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The heavy-electron compound URu2Si2 exhibits two electronic phase transitions: superconductivity at T, =1.5 K and a second transition at To--17.5 K. The specific-heat anomaly associated with the transition at To has a mean-field BCS-like form which suggests the formation of a chargeor spin-density wave that partially gaps the Fermi surface. Through electrical-resistivity measurements, we have studied the influence of pressure P up to -15.4 kbar on these transitions. For pressures less than 12 kbar, To increases linearly at a rate -130 mK/kbar, while T, decreases linearly at a rate -95 mK/kbar. The nearly equal but opposite P dependences of To and T, suggest a cornpetition for electronic density of states at the Fermi level. The resistivity, when normalized to its maximum value p(T "),scales as a function of reduced temperature T/T, "(P) from T, to well above T a"except in the immediate vicinity of To.

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We have used resistance measurements at pressures to 95 kbar and magnetic fields to 8 T to probe the estabHshment of Kondo coherence in the heavy-fermion Kondo lattice UBeia. With increasing pressure, the negative magnetoresistance characteristic of noninteracting Kondo impurities evolves into the positive magnetoresistance expected for a coherent Kondo lattice. We show that coherence is established by the suppression of inelastic scattering, over a temperature range which expands as pressure decreases the superconducting transition temperature Tc and increases the Kondo temperature TV.PACS numbers: 71.28.+d, 72.15.Qm, 74.70.Tx, 75.30.Mb Heavy-fermion materials display many of the characteristics of Kondo lattices, w^hose properties are a problem of continued experimental and theoretical interest. At high temperature, a Kondo lattice behaves like an ensemble of noninteracting Kondo impurities. In this regime, the resistivity increases and the magnitude of the local moments decreases as the temperature is lowered, with a characteristic temperature scale of the Kondo temperature TK> At the lowest temperatures, the Kondo lattice can be described as a periodic Fermi liquid. * The crossover from incoherent Kondo impurity scattering at high temperatures {T^TK)to the formation of a Fermi-liquid regime at low temperatures (T<^TK) is commonly referred to as the establishment of Kondo coherence. In addition to the appearance of bandlike character in the transport properties, sign changes in the magnetoresistance^ and Hall coefficient,^ as well as peaks in the temperature dependence of the thermoelectric power"* and specific heat,^ are characteristic of the Fermi liquid and suggest the presence of low-energy features and gaps at the Fermi surface. Despite extensive theoretical work on the high-temperature state, ^ and more recently on the description of the low-temperature state as a Fermi liquid,^ a complete theoretical understanding of the Fermi-surface features reflected in these transport measurements remains lacking. We will demonstrate that, in contrast, comparison of elastic and inelastic scattering rates determined from magnetoresistance measurements provides a clear microscopic criterion for the onset of coherence. Our interest has focused on UBei3 primarily because of all the known heavy-fermion systems it shows the least evidence for the existence of a Fermi-liquid regime above the superconducting transition temperature at 1 bar. We have studied the crossover in UBen between incoherent and coherent scattering in a controlled way by variation of pressure.Polycrystalline samples of UBen were prepared by arc melting high-purity uranium and beryllium in an argon atmosphere.^ The sample and a lead manometer were placed in series in the Bridgman anvil cell.^ Platinum leads are attached to both with silver filled epoxy, allowing simultaneous four-probe measurements of the lead and sample resistances. The average pressure in the completed cell was determined from the depression of the lead superconducting transition t...

We report results of neutron powder diffraction experiments on the heavy-electron magnet UCdll at temperatures above and below its ordering temperature. These measurements set an upper limit on the ordered moment of 1.5ktB/U.