Glassy Spin Dynamics in Non-Fermi-Liquid UCu_(5-x)Pd_x, x = 1.0 and 1.5

Local f-electron spin dynamics in the non-Fermi-liquid heavy-fermion alloys UCu_{5-x}Pd_x, x = 1.0 and 1.5, have been studied using muon spin-lattice relaxation. The sample-averaged asymmetry function Gbar(t) indicates strongly inhomogeneous spin fluctuations, and exhibits the scaling Gbar(t,H) = Gbar(t/H^\gamma) expected from glassy dynamics. At 0.05 K \gamma(x=1.0) = 0.35 \pm 0.1, but \gamma(x=1.5) = 0.7 \pm 0.1. This is in contrast to inelastic neutron scattering results, which yield \gamma = 0.33 for both concentrations. There is no sign of static magnetism \gtrsim 10^{-3} \mu_B/U ion in either material above 0.05 K. Our results strongy suggest that both alloys are quantum spin glasses.