Spin-Dependent Phenomena in Superconductor Hybrid Structures

PhD Program

Speaker

Tim Kokkeler

When

2025/06/03

Place

Waaier Building, University of Twente, Enschede, The Netherlands

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PhD Thesis defense by Tim Kokkeler

Supervisors: F. Sebastian Bergeret (DIPC associate) & Hans Hilgenkamp

Condensed Matter Theory - Superconductivity and magnetism

This thesis describes the interplay of the superconducting proximity effect of conventional and unconventional superconductors with materials that are magnetic or have spin-orbit coupling. The different Chapters in this thesis predict that this interplay yields several interesting emergent phenomena, such anomalous currents and superconducting diode effects. The three main achievements obtained within this thesis are the following. A formalism is developed to derive the transport equations and corresponding boundary conditions for a large range of materials through the construction of the quasiclassical action, called the nonlinear sigma model (NLSM), based on phenomenological symmetry considerations. Using this approach the theory of transport in the presence of a broad range of magnetic and spin-orbit interactions is developed. This theory is used for the prediction of several effects that can be used to characterize materials in which these phenomena are present. The second main achievement is the characterization of gyrotropy on multiple scales, which explains that non-reciprocal transport may appear in junctions in which none of the materials contains microscopic gyrotropy. To this end we have shown that superconducting diode effects exist even in cubic normal metals with a nonzero spin-Hall angle, when in contact with a ferromagnetic insulator on one side and a superconductor on another side. The third main achievement is the development of the theory of noise in junctions with unconventional superconductors. This theory is subsequently used to show that noise is a suitable tool for the clarification of the pairing symmetry of unconventional superconductors, because surface Andreev bound states yield distinctive signatures in the differential Fano factor, the ratio of the differential noise power and the conductance.