicting requirements of a large magnetic field (to enter the QH regime), induced superconductivity and large superconducting coherence length of the strip. As a promising platform to implement this proposal, we use an InSb two-dimensional electron gas, which has high mobility and large g-factor, allowing to access ν=1 regime at fields around 3T. In addition, it has a large spin-orbit interaction, which is required to couple spin-polarized electrons through a superconductor. We implement and test several types of surface and side contacts of a high critical field Nb-based superconductors. We then explore behavior of the system in QH regime, where signatures of CAR are expected. |
Monday, March 15, 2021 9:12AM - 9:24AM Live |
A52.00005: Proximity induced superconductivity in epitaxial Nb/Au(111) bilayers Cliff Chen, Patrick A Lee, Jagadeesh S Moodera, Peng Wei Majorana zero modes (MZM) are novel quasi-particle excitations that are most accessible to experiments through hybrid systems combining spin-orbit coupling, superconductivity, and magnetism. Recent work demonstrates that a pair of MZMs can emerge in a heterostructure of epitaxial Au(111) sandwiched between superconducting vanadium and the magnetic insulator EuS [1,2]. The induced superconducting gap in the Au (111) layer topologically protects the MZMs. In this talk, we present our work in swapping the vanadium layer with niobium (Nb) in order to increase the size of the induced gap. We use molecular beam epitaxy to grow epitaxial Au (111) of various thicknesses on 8nm of epitaxial Nb (110). The sample quality is confirmed by x-ray diffraction and in-situ reflection high energy electron diffraction. In addition, evolution of the induced gap as a function of the film thickness is examined by point contact Andreev reflection (PCAR). Magnetic field dependent PCAR measurements are also discussed. |
Monday, March 15, 2021 9:24AM - 9:36AM Live |
A52.00006: Topological Superconductivity in Ferromagnetic Insulator-Superconductor-Semiconductor Heterostructures Aleksei Khindanov, Andrey Antipov, William S Cole, Jason F. Alicea, Patrick Lee Superconductor-semiconductor heterostructures in magnetic field are widely viewed as a promising platform for realizing topological superconductivity. In this work, using Usadel equation we theoretically investigate ferromagnetic insulator-superconductor-semiconductor heterostructures taking into account spin-orbit scattering, spin-flip scattering and Zeeman field in the parent superconductor. We find that critical field needed to induce topological superconductivity can be reduced by the presence of Zeeman field in the parent superconductor while preserving value of the topological gap. Besides, we find that spin-orbit scattering, if present in the parent superconductor, can substantially increase critical Zeeman field of the parent superconductor, thus potentially allowing for observation of topological phase at higher fields. Our results could be of use for understanding recent and future experiments with EuS-Al-InAs heterostructures [1]. |
Monday, March 15, 2021 9:36AM - 9:48AM Live |
A52.00007: Topological superconductivity in ferromagnetic hybrid nanowires Samuel D. Escribano, Elsa Prada, Yuval Oreg, Alfredo Levy-Yeyati Hybrid nanowire devices combining expitaxial ferromagnetic insulator and superconducting layers have been recently proposed [1-3] as a platform for topological superconductivity without the need of applying an external magnetic field. In this theoretical work we show that the topological regime can arise in these devices provided that certain geometrical and electrostatic conditions are met. To this end we perform detailed microscopic simulations in which we take into account both the magnetic and superconducting proximity effects. We also include self-consistently the interaction with the electrostatic environment that typically surrounds these hybrid wires in the experiments. Our calculations indicate that there is a strong magnetization in the nanowire regions close to the interface with the ferromagnet and a weaker one inside the superconductor. As a consequence, the topological phase depends critically on the wavefunction location across the wire’s cross-section, something that can be properly controlled by tuning the external gates. |
Monday, March 15, 2021 9:48AM - 10:00AM Live |
A52.00008: Single-shot fabrication of semiconducting-superconducting nanowire devices Francesco Borsoi, Grzegorz Mazur, Nick van Loo, Michal Nowak, Leo Bourdet, Kongyi Li, Svetlana Korneychuk, Alexandra Fursina, Elvedin Memisevic, Ghada Badawy, Sasa Gazibegovic, Kevin Van Hoogdalem, Erik P. A. M. Bakkers, Leo Kouwenhoven, Sebastian Heedt, Marina Quintero-Peréz Semiconducting-superconducting nanowires attract widespread interest owing to the presence of elusive Majorana zero modes, which hold promise for topological quantum computation. The systematic search for Majoranas signatures is challenging because it requires reproducible hybrid devices and reliable fabrication methods. Here, we exploit a fabrication platform based on shadow walls that enables the in-situ, selective and consecutive depositions of superconductors and normal metals to form normal-superconducting junctions. Crucially, this method allows realizing devices in a single shot, eliminating fabrication steps after the synthesis of the fragile semiconductor/superconductor interface. At the atomic level, all investigated devices reveal a sharp and defect-free semiconducting-superconducting interface and, correspondingly, we measure electrically a hard induced superconducting gap. While the cleanliness of our technique enables systematic studies of topological superconductivity in nanowires, it also allows for the synthesis of advanced nano-devices based on a wide range of material combinations and geometries while maintaining an exceptionally high interface quality. |
Monday, March 15, 2021 10:00AM - 10:12AM Live |
A52.00009: Enhanced Majorana bound states in magnetic chains on superconducting topological insulator edges Luis Dias Da Silva, Raphael L. R. C. Teixeira, Dushko Kuzmanovski, Annica M Black-Schaffer The most promising mechanisms for the formation of Majorana bound states (MBSs) in condensed matter systems involve 1D systems (magnetic chains, nanowires, quantum spin-Hall (QSHI) edge states) proximitized to superconducting materials. In this contribution, we propose that a magnetic chain deposited on a QSHI edge in contact with a superconducting surface, offers a better choice of tunability and MBS robustness compared to the same magnetic chain deposited on the QSHI bulk or a superconduting surface. We show that MBSs near the QSHI edge can be realized with lower chemical potential and Zeeman field than the ones inside the bulk, independently of the chain's magnetic order. Moreover, we quantify the "quality" of MBSs by calculating the Majorana Polarization (MP) for different configurations. For chains located at the edge, the MP is close to its maximum value already for short chains. For chains located away from the edge, longer chains are needed to attain the same quality as chains located at the edge. |
Monday, March 15, 2021 10:12AM - 10:24AM Live |
A52.00010: Polarized Majorana fermions in double Kitaev zigzag honeycomb nanoribbons Renan Bento Ribeiro, Jorge Huamani Correa, Luciano Siliano Ricco, Antonio Carlos Seridonio, Marcos Figueira In this work, we study Majorana fermions' formation at the ends of a finite double spin-Kitaev zigzag honeycomb nanoribbon (KZHNR). We characterize the system using a tight-binding minimal model that consists of the first nearest neighbor hopping, the intrinsic Rashba spin-orbit coupling, the triplet superconducting pairing between the atomic spins of the double ZHNR edges, and an external exchange magnetic field. |
Monday, March 15, 2021 10:24AM - 10:36AM Live |
A52.00011: Majorana Zero Modes in Magnetic Texture Vortices Panagiotis Kotetes, Daniel Steffensen, Brian Møller Andersen We propose a novel route to engineer Majorana zero modes (MZMs), which relies on inducing shift or spin vortex defects in magnetic textures which microscopically coexist or are in proximity to a superconductor. The underlying superconducting host is required to contain robust point nodes in its bulk energy spectrum, which become all gapped by the magnetic texture. Our mechanism is generic and it applies to nodal superconductors of spin-singlet, -triplet, or mixed type of pairing. Moreover, it provides a new framework to understand the recent observations of pairs of MZMs in superconductor - magnetic adatom systems, and it can further inspire the experimental development of new platforms which consist of nanowires in proximity to conventional superconductors with strong Rashba spin-orbit coupling. |
Monday, March 15, 2021 10:36AM - 10:48AM Live |
A52.00012: Self consistent order parameter calculations in superconductor-nanowire hybrid systems Atri Dutta, Praveen Sriram, Bhaskaran Muralidharan Semiconducting nanowire-superconductor hybrid systems provide a promising platform for realizing topological superconductivity that is necessary to host Majorana bound states (MBS). In this study, we employ the Keldsyh nonequilibrium Green's function (NEGF) formalism [1] coupled self consistently with order parameter calculations [2], in order to elucidate the subtleties involved in the issue of current conservation in the N (normal metal)-S (topological superconductor)-N (normal) link. By considering the Kitaev chain as the model system of a topological superconductor, we analyze the transport characteristics pointing out significant variations in the order parameter away from the topological region, and near the topological phase transition point. Our calculations reveal that the bias set up of the N-S-N set up significantly changes the local and non-local contributions to the currents and hence conductances. These results point toward the importance of self consistent evaluation of the order-parameter for accurate and realistic descriptions of the Majorana nanowire set up. |
Monday, March 15, 2021 10:48AM - 11:00AM Live |
A52.00013: Electronic properties of InAs/Al/EuS hybrid nanowires: Effective Zeeman spin splitting and topological phase diagram Chun-Xiao Liu, Yu Liu, Sergej Schuwalow, Peter Krogstrup, Michael Wimmer Motivated by recent experiments on InAs/Al/EuS heterostructures, we theoretically study the electronic properties of semiconductor-superconductor-ferromagnetic insulator hybrid nanowires. Our focus is on the physical mechanisms for inducing the effective Zeeman energy and on whether topological superconductivity can be achieved in these devices. We show that the magnetic proximity effects at the Al/EuS as well as the InAs/EuS interfaces are both essential for inducing a sufficiently large total Zeeman energy. Such a Zeeman energy would also depend on the applied gate voltages, the effect of which is included via an electrostatic potential calculated by the Thomas Fermi-Poisson method. Finally we map out the topological phase diagram of the hybrid system as a function of both the gate voltages and the proximity-induced exchange coupling. |
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