Dynamic $0-\pi$ transition induced by pumping mechanism

TL;DR

This paper develops a nonequilibrium theory for charge and spin pumping in a quantum dot coupled to superconductors, revealing a laser-induced dynamic $0-ppa$ transition unaffected by Coulomb interactions and highlighting spin current behavior.

Contribution

It introduces a novel nonequilibrium Green's function approach to analyze laser-induced dynamic $0-ppa$ transitions in superconducting quantum dots, extending non-superconducting spintronics.

Findings

Supercurrent exhibits a laser-induced dynamic $0-ppa$ transition.

Spin current appears and is symmetric with phase difference at high laser frequencies.

The transition is unaffected by Coulomb interactions.

Abstract

Using Nambu$\otimes $spin space Keldysh Green's function approach, we present a nonequilibrium charge and spin pumping theory of a quantum dot in the mico-cavity coupled to two superconducting leads. It is found that the charge currents include two parts: The dissipationless supercurrent standing for the transfer of coherent Cooper pairs and the pumped quasi-particle current. The supercurrent exhibits a dynamic $0-\pi $ transition induced by the frequency and strength of the $\sigma_{-}$ polarized laser field. This dynamic transition is not affected by the strong Coulomb interaction. Especially, the spin current appears and is an even function of the phase difference between two superconductors when the frequency of the polarized laser field is larger than two times superconducting energy gap. Our theory serves as an extension to non-superconducting spintronics.