HIGH-RESOLUTION ELECTRON-ENERGY-LOSS STUDY OF THE SURFACES AND ENERGY GAPS OF CLEAVED HIGH-TEMPERATURE SUPERCONDUCTORS

High-resolution electron-energy-loss spectroscopy (HREELS) is used to characterize the resistivity, vibrational, and electronic structure of YBa2Cu3O7, Bi2Sr2CaCu2O8, and TiS2 single crystals cleaved in ultrahigh vacuum. The HREELS data are analyzed using dipole-scattering theory for a semi-infinite sequence of conducting sheets separated by dielectric slabs. Cleaved surfaces show strong lateral inhomogeneities and a variety of terminations with different properties. Spectroscopy on superconducting regions for a variety of these Cu-Obased superconductors reveals an energy gap in the ab plane that corresponds to (7.80.3)kBTc. The superconducting gap of the 1:2:3 material shows a non-BCS temperature dependence. We also analyze the infrared reflection data from the 1:2:3 material obtained by different groups and find good agreement with our HREELS results. Based on these experimental results, we suggest that the transition to superconductivity is due to a Bose-Einstein condensation of the preexisting or real space pairs. © 1990 The American Physical Society.