PSEUDOGAP AND ORDER-PARAMETER SYMMETRY IN THE UNDERDOPED HIGH-T-C COMPOUNDS HOBA2CU4O8 AND ER2BA4CU7O14.92 - A NEUTRON CRYSTAL-FIELD STUDY

The temperature dependence of the relaxation rate of crystal-field exc itations in the slightly underdoped HoBa2 Cu4O8 and Er2Ba4Cu7O14.92 hi gh-temperature superconductors has been investigated by means of neutr on scattering measurements. Our data show clear evidence for the openi ng of an electronic gap in the normal state at T approximate to 200 K far above T-c. The temperature evolution of the relaxation rate appea rs to be extremely dependent upon the energy at which the susceptibili ty is being probed. This points to a strongly anisotropic gap function which is tentatively modelled to be of the form s + id(3), with Delta (s)/Delta(d) approximate to 0.2.