Evidence for Extreme Gap Anisotropy in H${\mathrm{o}}_{0.1}$${\mathrm{Y}}_{0.9}$B${\mathrm{a}}_2$C${\mathrm{u}}_3$${\mathrm{O}}_7$ from Neutron Spectroscopy of ${\mathrm{Ho}}^{3+}$

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 Γ and peak position over the temperature range 1.5–160 K. The data show an anomaly at $\sim 115\mathrm{K}$ 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.