Spin phase regulated spin Josephson supercurrent in topological superconductor

Abstract

Without applied bias voltage, a superconducting phase difference can drive a charge Josephson supercurrent in a superconductor junction. In analogy, we here theoretically propose a spin phase that intrinsically generates spin Josephson supercurrent, and this spin Josephson effect is studied in a junction of superconducting nanowire (SNW). We show that spin-orbit coupling and magnetic field give rise to spin-triplet superconductivity in the SNW, thus allowing the superfluid of both charge and spin. Next we introduce the concept of a spin phase that can be generated by controls of spin current, magnetic field, or electric field. By the analysis of pairing correlations and Ginzburg-Landau-type theory, it is shown that the spin phase makes spin-up and spin-down $S=1$ Cooper pairs get opposite phases and move oppositely in a Josephson junction, so a dissipationless pure spin current is induced. We also derive the formula that the spin current is equal to the derivative of an Andreev bound state to the spin phase, which is analogous to that of charge Josephson effect. At last, our calculation verifies the existence of spin supercurrent inside the superconducting gap in the topologically nontrivial phase. Our paper provides a view on spin Josephson effect and indicates the potential combination of topological superconductivity and spintronics.