Triplet supercurrents in clean and disordered half-metallic ferromagnets

TL;DR

This paper explores how supercurrents can pass through half-metallic ferromagnets by converting singlet pairs into triplet pairs at interfaces, revealing new physics and potential for exotic Josephson devices.

Contribution

It proposes a novel mechanism involving spin precession and triplet pair rotation for supercurrent conversion in half-metallic ferromagnets.

Findings

Triplet supercurrents are dominated by odd-frequency s-wave and even-frequency p-wave pairs.

Additional symmetry components emerge in the ballistic limit.

Interfaces exhibit a mixed-spin superconducting state with unusual symmetry properties.

Abstract

Interfaces between materials with differently ordered phases present unique opportunities to study fundamental problems in physics. One example is the interface between a singlet superconductor and a half-metallic ferromagnet, where Cooper pairing occurs between electrons with opposite spin on one side, while the other displays 100% spin polarisation. The recent surprising observation of a supercurrent through half-metallic CrO_2 therefore requires a mechanism for conversion between unpolarised and completely spin polarised supercurrents. Here we suggest a conversion mechanism based on electron spin precession together with triplet pair rotation at interfaces with broken spin-rotation symmetry. In the diffusive limit the triplet supercurrent is dominated by inter-related odd-frequency s-wave and even-frequency p-wave pairs. In the crossover to the ballistic limit additional symmetry components become relevant. The interface region exhibits a superconducting state of mixed-spin pairs with highly unusual symmetry properties that opens up new perspectives for exotic Josephson devices.