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Title: The application of spin polarised neutron scattering to superconductors
Authors: Smith, Timothy J.
Issue Date: 1999
Publisher: © T.J. Smith
Abstract: The use of spin polarised neutron scattering as an experimental probe for magnetic phenomena within solid state physics has been demonstrated in two separate studies of superconductors. With the use of three-dimensional polarisation analysis, the magnetic response within a thermal energy window -30meV<Iiw<+30meV has been measured for both insulating and superconducting compositions of the high T c material YBa2Cu306+x. The observed magnetic scattering from the superconducting composition (x=0.9S, T c-90K) at T =300K shows conclusively that there is negligible magnetic scattering in the normal state below 30meV. What little scattering is observed corresponds to a paramagnetic cross-section of (0.048±0.008)barns or to -3.2% of the Cu atoms carrying a spin 1/2. Similar polarised neutron measurements performed on insulating YBa2Cu306.11, above and below TN, reveal a small but finite level of scattering corresponding to -18% of the Cu atoms carrying a spin 1/2 in the paramagnetic state. The observed increase in paramagnetic scattering above T N corresponds to just 20% of that expected due to the thermal disordering of the observed ordered magnetic moment at 300K. These measurements are consistent with a magnetic excitation spectrum driven by quantum rather than thermal fluctuations for which the notion of separate magnetic and charge degrees of freedom (and therefore also the concept of local moments) may not be valid. In addition, measurements conducted on single crystal YBa2Cu307 in its normal state revealed no indication of magnetic enhancement in the vicinity of the (It, It) point up to 30meV. This is in disagreement with theoretical predictions of an enhanced magnetic scattering in this region for spin fluctuation energies of order 10 to 20meV. The second study demonstrates the feasibility of spin polarised neutron scattering as a novel tool for the investigation of the flux line lattice in type 11 superconductors. Measurements conducted in the mixed state of niobium (T =4.SK, B=O.22T) constitute the first experimental observation of lattice distortions due to the presence of flux line vortices. The experimentally determined magnitude of the lattice distortion yields a value approximately three orders of magnitude greater than that expected due to the volume anomaly between the coexisting normal and superconducting regions. An alternative mechanism is suggested on the basis of electron redistribution between the normal and superconducting regions in which electrons are trapped by the flux lines. As a result of this process, a response in the form of a lattice distortion is induced in order to maintain charge neutrality.
Description: A Doctoral Thesis. Submitted in partial fulfilment of the requirements for the award of Doctor of Philosophy of Loughborough University.
URI: https://dspace.lboro.ac.uk/2134/10465
Appears in Collections:PhD Theses (Physics)

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