Proximity effects, spin transport, and phase transitions in superconducting hybrids

Abstract

This thesis presents our work on the theory of spin-dependent effects and transport in heterostructures involving superconductors, magnets, and non-centrosymmetric materials. The results are roughly divided into those concerning i) the superconducting phase transition and its influence on adjacent materials, and ii) spin transport. Our study of the superconducting phase transition is focused around spin-valve effects where the superconducting critical temperature can be controlled by the magnetic configuration or inversion symmetry breaking in materials in proximity to the superconductor. We also consider the reciprocal effect where decreasing the temperature below the superconducting transition temperature alters the magnetic anisotropy of an adjacent magnet. Furthermore, we consider how the superconducting critical temperature of an unconventional p-wave superconductor can be enhanced by proximity-coupling it via a ferromagnetic interlayer to a conventional superconductor with a higher critical temperature. Finally, we study the magnetic field-driven superconducting transition in a highly disordered hole-overdoped d-wave cuprate superconductor where superconducting pairing remains above the critical field where phase coherence is lost. Our work on spin transport includes the study of both dissipationless Cooper pair and resistive quasi-particle transport. We predict that due to interfacial exchange coupling, a supercurrent of spin-polarized Cooper pairs can induce a non-reciprocity in the magnon dispersion of a ferromagnetic insulator giving rise to magnon spin currents. We also study how non-equilibrium quasi-particle transport is affected by the pairing symmetry of the superconductor, how the inverse spin-Hall effect and spin-swapping are renormalized by a spin-splitting field, and how the coupling between two ferromagnetic insulators can be mediated by a superconductor thus affecting spin-pumping.