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

TL;DR

Impurities near a half-metal-superconductor interface enable finite Andreev reflection at zero energy, significantly affecting low-bias conductance and Josephson current in such junctions.

Contribution

This work demonstrates that impurities induce finite Andreev reflection at zero energy, which was previously suppressed in clean interfaces with magnetization gradients.

Findings

Impurities lead to finite Andreev reflection at zero energy.

Impurity-assisted reflection dominates low-bias conductance.

Enhancement of Josephson current in long junctions due to impurities.

Abstract

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 $\varepsilon=0$ [B\'{e}ri {\em et al.}, Phys.\ Rev.\ B {\bf 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 a HS junction and the Josephson current of an SHS junction in the long-junction limit.