Bulletin of the American Physical Society
APS March Meeting 2020
Volume 65, Number 1
Monday–Friday, March 2–6, 2020; Denver, Colorado
Session R49: Superconducting Proximity Effect and Josephson Junctions IV |
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Sponsoring Units: DCMP Chair: Thomas Bullard, UES, Inc. Room: Mile High Ballroom 1B |
Thursday, March 5, 2020 8:00AM - 8:12AM |
R49.00001: Search for Majorana bound states in niobium-Bi2Se3-niobium arrays and junctions Xiangyu Song, Yang Bai, Guang Yue, James N Eckstein, Dale J Van Harlingen, Alexey Bezryadin Under the right conditions, Majorana bound states (MBS) are believed to live in lateral Josephson junctions formed by depositing s-wave superconductors onto the surface of topological insulator thin films, e.g., Bi2Se3. |
Thursday, March 5, 2020 8:12AM - 8:24AM |
R49.00002: Growth and Characterization of (BixSb1-x)2Te3 /MgB2 heterojunctions Long Cheng, Lin Li, Hui Zhang, Xiaodong Ma, George Nichols, Peter Sprenger, Deler Langenberg, David Cory, Guoxing Miao Topological insulator-superconductor heterostructure, which was predicted to be an ideal system to search Majorana Fermions, has been extensively studied in recent years. Zero bias conductance peak (ZBCP) in such system was reported as a signature of Majorana bound states by several groups. Here we report the growth of epitaxial (BixSb1-x)2Te3 (BST) thin film on MgB2 and study the vertical Josephson junction based on BST/MgB2. The interface of BST/MgB2 was found to be chemically reactive. Substrate temperature has to be lowered than typical growth temperature of BST to inactivate the reaction. In addition, a 2.5nm thin Nb layer inserted in between BST and MgB2 can significantly improve the interface quality with little reduction to the pairing gap. Finally, 50nm Nb thin film was deposited on top to form the Nb/BST/MgB2 and Nb/BST/Nb/MgB2 junction. The former one shows tunneling behavior due to the formation of insulating layer at the interface of BST/MgB2, while the latter one behaves more like a Josephson junction. ZBCPs were observed in both the junctions, which survive even above 4.2K, much higher than previously reported. Our study provides a new design for searching Majorana Fermions which may exist at a higher temperature than before. |
Thursday, March 5, 2020 8:24AM - 8:36AM |
R49.00003: Shapiro steps as a thermal probe in superconducting nanobridges Connor Shelly, Patrick See, Ivan Rungger, jonathan williams We present measurements of microwave-induced Shapiro steps in a superconducting nanobridge weak link in the dissipative branch of a hysteretic current-voltage characteristic. Typically the hot-spot Joule heating in nanobridges is suggested to extend many micrometers beyond the nanobridge itself and into the superconducting electrodes heating the system above the critical temperature. We demonstrate that the Shapiro steps can be used to infer a reduced critical current and an associated local temperature. The observation of the Shapiro steps in the dissipative branch of the IVC show that a finite Josephson coupling exists in the dissipative state, and although the nanobridge is heated, our thermal model shows that the temperature remains always below the critical temperature. This work provides evidence that Josephson behaviour can remain in thermally-hysteretic nanobridges and allows extension of the temperature range that nanobridge devices (e.g., SFQ circuits, nanoSQUIDs) can be operated in. |
Thursday, March 5, 2020 8:36AM - 8:48AM |
R49.00004: Towards Perfect Andreev Reflection in Graphene-NbSe2 planar Josephson Junctions Adam Kristopher Williams, Mitali Banerjee, Cory Dean Several theoretical proposals suggest that combining 2D superconductors with 2D semiconductors could provide a rich platform to study novel quantum phases such as Majorana zero modes — fractional quasiparticles localized at boundaries of topological superconductors. However, interfacing these two phenomena with high interfacial transparency remains a longstanding challenge. The rapidly growing catalogue of 2D Van der Waals materials provides a platform for making atomically clean interfaces between materials with a variety of properties. In this talk I will present newly developed techniques to assemble NbSe2-graphene Josephson junctions such that we simultaneously improve the junction transparency while also maintaining high mobility in the graphene channel. Transparent graphene-NbSe2 contacts as a platform to explore planar topological Josephson junctions will be discussed. |
Thursday, March 5, 2020 8:48AM - 9:00AM |
R49.00005: Tunneling spectroscopy of c-axis epitaxial cuprate junctions Douglas Natelson, Panpan Zhou, Liyang Chen, Ilya Sochnikov, Tsz Chun Wu, Matthew Foster, Anthony Travis Bollinger, Xi He, Ivan Bozovic Tunnel junctions are valuable tools for electronic spectroscopy, with epitaxial structures minimizing extrinsic effects of disorder. We report tunneling spectroscopy measurements, over a broad temperature and bias range, in c-axis La2−xSrxCuO4/La2CuO4/La2−xSrxCuO4 heterostructures grown via atomic layer-by-layer molecular beam epitaxy. Above Tc of the LSCO layers, these structures show the pseudogap, while below Tc, the devices show superconductor/insulator/superconductor (SIS) response. Down to 20 mK there is no Josephson critical supercurrent Ic, indicating strongly incoherent transport. Tunneling spectra show greatly suppressed coherence peaks. The T=0 zero-bias conductance remains at approximately 20-30% that of the normal-state, implying a substantial population of unpaired carriers. As temperature is raised, the superconducting gap fills in rather than closing at Tc. The spectra show an inelastic tunneling feature at ∼0.08 eV, suppressed as T exceeds Tc. We discuss how these properties are inconsistent with standard d-wave tunneling expectations, despite the structural perfection of the epitaxial stack. |
Thursday, March 5, 2020 9:00AM - 9:12AM |
R49.00006: Scanning Tunneling Spectroscopy Study of the Superconducting Proximity Effect in (110)-oriented Manganite/Cuprate and Nickelate/Cuprate Thin Films Rainni Chen, Chao C Zhang, Chris Granstrom, John Y.T. Wei An anomalously long-ranged (~ 30 nm) superconducting proximity effect was previously reported in thin-film heterostructures of ferromagnetic La2/3Ca1/3MnO3 (LCMO) and superconducting YBa2Cu3O7−δ (YBCO), and attributed to the induction of spin-triplet pairing [1,2]. However, there remains debate over the existence of this long-ranged proximity effect, since scanning tunneling spectroscopy (STS) on c-axis LCMO/YBCO bilayers has failed to observe any direct and microscopic evidence of this effect [3]. In this work, we extend prior STS studies to (110)-oriented films, the orientation most likely to produce d-wave Andreev resonances that show up as zero-bias conductance peaks (ZBCP). We observe these ZBCPs ubiquitously on our unilayer YBCO films, but do not observe them on our bilayer LCMO/YBCO films down to an LCMO overlayer thickness of ~ 5 nm. The ZBCPs also do not appear on (110)-oriented LaNiO3/YBCO bilayers, measured as a non-magnetic control. Our results are inconsistent with proximity-induced spin-triplet superconductivity in LCMO/YBCO structures. |
Thursday, March 5, 2020 9:12AM - 9:24AM |
R49.00007: Absence of proximity induced superconductivity in Bi/Bi2212 heterostructure Asish Kundu, Ilya Drozdov, Zebin Wu, Genda Gu, Tonica Valla Mixing of topological states with superconductivity could result in topological superconductivity, where the elusive Majorana fermions could be applied in fault-tolerant quantum computing. Recent study(1) suggests that topological superconductivity might be possible to realize in a Bi /Bi2Sr2CaCu2O8+δ (Bi2212) heterostructure by inducing a pairing in the Rashba-spin−orbit-coupled states of Bi by proximity effect. Here, we present an angle-resolved photoemission spectroscopy study of ultrathin Bi films grown in-situ on Bi2212 that show the absence of proximity effect. We find that the electron transfer from the film to the substrate and the resulting severe underdoping of Bi2212 interface is a likely origin for the absence of proximity effect. We also propose a possible way of preventing a total loss of proximity effect in this system. |
Thursday, March 5, 2020 9:24AM - 9:36AM |
R49.00008: The proximity effect in disordered and quasiperiodic systems Gautam Rai, Stephan Wolfgang Haas, Anuradha Jagannathan We investigate the superconducting proximity effect in one-dimensional hybrid rings with a superconducting and a normal part. We consider two cases: when the normal part is a) a weakly disordered crystal and b) quasiperiodic. We carry out self-consistent calculations using a real space mean field Bogoliubov-de Gennes type approach within the tight binding framework. For the disordered case we find that the decay of the superconducting order parameter into the normal region reflects the the band center anomaly—a cusp like deformation in the density of states around E=0 known to exist for weak disorder. The order parameter decay when the normal chain has a Fibonacci hopping sequence will be discussed as well, along with the key features that differentiate the proximity effect in the two cases. |
Thursday, March 5, 2020 9:36AM - 9:48AM |
R49.00009: Proximitized Josephson junctions in highly-doped InAs nanowires robust to optical illumination Lily Yang, Stephan Steinhauer, Elia Strambini, Thomas Lettner, Lucas Schweickert, Marijn Versteegh, Francesco Giazotto, Valentina Zannier, Lucia Sorba, Dmitry Solenov We have studied the effects of optical-frequency light on proximitized InAs/Al Josephson junctions based on highly n-doped InAs nanowires at varying incident photon flux and at three different photon wavelengths. The experimentally obtained IV curves were modeled using a shunted junction model which takes scattering at the contact interfaces into account. The Josephson junctions were found to be surprisingly robust, interacting with the incident radiation only through heating, whereas above the critical current our devices showed non-thermal effects resulting from photon exposure. Our work provides guidelines for the co-integration of Josephson junctions alongside quantum photonic circuits and lays the foundation for future work on nanowire-based hybrid photon detectors. |
Thursday, March 5, 2020 9:48AM - 10:00AM |
R49.00010: Quantum phenomona in Unconventional 1D Josephson Arrays Wen-Sen Lu, Konstantin Kalashnikov, Plamen Kamenov, Thomas J DiNapoli, Michael Gershenson The largely unexplored regime of low transparency Josephson junction chains with low charging energy EC has attracted theoretical interest since it offers a platform to realize many-body effects in superconducting circuits [1] and quantum phase slip devices [2]. In this work we study the current-voltage characteristics (IVc) of Josephson junction with Josephson energy EJ ~ EC ~ 0.01-1K and temperature range 0.03-1K. We observed the suppression of switching current ISW and finite zero-biased-resistance R0 for different EJ/EC and EJ/T ratios due to thermally activated phase slips and macroscopic quantum phase tunneling. |
Thursday, March 5, 2020 10:00AM - 10:12AM |
R49.00011: Proximity effects of niobium nano-hydrides on state of the art superconducting radio frequency Nb cavity ZuHawn Sung, Alexander Romanenko, Anna Grassellino Nb hydrides are responsible for performance degradation at high accelerating field regime in a superconducting radio frequency cavity for the linear particle accelerator, known as high field Q slope (HFQS). Normal conductive Nb hydride precipitates at the cavity operation temperature , ~2K, and leads to breakdown of surface superconductivity due to proximity effect under RF magnetic field, from which significant power dissipation occurs on the cavity surface. In this study, precipitation of nano-meter scale Nb hydrides on a SRF Nb cavity surface is first directly observed using cryogenic atomic force microscopy, and statistical morphological features of Nb hydrides are compared with HFQS behaviors of state of the art SRF Nb cavities that have shown high accelerating gradient close to the theoretical limit, 50MV/m. Additionally, RF magnetic field enhancement due to Nb hydride topological properties are discussed. |
Thursday, March 5, 2020 10:12AM - 10:24AM |
R49.00012: Low Temperature Annealing and the Generation of Superconductivity in Nickel-Bismuth Bilayers Matthew Vaughan, Nathan Satchell, Mannan Ali, Christy Kinane, Gavin Stenning, Sean Langridge, Gavin Burnell Unconventional superconductivity has been suggested to be present at Ni/Bi interfaces in thin-film bilayers. We report a study of the structural, magnetic and superconducting properties of sputtered Bi/Ni bilayers. As-grown, our films do not display a superconducting transition. However, when stored at room temperature, after about 14 days our bilayers do develop a superconducting transition up to 3.8 K. |
Thursday, March 5, 2020 10:24AM - 10:36AM |
R49.00013: Superconducting Phase Slip Oscillations on Demand Iris Dorn, Armen Gulian Remarkable properties of phase slip centers (PSCs) have direct analog with weakly coupled superconductors (Josephson contacts, SNS-junctions, ScS systems, etc.). In typical 1-D wire models of these types, PSC oscillations arise in the geometric wire centers or, if more than one PSC, symmetrically to this center location. We add weak spots in the model, which shift the position of the PSCs away from the symmetric location in a degree dependent on the imperfection’s character, by taking into account enhanced phonon feedback onto the Cooper-pair/normal metal system of the wire which generates these phonons. The inclusion of this effect into the time-dependent Ginzburg-Landau (TDGL) system of equations requires an additional function and corresponding equation in the TDGL system. Interestingly, the system finds there is a competition between the induced weak points and symmetry-induced locations. Thus using our approach the PSCs can be generated on demand and provides the opportunity to initiate PSCs at the required time and required place in the wire without changing its biasing properties with high accuracy theoretical modeling. |
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