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

Abstract

Numerous theoretical models have been proposed as a possible explanation for the high superconducting transition temperatures T_c 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-T_c’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 YBa₂Cu₃O_6.8. 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⁺⁺.