Subharmonic Gap Structure in Superconductor/Ferromagnet/Superconductor Junctions

Abstract

The behavior of dc subgap current in magnetic quantum point contact is discussed for the case of low-transparency junction with different tunnel probabilities for spin-up ($D_{\uparrow }$) and spin-down ($D_{\downarrow }$) electrons. Due to the presence of Andreev bound states $\pm {\epsilon }_0$ in the system the positions of subgap electric current steps $e{V}_n=(\Delta \pm {\epsilon }_0)/n$ are split at temperature $T\ne 0$ with respect to the nonmagnetic result $e{V}_n=2\Delta /n$. It is found that under the condition $D_{\uparrow }\ne D_{\downarrow }$ the spin current also manifests subgap structure, but only for odd values of $n$. The split steps corresponding to $n=1,2$ in subgap electric and spin currents are analytically calculated and the following steps are described qualitatively.

4 pages, 1 figure, minor stylistic changes, journal-ref added