| The aim of this thesis is to deduce the symmetry of the heavy fermion super-conductor UPt;Based on Ginzburg-Landau theory, two main classes of models have been proposed to explain the phase diagram. The first class, referred to as E models, assumes a single two-dimensional order parameter. These models require a symmetry-breaking field to account for the splitting of the transition in zero field, and the kink in the upper critical field in the basal plane. In this thesis, we show that, as originally proposed, these models fail to explain the observed phase diagram for fields along the c axis. A modified form of these models proposed by Sauls is not obviously inconsistent with observations, but requires special assumptions. We therefore propose another class of models, now known as AB models, consisting of two independent one-dimensional order parameters. We study the bulk phase diagram of the AB models, and show that it is in complete qualitative accord with observations. We deduce all possible stable vortex lattice structures. Our deductions agree with the single available observation of the vortex lattice orientation at high fields and low temperatures. But, the bulk phase diagram alone can not definitely eliminate any of the AB models or the modified E model. We thus study surface superconductivity to further limit the order parameter symmetry. Comparison of our results with recent experiments by Keller et al. on a UPt... |
