The specific heats of two samples of UPts have been measured in the vicinity of the transition to the superconducting state. In both cases the specific-heat anomalies are sharper than any previously observed, and two maxima are clearly resolved. The results are interpreted as evidence of a splitting of the transition and unconventional pairing. A model that is consistent with the known sample dependence of the superconducting-state specific heat is used to derive "intrinsic" values of the related parameters.PACS numbers: 74.70.Tx, 65.40.Em, 74.30.Ek, 74.60.Mj From the initial discoveries of superconductivity in CeCu2Si2/ UBei3,^ and UPts, ^ it has been clear that these heavy-fermion superconductors (HFS) are unusual, and it was soon recognized"^ that the coupling mechanism and the nature of the superconducting state might be unconventional. A number of differences between the properties of the superconducting state in HFS and in conventional BCS superconductors have been observed, for example, in the temperature dependences of both transport and thermodynamic properties. However, the interpretation of these results has been clouded by questions associated with sample quality, and particularly by the inhomogeneity implied by the broad superconducting transitions generally observed. In the case of UPts, the temperature dependences of the upper critical field ^ and the rf susceptibility,^ as well as the field dependence of the ultrasonic attenuation,^ suggest the existence of two distinct superconducting states that occur at different fields. It has since been pointed out that this is to be expected for rf-wave pairing, and that even in zero field there should be two transitions occurring at different temperatures.^ It seems possible that these transitions would appear as two separate anomalies in the specific heat, C, as is observed in the case of liquid ^He. The discontinuity in C at the critical temperature Tc, AC(Tc), can also be expected to give information about the nature of the pairing in the superconducting state (see Ref. 9 and others cited there). However, most measurements on UPt3 have been made on samples that showed broad transitions with no sign of structure in ^3,10-12 Y\iQ exceptions are measurements on a series of three samples that were prepared at Grenoble: Measurements there, ^^~^^ and also at Berkeley on one of the samples, ^^ have shown "shoulders" on the high-temperature sides of the anomalies at Tc. The recurrence of that feature from sample to sample was highly suggestive, but, in view of the known dependence of Tc on sample quality, ^' the results were interpreted cautiously, and until now it has not been claimed that this structure was an intrinsic property of UPts.Specific-heat measurements on two new samples of UPt3, each of which shows two distinct maxima near Tc that correspond to two transitions separated by approximately 60 mK, are presented in this Letter. The new samples were prepared in diflferent laboratories by different techniques, and in both cases their properties reflec...

No abstract

Tantalum carbide and niobium carbide samples with carbon-to-metal molar ratios varying from 0.754 to 0.987 for tantalum carbide and from 0.70 to 0.977 for niobium carbide were examined for superconductivity. Both niobium and tantalum carbides were found to be superconducting with transition temperatures which vary with composition. The major conclusion is that the highest temperature is reached for the stoichiometric composition and any deviation therefrom results in a lowering and eventual disappearance (less than 1.05°K) of T c .

The moderately heavy-electron compound URu&Si2 is known to exhibit two electronic phase transitions at low temperatures, one to an antiferromagnetically ordered state at TN =17.5 K followed by another to a superconducting state at T, -1.5 K. The shape of the specific-heat anomaly at TN, which is reminiscent of a second-order BCS-type mean-field transition, suggests the formation of a spin-or charge-density wave opening a gap over part of the Fermi surface. The efFect of chemical substitution of the transition metals M =Re, Tc, Os, Rh, and Ir for Ru in URuzSi2 has been investigated by means of electrical resistivity, magnetic susceptibility, and specific-heat measurements in URu2 "M"Si2for x 0.2. The anomaly associated with the 17.5 K transition involves a very small magnetic entropy and is smeared out by very small concentrations of the M substituent. An inverse correlation between T, and TN in the Rhand Ir-doped materials for x &0.01 and in the Os-doped series for x & 0. 1 is consistent with the picture of two electronic transitions competing for states at the Fermi level.

No abstract