Low-temperature heat-capacity study of the U6X (XMn, Fe, Co, Ni) compounds

Measurements of the superconducting- and normal-state heat capacity of U6X (XMn, Fe, Co, Ni) compounds have been performed over a temperature range 1 KT35 K. The U6X compounds have strong renormalizations of the free-carrier effective mass m* in the range 10mem*50me (mefree-electron mass). The unusual magnitude and temperature dependence of the U6X heat capacities suggest the presence of high densities of low-energy excitations of undetermined nature. The results are analyzed in terms of models appropriate to heavy-fermion liquids, and anisotropic or strong-coupled superconductors. The U6X compounds form a link between relatively low-m*, high-transition-temperature A15 compounds and the more extreme examples of heavy-fermion superconductors such as UBe13, UPt3, and CeCuSi2 for which m*102me.. AE