UC Irvine

We report neutron-scattering experiments for (Formula presented) that were performed in the time-of-flight mode at a neutron spallation source. Two independent methods were used to determine the nonmagnetic scattering and both gave comparable results for the magnetic scattering. The magnetic scattering was measured at temperatures (Formula presented) and 60 K, below and above the temperature of the valence transition (Formula presented) Our key result is that the spin dynamics in the mixed valent phase are well described over a broad range of energy transfer (Formula presented) and incident energies (Formula presented) by a Lorentzian power spectrum, centered at (Formula presented) and with halfwidth (Formula presented) The magnetic susceptibility derived from the fits is within 20% of the value measured for the dc susceptibility. The peak energy is in good agreement with the Kondo temperature derived from thermodynamic studies but the Lorentzian line shape fits the data significantly better (Formula presented) than the line shape expected for a Kondo impurity (Formula presented) In addition we report results for (Formula presented) For this compound, the scattering is inelastic at low temperature (at 10 K (Formula presented) and (Formula presented)) but is quasielastic at higher temperature where (Formula presented) and 13.2 meV at 150 and 300 K, respectively. This increase in linewidth is contrary to the 50% decrease in linewidth predicted by the Anderson impurity model for this temperature range, and is also smaller than the factor of four increase in characteristic energy suggested by application of the impurity model to the magnetic susceptibility. © 1999 The American Physical Society.