Magnetization near a constriction between BCS superconductors by spin-dependent tunneling

TL;DR

This paper explores how spin-dependent tunneling in a superconductor constriction can induce magnetization and spin currents, highlighting the role of bias and electron-hole asymmetry in enabling triplet Cooper pairs.

Contribution

It demonstrates that bias and asymmetry can break interference effects, allowing magnetization and spin currents to form in superconductor constrictions.

Findings

Bias enables triplet Cooper pair formation.

Symmetry breaking destroys destructive interference.

Magnetization can be electrically controlled.

Abstract

Spin-dependent electron tunneling through a voltage-biased micro-constriction between two bulk superconductors is shown to create a static magnetization near the constriction and an ac Josephson-like spin current. Although spin-dependent tunneling generates quantum spin fluctuations even in the absence of a bias, the formation of spin-triplet Cooper pairs necessary for the creation of magnetization is blocked by destructive interference between different quasi-electron and quasi-hole tunneling channels, unless there is an asymmetry between the tunneling densities of states for electrons and holes. Breaking the symmetry in the electron-hole tunnel density of states and creating electron-hole tunneling imbalance by biasing the device destroys the destructive interference and enables triplet Cooper-pair formation. As a result, magnetizing the superconductor becomes possible. The role of the voltage in lifting the blockade hindering the spin-triplet Cooper pair formation is an example of an electrically-controlled dissipation-less spintronic phenomenon.