Transport in mesoscopic superconducting devices

Licentiate thesis, 2019

A field of growing interest within the last few decades is the study of superconductivity in mesoscopic-scale heterostructures. Mesoscopic refers to sizes between the atomic and macroscopic scales. Here, the size of heterostructures can be comparable to the inherent scale of superconductivity, the superconducting coherence length, and give rise to new physical phenomena.

The focus of this work is on mesoscopic hybrid structures consisting of superconducting, normal-metal, and magnetic regions. The combination of these different types of materials and the competition between interactions such as magnetism and superconductivity can then be used to design structures with novel effects. This is not only interesting from a fundamental point of view but equally relevant for technological applications. The magnet-superconductor hybrid structures examined in this work, for example, give rise spin-polarized Andreev bound states, a promising ingredient to superconducting spintronics.

We study transport in such hybrid systems under current bias to investigate the effects of such Andreev bound states on nonequilibrium properties. As part of this work, we develop a general calculation scheme for current-bias nonequilibrium within the quasiclassical theory of superconductivity. We use this scheme to study charge and spin imbalance in a normal-metal/superconductor structure with a spin-active interface. Our results show that transport in systems with spatially extended tunnel barriers is more accurately described by this current-bias picture compared to a voltage-bias description traditionally used in the theoretical literature for narrow constrictions. We find that the presence of Andreev bound states at a spin-active interface between normal-metal and superconducting regions strongly influence the charge as well as spin transport in such structures.