ACCIDENTAL NEAR DEGENERACY OF THE ORDER PARAMETER FOR SUPERCONDUCTING UPT3

UPt3 appears to have a tetracritical point in the H-T plane for all orientations H of H. Such a point can exist only if the Ginzburg-Landau (GL) ''Hamiltonian'' has a conserved quantity Q. It is shown that the model of a two-component vector order parameter admits a Q for H parallel-to c, but has no tetracritical point. Hence, accidentally nearly degenerate order parameters are examined, particularly the model where one behaves as an A and the other as a B representation of D6h. This model can yield a tetracritical point for all H. Its physical properties are calculated and GL parameters are fitted to experiment.