Evidence for extreme gap anisotropy in Ho0.1Y0.9Ba2Cu3O7 from neutron spectroscopy of Ho3+

We have measured by neutron spectroscopy the relaxation behavior of the 0.5 meV ground state crystal held transition of Ho3+ in Ho0.1Y0.9Ba2Cu3O7. We studied the intrinsic linewidth Gamma and peak position over the temperature range 1.5-160 K. The data show an anomaly at similar to 115 K consistent with the formation of an energy gap. We extrapolate the normal state linewidth Gamma(n) to lower temperatures, and hence evaluate the reduced relaxation rate Gamma/Gamma(n). Both Gamma/Gamma(n) and the peak position show a significant increase with temperature well below T-c characteristic of a high degree of gap anisotropy.