Maki parameter and upper critical field of the heavy-fermion superconductor UBe₁₃

We have performed low-temperature specific-heat measurements in magnetic fields for a single crystal UBe₁₃. It has been observed that our sample exhibits a superconducting transition at an intermediate temperature (T_c ∼ 0.81 K) between previously reported values for two variant samples called H type and L type. The specific heat C(T) of our sample shows a T³ behavior below ∼0.7 T_c, which is similar to the behavior of the H-type sample, suggesting the existence of point nodes in the superconducting gap function. We have obtained the upper-critical-field curves H_c2 for the (001), (111), and (110) crystal axes, which show no anisotropy at least down to the lowest measured temperature of 0.5 K. We have also derived the Maki parameter k₂, and it has been revealed that the k₂ steeply decreases isotropically upon cooling just below T _c. Paramagnetic effects and the symmetry of Cooper pairing of UBe₁₃ are discussed.