Rashba splitting of Cooper pairs

TL;DR

This paper theoretically explores how a Rashba spin-orbit coupled nanowire weak link affects Cooper pair tunneling, revealing spin splitting and interference effects that modulate the Josephson current based on electric field and nanowire geometry.

Contribution

It introduces the concept of a spin splitter in a superconducting weak link with Rashba spin-orbit coupling, showing how it influences Josephson current interference patterns.

Findings

Josephson current exhibits periodic dependence on spin-orbit strength.

Spin splitting of Cooper pairs depends on electric field direction.

Interference effects are absent in normal metal weak links.

Abstract

We investigate theoretically the properties of a weak link between two superconducting leads, which has the form of a non-superconducting nanowire with a strong Rashba spin-orbit coupling caused by an electric field. In the Coulomb blockade regime of single-electron tunneling, we find that such a weak link acts as a "spin splitter" of the spin states of Cooper pairs tunneling through the link, to an extent that depends on the direction of the electric field. We show that the Josephson current is sensitive to interference between the resulting two transmission channels, one where the spins of both members of a Cooper pair are preserved and one where they are both flipped. As a result, the current is a periodic function of the strength of the spin-orbit interaction and of the bending angle of the nanowire (when mechanically bent); an identical effect appears due to strain-induced spin-orbit coupling. In contrast, no spin-orbit induced interference effect can influence the current through a single weak link connecting two normal metals.