Optical Excitations in CuO₂ Sheets With and Without Electron Doping: Relevance to Superconductivity Produced by Electron Doping

Abstract

Optical excitations, in particular charge-transfer(CT) excitons, plasmons and two-magnon excitations, have been investigated spectroscopically for single crystals of prototypical cuprate compounds with single-layer Cu-O sheets and chains. In all crystals investigated (for example Nd₂CuO₄, CaCuO₂, Ca₂CuO₃, Sr₂CuO₂Cl₂ and etc.), the strong CT transtion from O-2p to Cu-3d is observed at 1.5–1.9eV and its anisotropy depending on the Cu-O network is quite evident. Together with results of two-magnon Raman scattering and magntetic susceptibility, analysis of these optical data enables us to evaluate the values of fundamental physical parameters for Cu-O sheets(e.g. transfer energy, CT gap and exchange energy) as well as their depedence on the Cu-O network and dimension. To investigate change of electronic states upon electron-doping in CuO₂ and its relevance to electronsuperconcductivity, reflectance spectra of single crystals of Ce-doped Nd₂CuO₄ have been measured in the wide electron-concentration range where the compound undergoes the phase change from the CT insulator to nonsuperconducting metal through the high-T_c superconductor.