Two-order-parameter theory of the phase diagram of superconducting UPt3

The H-T phase diagram of UPt3 has previously been shown to be incompatible with a two-component-vector order parameter. It is studied in this paper within the ''AB'' model, consisting of two nearly degenerate order parameters, one transforming as an A and the other as a B representation of D6h. The nature of the different one- and two-order-parameter vortex-lattice phases and the possible transitions between them are examined in detail. A quantitative comparison is made with the experimental diagram, and the parameters in the Ginzburg-Landau free energy are deduced. It is found that the flux lattice evolves continuously in the low-H, low-T phase with no additional transitions between different types of two-order-parameter lattices, and that its orientation is in accord with neutron-diffraction studies near one of the inner transition lines.