Optical excitations inCuO2sheets and their strong dependence on Cu-O coordination and bond length

The charge-transfer (CT)-type optical excitations 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 ${\mathit{L}}_2$ ${\mathrm{CuO}}_4$ 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 ${\mathit{L}}_2$ ${\mathrm{CuO}}_4$ compounds with the T, ${\mathit{T}}^{\mathrm{*}}$, and T’ structure 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 ${\mathrm{CuO}}_2$ sheets.