Spin-flip Enhanced Thermoelectricity in Superconductor-Ferromagnet Bilayers

Abstract

We study the effects of Zeeman-splitting and spin-flip scattering in a superconductor (S) on the thermoelectric properties of a tunneling contact to a metallic ferromagnet (F) using the Green's function method. A giant thermopower has been theoretically predicted and experimentally observed in such structures. This huge thermoelectric effect is attributed to the spin-dependent particle-hole asymmetry in the tunneling density of states in the S/F heterostructure. Here, we evaluate the S density of states and thermopower for a range of temperatures, Zeeman-splitting, and spin-flip scattering. In contrast to the naive expectation, we find that the spin-flip scattering strongly enhances the thermoelectric performance of the system in the low-field and low-temperature regime. This is attributed to a complex interplay between the charge and spin conductances caused by the softening of the spin-dependent superconducting gaps. The maximal value of the thermopower exceeds kB/e by a factor of ~ 5 and has a nonmonotonic dependence on Zeeman-splitting and spin-flip rate. We also demonstrate that the incoherent broadening leads to a drastic reduction of the thermoelectric performance.