Heat capacity measurements on UBe_(13) in rotated magnetic fields: Anisotropic response in the normal state and absence of nodal quasiparticles

摘要

In order to gain insight into the superconducting (SC) gap of UBe_(13), we studied its quasiparticle excitations by means of heat-capacity measurements. Quite unexpectedly, we found the isotropic C(H) ∝ H behavior in low fields at low temperatures, implying the absence of nodal quasiparticle excitations. This result indicates that the SC gap in UBe_(13) 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 UBe_(13).