Spin-polarized superconductivity: order parameter topology, current dissipation, and multiple-period Josephson effect

TL;DR

This paper explores the unique transport phenomena in fully spin-polarized triplet superconductors, revealing topologically distinct order parameter configurations that lead to unconventional Josephson effects and potential applications in moiré systems.

Contribution

It introduces the concept of topologically equivalent phase windings in spin-polarized triplet superconductors and predicts an unusual AC Josephson effect with a multiple of the standard period.

Findings

Supercurrent relaxation can occur via smooth order parameter deformations.

An applied voltage induces an unconventional AC Josephson effect with multiple periods.

Theoretical model supports the existence of these phenomena in specific superconducting systems.

Abstract

We discuss transport properties of fully spin-polarized triplet superconductors, where only electrons of one spin component (along a certain axis) are paired. Due to the structure of the order parameter space, wherein phase and spin rotations are intertwined, a configuration where the superconducting phase winds by $4\pi$ in space is topologically equivalent to a configuration with no phase winding. This opens the possibility of supercurrent relaxation by a smooth deformation of the order parameter, where the order parameter remains non-zero at any point in space throughout the entire process. During the process, a spin texture is formed. We discuss the conditions for such processes to occur and their physical consequences. In particular, we show that when a voltage is applied, they lead to an unusual alternating-current Josephson effect whose period is an integer multiple of the usual Josephson period. These conclusions are substantiated in a simple time-dependent Ginzburg-Landau model for the dynamics of the order parameter. One of the potential applications of our analysis is for moir\'e systems, such as twisted bilayer and double bilayer graphene, where superconductivity is found in the vicinity of ferromagnetism.