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# High-field SANS studies of the vortex lattice in YBCO

Brown, Stephen Phillip (2006)
Ph.D. thesis, University of Birmingham.

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## Abstract

In this thesis is presented the first high-field structural study of the vortex lattice in YBa$$_2$$Cu$$_3$$O$$_7$$. Small-angle neutron scattering was used to deduce the configuration of vortices in a twinned sample of YBa$$_2$$Cu$$_3$$O$$_7$$ over the field range 1–11T applied parallel to the crystal c-axis. The experiments revealed a field-induced continuous transition from a distorted London-like vortex lattice at 1T, to an unconventional square vortex lattice with a slight rectangular distortion. It is clear from the data that some kind of unconventional intrinsic anisotropy is responsible for the high-field square lattice and the data are suggestive of two possible origins, Fermi surface anisotropy and d-wave effects, both of which may be described as non-local effects. However, from the present data it is not possible to determine which of these effects is more important. The dependence of the vortex lattice structure upon temperature and angle of applied field was also studied. In accordance with expectation, it was found that the unconventional anisotropy responsible for the stability of the square vortex lattice is diminished with increasing temperature and the distorted triangular configuration eventually reappears via a continuous transition. Similarly, the square vortex lattice is suppressed by rotating the applied field away from the crystal c-axis. However, in this case the transition between the triangular and square vortex lattices becomes firstorder. Moreover, the low-field triangular domains are very different to those observed with the field parallel to the c-axis. Three other neutron studies in which this author was involved were also briefly presented. Of these, two were small-angle neutron scattering studies of the vortex lattice (in Nd$$_{1.85}$$Ce$$_{0.15}$$CuO$$_4$$ and detwinned YBa$$_2$$Cu$$_3$$O$$_7$$−δ) and are thus intimately related to the main topic. In Nd$$_{1.85}$$Ce$$_{0.15}$$CuO$$_4$$, a square vortex lattice was observed, but at unprecedentedly low fields (20 mT). The detwinned YBa$$_2$$Cu$$_3$$O$$_7$$−δ samples have provided confirmation of the results observed in the twinned sample and hold prospect for further studies, particularly of the effect of doping on the transition to square. Lastly, vii measurements by inelastic neutron scattering of the spin fluctuations at the metamagnetic field in Sr$$_3$$Ru$$_2$$O$$_7$$ were shown. These important experiments have demonstrated the first microscopic evidence for a wholesale change in the character of the spin correlations at the metamagnetic field.

Type of Work: Ph.D. thesis. Forgan, E.M. Schools (1998 to 2008) > School of Physics & Astronomy Physics & Astronomy QC Physics University of Birmingham Check for printed version of this thesis 267
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