Phenomenological theory of the superconductivity phase diagram of U1-xThx

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Possible phenomenological theories to describe the phase diagram of ${\mathrm{U}}_{1\mathrm{\ensuremath{-}}\mathrm{x}}$${\mathrm{Th}}_{\mathrm{x}}$${\mathrm{Be}}_{13}$ based on a crossing of two different types of anisotropic superconductivity at x\ensuremath{\approxeq}0.018 are examined. In this description the second transition for xg0.018 is interpreted as a further superconducting transition. It is shown that measurements of the critical magnetic field support this assumption. The effects of uniform pressure and the specific-heat measurements are qualitatively in a good agreement with these theories. The large peak in the ultrasonic attenuation in the low-temperature phase in the region xg0.018 is explained by a dissipative domain-wall motion, induced by the sound wave, which in this case couples in both [001] and [111] directions. The theory predicts nonunitary superconducting states below the second phase transition. Such states have a finite local spin polarization in each unit cell, which leads to an explanation of zero-field relaxation rate data in muon-spin-rotation experiments.