Difficulty of probing the superconducting gap with relaxation measurementson 4f crystal-field transitions with neutron scattering

Inelastic neutron scattering results on the temperature dependence of the q uasi-elastic crystal-field transition of Tb in YBa2Cu3O7-delta for samples with delta = 0.03 (metallic, superconducting) and delta = 0.9 (insulating) are presented. The linewidth as a function of temperature does not show an anomaly at the superconducting transition temperature T-c and follows a 1/( exp(Delta/T) - 1) law with Delta the energy difference from an excited crys tal-field slate. The relaxation of the 4f magnetic moments can be described in terms of simple Orbach processes originating from the magneto-elastic i nteraction with lattice vibrations. These results, when put into context wi th reported measurements on the linewidth of crystal-field excitations and EPR, imply that previous neutron measurements must be re-examined.