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.