Density driven vortex motion in narrow channel superconductors

Watkins, Jonathan Stewart (2016). Density driven vortex motion in narrow channel superconductors. University of Birmingham. Ph.D.

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Abstract

We present simulations and continuum calculations of the rheology and structure of vortex matter confined to flow in narrow channels.
First, through the use of Langevin dynamics we perform two-dimensional Couette flow simulations of the vortex liquid in a homogeneous magnetic field. In this sheared geometry we report wall slip at the channel boundary for high shear rates. A result that contrasts with the Newtonian constitutive relation suggested by Marchetti and Nelson(Marchetti, M. C. & Nelson, D. R. Phys. Rev. B 42 , 9938 (1990)). We also find structural ordering near the solid channel boundaries above the bulk melting temperature, Tm bulk
We also present simulations and analysis of a novel field gradient applied to a narrow channel superconductor. Here the “solid” vortex lattice flows plastically by enlisting two distinct populations of dislocations. One that relaxes density along the channel and the other that relaxes shear stresses at the boundary. In spite of the lack of a commensurate field over the majority of the channel, the vortex glass remains commensurate with the width of the channel along its entire length by stretching parallel to the flow direction. This non-equilibrium system reaches a steady-state marked by a stable density profile and localised repeatable non-linear dislocation motion. We are further able to show the source of this non-linearity is due to image forces in the channel edges.
For wider channels we discuss and implement a novel vortex reservoir geometry de- signed to generate a continuous source of circular grain boundary scars - bubbles. The presence of the bubbles in the channel does not alter the density of the channel but does disturb the steady-state structure and motion of the other populations of dislocations.
Finally we discuss the details of a new versatile simulation software that was created for this research, VLSim. This software utilises object-orientation techniques to allow fast future prototyping of varied geometry and magnetic fields.

Type of Work:

Thesis (Doctorates > Ph.D.)

Award Type:

Doctorates > Ph.D.

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