Heslop, Richard William (2012)
Ph.D. thesis, University of Birmingham.
Results are reported of small-angle neutron scattering studies of the collective vortex behaviour from two overdoped YBa2Cu3O7-d samples, one 90% detwinned and one twin free. The results are discussed alongside other studies of similar samples over the years of SANS studies of
this material. Concerning the twin free sample, it was found that apparent indications of a discontinuity in the spot distribution axial ratio between the low and intermediate field phases as previously reported may have been influenced by the methodology of analysis. However, results with the same sample rotated off the axis of the applied field do show a clear difference in axial ratio. Excellent agreement in the field dependence of apex angle in the fully detwinned sample and that of a maximally overdoped yet twinned sample is seen, despite different orientation of the vortex lattice with respect to the crystal lattice. This is compelling evidence that Fermi surface effects are unlikely to be the cause of the high-field apex angle behaviour in these samples. Findings are also discussed from studies of an under doped sample YBa2Cu3O6.85, a calcium doped sample Y0.85Ca0.15Ba2Cu3O7 and a double copper oxygen chain sample YBa2Cu4O8. YBa2Cu3O6.85 and Y0.85Ca0.15Ba2Cu3O7 both displayed low field behaviour reminiscent of the optimally doped and over doped samples. There was no obvious intermediate field regime in these two samples and the high field rhombic phase was in both cases orthogonal to that seen in the optimally doped and over doped samples. YBa2Cu4O8 exhibited only the intermediate phase morphology. Since twinning is not possible in YBa2Cu4O8 the orientational pinning which was present in all other YBCO samples of this study was not present in this material. Also reported are results of the first studies of a vortex lattice in a KFe2As2 sample which was sufficiently ordered to yield Bragg spots instead of the ring-like diffraction intensity featured in all previous SANS studies of pnictides. The high degree of order was attributed to the stoichiometric nature of the sample. The most natural interpretation of the temperature dependence of the form factor is that the order parameter is nodal. However the field dependence of the structure is quite unlike that expected of a d-wave superconductor.
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