- Wednesday, 06 February 2019 18:00
Sam Wilkinson has submitted his PhD today, entitled "Quasicharge models of Josephson junction arrays". This thesis studies electronic response and transport in arrays of Josephson junctions, in 1-, (quasi)1- and 2-dimensions. A notoriusly difficult problem, transport in junction arrays combines all of the most difficult parts of condensed matter physics. It is a many-body problem, with long interaction lengths, disorder plays a key role in the dynamics, finite size effects are crucial, both Cooper-pairs and quasiparticles can transport current, the processes are partially coherent, and depending on the experimental conditions very different physics can be at play. An important part of Sam's thesis is that he has provided a unified theoretical framework of the mathematical description of these circuits. Starting from the detailed derivation of the Hamiltonian of the circuits, he shows how the quasicharge description can be used to derive effect equations of motion and how it links to the theory of depinning. He then applies this approach to the study of Coulomb drag in bilinear arrays, a first for the study of drag and depinning in the same system. In the case of 2D arrays, Sam has performed some of the first theoretical analysis of a new spectroscopic technique for studying vortex dynamics, using linear response theory and master equations. Another highlight of the thesis is chapter 3 where Sam surveys the many papers written by mathematicians on the Mathieu equation, chases down all the units and factors of two to link this to the physics literature and even shows that substracting 1 provides a better approximation than not! Throughout his PhD Sam has balanced his excursions into practically every branch of condensed matter physics with his love of B, C and even D grade movies, a pretty impressive amount of European travel and creating an imaginary world of the dungeons and/or the dragons to help distract his fellow students from their own 'real-life' quests.