Session F25: Superconductivity:JJ-I

The Role of Debye Relaxation in the I-V Characteristics of SNS junctions

We investigate a mechanism for dissipation in superconductor-normal metal-superconductor (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, I-V 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 non-reciprocal features in the I-V characteristics which can be characterized by the parameter ν(ε,χ)- ν(ε,-χ).   

Universal Josephson diode effect

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 time-reversal 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 phase-independent asymmetric current from the continuum. Last, we propose a simple scheme for achieving finite-momentum pairing, which does not rely on spin-orbit coupling and thus greatly expands existing platforms for the observation of supercurrent diode effects.   

Anomalous Josephson effect in planar noncentrosymmetric superconducting devices

In two-dimensional electron systems with broken inversion and time-reversal 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 double-barrier configurations of hybrid circuits, where the noncentrosymmetric normal region is described in terms of the paradigmatic Rashba model of spin-orbit coupling. We analyze this anomalous Josephson effect by means of both the Ginzburg-Landau formalism and the microscopic Green's functions approach in the clean limit. The magnitude of the critical current is calculated for an arbitrary in-plane magnetic field orientations, and anomalous phase shifts in the Josephson current-phase relation are determined in terms of the parameters of the model in several limiting cases.   

Probing spin--orbit coupling and in-plane Zeeman effects in SNS junctions of two-dimensional hole gases

We investigate theoretically a Josepshon junction consisting of a 2D hole gas with Rashba spin-orbit coupling and a Zeeman effect coupled to two superconducting leads. We develop a semi-classical theory to describe the critical current of the junction in response to an applied in-plane 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 spin-orbit interaction and the in-plane g-factor.   

Josephson junctions with charging energy

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.

Here we extend our model for a description of a Josephson junction with an embedded quantum dot, a model applicable to various realizations of superconducting qubits. Charging energy changes the superconducting state by collapsing the superposition with a well defined phase difference across the junction into a state with well defined charge difference. We investigate the nature of low energy states, particularly focusing on the effect of phase collapse with increasing charging energy.   

Monochromatic THz radiation caused by vortex-antivortex pair production in a current-driven Josephson stack

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 self-consistently 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 counter-propagating vortices and antivortices which get synchronized and eventually form large-amplitude 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∝ N⁶) with N at N<261 and tends to slow down at larger N. For a Bi-2212 mesa with 261 CuO₂ layers the peak power P can reach a few μW while overheating remains well below T_c.   

Topological phase transition by a tuning of superconducting phase in multi-terminal Josephson junctions

We theoretically investigate four- and five-terminal Josephson junctions with quantum point contact (QPC) structures.

$N$ superconductors can define $N-1$ independent superconducting phase differences.

The spectrum of Andreev bound states (ABSs) in the junction is $2pi$ periodic in

all the phase differences ${ varphi }$ and can be regarded as the ``band structure.''

In a previous studies, we have demonstrated presence of topologically protected singularities (Weyl points) at zero energy.

footnote{T. Yokoyama and Yu. V. Nazarov, PRB {f 92}, 155437 (2015).}

In this study, we investigate trajectories of Weyl points when the QPC voltages and the fifth superconducting phase are tuned.

Owing to the topological protection, the Weyl points do not disappear solely.

Hence parameter modulations cause the trajectories of Weyl points connecting

the pair annihilation and creation positions in ${ varphi }$-space.

Such pair annihilation and creation coincide with the topological phase transitions between four, two, and no topological charges.

Moreover, the trajectories show two categories:

One is closed type in the first ``Brillouin zone'' and the other is open type for the annihilation and creation with neighboring ``Brillouin zone.''

We consider a phase diagram for the four- and five-terminal junctions and propose a classification of topology by the trajectories.   

Chern number of Andreeev bound states with quasi-particle poisoning

Topological classification of closed quantum systems, known as the ten-fold 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 non-Hermitian Bernard-LeClair (BL) symmetry classes was proposed, which reduce to Altland-Zirnbauer classes in the closed limit. Here, we test and apply this classification scheme for multi-terminal 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.   

Microscopic Theory of Non-Reciprocal Dissipative Current in Diffusive SNS Junctions

We develop a microscopic theory of non-reciprocal currents in the dissipative state in diffusive SNS junctions. A new contribution to the non-linear conductance arises from the combination of a Zeeman field and Rashba spin-orbit 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.   

Transparent superconducting contact to both polarities of graphene

 

 Graphene has been a favorable platform for studying the superconducting proximity effect thanks to its high electronic mobility and high transparency with superconducting contacts.  It is known that many kinds of superconducting materials such as aluminium, niobium, tantalum and molybdenum-rhenium alloys, induce negative doping on graphene in contact with superconducting electrodes due to their work function difference. Thus, when the graphene channel is positively doped, p-n junction is developed near the superconducting contacts, which largely limits the various device scheme utilizing the bipolarity of graphene. Overcoming such issue, we fabricate two-dimensional superconducting contact which is highly transparent to both polarities of graphene. In this structure, the charge concentration of graphene beneath the superconductor and that of graphene channel are independently controlled to minimize the potential barrier between them. Measured contact transparency as well as the magnitude of Josephson critical current at zero magnetic field demonstrate the realization of transparent contact to both polarities of graphene. The contact transparency reaches about 0.6 for n-doped and 0.45 for p-doped regime. Our result paves a promising way to realize various theoretical propositions utilizing the bipolarity of graphene with superconductivity.   

Steady Floquet-Andreev States in graphene Josephson junction

Engineering quantum states through light–matter interaction has created a 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 realize 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 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–Andreev-state 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 non-equilibrium quantum states in nanodevices.   

Van der Waals Josephson junction as a gate tunable emitter

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 single-crystalline, ultra-low 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 sub-THz 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 gate-tunable vdW Josephson junctions and show that they can meet desirable performance criteria, including narrow linewidths < 1MHz and appreciable output power at the microwatt level.

    

Author not Attending

Local non-centrosymmetry in materials can lead to staggered spin-orbit coupling with interesting properties such as layer-dependent 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 s-wave superconducting order parameter. In this work, we establish the presence of a proximity-induced time-reversal symmetry breaking order parameter in a Josephson junction constructed from a two-dimensional 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 time-reversal symmetry breaking topological superconducting state in an air-stable van der Waals system provides a new and flexible avenue for research on topological superconductivity.

    

Limits of magnetic interactions in Ni-Nb ferromagnet-superconductor bilayers

Studies of ferromagnet-superconductor 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 Ni-Nb samples to be ±0.27 mT [3].

 

[1] Flokstra M G, et al. Nat. Phys. 12 57–61 (2016)

[2] Mironov S, et al. Appl. Phys. Lett. 113 022601 (2018)

[3] Satchell N, et al. arXiv:2209.15366 (2022)   

Thermal properties and overheating of the superconductor-quantum Hall interfaces

Low-frequency 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 Johnson-Nyquist 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.