Spin-flip enhanced thermoelectricity in superconductor-ferromagnet bilayers

Abstract

We study the effects of spin-splitting and spin-flip scattering in a superconductor (S) on the thermoelectric (TE) 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 is attributed to the spin-dependent particle–hole asymmetry in the tunneling density of states (DOS) in the S/F heterostructure. Here, we evaluate the S DOS and thermopower for a range of temperatures, Zeeman-splitting, and spin-flip scattering. In contrast to the naive expectation based on the negative effect of spin-flip scattering on Cooper pairing, we find that the spin-flip scattering strongly enhances the TE 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 k _B / e by a factor of ≈5 and has a non-monotonic dependence on spin-splitting and spin-flip rate.

Keywords

• ferromagnet-superconductor heterostructures
• thermopower
• figure of merit
• proximity effect
• quasiclassical theory
• spin-flip scattering