Fulde-Ferrel-Larkin-Ovchinnikov inhomogeneous superconducting state and phase transitions induced by spin accumulation in a ferromagnet-d_(x~2-y~(2-))-wave superconductor-ferromagnet tunnel junction

摘要

Fulde-Ferrel-Larkin-Ovchinnikov (FFLO) inhomogeneous superconducting (SC) state, first- and second-order phase transitions, and quantum criticality induced by spin accumulation in a ferromagnet-d_(x~2-y~(2-))-wave superconductor-ferromagnet tunnel junction are theoretically predicted. A complex phase diagram in the temperature-bias voltage plane is determined. It is found that the phase transitions from the homogeneous BCS state to the inhomogeneous FFLO state, and from the FFLO state with the momentum q's azimuthal angle θ_q=0 to that with θ_q=π/4, are of the first order, whereas the transitions from all SC states to the normal state at critical voltages are of the second order. A Lifshitz point, a bicritical point, and a quantum critical point are identified.