Spin-orbit coupling suppression and singlet-state blocking of spin-triplet Cooper pairs

TL;DR

This study explores how spin-orbit coupling affects the decay and coupling of spin-triplet Cooper pairs in superconductors, revealing suppression mechanisms and state interactions crucial for superconducting spintronics.

Contribution

It provides new experimental insights into the decay behavior of triplet supercurrents in Nb and their interaction with singlet states under spin-orbit coupling.

Findings

Triplet supercurrents decay rapidly in Nb's normal state due to SOC.

Triplet supercurrents are blocked in the superconducting state of Nb.

SOC significantly suppresses triplet pair decay in non-magnetic metals.

Abstract

An inhomogeneous magnetic exchange field at a superconductor/ferromagnet interface converts spin-singlet Cooper pairs to a spin-aligned (i.e. spin-polarized) triplet state. Although the decay envelope of such triplet pairs within ferromagnetic materials is well studied, little is known about their decay in non-magnetic metals and superconductors, and in particular in the presence of spin-orbit coupling (SOC). Here we investigate devices in which triplet supercurrents are injected into the s-wave superconductor Nb. In the normal state of Nb, triplet supercurrents decay over a distance of 5 nm, which is an order of magnitude smaller than the decay of spin singlet pairs due to the SOC interacting with the spin associated with triplet pairs. In the superconducting state of Nb, triplet supercurrents are not able to couple with the singlet wavefunction and thus blocked by the absence of available equilibrium states in the singlet gap. The results offer new insight into the dynamics between s-wave singlet and s-wave triplet states.