Session B41: Nonreciprocal Superconductivity

Superconducting diode effect and spontaneous symmetry breaking in multi-layer graphen

The zero-field superconducting diode effect opens a unique window into the nature of the superconducting phase. Nonreciprocity in the critical superconducting transport indicates that inversion and time-reversal symmetries are broken simultaneously. In this talk, I will discuss the potential origin of the zero-field superconducting diode effect in multi-layer graphene structures. We utilize the angle-resolved measurement of transport nonreciprocity in the normal phase to examine the nature of spontaneous symmetry breaking. The angular dependence in both longitudinal and transverse channels offers direct identification that in-plane rotational and inversion symmetries are broken simultaneously in the normal phase. By investigating the interplay between transport nonreciprocity, ferromagnetism, and superconductivity, our findings suggest that the exchange-driven instability in the momentum space plays a key role in the zero-field superconducting diode effect.   

Nonreciprocal effects in Josephson junction arrays

We investigate nonreciprocal effects in arrays of Josephson junctions, where a Zeeman field induces an anomalous phase shift in the current-phase relation. We show that in square-symmetry 2D-arrays nonreciprocity is given by ratchet-like deformation of the egg-crate potential which pins Josephson vortices. A simple analysis reveals that second-nearest neighbor coupling between the array islands is necessary to break the symmetry. In experiments, the nonreciprocal effect is demonstrated by measuring both the critical current and the array inductance-versus-current asymmetry. For dilute vortices, the experimental results match what is expected from simple models. For finite frustration, measurements show a similar nonreciprocity for all major commensurate fractions, but with an intriguing exception: for f=1/3, the nonreciprocity changes sign. Our results open a new research direction in the field of Josephson junction arrays, namely, the possibility to control the real-space shape and symmetry of vortices and of their potential by an external Zeeman field.    

Universal Josephson and normal-superconductor diodes

In this work, we explore how nonreciprocal superconductivity can be probed through both nonreciprocity of a critical current through Josephson junctions as well as nonreciprocity of transport through N-S junctions. We propose a universal mechanism for the Josephson diode effect in short Josephson junctions. This 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 Ic+/Ic− ≈ 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. 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. 

We also show that nonreciprocal superconductivity reveals itself in asymmetric current-voltage characteristic of a transparent planar N-S junction. This leads us to a proposal of normal metal-superconductor (N-S) diode, which can exhibit twofold higher resistance in one direction than the other. N-S diode setup can be used as a smoking-gun tool for studying no reciprocal superconductivity.   

Nonreciprocal superconducting transport in non-centrosymmetric low-dimensional materials

Nonreciprocal transport is now recognized as a powerful probe of characteristic electronics states and charge/spin dynamics as well as being a useful functionality in quantum materials. Especially, nonreciprocal superconducting transport will provide unique information about paring symmetry, superconducting fluctuations, and charge/vortex dynamics.

In this presentation, I will talk about our work on the nonreciprocal superconducting transport in non-centrosymmetric low-dimensional superconductors under and without a magnetic field [1-6] and their possible microscopic origins including nonreciprocal paraconductivity and vortex Rachet motions. I will introduce the characteristic crossover behavior between these mechanisms and also quantum/classical vortex motions probed by the nonreciprocal transport. Possible future directions of nonreciprocal superconducting transport as a probe of exotic quantum dynamics in non-centrosymmetric superconductors will be also discussed.