Creation of spin-triplet Cooper pairs in the absence of magnetic ordering

TL;DR

This paper demonstrates a novel method to generate and isolate spin-triplet Cooper pairs without magnetic ordering by using a driven topological insulator-superconductor junction with strong spin-orbit interaction.

Contribution

It introduces a new approach to produce spin-triplet pairs in non-magnetic systems using out-of-equilibrium conditions in a topological insulator-superconductor setup.

Findings

Nonlocal singlet pairing is suppressed in the junction.

The junction acts as a perfect spin triplet filter.

Equal-spin Cooper pairs are generated via crossed Andreev reflection.

Abstract

In superconducting spintronics, it is essential to generate spin-triplet Cooper pairs on demand. Up to now, proposals to do so concentrate on hybrid structures in which a superconductor (SC) is combined with a magnetically ordered material (or an external magnetic field). We, instead, identify a novel way to create and isolate spin-triplet Cooper pairs in the absence of any magnetic ordering. This achievement is only possible because we drive a system with strong spin-orbit interaction--the Dirac surface states of a strong topological insulator (TI)--out of equilibrium. In particular, we consider a bipolar TI-SC-TI junction, where the electrochemical potentials in the outer leads differ in their overall sign. As a result, we find that nonlocal singlet pairing across the junction is completely suppressed for any excitation energy. Hence, this junction acts as a perfect spin triplet filter across the SC generating equal-spin Cooper pairs via crossed Andreev reflection.