NMR and mu SR studies of non-Fermi-liquid behaviour in disordered heavy-fermion systems

Evidence has emerged from nuclear magnetic resonance (NMR) and muon spin-rotation (mu SR) experiments that non-Fermi-liquid (NFL) behaviour in heavy-fermion alloys can in some cases be due to an inhomogeneous distribution of Kondo temperatures T-K arising from disorder in the random alloy. Such 'Kondo disorder' implies a broad distribution of the heavy-fermion spin polarization, which is reflected in the widths of NMR and mu SR lines. A simple model for the shape and width of the distribution of T-K accounts for the temperature and field dependence of the bulk susceptibility in the NFL alloys UCu5-xPdx, x = 1.0 and 1.5, and then agrees with NMR values of the width (delta chi)(rms) of the susceptibility distribution with no further adjustable parameters. Comparison of NMR and mu SR estimates of (delta chi)(rms) indicates that the susceptibility is disordered on an atomic length scale. In contrast, mu SR lines in CeCu5.9Au0.1 are too narrow for Kondo disorder to account for NFL properties unless, as seems unlikely, the correlation length is long in this alloy. Similarly, small low-temperature Y-89 NMR linewidths make it unlikely that Kondo disorder is the origin of NFL behaviour in Y0.8U0.2Pd3.