Single-ion effects in the formation of the heavy-fermion ground state inUBe13

We report a study of ${\mathrm{U}}_{1\mathrm{-}\mathit{x}}$ ${\mathit{M}}_{\mathit{x}}$ ${\mathrm{Be}}_{13}$ (0≤x≤0.995 with the nonmagnetic M=Hf, Zr, Sc, Lu, Y, Pr, Ce, Th, and La), with characterization by x-ray diffraction, resistivity, dc susceptibility, and specific heat. Hybridization, and the distance, between the U 5f electrons and the Be s and p electrons is found to control the specific heat γ and therefore the effective mass ${\mathit{m}}^{\mathrm{*}}$, while the outer electronic configuration of M plays no role. When the ${\mathrm{UBe}}_{13}$ lattice is not expanded by the dopant M, γ (normalized per U mole) reaches a constant value for x>0.15 that is about 40% that of pure ${\mathrm{UBe}}_{13}$. This dilute single-ion behavior is also observed in the resistivity. Surprisingly, the low-temperature magnetic susceptibility (normalized per U mole) is independent of doping for all x. Thus, the large magnetic susceptibility at low temperature in ${\mathrm{UBe}}_{13}$ is entirely attributable to single-ion effects. The source of the remaining 60% of the large ${\mathit{m}}^{\mathrm{*}}$ in ${\mathrm{UBe}}_{13}$ and the implications for heavy-fermion superconductivity are discussed.