Maki Parameter and Upper Critical Field of the Heavy-Fermion Superconductor UBe13

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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_{\text{c}} \sim 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 T^{3} behavior below {\sim}0.7 T_{\text{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_{\text{c2}} for the \langle 001\rangle, \langle 111\rangle, and \langle 110\rangle 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 \kappa_{2}, and it has been revealed that the \kappa_{2} steeply decreases isotropically upon cooling just below T_{\text{c}}. Paramagnetic effects and the symmetry of Cooper pairing of UBe13 are discussed.
2011-09-15