PROPERTIES OF YBAGCU4 AT HIGH-PRESSURES AND MAGNETIC-FIELDS

We have measured the electrical resistivity of YbAgCu4 at quasihydrostatic pressures to 190 kbar and in magnetic fields to 8 T. At ambient pressure, YbAgCu4 is weakly mixed valent and has a Sommerfeld coefficient of specific heat gamma of approximately 240 mJ/mole K-2. With applied pressure, the ground-state degeneracy is reduced from eightfold to twofold degenerate as the energy scale T-0 characterizing the strongly correlated electron system is reduced from 130 to similar to 0.9 K at 190 kbar and gamma is estimated to increase to approximately 4900 mJ/mole K-2. At intermediate pressures T-0 and crystal-field splittings are comparable, which accounts for the lack of resistive scaling at these pressures. Systematics in the pressure-dependent resistivity suggest that YbAgCu4 should order magnetically at pressures not far above 190 kbar. The ''residual resistivity'' rho(0) of YbAgCu4 increases dramatically with decreasing volume due to resonant scattering of heavy quasiparticles by lattice defects. At high pressures and low temperatures the magnetoresistance is large and negative and has the field dependence expected of a spin-1/2 Kondo impurity. The pressure and field dependences of rho(0) and the T-2 coefficient of resistivity raise the question of their functional relationship and how we are to understand impuritylike behavior in a band of strongly correlated quasiparticles.