Superconducting proximity effect in mesoscopic metals

Abstract

The relentless pace for miniaturization of electronic components has reached the limit where quantum-mechanical phenomena can be observed in the currents through submicron-size circuits. In this mesoscopic regime, the conduction electrons exhibit interference effects due to their wave nature, and inelastic scattering ceases to be efficient enough to break the intrinsic phase coherence of the electron waves. In macroscopic metallic wires, such phase-coherent phenomena can be found only in superconducting materials.

This dissertation considers the phenomena encountered in heterostructures of superconductors (S) and mesoscopic normal, nonsuperconducting (N) metals. There, the superconducting order parameter, the pairing amplitude, penetrates into the normal-metal side inducing superconductive features in the normal-metal region near the interface. This superconducting proximity effect alters the properties of the normal metal and makes it possible to transport nondissipative supercurrents through it. An alternative viewpoint is given by the phenomenon of Andreev reflection, which, on the single-particle level, provides the means for the penetration of the pairing amplitude.

Theoretically, SN heterostructures and especially their transport properties can be modelled using two different types of approaches, one based on scattering matrices and the other on quasiclassical Green's functions. In this work, both of these formalisms are utilized in the study of a range of phenomena. These include the mutual proximity effects between superconductors and ferromagnets, the influence of Andreev reflection on universal conductance fluctuations, and nonequilibrium effects in current noise and in supercurrent through normal-metal Josephson weak links.

This study is conducted in a close collaboration with experimental work carried out in different low-temperature laboratories.