Evidence for Extreme Gap Anisotropy in Ho0.1Y0.9Ba2Cu3O7 from Neutron Spectroscopy of Ho3+

Abstract

We have measured by neutron spectroscopy the relaxation behavior of the 0.5 meV ground state crystal field transition of ${\mathrm{Ho}}^{3+}$ in H${\mathrm{o}}_{0.1}$${\mathrm{Y}}_{0.9}$B${\mathrm{a}}_{2}$C${\mathrm{u}}_{3}$${\mathrm{O}}_{7}$. We studied the intrinsic linewidth \ensuremath{\Gamma} and peak position over the temperature range 1.5--160 K. The data show an anomaly at $\ensuremath{\sim}115\mathrm{K}$ consistent with the formation of an energy gap. We extrapolate the normal state linewidth ${\ensuremath{\gamma}}_{n}$ to lower temperatures, and hence evaluate the reduced relaxation rate $\ensuremath{\gamma}/{\ensuremath{\gamma}}_{n}$. Both $\ensuremath{\gamma}/{\ensuremath{\gamma}}_{n}$ and the peak position show a significant increase with temperature well below ${T}_{c}$ characteristic of a high degree of gap anisotropy.