The phase diagram of superconducting UPt 3 is explained in a GinzburgLandau theory starting from the hypothesis that the order parameter is a pseudo-spin singlet which transforms according to the E 1g representation of the D 6h point group. We show how to compute the positions of the phase boundaries both when the applied field is in the basal plane and when it is along the c-axis. The experimental phase diagrams as determined by longitudinal sound velocity data can be fit using a single set of parameters. In particular the crossing of the upper critical field curves for the two field directions and the apparent isotropy of the phase diagram are reproduced. The former is a result of the magnetic properties of UPt 3 and their contribution to the free energy in the superconducting state. The latter is a consequence of an approximate particle-hole symmetry. Finally we extend the theory to finite pressure and show that, in contrast to other models, the E 1g model explains the observed pressure dependence of the phase boundaries.

We examine the hypothesis that fluctuating local superconducting order exists well above the critical temperature in high-T, materials. This might explain anomalies in the magnetic susceptibility of some of these systems. It could also create a gap or pseudogap in the density of states, which could be measured by photoemission or tunneling experiments. We compute the density of states for a twodimensional tight-binding model with such local order in the coherent-potential approximation. The order parameter is taken to have a spatially random phase and constant magnitude. We calculate this magnitude by a Ginzburg-Landau method. The conclusion is that the disorder smooths out the gap in the density of states unless the local order parameter is larger than the zero-temperature gap.