No abstract

Electrical-resistivity, magnetic-susceptibility, and specific-heat data reveal that UBe f 3 is superconducting below 0.85 K. Highly anomalous low-temperature electronic properties in both the normal and superconducting states result in an enormous electronic specific-heat coefficient y = 1.1 J/mole K and a corresponding magnetic susceptibility y = 1.5&&10 emu/mole. The superconducting state appears to be extremely stable with an initial slope of the temperature derivative of the critical field (BH, 2/BT) q, =-257 kOe/K.

We study the competition between intersite and local correlations in a spinless two-band extended Hubbard model by taking an alternative limit of infinite dimensions. We find that the intersite density fluctuations suppress the charge Kondo energy scale and lead to a Fermi liquid to non-Fermi liquid transition for repulsive on-site density-density interactions. In the absence of intersite interactions, this transition reduces to the known Kosterlitz-Thouless transition. We show that a new line of non-Fermi liquid fixed points replace those of the zero intersite interaction problem.

We report the first measurements of the magnetic-field penetration depth )~ in the heavy electron superconductor UBe13, performed using a SQUID magnetometer. We find the temperature dependence of 2(T)-2(0) to follow a T 2 law at low temperatures, giving further evidence of extreme gap anisotropy in this compound. We calculate the temperature dependence expected for a variety of anisotropic states, including those representing certain classes of "exotic" pairing. In general situations, the supercurrent is not parallel to the vector potential, and a more complicated field penetration takes the place of the normal Meissner effect. We argue that the data are consistent with an energy gap with point nodes on the Fermi surface but inconsistent with the large value of the Landau parameter F~ expected for a translationally invariant Fermi liquid with large effective mass.

We further develop an extended dynamical mean field approach introduced earlier. It goes beyond the standard D = ∞ dynamical mean field theory by incorporating quantum fluctuations associated with intersite (RKKY-like)interactions. This is achieved by scaling the intersite interactions to the same power in 1/D as that for the kinetic terms. In this approach, a correlated lattice problem is reduced to a single-impurity Anderson model with additional self-consistent bosonic baths. Here, we formulate the approach in terms of perturbation expansions. We show that the two-particle vertex functions are momentum-dependent, while the single-particle self-energy remains local.In spite of this, the approach is conserving. Finally, we also determine the form of a momentum-dependent dynamical susceptibility; the resulting expression relates it to the corresponding Weiss field, local correlation function and (momentum-dependent) intersite coupling.

No abstract

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...

We report the discovery of a superconducting transition in the heavy-fermion compound CeRh 2 Si 2 under hydrostatic pressure. Superconductivity appears at pressures above about 9 kbar, near the critical pressure required to suppress antiferromagnetic order ͓T N ( Pϭ0)ϭ36 K͔. Onset of superconductivity occurs at a temperature of Ϸ350 mK. Resistivity measurements as a function of field at constant temperature allow us to build an H*-T phase diagram for the onset of superconductivity. A Ginzburg-Landau analysis of the initial slope of the critical field leads to an effective mass of m*/m 0 Ϸ200, supporting the heavy-fermion nature of superconductivity in CeRh 2 Si 2 . Magnetic ac susceptibility ( ac ) shows a diamagnetic response corresponding to about 1% of perfect diamagnetism. The size of the feature in ac also is strongly peaked at a pressure of Ϸ9 kbar.