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

TL;DR

This paper investigates how a ferromagnet with spin-dependent masses affects charge transport in a ferromagnet/insulator/superconductor junction, revealing potential spin-filtering effects due to mass mismatch rather than traditional band splitting.

Contribution

It introduces the concept that spin-dependent mass renormalization in ferromagnets can mimic spin-active barriers, influencing conductance and enabling spin-filtering in superconductor junctions.

Findings

Mass mismatch can enhance minority carrier conductance below the gap.

Spin-dependent conductance asymmetry is observed in d-wave superconductors.

The junction can act as a spin-filtering device under certain conditions.

Abstract

We analyze charge transport through a ballistic ferromagnet/insulator/superconductor junction by means of the Bogoliubov-de Gennes equations. We take into account the possibility that ferromagnetism in the first electrode may be driven by a mass renormalization of oppositely polarized carriers, i.e. by a spin bandwidth asymmetry, rather than by a rigid splitting of up-and down-spin electron bands as in a standard Stoner ferromagnet. By evaluating the averaged charge conductance for both an s- and a $d_{x^2-y^2}$-wave order parameter for the S side, we show that the mass mismatch in the ferromagnetic electrode may mimic a spin active barrier. Indeed, in the $s$-wave case we show that under suitable conditions the spin dependent conductance of minority carriers below the energy gap $\Delta_0$ can be larger than for majority carriers, and lower above $\Delta_0$. On the other hand, for a d_{x^2-y^2}-wave superconductor similar spin-dependent effects give rise to an asymmetric peak splitting in the conductance. These results suggest that the junction may work as a spin-filtering device.