CRYSTAL-FIELD-EXCITATION LINEWIDTH MEASUREMENTS IN HOXY1-XBA2CU3O7

We have studied the linewidth of the GAMMA3-GAMMA2 (0.5 MeV) crystal-field transition of Ho3+ in polycrystalline samples of HoxY1-xBa2Cu3O7 (x = 0.1 and 0.2) by high-resolution, inelastic neutron scattering. The linewidth increases with temperature, but shows an anomaly at, or slightly above, the superconducting transition temperature T(c). To investigate the connection with superconductivity we have applied the theory of Becker, Fulde, and Keller (BFK) to calculate the normal-state linewidth based on the assumption that the primary relaxation mechanism is the s-f interaction with a normal Fermi liquid on the CuO2 planes. The calculations indicate that there is a reduction in damping associated with the superconducting state, but that at low temperatures, where the relaxation behavior should reflect the symmetry of the gap function, the linewidth increases more rapidly with temperature than might be expected. We have also applied the BFK theory to the damping of crystal-field transitions of Tm3+ in TmxY1-xBa2Cu3O6+y measured recently by Osborn and Goremychkin. From the measured linewidths above T(c) derive reduced s-f coupling constants rho' = 0.014 for Ho3+, and rho' = 0.043 for TM3+. We discuss possible reasons for why these two values differ.