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[mu]SR measurement of the magnetic penetration depth and coherence length in the high-T superconductor YBa₂Cu₃O₆ 95 Riseman, Tanya M.

Abstract

This thesis is concerned with the temperature dependence and anisotropy of the magnetic penetration depth (A) and the Ginzburg-Landau parameter K A/e in the high temperature superconductor YBa2Cu306.95 as observed by OR measurements of the local field inhomogeneity produced by the vortex state. Measurements were taken on a mosaic of single crystals in applied fields in three field regimes: low (A < L), moderate (A > L) and high (A > L > e), where L is the intervortex spacing and e is the coherence length. The anisotropy of A leads to variation in the measured field inhomogeneity as a function of the angle between the applied field and e axis of the crystal. Measurements in 100 G are consistent with an anisotropy of five. If the applied field is not parallel to one of the superconductor's principal axes, an attractive force between vortices along one direction arises when the superconductor is cooled in a constant and low field. When the net intervortex repulsion exceeds the forces pinning the vortices in place, the aspect ratio of the isosceles triangle defining the vortex lattice will change. Unfortunately, the degree of disorder, where disorder is defined as the mean deviation of the vortices from their ideal positions in the lattice relative to the distance between vortices, makes it impossible to determine the low temperature aspect ratio in YBa2Cu306.95 using OR. In the high field data, the degree of disorder has an upper limit of 5.5%. In the vortex lattice, the maximum field is reached at the vortex core, which has a radius of e. In the low and moderate field regimes, the core occupies a very small percentage of the unit cell of the lattice, so the maximum field is not observable by ASR. In the high field regime, the core results in a high field cutoff in the distribution of local fields, which can be observed directly in the Fourier transform of ttSR data. Fitting both Aab and eab simultaneously in fields of 1.9 T,4.1 T, 4.7 T and 6.5 T applied parallel to the e axis of YBa2Cu306.95 yields Kai) = 70+ 6 between30 K and 75 K. At 10 K, Aab is measured to be 0.1490 + 0.0120 pm.

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