Small amounts of Th and Lu impurities considerably affect both the low-temperature normal and the superconducting states of ${\mathrm{UBe}}_{13}$. Th enhances the electronic specific-heat parameter γ for T→0 K and leads to a second phase transition in the superconducting state. Lu substitution for U suppresses superconductivity quite effectively, and also γ is sizably reduced. Bulk measurements confirm the unusual $H_{c2}$(T) curve of ${\mathrm{UBe}}_{13}$ that was previously obtained from electrical resistivity experiments.

• Received 30 May 1985

DOI:https://doi.org/10.1103/PhysRevB.33.126