Bulletin of the American Physical Society
APS March Meeting 2019
Volume 64, Number 2
Monday–Friday, March 4–8, 2019; Boston, Massachusetts
Session L09: Superconductivity: Proximity Effect and Josephson Junctions I |
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Sponsoring Units: DCMP Chair: Michael Osofsky, United States Naval Research Laboratory Room: BCEC 151A |
Wednesday, March 6, 2019 11:15AM - 11:27AM |
L09.00001: Quantum Vortex Melting and Superconductor Insulator Transition in a 2D Josephson Junction Array in a Perpendicular Magnetic Field via Diffusion Monte Carlo Pragalv Karki, Yen Lee Loh In this study [1], we simulated a quantum rotor model describing a Josephson junction array (JJA) in the phase representation at zero temperature in a perpendicular magnetic field B=0.1,0.2,0.3,0.4 (in units of h/4πea2) on a LxL square lattice with spacing a for L=6,8,10,12. The superconductor-insulator transition (SIT) is tuned by the ratio of charging energy to Josephson coupling, U/J. Abrupt drops in the magnetization values were observed in the bigger lattices at certain values of B and U/J caused by the formation of vortices. Increasing U/J at a fixed B field causes quantum vortex melting. The magnetization drops to zero around U/J ∼ 5 indicating SIT. For B=0.1 the SIT occurs without an intermediate vortex state and the magnetization scales as M ∼ L4, whereas for B=0.4 the scaling is M ∼ L4 during the vortex melting. For B between 0.1 and 0.4 the scaling is not clear. We used the diffusion Monte Carlo (DMC) method with a guiding wavefunction optimized using the variational Monte Carlo (VMC) method. |
Wednesday, March 6, 2019 11:27AM - 11:39AM |
L09.00002: Protected gap closing in Josephson junctions constructed on Bi2Te3 surface Zhaozheng Lyu, Yuan Pang, Junhua Wang, Guang Yang, Guangtong Liu, Fanming Qu, Li Lu, Jie Fan, Zhongqing Ji, Xiunian Jing On the road of searching for Majorana zero modes(MZMs) in topological insulator-based Josephson junctions, a highly-sought signature is the protected full transparency of electron transport through the junctions due to the existence of the MZMs, associated with complete gap closing between the electronlike and holelike Andreev bound states (ABSs). We fabricated Pb-Bi2Te3-PbJosephson junctions and developed the method of using nonsuperconducting Pd electrodes to detect the ABS spectra in the junctions area. We generalized the Blonder-Tinkham-Klapwijk theory to describe the measured contact resistance of the Pd-Bi2Te3 interface. Our results provide direct experimental evidence of complete gap-closing and full transparent transport in single Josephson junctions constructed on the surface of Bi2Te3. |
Wednesday, March 6, 2019 11:39AM - 11:51AM |
L09.00003: A Mesoscopic Spectrometer Based on the Josephson Effect Joel Griesmar, Vincent Benzoni, Fabien Lafont, Leo Peyruchat, Jean-Loup Smirr, Caglar Girit A key element of mesoscopic topological systems, such as hybrid semiconductor-superconductor circuits, are Andreev Bound States, single quasiparticles localized at superconducting weak links. The characteristic transition energy of these states is twice the superconducting gap (90 GHz in aluminum). Conventional microwave techniques allow probing these states but only in a limited bandwidth. We propose a new broadband spectrometer operating at frequencies up to 180 GHz based on the Josephson effect which converts a DC voltage to microwave oscillations at a frequency proportional to this voltage. Using a symmetrical SQUID biased at half a flux quantum allows decoupling the spectrometer from environmental modes. In addition, careful design of the biasing circuit reduces the number of remaining modes and damps them. The fabricated mesoscopic spectrometer has a linewidth of 2 MHz, a bandwidth of 180 GHz and a minimal theoretical sensitivity of 5 kHz. |
Wednesday, March 6, 2019 11:51AM - 12:03PM |
L09.00004: Quantum Hall Supercurrent in a locally gated Graphene Josephson Junction - Part 1: Induction of Local Filling Factors and Quantum Hall Supercurrent Andrew Seredinski, Anne M Draelos, Ethan Arnault, Ming-Tso Wei, Hengming Li, Tate Fleming, Kenji Watanabe, Takashi Taniguchi, Francois Amet, Gleb Finkelstein We present a study of a graphene-based Josephson junction with local gates modulating the carrier density along either edge of the junction in a wide range. In magnetic fields in the 1-2 Tesla range, we populate the next Landau level, resulting in Hall plateaus with conductance that differs from the bulk filling factor. We observe robust supercurrent when the gating introduces counter-propagating quantum Hall edge states along either edge of the junction. |
Wednesday, March 6, 2019 12:03PM - 12:15PM |
L09.00005: Quantum Hall Supercurrent in a locally gated Graphene Josephson Junction - Part 2: Evolution of Interference patterns with Carrier Density and Field Ethan Arnault, Andrew Seredinski, Anne M Draelos, Ming-Tso Wei, Hengming Li, Tate Fleming, Kenji Watanabe, Takashi Taniguchi, Francois Amet, Gleb Finkelstein We explore supercurrent induced along individual edges in a locally gated graphene-based Josephson junction in the quantum Hall regime. When supercurrents are present along both sides of the junction, they produce interference pattern as a function of magnetic field, bulk and edge carrier densities. This behavior allows us to infer information about the position of the edge states. We present electrostatic models relating the evolution of the interference patterns to the physical location of quantum Hall edge states in the device. |
Wednesday, March 6, 2019 12:15PM - 12:27PM |
L09.00006: Realization of Hybrid Superconductor–Semiconductor Systems by Homoepitaxial Growth of Non-equilibrium P-doped Si(111) Kasra Sardashti, Kaushini Wickramasinghe, Tri Nguyen, William Andrew Mayer, Mehdi Hatefipour, Joseph Yuan, Javad Shabani To fulfill the long-term vision of reliable quantum computation, the ideal hybrid platform allows quantum information to be processed, stored and transmitted in the same materials system. Silicon can be a promising candidate for fault-tolerant hybrid quantum systems due to its multifunctionality, reliability and long coherence times. Superconducting Si could provide a viable pathway for realization of Si qubit circuits, by forming homogeneous Superconductor (SC)–Semiconductor–SC Josephson junctions. In this work, non-equilibrium p-doping of Si thin-films by homoepitaxial growth of Ga-rich Si on Si(111) is studied. Surface structure and morphology of the p-doped films were evaluated by reflection high-energy electron diffraction and atomic force microscopy. Doping levels and carrier concentrations were determined by temperature-dependent resistivity (ρ–T) measurements in van der Pauw configuration and on Hall bars. Upon increase in Ga incorporation, ρ–T characteristics of the p-doped films transitioned from insulating to quasi-reentrant superconductivity with Rmin/RN of 0.82–0.89. Based on the known theories, mechanisms and strategies to go beyond the quasi-reentrant regime will be discussed. |
Wednesday, March 6, 2019 12:27PM - 12:39PM |
L09.00007: Spin-orbit splitting of Andreev states revealed by microwave spectroscopy Leandro Tosi, Cyril Metzger, Marcelo Goffman, Cristian Urbina, Hugues Pothier, sunghun park, Alfredo Levy Yeyati, Peter Krogstrup, Jesper Nygård The Josephson supercurrent that flows through a weak link between two superconductors is mediated by fermionic quasiparticle states localized at the weak link: the Andreev bound states. To explore the role of the spin of these states, we have performed their microwave absorption spectroscopy in superconducting weak links with strong spin-orbit coupling: an InAs-Al (core-full shell) epitaxially-grown nanowire. The spectra present distinctive features that we interpret as arising from zero-field spin-split Andreev states. A simple empirical model, which takes into account the Rashba spin-orbit interaction in a multichannel nanowire, explains these features and their evolution with magnetic field. Our results show that the spin of quasiparticles can be a relevant degree of freedom in Josephson weak links. |
Wednesday, March 6, 2019 12:39PM - 12:51PM |
L09.00008: Anomalous interference in planar WTe2 Josephson junctions Andrew Pierce, Johannes Gooth, Michael Kosowsky, Claudia Felser, Amir Yacoby Josephson junctions fabricated on materials with strong spin-orbit coupling (SOC) have driven substantial research efforts due to the possibility of realizing topological superconductivity (TSC) in such devices. Layered materials with strong SOC, especially those which can be isolated via mechanical exfoliation, are particularly attractive due to their natural compatibility with large in-plane magnetic fields. This talk discusses the evolution of the critical current of WTe2 Josephson junctions as a function of in-plane magnetic field. Thin (<100 nm) samples of WTe2 are of interest for applications to TSC as they provide strong SOC with the required type of Fermi-surface spin texture. Under certain conditions, the interference patterns observed in these devices diverge from the standard Fraunhofer-like pattern that follows from the simplest set of assumptions, suggesting the presence of important orbital effects. |
Wednesday, March 6, 2019 12:51PM - 1:03PM |
L09.00009: WITHDRAWN ABSTRACT
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Wednesday, March 6, 2019 1:03PM - 1:15PM |
L09.00010: Manipulation of Phases and Supercurrents in S-TI-S Lateral Josephson Junction Networks Erik Huemiller, Can Zhang, Guang Yue, Seongshik Oh, Dale J Van Harlingen Quantum computing using topological Q-bits composed of Majorana Fermions (MF) shows promise for its robustness against disorder and its scalability with existing lithography infrastructure. In lateral Josephson junction networks, the MFs are localized to points in the junction where the phase difference of the superconducting order parameter across the junction is equal to an odd multiple of π. Quantum computations are performed by physically moving MFs pinned to odd π crossings around one another, encoding the quantum state of the system in the historical locations of the MFs relative to each other. Understanding the phase distributions around multi-junction nodes in the network is one key hurdle that needs to be overcome before the implementation of this architecture is a reality. Measurements and modeling of Nb-BiSe-Nb lateral Josephson junctions in three and four junction geometries will be presented. The results will be discussed with their implication for the realization of a topological quantum architecture. |
Wednesday, March 6, 2019 1:15PM - 1:27PM |
L09.00011: Transport Studies in Gate-Tunable Multi-Terminal Josephson Junctions Gino Graziano, Joon Sue Lee, Mihir Pendharkar, Chris Palmstrom, Vlad S Pribiag Josephson junctions with three or more superconducting leads are predicted to exhibit topological physics in the presence of few conducting modes within the interstitial normal material.[1][2] Such topological behavior manifests itself as signatures in the complex transport properties between the different terminals, with topological phase transitions occurring as a function of phase and voltage bias.[3] Here we study the superconducting and resistive properties of top-gated multi-terminal Josephson devices, based on an InAs 2DEG proximitized with epitaxial aluminum. The top gate is used to deplete the 2DEG, and resistances are analyzed under various bias currents and magnetic fields. |
Wednesday, March 6, 2019 1:27PM - 1:39PM |
L09.00012: Spin-triplet supercurrents in Josephson junctions with Co/Py exchange-spring interfaces Ekta Bhatia, James Devine Stoneman, Sachio Komori, Anand Srivastava, Zoe Barber, Kartik Senapati, Jason Robinson Conventional (spin-singlet) s-wave superconductivity (S) and ferromagnetism (F) are incompatible. However, it is now established that s-wave S can coexist with F through the conversion of spin-singlet to spin-triplet Cooper pairs at a magnetically inhomogeneous interface [1]. Here we report Josephson coupling in Nb/Co/Py/Nb Josephson junctions in which the total bilayer (Co/Py) thickness exceeds the singlet pair coherence length [1nm in Py [2]). At a Co/Py interface, the interface exchange coupling is strong compared to the weak magnetic anisotropy of Py and so, depending on the direction and magnitude of an external magnetic field, an in-plane Bloch domain wall can form in Py creating the necessary magnetic non-collinearity for pair conversion [3]. We observe Josephson coupling for Py layer thicknesses up to 11 nm which greatly exceeds the singlet coherence length, strongly suggesting the propagation of spin-polarized triplet supercurrents. Furthermore, through magnetic field history we are able to programme the magnetic state, which offers the potential for active control of triplet supercurrents. |
Wednesday, March 6, 2019 1:39PM - 1:51PM |
L09.00013: Non-local Josephson effect in Andreev molecules Caglar Girit, Jean-Damien Pillet, Vincent Benzoni, Joel Griesmar, Jean-Loup Smirr We propose the Andreev molecule, an artificial quantum system comprised of two closely spaced Josephson junctions. As in a real molecule, the coupling between Josephson junctions in an Andreev molecule occurs through the overlap and hybridization of the junction's "atomic" orbitals, the Andreev Bound States. One of the striking consequences of molecular hybridization is that the supercurrent flowing through one junction depends on the superconducting phase difference across the other junction. The energy spectrum of Andreev molecules have gaps which open as the inter-junction separation is reduced. The current-phase relations are non-local and demonstrate the possibility of a non-zero supercurrent at zero phase difference, a φ-junction. In order to synthesize and detect Andreev molecules, we propose experiments on devices fabricated only with conventional materials and standard nanofabrication techniques. Andreev molecules are a new class of superconducting quantum devices with potential applications in quantum information, metrology, sensing, and molecular simulation. |
Wednesday, March 6, 2019 1:51PM - 2:03PM |
L09.00014: Phase transitions in current biased superconductors Vadim Oganesyan, Negin Moharrami Allafi We consider models of Josephson coupled superconducting wires and sheets in the presence of current biasing. We study signatures of a phase transition between fully locked ("locked") and unlocked ("soliton") phases akin to the conventional commensurate-incommensurate transition in mutual inductance and tunnelling spectroscopy. |
Wednesday, March 6, 2019 2:03PM - 2:15PM |
L09.00015: Probing the magnetic screening properties of superconductor-ferromagnet hybrids. Nathan Satchell, Patrick Quarterman, Reza Loloee, Brian Kirby, Julie Borchers, Norman Owen Birge Ferromagnetic Josephson junctions are a strong candidate for a dissipationless cryogenic memory alternative to dissipative CMOS technologies. Much attention has been focused on what happens to the supercurrent propagating through the Josephson junction, including the discovery of spin aligned triplet Cooper pairs in these systems [1]. In parallel, the magnetic screening properties of superconductors in electronic proximity to ferromagnetic materials have been studied, as an additional component of the screening is expected to be induced by the proximity effect [2]. These additional screening currents may account for several experimental observations, most notably in Co/Nb/Cu trilayers where an anomalous Meissner effect is reported by the low energy muon spin rotation technique [3]. In this work, we report our progress characterising the magnetic screening properties of isolated Nb (200 nm) and a bilayer of Ni (x nm)/Nb (200 nm) using polarized neutron reflectometry and the Fraunhofer characteristic of Josephson junctions. |
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