Abstract
The temperature dependence of the relaxation rate of crystal-field excitations in the slightly underdoped HoBa2Cu4O8 and Er2Ba4Cu7O14.92 high-temperature superconductors has been investigated by means of neutron scattering measurements. Our data show clear evidence for the opening of an electronic gap in the normal state at T* ≈ 200 K far above Tc. The temperature evolution of the relaxation rate appears to be extremely dependent upon the energy at which the susceptibility is being probed. This points to a strongly anisotropic gap function which is tentatively modelled to be of the form s + id3, with Δs/Δd ≈ 0.2.