This thesis reports SR measurements of the internal magnetic field distribution n(B) in LuNi2B2C at temperatures T between and , under a magnetic field applied parallel to the crystal c axis. The SR data are analysed with a nonlocal London model  developed specifically for borocarbide superconductors, assuming the square vortex lattice appropriate for these temperature T and external field H conditions. The results of this analysis enable a number of conclusions to be drawn regarding nonlocality and the behaviour of the penetration depth and core radius with temperature T in LuNi2B2C.
Nonlocality plays an important role in the vortex state of LuNi2B2C. The incorporation of first order nonlocal corrections into the traditional London model improves the fit quality dramatically by qualitatively modifying the fitted internal magnetic field distribution n(B). In comparison to the field distribution n(B) produced for a square vortex lattice by the basic London model, the inclusion of these nonlocal terms considerably diminishes the spectral weight of the low field shoulder and generates a small peak at the lowest field B in the distribution n(B).
The penetration depth in LuNi2B2C increases slightly from at temperature to at . The form of the measured penetration depth temperature variation agrees with that expected for a BCS s-wave superconductor, although the error bars suffice in size for the observed temperature dependence to be consistent with weak linear growth. Such a linear rise in the penetration depth at low temperatures would imply the presence of low energy delocalised quasiparticles. However the considerably reduced steepness of the possible linear growth in LuNi2B2C relative to that observed for YBa2Cu3O6.95 means that the energy gap anisotropy in LuNi2B2C is much less than would be generated by line nodes.
The core radius in LuNi2B2C contracts linearly upon cooling through the investigated temperature interval. The rate of core shrinkage is remarkably slower than anticipated from the predicted Kramer-Pesch effect. The zero temperature core radius , as determined by comparison with NbSe2 data, greatly exceeds the proposed . However, the extrapolated quantum limit temperature for LuNi2B2C agrees well with the expected value . Surprisingly, the behaviour of the core radius with reduced temperature T/Tc is almost identical in nearly three-dimensional LuNi2B2C and quasi two-dimensional NbSe2. This similarity indicates that longitudinal disorder of vortices exerts negligible influence on SR measurements of the vortex core radius , and that quasiparticles in these two superconductors act in much the same manner. As is the case for NbSe2, the weakness of the observed Kramer-Pesch effect in LuNi2B2C points to the need for theoretical work on the temperature dependence of vortex structure to take into account zero point motion of vortices and vortex-vortex interactions.