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Evidence for a New Type of Re-entrant Phase Transition in the Unconventional Superconductor UPt3

The unconventional heavy-fermion superconductor UPt3 exhibits a third superconducting transition at Tc3 = 18 mK in addition to those already known at Tc1 = 0.5 K and Tc2 = 0.45 K. The splitting of Tc1 and Tc2 is supposed to be the result of a coupling of a two-dimensional superconducting and a magnetic order parameter. It is shown that the new transition at Tc3 can be incorporated in such a model. It is interpreted as a new type of superconducting re-entrant transition caused by higher-order terms in the Landau free energy. The conditions for the existence of such a transition are discussed. Temperature dependence of the order parameter and specific heat are calculated within a Landau approach. The significance of hydrostatic-pressure experiments is discussed.