Cu d-d Orbital Transitions and Charge-Transfer Excitations in High-Tc Superconductors

Numerous theoretical models have been proposed as a possible explanation for the high superconducting transition temperatures Tc of the superconducting cuprates. Electron-phonon coupling, magnetic-exchange mechanisms, or coupling by low-lying electronic excitations such as plasmons or charge-transfer excitations have been considered as explanations for the high-Tc’s. In order to obtain information on the nature of low energy electronic excitations, we have investigated such excitations by optical reflectivity and absorption measurements and by high-energy electron energy-loss spectroscopy (EELS) in transmission. The optical data have been published already previously.1 The EELS measurements have been extended to 50 eV in order to obtain the dielectric constants of single-crystalline YBa2Cu3O6.8. Thus information on the stronger bonded electronic states could be derived. The data are interpreted by a large charge-transfer gap of about 4 eV. In this gap, weak absorption maxima appear which are analyzed in terms of valence conserving d-d transitions of Cu++.