Nonreciprocal superconducting transport and the spin Hall effect in gyrotropic structures

TL;DR

This paper presents a theoretical study of nonreciprocal superconducting transport and the spin Hall effect in gyrotropic structures, unifying superconducting diode effects with classical spintronic phenomena.

Contribution

It develops a comprehensive theory linking nonreciprocal superconducting transport with spin Hall and related effects in gyrotropic structures without external magnetic fields.

Findings

Dependence of anomalous current on spin relaxation length

Unified description of nonreciprocal transport and spintronic effects

Proposal for experiments to measure magnetoresistance and nonreciprocal transport

Abstract

The search for superconducting systems exhibiting nonreciprocal transport and, specifically, the diode effect, has proliferated in recent years. This trend encompasses a wide variety of systems, including planar hybrid structures, asymmetric SQUIDs, and certain noncentrosymmetric superconductors. A common feature of such systems is a gyrotropic symmetry, realized on different scales and characterized by a polar vector. Alongside time-reversal symmetry breaking, the presence of a polar axis allows for magnetoelectric effects, which, when combined with proximity-induced superconductivity, results in spontaneous non-dissipative currents that underpin the superconducting diode effect. This symmetry established, we present a comprehensive theoretical study of transport in a lateral Josephson junctions composed of a normal metal supporting the spin Hall effect, and attached to a ferromagnetic insulator. Due to the presence of the latter, magnetoelectric effects arise without requiring external magnetic fields. We determine the dependence of the anomalous current on the spin relaxation length and the transport parameters commonly used in spintronics to characterize the interface between the metal and the ferromagnetic insulator. Therefore, our theory naturally unifies nonreciprocal transport in superconducting systems with classical spintronic effects, such as the spin Hall effect, spin galvanic effect, and spin Hall magnetoresistance. We propose an experiment involving measurements of magnetoresistance in the normal state and nonreciprocal transport in the superconducting state. Such experiment, on the one hand, allows for determining the parameters of the model and thus verifying with a greater precision the theories of magnetoelectric effects in normal systems. On the other hand, it contributes to a deeper understanding of the underlying microscopic origins that determine these parameters.