Impurity-assisted Andreev reflection at a spin-active half metal-superconductor interface

The Andreev reflection amplitude at a clean interface between a half-metallic ferromagnet (H) and a superconductor (S) for which the half metal's magnetization has a gradient perpendicular to the interface is proportional to the excitation energy $\varepsilon$ and vanishes at $\varepsilon =0$ [Béri et al., Phys. Rev. B 79, 024517 (2009)]. Here we show that the presence of impurities at or in the immediate vicinity of the HS interface leads to a finite Andreev reflection amplitude at $\varepsilon =0$. This impurity-assisted Andreev reflection dominates the low-bias conductance of an HS junction and the Josephson current of an SHS junction in the long-junction limit.

Figure 1

HS interface with a spin-active interface, originating from a magnetization gradient perpendicular to the interface. Arrows indicate the magnetization direction.

Figure 2

SHS junction of length $L$ with two spin-active interfaces.