Glassy Spin Dynamics in Non-Fermi-LiquidUCu5−xPdx,x=1.0 and 1.5

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