Two-component order-parameter model for pure and thorium-doped superconductingUBe13

We discuss a Ginzburg-Landau model of two even-parity superconducting order parameters (e.g., of s-wave and d-wave symmetry with transition temperatures $T_0$ and $T_2$, respectively) as a model for ${\mathrm{U}}_{\mathrm{x}}$ ${\mathrm{Th}}_{1\mathrm{-}\mathrm{x}}$ ${\mathrm{Be}}_{13}$. The critical temperatures for the pure system are assumed such that $T_2$>$T_0$. We suggest that impurity scattering strongly suppresses $T_2$ but barely affects $T_0$ so that the above inequality is reversed for an impurity concentration x>$x_c$. For special choices of order parameters, the second component is switched on continuously in the pure system (x<$x_c$) but via a second-order phase transition in the impure system (x>$x_c$).