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We have performed low-temperature specific-heat measurements in magnetic fields for a single crystal UBe 13 . 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 T 3 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], [110], and [111] 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 κ 2 , and it has been revealed that the κ 2 steeply decreases isotropically upon cooling just below T c . Paramagnetic effects and the symmetry of Cooper pairing of UBe 13 are discussed. KEYWORDS: Unconventional Superconductor, Heavy Fermion, Paramagnetic EffectThe heavy-fermion compound UBe 13 has attracted much attention since the discovery of unconventional superconductivity 27 years ago. 1) UBe 13 has a cubic structure with the space group O h 6 (Fm−3c), and exhibits a superconducting (SC) transition at around 0.85 K with a large specific-heat jump ∆C. Despite prolonged scientific efforts to unravel the superconductivity of this material, many open questions remain, in particular concerning the symmetry of Cooper pairing and the unusual SC phase diagram. 2, 3)As for the SC gap structure, which is related to the parity of Cooper pairing, the temperature dependence of low-T specific heat C(T ) 4) and magnetic field penetration depth λ(T ) 5) suggest the presence of point nodes, whereas nuclear magnetic resonance (NMR) spin-relaxation rate 1/T 1 6) and ultrasonic attenuation α(T ) 7) suggest a line node. A p-wave AndersonBrinkman-Morel (ABM) state, which is identified as the A phase in superfluid 3 He,8) has been proposed for the SC of UBe 13 from the T 3 -like behavior observed in the low-T specific heat.4) An early NMR study also suggests the occurrence of the ABM state, because the Knight shift does not decrease below T c . 9) However, recent muon spin-rotation (µSR) measurements performed in field, H || [001], have revealed that the muon Knight shift shows a clear decrease below T c . 10)Furthermore, a recent NMR study by Tou et al. has suggested that the Knight shift decreases below T * ( ∼ 0.7 K < T c ) for [001], whereas increases below T * for [111]. 11) Therefore, no consensus has been achieved regarding the behavior of Knight shift below T c as well as the SC-gap structure. Namely, it is still debated whether the Cooper-pairing state of UBe 13 is triplet or singlet.In order to obtain further information about the parity of Cooper pairing, it is important to investigate the presence (or absence) of paramagnetic effects. The value of H c2 estimated experimentally by extrapolation to T = 0 for UBe 13 is about 80-130 kOe, which is much larger than the Pauli limit * E-mai...

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Low-temperature specific heat and static dc magnetizetion measurements have been performed for the superconducting state of the heavy-fermion compound UBe 13 . The superconducting phase diagram B c2 -T (B || [110]) has been obtained by our specific heat and magnetization measurements. From the slope of the upper critical field and specific heat jumps at superconducting transition temperatures in magnetic fields, we have also obtained a temperature dependence of Ginzburg-Landau parameter κ 2 along [110]. It has been revealed that κ 2 decreases with decreasing temperature down to 0.5 K, which indicates that a paramagnetic effect may be present at least down to 0.5 K for [110].