Engineering nonequilibrium superconducting phases in a voltage-driven superconductor under an external magnetic field

Abstract

We theoretically investigate nonequilibrium properties of a normal metal-superconductor-normal metal (NSN) junction under an external magnetic field. When a bias voltage is applied between the normal-metal leads, the confined superconductor is driven out of equilibrium, resulting in a nonequilibrium quasiparticle distribution function having a two-step structure. Using the nonequilibrium Green's function technique, we determine a comprehensive phase diagram of the nonequilibrium superconductor. Our analysis reveals that the interplay between Zeeman-split energy bands and the nonequilibrium distribution function gives rise to a rich phase structure. Notably, we find that superconductivity destroyed by a strong external magnetic field revives by applying the bias voltage. This reentrant phenomenon is shown to originate from four effective "Fermi surfaces" that result from the combination of Zeeman-split energy bands and the two-step structure in the nonequilibrium distribution function. Our results demonstrate the possibility of controlling quantum states of matter through the combined engineering of energy band structures and distribution functions.