Vonka, Jakub and Fisher, Shaun and Bradley, David and Haley, Richard (2018) Demagnetisation of solid 3He and supercritical superflow. PhD thesis, Lancaster University.
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Abstract
This work describes the efforts with two ultra low temperature experiments with superfluid 3He as medium of interest. The experiments are mostly performed at temperatures below 200uK, in the regime where superfluid quasiparticle excitations are ballistic. Recently, a novel experimental tool has been built in Lancaster - a superconducting goalpost-shaped wire that can be moved trough the superfluid in oscillatory as well as in uniform linear motion. An object moving with high enough velocity that the excitation spectrum becomes gapless can create excitations at no energy cost and initiate the breakdown of the condensate - this limit is the well-known Landau velocity. In superfluid 3He, flow around an oscillating body displays a very clear onset of such dissipation. However, with this experiment it was found that for a uniform linear motion there is no discontinuity whatsoever in the dissipation as the Landau critical velocity is passed and exceeded, entering a supercritical flow regime. This regime allows for studying the dynamics of the Andreev bound states on the surface of the wire. This work presents a recent experimental estimation of the relaxation time of the bound states and a description of the relaxation mechanism. Next, the work describes the design and initial testing of a new experiment. Here a layer of solid 3He formed on the surface of a large aerogel sample submersed in superfluid 3He will be cooled down to below 100uK in a double nuclear demagnetisation process. NMR on the solid 3He will be used to search for a possible magnetic phase transition as well as to study superfluid 3He virtually free of quasiparticle excitations. The work reports progress to the present state of the experiment and discusses setbacks due to a large unexpected heating which appeared during the demagnetisation of the copper stage.