Bulletin of the American Physical Society
APS March Meeting 2022
Volume 67, Number 3
Monday–Friday, March 14–18, 2022; Chicago
Session D61: Superconductivity: Tunneling, Proximity & JosephsonRecordings Available
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Sponsoring Units: DCMP Chair: Stephan Haas, University of Southern California Room: Hyatt Regency Hotel -Field |
Monday, March 14, 2022 3:00PM - 3:12PM |
D61.00001: Tunneling spectroscopy of multi-phase-dependent Andreev bound states in three-terminal Josephson junctions Hanho Lee, Roman Kuzmin, Mehdi Hatefipour, Javad Shabani, Maxim G Vavilov, Vladimir Manucharyan A Josephson junction (JJ) hosts localized subgap excitations called Andreev bound states (ABSs) whose energies depend on the superconducting phase difference across the junction. When more than two superconductors form a “multi-terminal” JJ, multiple independent phase differences are defined across the junction and its ABSs may depend on those multiple phases. In this talk, we will discuss the energy spectrum of the multi-phase-dependent ABSs in three-terminal JJs obtained by tunneling spectroscopy measurements. Since the ABS energy spectrum mimics electronic band structure in solid, multi-terminal JJs provide a new platform to study band topology in higher dimensional parameter space. |
Monday, March 14, 2022 3:12PM - 3:24PM |
D61.00002: Proximity-induced superconducting collective modes Jonathan B Curtis, Nicholas R Poniatowski, Amir Yacoby, Prineha Narang The proximity effect has been widely studied in superconducting systems as a means of inducing novel superconducting ground states, and studying interesting quasiparticle dynamics. Here we consider a heterostructure comprised of a conventional s-wave superconducting substrate and an unconventional superconducting monolayer, forming an S-S heterostructure. In the case where the unconventional superconducting order is orthogonal to that of the substrate, the mean-field transition of the monolayer superconductivity is depressed by the substrate, which quenches the density of states at the Fermi level. Nevertheless, the attractive interaction in the monolayer remains intact, and by going beyond the standard quasiparticle picture, we show that this can give rise to distinct collective modes which are effectively remnants of the intrinsic monolayer order. This collective mode is essentially a Bardasis-Schrieffer mode, and corresponds to a bound state of proximitized electrons. Upon further lowering the sample temperature this mode progressively sharpens as it emerges from the proximitized continuum, and eventually softens to zero-frequency at the heterostructure transition temperature. We hypothesize that below this transition temperature, the system spontaneously breaks time-reversal symmetry and enters a mixed superconducting state where the monolayer order parameter is fixed out-of-phase with the substrate. Finally, we speculate about applications to various monolayer unconventional superconductors and superconductors which are subjected to high-levels of non-magnetic disorder. |
Monday, March 14, 2022 3:24PM - 3:36PM |
D61.00003: Observation of secondary gap-like structure in Nb/Al bilayers with thick Al Zac S Barcikowski, Joshua Pomeroy A second, gap-like structure emerges in the sub-gap region of the tunneling density of states (TDOS) above ~60 nm Al thickness in Nb/Al bilayers with 40 nm Nb. Additionally, with increasing Al thickness, the primary gap decreases linearly in energy and the normalized zero-bias conductance increases linearly. Nb and Al are principal materials for superconducting electronics, as Nb provides the highest Tc of the elemental superconductors and Al enables the growth of high quality AlOx tunnel barriers. To investigate the evolution of the superconducting state with Al thickness, we fabricated superconducting Nb/Al-based normal-insulator-superconductor (NIS) tunnel junctions with different Al thicknesses and measured the tunnel junction conductance spectra. We model the data using the Blonders-Tinkham-Klapwijk theory using barrier strength (Z), superconducting energy gap (Δ), and Dynes broadening (Γ) and interpret the implications of the best fit values. |
Monday, March 14, 2022 3:36PM - 3:48PM |
D61.00004: Conductivity spectrum for impurity induced spin-charge coupled Luttinger liquid in an SNS junction Tamoghna Barik, Jay D Sau, Yang-Zhi Chou The problem of interacting electrons in a one dimensional metal is well described by the Luttinger liquid (LL) theory at low energies around the Fermi level. The density-density interactions can be treated exactly with a non-interacting bosonic Hamiltonian which is separated into charge and spin density modes of excitations. However, presence of back-scattering potential in the system couples the unperturbed spin and charge modes. We study the effects of impurity induced spin-charge coupling on the conductivity spectrum of a metal as the normal region in an SNS junction where the normal region is much longer than the superconducting coherence length. Treating the impurity with a disorder potential perturbatively, we show that the resultant conductivity spectrum not only contains a peak at the energy of the charge mode excitation but also contains peaks corresponding to spin density wave excitations at neighboring energies which is a manifestation of the impurity induced spin charge coupling. With varying interaction strengths of the LL the locations of the peaks on the energy axis shift due to the re-normalization of the spin and charge velocities. Thus, in presence of impurities the spin mode excitations can be probed via conductivity measurement setup with a long SNS junction. |
Monday, March 14, 2022 3:48PM - 4:00PM |
D61.00005: Tuning current-phase relations with spin-orbit coupling in planar Josephson junctions David Monroe, Mohammad Alidoust, Igor Zutic While current-phase relations (CPRs) in Josephson junctions (JJ) based on conventional s-wave superconductor are commonly considered to have a harmonic dependence, the breaking of spatial inversion symmetry and the tunability of the resulting spin-orbit coupling (SOC) provide a much more complex picture. At a fixed Zeeman field, a gate-controlled (SOC) in a JJ is responsible for the 0-pi ground-state phase difference or an anomalous intermediate phase, such that even at the zero-phase difference the supercurrent will flow in the junction [1,2]. The resulting CPRs can have strong anharmonicities and support spin-triplet superconductivity sought for both superconducting spintronics and Majorana bound states [2,3]. Here we examine various implications of the gate-controlled SOC on the evolution of CPRs and distinguish experimentally-relevant outcomes for the cases of the small and large chemical potential in JJs based on a two-dimensional electron gas. |
Monday, March 14, 2022 4:00PM - 4:12PM |
D61.00006: Model for missing Shapiro steps due to bias-dependent resistance and Shapiro steps experiments in topologically trivial planar Josephson junctions Sanchayeta Mudi, Po Zhang, Joon Sue Lee, Mihir Pendharkar, Connor Dempsey, Anthony McFadden, Sean Harrington, Ghada Badawy, Sasa Gazibegovic, Roy Op het Veld, Moira Hocevar, Erik P. A. M. Bakkers, Chris J Palmstrom, Sergey M Frolov In this talk, we introduce a new phenomenological model to explain the suppression of Shapiro steps, thus giving an alternative explanation for missing Shapiro steps. We introduce non-linearity in the RSJ model by introducing peaks in the differential resistance as a function of the dc current bias. We demonstrate that such peaks can suppress odd Shapiro steps in the presence of only a 2π periodic current and thus mimic the 4π Josephson effect which is considered a signature of topological superconductors hosting Majorana fermions. We also show that these resonance peaks can be used to suppress any Shapiro step. We finally discuss the limitations of our model and its applicability to recent experiments where such resonances have been observed. |
Monday, March 14, 2022 4:12PM - 4:24PM |
D61.00007: Superconductivity at ferromagnetic domain walls in hybrid InAs/EuS/Al nanowires, part 1: studied by scanning SQUID imaging Nabhanila Nandi, Irene P Zhang, Juan Carlos Estrada Saldaña, Alexandros Vekris, Michelle Turley, Yu Liu, Mario Castro, Martin Bjergfelt, Sabbir A Khan, Sebastian Allende, Peter Krogstrup, Kasper Grove-Rasmussen, Jesper Nygård, Kathryn Moler Ferromagnet/superconductor heterostructures are playgrounds for novel quantum phases that rarely occur naturally in nature, e.g. topological, spin-triplet, etc. One particularly intriguing example is the possible signature of Majorana fermions recently observed in EuS/InAs/Al nanowires. While ferromagnetism normally precludes superconductivity, Cooper pairing may survive in proximity to magnetic domain walls. Heterostructure nanowires are particularly suitable for studying such domain wall superconductivity as their high aspect ratio encourages domain formation only along the length alleviating averaging effects from neighbouring domains. In this talk I will present magnetometry images taken using scanning Superconducting QUantum Interference Device (SQUID) to explore magnetic domain formation in full shell InAs/EuS/Al nanowires as we sweep a full in-plane hysteresis loop at 4 K at a range of field angles. We use our magnetometry images to explain the observation of superconductivity in magneto-transport measurements, which will be presented in the second part of this talk. |
Monday, March 14, 2022 4:24PM - 4:36PM |
D61.00008: Persistent photoconductance effects in cuprate-based superconducting tunnel junctions Ralph El Hage, Vincent Humbert, Anke Sander, Jerome Charliac, Salvatore Mesoraca, Juan Trastoy, Jacobo Santamaria, Javier E Villegas It is well known that the normal-state conductance and superconducting critical temperature of underdoped YBa2Cu3O7-d (YBCO) can be persistently enhanced upon illumination with visible or UV light. These effects, called persistent photoconductivity (PPC) and photo-superconductivity (PPS), were investigated using YBCO thin films and attracted a great deal of attention over three decades ago. Despite a significant amount of work, a clear picture of the governing microscopic mechanism did not emerge, leaving open an ongoing debate between theories based on doping due to photocarrier excitation and those based on doping by the photoinduced oxygen ordering in the crystal structure. Motivated by this, we studied photoinduced in a different type of device, namely in micrometric tunnel junctions between a degenerate semiconductor transparent to visible and UV light (ITO) and a YBCO electrode. We found that these junctions show a very unique and rich behavior, with different regimes in which either persistent increase or a decrease of the conductance is produced after illumination depending on the electrical and optical history. Interestingly, as it will be discussed, these effects reflect the interplay of various mechanisms in addition to PPC, including photovoltaic effects and photo-activated oxygen migration across the tunnel barrier. |
Monday, March 14, 2022 4:36PM - 4:48PM |
D61.00009: Crossed Andreev reflection of a quantum Hall edge Vladislav Kurilovich, Zachary M Raines, Leonid Glazman We develop the theory of charge transport along the quantum Hall edge proximitized by a superconductor. An electron incident on the proximitized segment of the edge may be transmitted across it either normally or, upon experiencing an Andreev reflection, as a hole. The amplitudes of these processes are sensitive to disorder in the structure, which leads to large mesoscopic fluctuations of the segment’s transconductance with magnetic field and carrier concentration. We show that, for a long segment, the fluctuations are alternating in sign and have a zero mean. We quantify the dependence of the average conductance, its variance and correlation function on the length of the proximitized segment, as well as on properties on the superconducting electrode and the NS interface. Our analytical theory explains key observations of a recent experiment [L. Zhao et. al., Nature Physics 16, 862 (2020)]. |
Monday, March 14, 2022 4:48PM - 5:00PM |
D61.00010: Scanning d-wave Andreev reflection probe of 2H-NbSe2 Rainni K Chen, Chao C Zhang, Chris Granstrom, Cedomir Petrovic, Ruixing Liang, Walter N Hardy, John Y Wei 2H-NbSe2 has been heavily studied by scanning tunneling spectroscopy (STS) for both its multiband superconductivity and coexisting charge density waves. Recent STS advances using s-wave superconducting tips [1] have revealed pair density-wave order in its superconducting state [2]. Recent theoretical studies have also predicted ferromagnetic instability leading to singlet-triplet mixing, due to spin-orbit coupling and broken inversion symmetry [3]. We present STS measurements of 2H-NbSe2 using a d-wave superconducting tip, a novel technique that exploits resonant d-wave Andreev reflection (AR) to achieve atomic-scale resolution with potential sensitivity to spin-polarization. Our scanning d-wave AR data were taken in the normal, superconducting and mixed state of 2H-NbSe2, to probe bound states within vortex cores or near impurities. We discuss our results in light of the recent studies, and also address the possibility for high-impedance proximity coupling across s-wave/insulator/d-wave junctions [4]. |
Monday, March 14, 2022 5:00PM - 5:12PM |
D61.00011: Steady Floquet-Andreev States Probed by Tunnelling Spectroscopy Sein Park, Wonjun Lee, Seong Jang, Yong-Bin Choi, Jinho Park, Woochan Jung, Kenji Watanabe, Takashi Taniguchi, Gil Young Cho, Gil-Ho Lee Engineering quantum states through light-matter interaction has created a new paradigm in condensed matter physics. A representative example is the Floquet-Bloch state, which is generated by time-periodically driving the Bloch wavefunctions in crystals. Previous attempts to realise such states in condensed matter systems have been limited by the transient nature of the Floquet states produced by optical pulses, which masks the universal properties of non-equilibrium physics. Here, we report the generation of steady Floquet Andreev (F-A) states in graphene Josephson junctions by continuous microwave application and direct measurement of their spectra by superconducting tunnelling spectroscopy. We present quantitative analysis of the spectral characteristics of the F-A states while varying the phase difference of superconductors, temperature, microwave frequency and power. The oscillations of the F-A state spectrum with phase difference agreed with our theoretical calculations. Moreover, we confirmed the steady nature of the F-A states by establishing a sum rule of tunnelling conductance, and analysed the spectral density of Floquet states depending on Floquet interaction strength. This study provides a basis for understanding and engineering non-equilibrium quantum states in nano-devices. |
Monday, March 14, 2022 5:12PM - 5:24PM |
D61.00012: Andreev bound states, molecules and fusion Andreas Baumgartner, Christian Jünger, Christian Schonenberger, Kimberly A Dick, Sebastian Lehmann, Claes Thelander In a sequence of tunneling spectroscopy measurements we show intermediate states in the process of merging two Andreev bound states (ABSs), separated by an in-situ grown quantum dot (QD), previously used to investigate the superconducting proximity effect [1] and magnetic field independent subgap states in semiconducting InAs nanowires (NWs) [2]. The QD couples the two adjacent NW segments carrying individual ABSs, or “Andreev atoms”. For decreasing barrier strengths, these ABSs sequentially hybridize with the central QD, forming different types of “Andreev molecules”, with zero-bias conductance peaks as precursors of the Josephson effect. At even larger gate voltages, we then find gate tunable Josephson currents, suggesting an ABS spanning the complete NW, which one might call a "fused" ABS or “Andreev helium”. Our results illustrate the merging of subgap states and may serve as a guide in future Majorana bound state fusion experiments. |
Monday, March 14, 2022 5:24PM - 5:36PM |
D61.00013: Capacitative measurement of an Andreev bound state in a quantum dot coupled to a floating superconducting island Filip K Malinowski, Rok Zitko, Damaz De Jong, Christian G Prosko, Zoltan Guba, R. K. Rupesh, Luka Pavesic, Lin Han, Michael Chan, Liu Yu, Peter Krogstrup, Andras Palyi, Jonne V Koski We study the response of a resonator attached to a gate of a floating quantum dot coupled to a superconducting island, defined in an InAs nanowire with an epitaxial Al shell. The reflectometry signal distinguishes between the superconducting island hosting no subgap states and discrete subgap states. In the case of the superconducting island hosting no subgap states, we vary the applied magnetic field and tunnel coupling. The resonance frequency shift and changes in the quality factor at the charge transitions are simulated using a projected numerical renormalization group method, and show good quantitative agreement with the experiment. The established measurement method, as well as the simulation framework, are suitable for the study of devices consisting of multiple superconducting islands and quantum dots, such as Andreev molecules or a Kitaev chain. |
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