Phase-sensitive Interface and Proximity Effects in Superconducting Spintronics

Spintronics with superconductors has been developing speedily since a number of pivotal experiments proved its feasibility in the end of the 1990’s and beginning of the 2000’s. It is based on a number of theoretical predictions well before that, and has proved of interest both from the fundamental point of view in realizing new states of matter as well as from the practical point of view via the current urge to develop an energy-saving technology of large-scale computing and data storage centers.
The main driving forces are along three directions. The first is control of the phase of the order parameter via frustration between two types of Cooper pairs, spin-singlet and spin-triplet Cooper pairs. The second consists in utilizing equal- spin Cooper pairs in ferromagnetic devices, which allows for long-ranged proxim- ity effects with penetration depths comparable to those that would occur in normal metals. The third is the production and control of pure spin currents utilizing the spin-polarized single-particle excitation spectrum in superconductors.
A number of spin-offs are of strong interest for fundamental research. Among those is the possibility to study so-called odd-frequency pairing, an elusive type of pairing that has never been observed in bulk superconductors, and, indeed, arguments have been put forward that it is not thermodynamically stable in a bulk superconductor.
The developing field of superconducting spintronics has at its goals the devel- opment of memory and logic devices based on unique properties resulting from the combination of superconductivity with spin-ordered states.