The flux line lattice (FLL) of Type-II superconductors is a topic of much recent interest. On the one hand, the FLL orientation and shape provide insights into the properties of the underlying superconducting state. On the other hand, the quasi-2D flux line system itself provides a test-bed for theories of structural order in solids and glasses. We use small-angle neutron scattering (SANS) to study FLLs in elemental niobium, a conventional superconductor. Applying the magnetic field along the fourfold [001] direction of pure Nb crystals, we discover surprising new FLL shapes and orientations that spontaneously break the fourfold symmetry. We examine how the crystal symmetry affects the FLL as the field is rotated away from [001]. Pursuing the second theme, we develop a reverse Monte Carlo (RMC) method to uncover the correlations between flux lines from SANS data. For FLLs perturbed by weak disorder, contemporary theories predict the survival of. quasi-long range order in a Bragg glass phase. Strong disorder is anticipated to generate a variety of vortex glass phases.' . Our results show that complex FLL shapes are not exclusive to unconventional superconductors. The potential of our RMC technique for revealing unambiguously the structural nature of flux line glasses is demonstrated.
| Identifer | oai:union.ndltd.org:bl.uk/oai:ethos.bl.uk:487517 |
| Date | January 2007 |
| Creators | Laver, Mark |
| Publisher | University of Birmingham |
| Source Sets | Ethos UK |
| Detected Language | English |
| Type | Electronic Thesis or Dissertation |
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