Influence of spin filtering and spin mixing on the subgap structure ofI−Vcharacteristics in a superconducting quantum point contact

Abstract

The effect of spin filtering and spin mixing on the dc electric current for voltage biased magnetic quantum point contact with superconducting leads is theoretically studied. The $I\text{\ensuremath{-}}V$ characteristics are calculated for the whole range of spin filtering and spin mixing parameters. It is found that with increasing of spin filtering, the subharmonic step structure of the dc electric current, typical for low-transparency junction and junction without considerable spin filtering, qualitatively changes. In the lower voltage region and for small enough spin mixing the peak structure arises. When spin mixing increases the peak subgap structure evolves to the step structure. The voltages where subharmonic gap features are located are found to be sensitive to the value of spin filtering. The positions of peaks and steps are calculated analytically and the evolution of the subgap structure from well-known tunnel limit to the large spin filtering case is explained in terms of multiple Andreev reflection (MAR) processes. In particular, it is found that for large spin filtering the subgap feature at ${\mathit{eV}}_{k}$ arises from $2k\text{th}$ and $(2k\ifmmode\pm\else\textpm\fi{}1)\text{th}$ order MAR processes, while in the tunnel limit the step at ${\mathit{eV}}_{n}$ is known to result from $n\text{th}$ order MAR process.