Thermodynamic features in the H-T plane of superconducting UBe13.

The spin degrees of freedom play an essential role in heavy-fermion superconductivity. In UPt3, not only may spin fluctuations provide the pairing mechanism, but the existence of antiferromagnetic order below T„(with a magnetic moment —0.02ptt) ' allows the possibility of multiple superconducting phases. Similarly, recent magnetic x-ray and neutron-scattering studies of URuzSi2 establish the microscopic coexistence of antiferromagnetism (with a magnetic moment —0.04Ittt) and superconductivity. Magnetic-susceptibility and heatcapacity measurements on thoriated CeCu2Si2 show a significant bulk ordered moment coexisting with the superconductivity in that heavy-fermion system. The situation for UBe~3 is less settled. When doped with thorium, magnetic correlations arise and for Th substitution between 2% and 4% two transitions are observed in the low-temperature specific heat. For pure UBe) 3 magnetostriction data indicate the onset of antiferromagnetic order at T~ =8.8 K —10T„but there is no direct evidence to date for magnetic ordering below T, . On the basis of muon spin resonance and lower criticalfield measurements of superconducting UBe~3, HeAner et a/. rule out either magnetism or a second phase transition at low fields below T, . In contrast, Rauchschwalbe' suggests two superconducting order parameters for UBe~3 from an analysis of a break in the upper critical-field slope at H —20 kG. We report here multiple thermodynamic signatures in pure UBe~3 for T/T, & 0.6 at H = 20 kG. The key diAerence in our heat-capacity measurements is that we fix temperature T and sweep magnetic field H, in contrast to the conventional approach of fixing H and varying T. Varying the magnetic field introduces the technical complexity of recalibrating the thermometer attached to the sample at every one of typically 80 magnetic-field points. It provides, however, sensitivity to thermodynamic features when the phase boundary of interest is essentially parallel to the temperature axis in the H-T plane, features which would be smeared beyond recognition in a temperature sweep at fixed H. The specific heat C(H) at fixed T has proved successful in revealing'' sharp features in the H-T plane of UPt3, even in a sample with one broad peak in C/T vs T at any H. Furthermore, the functional form of C(H) for T & T, provides information about the nature of the excitation spectrum in the su0 E