Bulletin of the American Physical Society
APS March Meeting 2020
Volume 65, Number 1
Monday–Friday, March 2–6, 2020; Denver, Colorado
Session L49: Superconducting Proximity Effect and Josephson Junctions III |
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Sponsoring Units: DCMP Chair: Keith Taddei, Oak Ridge National Lab Room: Mile High Ballroom 1B |
Wednesday, March 4, 2020 8:00AM - 8:12AM |
L49.00001: Novel superconducting effects in multiterminal Josephson junctions, Part 1 Andrew Seredinski, Ethan Arnault, Trevyn Larson, Lingfei Zhao, Hengming Li, Kenji Watanabe, Takashi Taniguchi, Francois Amet, Ivan Borzenets, Gleb Finkelstein We present comprehensive transport measurements of multiterminal graphene Josephson junctions. We have previously found that the coupling between different pairs of contacts results in nontrivial distribution of supercurrent across the device. Here, we isolate supercurrent mediated entirely by continuum contributions with energies above the superconducting gap. We also characterize the superconducting behaviors induced by out-of-plane magnetic fields and RF radiation. |
Wednesday, March 4, 2020 8:12AM - 8:24AM |
L49.00002: Novel superconducting effects in multiterminal Josephson junctions, Part 2 Ethan Arnault, Andrew Seredinski, Trevyn Larson, Lingfei Zhao, Hengming Li, Kenji Watanabe, Takashi Taniguchi, Francois Amet, Ivan Borzenets, Gleb Finkelstein We present comprehensive transport measurements of multiterminal graphene Josephson junctions. We have previously found that the coupling between different pairs of contacts results in nontrivial distribution of supercurrent across the device. Here, we isolate supercurrent mediated entirely by continuum contributions with energies above the superconducting gap. We also characterize the superconducting behaviors induced by out-of-plane magnetic fields and RF radiation. |
Wednesday, March 4, 2020 8:24AM - 8:36AM |
L49.00003: Radio-frequency response of a graphene Josephson junction based superconducting oscillator circuit Subhamoy Ghatak, Joydip Sarkar, Pratap Adak, Biswajit Datta, SUPRIYA MANDAL, Lucky N. Kapoor, Kishor V Salunkhe, Suman Kundu, R Vijay, Mandar M Deshmukh Ballistic transport in a graphene-based Josephson junction provides a great opportunity to realize different superconducting quantum circuits. Such circuits are tunable by various externally controllable knobs like - doping, electric field, magnetic field; hence, they can have the potential to be used as superconducting qubits over the existing ones which do not have such variety of tunability. We have devised a superconducting oscillator circuit, employing graphene-based Josephson junction. The analogous electrical circuit is a simple parallel LC oscillator, where the oscillator frequency is tunable through magnetic flux variation and carrier density modulation in graphene. The response of a Josephson junction as an inductor and its non-linearity is vital. The readout of the device involves coupling of an external RF signal to the oscillator through a reflection based measurement. Such a response can be promising for the future direction of graphene-based quantum devices. |
Wednesday, March 4, 2020 8:36AM - 8:48AM |
L49.00004: Gate Controlled Anomalous Phase Shift in Al/InAs Josephson Junctions William Mayer, Matthieu Dartiailh, Joseph Yuan, Kaushini S Wickramasinghe, Enrico Rossi, Javad Shabani In a standard Josephson junction the current is zero when the phase difference between superconducting leads is zero. This condition is protected by parity and time-reversal symmetries. However, the combined presence of spin-orbit coupling, and in-plane magnetic field breaks these symmetries, which can lead to a finite supercurrent even when the phase difference is zero. This is the anomalous Josephson effect which can be characterized by the corresponding anomalous phase shift. We report the observation of a tunable anomalous Josephson effect in Al/InAs Josephson junctions measured via a superconducting quantum interference device. By gate controlling the density of InAs, we are able to tune the spin-orbit coupling in the Josephson junction. This gives us the ability to tune the anomalous phase using both in-plane magnetic field and gate voltage. We observe anomalous phase shifts larger than expected from theory for our material parameters. These results open new opportunities for superconducting spintronics, and new possibilities for realizing and characterizing topological superconductivity. |
Wednesday, March 4, 2020 8:48AM - 9:00AM |
L49.00005: Superconductivity induced in InSb nanowires from new thin film superconductors Bomin Zhang, Mihir Pendharkar, Po Zhang, Hao Wu, Azarin Zarassi, connor dempsey, Joon Sue Lee, Sean Harrington, Ghada Badawy, Sasa Gazibegovic, Roy Op het Veld, Jason Jung, Marcel Verheijen, Moira Hocevar, Erik Bakkers, Chris J Palmstrom, Sergey M Frolov Previous research on superconductor-semiconductor nanowire devices focused heavily on aluminum and niobium alloys. Here we study superconductivity from new thin film superconductors in InSb nanowires. We observe hard superconducting gap and quantized conductance plateau at zero field. The magnetic field evolution of superconducting gap and of supercurrent are studied. New super-semi hybrid structures offering larger critical field and increased energy gap, as compared to aluminum, are interesting as they provide a powerful platform to study topological superconductivity, spin-orbit and Zeeman effects in Superconductor-Semiconductor Josephson Junctions. |
Wednesday, March 4, 2020 9:00AM - 9:12AM |
L49.00006: Magnetic field and flux driven 0-π phase transition in a spinful Josephson junction Alexander Whiticar, Antonio Fornieri, Felix Passmann, Abhishek Banerjee, Asbjorn Drachmann, Tiantian Wang, Candice Thomas, Sergei Gronin, Geoff C Gardner, Michael Manfra, Charles Marcus The coexistence of magnetism and superconductivity in a Josephson junction can lead to a 0-π phase transition. Quantum dots (QDs) can serve as spin impurities that can be controllably coupled to a superconductor (S). Here, we probe the Andreev spectrum of a hybrid S-QD-S Josephson junction by performing tunneling spectroscopy with a weakly coupled normal lead. We identify a gate-voltage-induced transition from singlet to doublet ground state, where the Andreev spectrum develops a π-phase shift. We demonstrate control of the 0−π transition using superconducting phase difference across the junction and an external magnetic field. We identify parity transitions by measuring zero-bias crossings induced by a magnetic field, phase difference, and gate voltage. This research was supported by Microsoft and the Danish National Research Foundation. |
Wednesday, March 4, 2020 9:12AM - 9:24AM |
L49.00007: A Mesoscopic Spectrometer Based on the Josephson Effect* Joël Griesmar, Fabien Lafont, Ramiro Rodriguez, Vincent Benzoni, Léo 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 Al). Conventional microwave techniques allow probing these states but only in a limited bandwidth. We implement a new broadband spectrometer operating at frequencies up to 180 GHz, with a 2 MHz linewidth and a minimal theoretical sensitivity of 5 kHz, based on the Josephson effect which converts a DC voltage to microwave oscillations at a frequency proportional to this voltage. Conveniently the absorption of the emitted photons is measured in the spectrometer DC current-voltage characteristic. Using a symmetrical SQUID biased at half a flux quantum allows decoupling the spectrometer from parasitic environmental modes. We demonstrate this spectroscopy technique by detecting the plasma frequency, near 100 GHz, of an RF-SQUID, fabricated both on- and off-chip, inductively coupled to the spectrometer. |
Wednesday, March 4, 2020 9:24AM - 9:36AM |
L49.00008: Graphene Josephson junctions in high in-plane magnetic field Tom Dvir, Ayelet Zalic, Kenji Watanabe, Takashi Taniguchi, Hadar Steinberg In recent years, there is a growing interest in graphene as the main ingredient in Josephson junctions (JJs). The ability to control the concentration and the sign of the charge carriers allows the fabrication of bipolar JJ with critical currents that can be varied in three orders of magnitude in the same device. Also, using clean graphene allows studying the interplay between quantum Hall effect and superconductivity. The study of graphene JJs in high magnetic fields was so far limited by the response of the host superconductor to the application of field. We fabricate JJs using ultra-thin NbSe_{2} as the superconductor, allowing us to apply very high in-plane magnetic fields without significantly affecting the superconducting gap. We show that such JJs, NbSe_{2}- graphene-NbSe_{2}, are highly transparent, and survive to in-plane fields up-to 8T. Due to the two-dimensional nature of the system, the interaction of the in-plane field with the system occurs only with the spin degree of freedom, imitating S-ferromagnet-S JJs, with effective exchange energy determined by the external field. |
Wednesday, March 4, 2020 9:36AM - 9:48AM |
L49.00009: Interference of chiral Andreev edge states in a multi-terminal Josephson junction Lingfei Zhao, Ethan Arnault, Andrew Seredinski, Trevyn Larson, Hengming Li, Kenji Watanabe, Takashi Taniguchi, Francois Amet, Gleb Finkelstein We study the interference effects of chiral Andreev edge states (CAES) in a multiterminal Josephson junction operated in the quantum Hall regime. On well quantized QH plateaus, we observe CAES interference in the form of voltage fluctuations downstream from the grounded superconducting contact. We have previously attributed this interference to successive electron-hole conversion along the quantum Hall-superconducting interface. Here, we show that these flucuations also exist in the Hall voltage, indicating a second order effect whereby the downstream particle experiences an additional electron-hole conversion along more than one superconducting contact. These observations indicate the potential for coherent control of the chiral Andreev edge states. |
Wednesday, March 4, 2020 9:48AM - 10:00AM |
L49.00010: Proximity effects in superconducting heterostructures Balazs Ujfalussy, Gabor Csire, Kyungwha Park The overlayer of metallic thin films on top of an s-wave superconducting substrate is studied by the numerical solution of the fully relativistic Kohn-Sham-Bogoliubov-de Gennes equations via multiple scattering methods. We briefly describe the methodology which allows for a material-specific calculation of the Andreev band structure. We discuss the relation of the effective pair interaction, the anomalous charge density and the induced superconducting gap in heterogeneous systems. Taking Au/Nb films as an example, we also show how surface and interface states behave in such systems and study the formation of the superconducting gap in one-dimensional impurity systems. |
Wednesday, March 4, 2020 10:00AM - 10:12AM |
L49.00011: Visibility of a topological qubit quantum dot measurement Aleksei Khindanov, Dmitry I. Pikulin Topological superconductor-quantum dot-topological superconductor (TS-QD-TS) system is of current interest for topological quantum computing, being a proposed platform for detecting Majorana zero modes (MZMs) and for measuring a topological qubit state. Here, using perturbation theory we derive expression for Josephson energy in this system, with a particular emphasis on a term associated with a coupling of the QD to quasiparticle continuums in each TS, arising in the fourth order. Furthermore, we study how 1/f flux noise, typical for SQUID-type devices, interferes measurement visibility of the qubit. We find that even though coupling of the QD to quasiparticle continuums does not directly affect measurement outcomes, the flux noise might considerably decrease the signal to noise ratio. We calculate experimental parameter values for which the effect is minimized and a best signal to noise ratio can be achieved. |
Wednesday, March 4, 2020 10:12AM - 10:24AM |
L49.00012: Domain wall based spin-triplet Josephson junction and SQUID Ekta Bhatia, Anand Srivastava, James Devine Stoneman, Nadia A Stelmashenko, Zoe Barber, Jason Robinson, Kartik Senapati Spin-singlet Cooper pairs convert to spin-triplet Cooper pairs on passing through a magnetic non-collinear structure at superconductor(S)/ferromagnet(F) interface [1]. In this context, the generation of triplet supercurrents through intrinsic ferromagnetic domain walls, which are naturally occurring non-collinear magnetic structure, was proposed theoretically in the last decade [2]. However, an experimental demonstration has been lacking in the literature, particularly due to the difficulty in accessing a single domain wall which is typically buried between two domains in a ferromagnetic material. In this report, by pinning a magnetic domain wall at the barrier of a nanoscale S/F/S planar junction, we have been able to demonstrate the predicted long-range triplet supercurrent due to a domain wall. The spin-triplet current was measured over a ferromagnetic barrier width exceeding 70 nm. Using the same concept of domain wall pinning at the Josephson junction barrier, we have also demonstrated a planar Nb/Ni/Nb triplet SQUID device. |
Wednesday, March 4, 2020 10:24AM - 10:36AM |
L49.00013: Unconventional magnetic flux response of Josephson junctions with a weak link made from alternating normal and ferromagnetic interlayers Ivan P Nevirkovets, Mikhail Belogolovskii We present the first systematic study of critical current-vs-external magnetic flux dependences in Josephson junctions with a weak link formed by alternating normal (N=Al) and ferromagnetic (F=Ni or Py) nominally identical nm-thick films coupled to superconducting (S=Nb) electrodes. In contrast to the well-known Fraunhofer-type behavior for conventional Josephson trilayers, the S/(NF)_{n}/S hybrids with n > 5 (with or without an ultra-thin Al-oxide barrier) have revealed anomalous patterns. In the most samples, with increasing magnetic flux, the supercurrent underwent periodic, constant-amplitude modulation resembling that in SQUIDs, see preliminary data [1]. The oscillatory pattern was superposed on a background supercurrent having much larger oscillation period in an externally applied magnetic field. We explain the origin of the non-Fraunhofer critical current curves as being associated with a special type of diffusive charge trajectories confined to the edges of the periodic (NF)_{n} weak link. The Josephson devices in the surface-dominated transport regime can be exploited for nanoscale low-field detection. |
Wednesday, March 4, 2020 10:36AM - 10:48AM |
L49.00014: Enhanced triplet pairing in magnetic junctions with s-wave superconductors Chenghao Shen, Thomas Vezin, Jong E Han, Igor Zutic A common path to Majorana fermions and topologically-protected quantum computing relies on spin-triplet superconductivity[1]. While spin-triplet pairing is elusive in nature and even common spin-triplet candidates, such as Sr2RuO4, could support alternative explanations[2], proximity effects in heterostructures can overcome these limitations. Specifically, a common expectation is that robust spin-triplet superconductivity in magnetic junctions should rely on highly spin-polarized magnets or complex magnetic multilayers[3]. Instead, we predict that the interplay of interfacial spin-orbit coupling and the barrier strength in simple magnetic junctions with s-wave superconductors can lead to nearly complete spin-triplet superconducting proximity effects when magnets have only a small spin polarization[4]. We show that this enhanced spin-triplet regime is characterized by a huge increase in conductance magnetoanisotropy[4-6], orders of magnitude larger than in the normal state. |
Wednesday, March 4, 2020 10:48AM - 11:00AM |
L49.00015: Measurement of Current-Phase Relationship in Josephson Junctions with Nb_{x}Si_{1-x} Barriers Miranda Thompson, Anna Fox, Manuel Castellanos-Beltran, Grace Butler, Michael Schneider, Paul Dresselhaus, Samuel P Benz The current-phase relationship of Josephson junctions is often assumed to be a sinusoid. However, this is not always the case. In order to directly measure the current-phase relationship of junctions with a range of characteristics, including both SIS and SNS junctions, we fabricated Josephson junctions having Nb_{x}Si_{1-x} barriers with varied Nb concentration. We constructed a superconducting SQUID-sensor-based circuit to extract their current-phase relationships. SIS-like junctions had the expected near-sinusoidal current-phase relationship, but the current-phase relationship gained additional harmonics as the barriers became less insulating. Thus, SNS-like junctions with normal barriers and high Nb content had sawtooth-like current-phase relationships. This deviation is likely due to additional Andreev reflections. The effects of this non-sinusoidal current phase relationship were observed in current-voltage characteristics through the appearance of non-integer Shapiro steps. Additionally, the same techniques were applied to Si-Mn nanocluster junctions to measure the changes in the current-phase relationship caused by the dynamic tuning of the critical currents of these junctions. |
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