We have measured the specific heat of single-crystal YbBe13 from 0.4 to 25 K and compared it to Schottky anomalies based on two different previously reported Yb3+ crystal-field spectra. Qualitative agreement is found with the level scheme determined by inelastic neutron scattering, and small quantitative discrepancies are consistent with a strongly enhanced electronic contribution (30 mJ/mole K2). TN is determined to be 1.1150.016 K. © 1986 The American Physical Society.

Single crystals of CeCu2Si2 were grown from metal solvents and their magnetic properties were systematically varied by partial substitution of Cu by Sn or In. The susceptibility χ is anisotropic and at high temperatures it is dominated by the crystal field split 4f state. At low temperatures the magnetic moments are drastically reduced. This reduction, as well as the Curie-Weiss temperature, depend sensitively upon the chemical composition. The observed variations both in the magnetic and the resistance behavior point to a Kondo-like coupling between local moments and conduction electrons. The strength of this interaction varies as the Fermi level is shifted by chemical means.

The low temperature resistivity of Ce0.9-xLaxTh 0.1 alloys is known to vary as ρ0+αT 2. We have investigated the variation of ρ0 and α with pressure for several concentrations x. An unusually strong nonlinear decrease of the residual resistivity with pressure occurs; the magnitude of the decrease is an order-of-magnitude larger than in the isostructural nonmagnetic alloy La0.8Th0.2. The temperature coefficient α(P) also decreases strongly. These results are in qualitative accord with recent theories of the resistivity of disordered valence fluctuation compounds.

The low-temperature behavior of the known heavy-fermion uranium compounds is discussed, and the current situation with respect to unusual superconducting and magnetic states in these compounds is reviewed. © 1985.

The formation of a heavy-electron state in a magnetically ordered material is established by measurements of the low-temperature specific heat of UCu5 and UAgCu4. In UCu5 this state undergoes a continuous but hysteretic phase transition which removes parts of the Fermi surface with a high density of electronic states and leads to a resistivity increase of almost an order of magnitude. © 1985 The American Physical Society.

Small amounts of Sc, Lu, Gd, Np, Ce, Th, La, and Ba have been substituted for uranium in UBe13 to observe their effects on the superconducting and normal state properties. The thorium, which was the most complete study, resulted in an extremely unusual nonmonotonic depression of the transition temperature for a nonmagnetic impurity. This comes from an interplay that exists between the lowest temperature resistivity peak and the transition temperature, as the peak is depressed. These results suggest that heavy Fermion superconductivity is only one of the possible ground states for heavy mass electron systems. All of the impurities tested resulted in a transition temperature depression.

We report the results of resistivity measurements on Ce0.9-xLaxTh0.1 alloys for x=0.10,0.11,0.14, and 0.17 in the temperature interval 4.2-300 K and for the pressure range 0-12 kbar. Using these results we have semiquantitatively determined the P-T phase boundary for the isomorphic - phase transition for each concentration x. For x=0.10,0.11, and 0.14 the transitions are continuous at zero pressure, first order for pressures within the interval PcL(x)Pcu(x). Hence, for these concentrations the phase boundaries terminate at two critical points. For x=0.17, all observed transitions are continuous. Hence, in x-P-T space the surface of first-order transitions terminates at a critical edge (Tc(x),Pc(x)) such that, over an interval of x,Pc(x) is double valued and such that there is a "critical inflection point." These unusual features can be understood qualitatively as following from very general Fermi-liquid properties of the cerium 4f electronic system, together with the dilution and negative-pressure effects of alloying with lanthanum. We show that this qualitative description is consistent with existing experimental data. © 1984 The American Physical Society.

The pressure-temperature phase diagram of Ce0.9-xLaxTh0.10 alloys is presented for 0.1≤x≤0.17. Two critical points are found in this range of compositions. The data are shown to be consistent with an analysis based on the known Fermi-liquid behavior of Ce in compounds and alloys. The possible appearance of β phase at intermediate pressures is noted.

Sound propagation in the heavy-fermion superconductor UBe13 is studied at frequencies from 0.9 to 2.4 GHz and temperatures from 0.01 to 100 K. A peak in the acoustic attenuation is observed below the superconducting transition of 0.9 K, instead of the rapid drop expected for a BCS superconductor. Absorption into a collective mode of a nonsinglet, anisotropic superconductor is proposed. © 1985 The American Physical Society.