Spin supercurrent and phase-tunable triplet Cooper pairs via magnetic insulators

TL;DR

This paper theoretically demonstrates that magnetic insulators can enable long-range, phase-tunable spin supercurrents in normal metals via triplet Cooper pairs, with potential for quantum state control.

Contribution

It introduces a novel mechanism for controlling dissipationless spin currents using magnetic insulators and superconducting phase differences, highlighting phase-tunability and triplet correlations.

Findings

Spin supercurrent can flow through diffusive normal metals without dissipation.

Magnetization orientation controls the supercurrent magnitude.

Superconducting phase difference enhances triplet correlations and affects density of states.

Abstract

We demonstrate theoretically that a dissipationless spin current can flow a long distance through a diffusive normal metal by using superconductors interfaced with magnetic insulators. The magnitude of this spin supercurrent is controlled via the magnetization orientation of the magnetic insulators. The spin supercurrent obtained in this way is conserved in the normal metal just like the charge-current and interestingly has a term which is independent of the superconducting phase difference. The quantum state of the system can be switched between 0 and $\pi$ by reversing the insulators from a parallel to antiparallel configuration with an external field. We show that the spin-current is carried through the normal metal by superconducting triplet (odd-frequency) correlations and that the superconducting phase difference can be used to enhance these, leaving clear spectroscopic fingerprints in the density of states.