Non-equilibrium and spin transport in hybrids of superconductors and magnets

Abstract

BCS superconductivity and ferromagnetism rarely coexist in bulk materials due to their mutually antagonistic spin orderings. Many exciting new phenomena which are not present in each system alone, however, could occur in proximity-coupled superconductor/ferromagnet (S/F) heterostructures. The goal of this dissertation is to theoretically present a variety of fascinating but still not experimentally reported phenomena occurring in magnetic superconducting hybrid devices, which in addition to serving as a platform for novel fundamental spin physics, could become essential building blocks in superconducting spintronics.One of the key concepts in quantum transport is the controlled creation and interaction of charge and spin in nanoscale hybrid structures. Conventional S is composed of singlet Cooper pairs that carry no net spin. Achieving exotic superconducting phases as well as exploiting zero-dissipation supercurrents for switching magnetic memories, for example, requires equal-spin triplet Cooper pairs which are challenging to obtain due to their elusive nature. We will demonstrate a minimal and experimentally achievable setup of a four-terminal S/F hybrid that allows for the generation, control, and detection of these pairs. In particular, we propose a detection scheme based on charge current measurements and show that equal-spin triplet correlations establish a tunable spin current flow into the magnets.A very large thermopower is not directly obtainable in conventional S due to the presence of an almost perfect spin-dependent electron-hole symmetry around the Fermi energy. However, a thin S with a spin-split density of states, could break this symmetry and thereby provides a giant thermoelectric effect. We will show that spinflip scattering could strongly enhance the thermoelectric response of an S/F bilayer via creating a sizeable figure of merit at low values of temperature and spin-splitting. The scattering rate shifts the maximum of the thermopower from a higher to a weaker magnetic field, which turns out to be of crucial importance for reducing the necessary spin-splitting in these structures and thereby avoiding additional detrimental effects related to external magnetic fields.Inducing a spin-splitting field in an adjacent S usually has been achieved via employing ferromagnets or externally applied magnetic fields. However, using ferromagnets in these designs is marred with numerous detrimental and parasitic effects which lower the practical feasibility of the proposed models. Deriving the interfacial self-energy, we will confirm that an insulating antiferromagnet in proximity to a thin S layer, can indeed more conveniently, be used to induce a sizeable, and disorder resistant spin-splitting in the S than ferromagnets or applied magnetic fields. This prediction is a promising step towards eliminating the limiting impacts of ferromagnets employed to this end and allows realizing several concepts involving spin-split superconductors.