Heat capacity measurements on UBe₁₃ in rotated magnetic fields: Anisotropic response in the normal state and absence of nodal quasiparticles

In order to gain insight into the superconducting (SC) gap of UBe₁₃, we studied its quasiparticle excitations by means of heat-capacity measurements. Quite unexpectedly, we found the isotropic C(H)αH behavior in low fields at low temperatures, implying the absence of nodal quasiparticle excitations. This result indicates that the SC gap in UBe₁₃ is fully open over the Fermi surfaces. Furthermore, we observed a characteristic oscillation of heat capacity both in the SC and non-Fermi-liquid normal states above ∼2 T, and the angular variation of heat capacity possibly originates from anisotropic magnetic response of the heavy-electron state. Our result regarding the low-energy quasiparticle excitations in the SC and normal states will be a clue to understand the unusual nature of UBe₁₃.