s-fhybridization model ofUBe13

We assume that in ${\mathrm{UBe}}_{13}$ the 5$f_{5/2\mathrm{-}}$ ${\Gamma }_8$ crystal-field level of U is filled and that the ${\Gamma }_7$ (Kramer’s doublet) lies exactly at the Fermi surface, so as to be half full. Resonant hybridization of the ${\Gamma }_7$ state with the Be(2s)-U(7s) conduction band leads to a Lorentzian density-of-states peak at $E_F$. Its width is 12 K and its peak height is ∼250 times that of the conduction band. This model explains the (normal-state) heat capacity, magnetic susceptibility, and nuclear-spin relaxation versus T. Curie-Weiss behavior is obtained even though no local moments are presumed. We show that the hybridization matrix elements are extremely anisotropic–being zero for conduction electrons having k→ along the cubic axes. It follows that the superconducting energy gap is also anisotropic in k→ space, and that the heat capacity below $T_c$ can be explained with singlet pairing caused by phonon-mediated interactions. Two key experiments are proposed.