CRYSTAL-FIELD-EXCITATION LINEWIDTH MEASUREMENTS IN HOXY1-XBA2CU3O7

We have studied the linewidth of the GAMMA3-GAMMA2 (0.5 MeV) crystal-f ield transition of Ho3+ in polycrystalline samples of HoxY1-xBa2Cu3O7 (x = 0.1 and 0.2) by high-resolution, inelastic neutron scattering. Th e linewidth increases with temperature, but shows an anomaly at, or sl ightly above, the superconducting transition temperature T(c). To inve stigate the connection with superconductivity we have applied the theo ry of Becker, Fulde, and Keller (BFK) to calculate the normal-state li newidth 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 associa ted with the superconducting state, but that at low temperatures, wher e the relaxation behavior should reflect the symmetry of the gap funct ion, the linewidth increases more rapidly with temperature than might be expected. We have also applied the BFK theory to the damping of cry stal-field transitions of Tm3+ in TmxY1-xBa2Cu3O6+y measured recently by Osborn and Goremychkin. From the measured linewidths above T(c) der ive 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 di ffer.