Superconducting symmetries and magnetic responses of uranium heavy-fermion systems UBe₁₃ and UPd₂Al₃

Low-temperature thermodynamic investigation for UBe₁₃ and UPd₂Al₃ were performed in order to gain insight into their unusual ground states of 5f electrons. Our heat-capacity data for the cubic UBe₁₃ strongly suggest that nodal quasiparticles are absent and its superconducting (SC) gap is fully open over the Fermi surface. Moreover, two unusual thermodynamic anomalies are also observed in UBe₁₃ at ∼ 3 T and ∼ 9 T; the lower-field anomaly is seen only in the SC mixed state by dc magnetization M(H) as well as heat-capacity C(H), while the higher-field anomaly appears for C(H) in the normal phase above the upper critical field. On the other hand, field-orientation dependence of the heat capacity in the hexagonal UPd₂Al₃ shows a significantly anisotropic behavior of C(H)∝H^1/2, reflecting the nodal gap structure of this system. Our result strongly suggests the presence of a horizontal line node on the Fermi surface with heavy effective mass in UPd₂Al₃.