Low-temperature heat-capacity study of theU6X (X≡Mn, Fe, Co, Ni) compounds

Measurements of the superconducting- and normal-state heat capacity of ${\mathrm{U}}_6$X (X≡Mn, Fe, Co, Ni) compounds have been performed over a temperature range 1 K≲T≲35 K. The ${\mathrm{U}}_6$X compounds have strong renormalizations of the free-carrier effective mass $m^{\mathrm{*}}$ in the range 10$m_e$≲$m^{\mathrm{*}}$≲50$m_e$ ($m_e$≡free-electron mass). The unusual magnitude and temperature dependence of the ${\mathrm{U}}_6$X 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 ${\mathrm{U}}_6$X compounds form a link between relatively low-$m^{\mathrm{*}}$, high-transition-temperature A15 compounds and the more extreme examples of heavy-fermion superconductors such as ${\mathrm{UBe}}_{13}$, ${\mathrm{UPt}}_3$, and ${\mathrm{CeCuSi}}_2$ for which $m^{\mathrm{*}}$∼${10}^2$ $m_e$. .AE