OPTICAL-EXCITATIONS IN CUO2 SHEETS AND THEIR STRONG DEPENDENCE ON CU-O COORDINATION AND BOND LENGTH

The charge-transfer (CT)-type optical "citations from O 2p to Cu 3d and to Cu 4s states have been observed around 1.5 and 5.0 eV, respectively, in undoped T'-phase L2CuO4 single crystals, where L = Pr, Nd, Sm, Eu, or Gd. The spectral shapes of these CT excitations are greatly modified upon electron doping by Ce substitution for the L sites. The CT excitation energies in a series of L2CuO4 compounds with the T, T*, and T' structures strongly depend on the L species, indicating the crucial effect of the in-plane Cu-O bond-length variation as well as of the Cu-O coordination on the electronic states in the CuO2 sheets.