Bulletin of the American Physical Society
APS March Meeting 2023
Las Vegas, Nevada (March 510)
Virtual (March 2022); Time Zone: Pacific Time
Session F25: Superconductivity:JJI 
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Sponsoring Units: DCMP Chair: Kush Saha, National Institute of Science Education and Research (NISER) Room: Room 217/218 
Tuesday, March 7, 2023 8:00AM  8:12AM 
F25.00001: The Role of Debye Relaxation in the IV Characteristics of SNS junctions Tony Liu, Boris Spivak, Anton Andreev, Michael Smith We investigate a mechanism for dissipation in superconductornormal metalsuperconductor (SNS) junctions related to the energy relaxation of quasiparticles due to an adiabatic flow of energy levels, similar to Debye relaxation in a gas. At small voltages, this Debye mechanism gives a contribution to the conductance which is proportional to the inelastic relaxation time τ_{in}, and is thus much larger than the conventional contribution to the conductance which is proportional to the elastic relation time τ_{el}. In this regime, IV characteristics in SNS junctions can be expressed in terms of the density of states in the normal region of the junction ν(ε,χ), where χ is the superconducting phase difference across the junction. In junctions with broken time reversal and inversion symmetry, the density of states can have terms which are odd in the phase difference chi. This generates nonreciprocal features in the IV characteristics which can be characterized by the parameter ν(ε,χ) ν(ε,χ). 
Tuesday, March 7, 2023 8:12AM  8:24AM 
F25.00002: Universal Josephson diode effect Margarita Davydova, Saranesh Prembabu, Liang Fu We propose a universal mechanism for the Josephson diode effect in short Josephson junctions. The proposed mechanism is due to finite Cooper pair momentum and is a manifestation of simultaneous breaking of inversion and timereversal symmetries. The diode efficiency is up to 40%, which corresponds to an asymmetry between the critical currents in opposite directions I_{c+}/I_{c−} ≈ 230%. We show that this arises from both the Doppler shift of the Andreev bound state energies and the phaseindependent asymmetric current from the continuum. Last, we propose a simple scheme for achieving finitemomentum pairing, which does not rely on spinorbit coupling and thus greatly expands existing platforms for the observation of supercurrent diode effects. 
Tuesday, March 7, 2023 8:24AM  8:36AM 
F25.00003: Anomalous Josephson effect in planar noncentrosymmetric superconducting devices S M Jaglul Hasan, Konstantin N. Nesterov, Songci Li, Manuel Houzet, Julia Meyer, Alex Levchenko In twodimensional electron systems with broken inversion and timereversal symmetries, a Josephson junction reveals an anomalous response: the supercurrent is nonzero even at zero phase difference between two superconductors. We consider details of this peculiar phenomenon in the planar doublebarrier configurations of hybrid circuits, where the noncentrosymmetric normal region is described in terms of the paradigmatic Rashba model of spinorbit coupling. We analyze this anomalous Josephson effect by means of both the GinzburgLandau formalism and the microscopic Green's functions approach in the clean limit. The magnitude of the critical current is calculated for an arbitrary inplane magnetic field orientations, and anomalous phase shifts in the Josephson currentphase relation are determined in terms of the parameters of the model in several limiting cases. 
Tuesday, March 7, 2023 8:36AM  8:48AM 
F25.00004: Probing spinorbit coupling and inplane Zeeman effects in SNS junctions of twodimensional hole gases Jonas Lidal, Jeroen Danon We investigate theoretically a Josepshon junction consisting of a 2D hole gas with Rashba spinorbit coupling and a Zeeman effect coupled to two superconducting leads. We develop a semiclassical theory to describe the critical current of the junction in response to an applied inplane magnetic field and we investigate the resulting dependence of the supercurrent, both numerically and analytically. We find qualitatively different patterns of critical current in different physically relevant regimes and we connect the observed features to the underlying model parameters. Our theory could thus be used for characterization of 2D hole gases, potentially even revealing quantitative information about parameters such as the effective strength of the spinorbit interaction and the inplane gfactor. 
Tuesday, March 7, 2023 8:48AM  9:00AM 
F25.00005: Josephson junctions with charging energy Luka Pavešic Superconducting (SC) islands coupled to semiconducting quantum dots are important building blocks of hybrid quantum devices, including superconducting qubits. To realistically model microscopic SC islands, their charging energy has to be included in the description. This is at odds with the standard BCS description where charge is not well defined. We developed a charge conserving model for quantum dots coupled to SC islands with charging energy and found that its competition with superconductivity qualitatively changes the nature of the subgap states. This understanding is important as modern devices are typically small enough that the effects of charging energy should not be neglected. The model is successful at describing spectra of InAs nanowires with Al SC islands. 
Tuesday, March 7, 2023 9:00AM  9:12AM 
F25.00006: Monochromatic THz radiation caused by vortexantivortex pair production in a currentdriven Josephson stack Ahmad K Sheikhzada, Alexander V Gurevich We report numerical simulations of the Cherenkov instability of Josephson vortices driven by a dc current density J in layered superconductors. Dynamic equations for a stack of up to 321 coupled interlayer junctions were solved selfconsistently together with an equation for a mean temperature T(N,J) of the stack. It is shown that the Cherenkov wake behind a trapped vortex bouncing back and forth along the stack can trigger proliferation of multiple counterpropagating vortices and antivortices which get synchronized and eventually form largeamplitude standing electromagnetic waves causing temporal oscillations of the magnetic moment M(t) of the stack. This happens as J exceeds a critical value J_{p}(η) which can be well below the Josephson interlayer critical current density J_{c} if the quasiparticle damping parameter η is small. The magnitude of oscillating M(t) has sharp peaks at certain values of J at which the spectrum of M(t) in the THz region is nearly monochromatic. The peak radiation power P caused by the oscillating M(J,t) increases rapidly (P∝ N^{6}) with N at N<261 and tends to slow down at larger N. For a Bi2212 mesa with 261 CuO_{2} layers the peak power P can reach a few μW while overheating remains well below T_{c}. 
Tuesday, March 7, 2023 9:12AM  9:24AM 
F25.00007: Topological phase transition by a tuning of superconducting phase in multiterminal Josephson junctions Tomohiro Yokoyama, Kento Takemura We theoretically investigate four and fiveterminal Josephson junctions with quantum point contact (QPC) structures. 
Tuesday, March 7, 2023 9:24AM  9:36AM 
F25.00008: Chern number of Andreeev bound states with quasiparticle poisoning Mohammad Atif Javed, RomanPascal Riwar Topological classification of closed quantum systems, known as the tenfold way, is a well established formalism by now. On the other hand the generalization of this classification to open quantum systems is still a matter of ongoing research, with many different approaches. Recently, a classification for fermionic systems governed by quadratic Lindbladians into ten nonHermitian BernardLeClair (BL) symmetry classes was proposed, which reduce to AltlandZirnbauer classes in the closed limit. Here, we test and apply this classification scheme for multiterminal Josephson junctions, which have the special property of harbouring a closed system Chern number in spite of all the materials being trivial. For this circuit, it is known that quasiparticle poisoning destroys the closed system topology, as it drives the system out of the topological subspace. In this work we show that a classification of the circuit in presence of quasiparticle poisoning is possible along the lines of the BL symmetry classes. We identify a generalized Chern number, which retains its quantization in spite of the poisoning. Investigations into how this Chern number shows up in the observables are currently underway. 
Tuesday, March 7, 2023 9:36AM  9:48AM 
F25.00009: Microscopic Theory of NonReciprocal Dissipative Current in Diffusive SNS Junctions Tony Liu, Michael Smith, Anton Andreev, Boris Spivak We develop a microscopic theory of nonreciprocal currents in the dissipative state in diffusive SNS junctions. A new contribution to the nonlinear conductance arises from the combination of a Zeeman field and Rashba spinorbit coupling in the normal metal. Further, this contribution arises due to inelastic scattering and is proportional to the inelastic scattering time τ_{in}. Therefore it can be much larger than the conventional contribution which is proportional to the elastic scattering time τ_{el}. We study this new contribution for both high and low transmission coefficient of the boundary. 
Tuesday, March 7, 2023 9:48AM  10:00AM 
F25.00010: Transparent superconducting contact to both polarities of graphene Seong Jang, Kenji Watanabe, Takashi Taniguchi, GilHo Lee

Tuesday, March 7, 2023 10:00AM  10:12AM 
F25.00011: Steady FloquetAndreev States in graphene Josephson junction Sein Park, Wonjun Lee, Seong Jang, YongBin Choi, Jinho Park, Woochan Jung, Kenji Watanabe, Takashi Taniguchi, Gil Young Cho, GilHo Lee Engineering quantum states through light–matter interaction has created a paradigm in condensedmatter physics. A representative example is the Floquet–Bloch state, which is generated by timeperiodically driving the Bloch wavefunctions in crystals. Previous attempts to realize such states in condensedmatter systems have been limited by the transient nature of the Floquet states produced by optical pulses, which masks the universal properties of nonequilibrium physics. Here we report the generation of steady Floquet–Andreev 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 Floquet–Andreev states while varying the phase difference of the superconductors, the temperature, the microwave frequency and the power. The oscillations of the Floquet–Andreevstate spectrum with phase difference agreed with our theoretical calculations. Moreover, we confirmed the steady nature of the Floquet–Andreev 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 nonequilibrium quantum states in nanodevices. 
Tuesday, March 7, 2023 10:12AM  10:24AM 
F25.00012: Van der Waals Josephson junction as a gate tunable emitter Xuanjing Chu, Jesse Balgley, Ethan G Arnault, James C Hone, Kin Chung Fong This talk will explore the use of van der Waals (vdW) materials as opposed to conventional materials as Josephson junction (JJ) emitters. VdW materials are uniquely apt for superconducting quantum devices due to their singlecrystalline, ultralow defect density, and lack of dangling bonds. Even more intriguingly, the electronic properties of vdW materials, including carrier density and superconducting gap energy, can be tuned by electrostatic gating. JJs made from vdW materials can be homogeneous across large, ~1000 µm2 areas, making them practical as novel subTHz JJ emitters whose frequency can be tuned with a gate voltage as opposed to changing the bias current. We present numerical calculations of the emitting properties of gatetunable vdW Josephson junctions and show that they can meet desirable performance criteria, including narrow linewidths < 1MHz and appreciable output power at the microwatt level. 
Tuesday, March 7, 2023 10:24AM  10:36AM Author not Attending 
F25.00013: Proximityinduced timereversal symmetry breaking superconductivity in a transition metal dichalcogenide Pranava Keerthi Sivakumar Local noncentrosymmetry in materials can lead to staggered spinorbit coupling with interesting properties such as layerdependent spin momentum locking, which is hidden within the system. In the context of superconductivity, this structural property can have more profound implications and lead to the formation of unconventional superconducting order parameters in the system beyond the conventional swave superconducting order parameter. In this work, we establish the presence of a proximityinduced timereversal symmetry breaking order parameter in a Josephson junction constructed from a twodimensional van der Waals transition metal dichalcogenide. We observe a large asymmetry in critical currents of the junction in zero applied magnetic fields, the sign of which switches upon reversing the sweep direction of the magnetic field leading to a clear magnetic hysteresis in the observed asymmetry which has been reported so far only in ferromagnetic Josephson junctions. This observation of a possible timereversal symmetry breaking topological superconducting state in an airstable van der Waals system provides a new and flexible avenue for research on topological superconductivity. 
Tuesday, March 7, 2023 10:36AM  10:48AM 
F25.00014: Limits of magnetic interactions in NiNb ferromagnetsuperconductor bilayers Nathan Satchell, Patrick Quarterman, Julie A Borchers, Gavin Burnell, Norman O Birge Studies of ferromagnetsuperconductor hybrid systems have uncovered magnetic interactions between the competing electronic orderings [1]. The Electromagnetic Proximity Effect predicts the formation of a spontaneous vector potential inside a superconductor placed in proximity to a ferromagnet [2]. In this work, we use a Nb superconducting layer and Ni ferromagnetic layer to test for such magnetic interactions. We use the complementary, but independent, techniques of polarised neutron reflectometry and detection Josephson junctions to probe the magnetic response inside the superconducting layer at close to zero applied field. In this condition, Meissner screening is negligible, so our measurements examine only additional magnetic and screening contributions from proximity effects. We report that any signals attributable to such proximity effects are below the detection resolution of our experimental study. From our measurements, we estimate an upper limit of the size of the zero field Electromagnetic Proximity Effect in our NiNb samples to be ±0.27 mT [3]. 
Tuesday, March 7, 2023 10:48AM  11:00AM 
F25.00015: Thermal properties and overheating of the superconductorquantum Hall interfaces Zubair Iftikhar, Trevyn Larson, Lingfei Zhao, Gleb Finkelstein, Francois Amet, Kenji Watanabe, Takashi Taniguchi Lowfrequency noise measurements have been successfully used to probe such phenomena as the fractional charges and the quanta of heat conduction of fermions or anyons in the quantum Hall (QH) regime. Here we study the excess noise at the interface of a superconducting film and an encapsulated graphene in the QH regime. We find that the primary source of the excess noise is local heating, which allows one to convert the noise to electron temperature using JohnsonNyquist noise calibration. As the magnetic field is increased, the temperature of the interface is reduced, eventually approaching the behavior expected for a normal metal. We further use our sample to study the heat conductivity across the superconducting film subject to a high magnetic field. 
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