Investigation of strongly correlated electron behavior in uranium- and praseodymium-based intermetallic compounds

The physical properties of several U- and Pr-based heavy fermion compounds have been investigated. Superconductivity has been observed in PrOs4Sb12 at TC = 1.85 K and appears to involve heavy fermion quasiparticles with an effective mass m* &sim 50me as inferred from the jump in the specific heat at TC, the upper critical field near TC, and the normal state electronic specific heat. Thermodynamic and transport measurements suggest that the heavy fermion state has a quadrupolar origin.Electrical resistivity measurements under pressure have been made on ferromagnetic UGe2, UxPt1- x (0.50 &le x &le 0.54), and UIr compounds. Superconductivity coexists with ferromagnetism in UGe 2 between 8 kbar < P < 14 kbar with a maximum onset temperature of 1.2 K at P &sim 13 kbar. These polycrystalline specimens have a residual resistivity rho0 up to &sim3 muOcm corresponding to an electron mean free path smaller than or of the order of the superconducting coherence length. These results suggest superconductivity in UGe2 may be s-wave in nature. The U xPt1-x materials order magnetically at TC1 &sim 18 K and TC2 &sim 27 K and the ordering temperatures exhibit a small pressure dependence. The Curie temperature of UIr, on the other hand, decreases from 47 K at ambient pressure to 27 K by 19 kbar.The URu2-xRe xSi2 system exhibits ferromagnetic (FM) order for Re concentrations 0.3 < x &le 1.0 and antiferromagnetic order for 0 &le x < 0.15. Magnetic measurements indicate the suppression of FM order occurs at xFMc = 0.3. The specific heat of samples with Re concentrations 0.15 < x &le 0.6 can be described by C/T &prop - ln T at low temperatures, typical of many non-Fermi liquid (NFL) systems. The resistivity also exhibits an NFL power law T-dependence (rho(T) &prop Tn) with an exponent n < 2 for 0.15 &le x < 0.8. The NFL behavior observed in the URu2- xRexSi2 system appears to be most consistent with proximity to a T = 0 K phase transition.