Does a ferromagnet with spin-dependent masses produce a spin-filtering effect in aferromagnetic/insulator/superconductor junction?

Abstract

We analyze charge transport through a ballistic ferromagnet/insulator/superconductorjunction by means of the Bogoliubov–de Gennes equations. We take into account thepossibility that ferromagnetism in the first electrode may be driven by a massrenormalization of oppositely polarized carriers, i.e. by a spin bandwidth asymmetry, ratherthan by a rigid splitting of up- and down-spin electron bands as in a standard Stonerferromagnet. By evaluating the averaged charge conductance for both an s- and adx2 − y2-wave order parameter for the S side, we show that the mass mismatch in the ferromagneticelectrode may mimic a spin active barrier. Indeed, in the s-wave case we show that undersuitable conditions the spin-dependent conductance of minority carriers below the energy gapΔ0 can be larger than for majority carriers, and lower aboveΔ0. On the otherhand, for a dx2 − y2-wave superconductor similar spin-dependent effects give rise to an asymmetric peaksplitting in the conductance. These results suggest that the junction may work as aspin-filtering device.