Superconductivity in heavy-fermion U(Pt, Pd)₃ and its interplay with magnetism

The effect of Pd doping on the superconducting phase diagram of the unconventional superconductor UPt₃ has been measured by (magneto)resistance, specific heat, thermal expansion, and magnetostriction. Experiments on single- and polycrystalline U(Pt_1-xPd_x)₃ for x<~0.006 show that the superconducting transition temperatures of the A phase T_c⁺, and of the B phase T_c^- both decrease, while the splitting ΔT_c increases at a rate of 0.30±0.02K/at. % Pd. The B phase is suppressed first, near x=0.004, while the A phase survives until x≅0.007. We find that ΔT_c(x) correlates with an increase of the weak magnetic moment m(x) upon Pd doping. This provides further evidence for Ginzburg-Landau scenarios with magnetism as the symmetry-breaking field, i.e., the two-dimensional (2D) E representation and the 1D odd-parity model. Only for small splittings ΔT_c∝m²(T_c⁺) (ΔT_c<~0.05K) as predicted. The results at larger splittings call for Ginzburg-Landau expansions beyond fourth order. The tetracritical point in the B-T plane persists until at least x=0.002 for B⊥c, while it is rapidly suppressed for B∥c. Upon alloying the A and B phases gain stability at the expense of the C phase.