Behaviour of vortex matter in the presence of periodic pinning

Filby, Ed (2007) Behaviour of vortex matter in the presence of periodic pinning. University of Southampton, Doctoral Thesis.

Abstract

High temperature superconductors YBa₂Cu₃O_7-δ and YBa₂Cu₄O₈ are studied for the case that field is aligned with the intrinsically layered structure.  A novel susceptibility technique is employed, revealing pronounced commensurability oscillations, including the first observation of commensurability phenomena in Y124.  Oscillations are periodic in B^-1/2 and include distinct hysteretic features.  History dependences reveal reversible regions that are attributed to commensurate states.  A study of statistical variations between repeated measurements is used to quantify correlation between irreproducibility and irreversibility, providing evidence for transitions between commensurate states.  Oscillation doubling is seen for high fields in Y123, and attributed to the effect of twins.

A study of the phase diagram of underdoped YBa₂Cu₃O_7-δ is presented, in fields up to 28T, made using both four-probe and flux transformer transport measurements, the latter technique enabling the measurement of the elimination of flux motion perpendicular to the crystal layers.  The initial, diagonal part of the melting line at high temperature is seen to continue past the upturn at T*.  The resulting phase diagram contains both this diagonal line, similar to the melting line away from alignment, and two vertical phase lines corresponding to the formation of Josephson vortices below T*, and the lower temperature T_L, detected using the flux transformer, and below which vortex movement is locked into the planes.

Pb hemispherical anti-dot arrays fabricated by a new technique of electroplating around self-assembled arrays of micron sized spheres are studied using AC susceptibility and DC magnetisation measurements.  Pronounced commensurability effects are seen, with large increase in critical current and temperature at fields where there is an integer number of flux quanta per hole.  Comparisons are drawn between these and arrays created using conventional lithographic techniques.  Whilst both demonstrate commensurability effects, differences are marked.  The large difference in the shape of holes in the arrays results in large differences in the qualitative commensurability behaviour.  The weaker vortex pinning in the spherical arrays leads to a greater influence of vortex lattice forces, resulting in more collective type vortex pinning than individual.  Unusual dynamic effects are also seen, including channelling effects for large drives.

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