Role of the Zhang-Rice-like exciton in optical absorption spectra of CuGeO3 and CuGe1-xSixO3 single crystals

The optical excitations in pure and Si-substituted CuGeO3 are investigated in the 15-300 K temperature range. A structure, detected for the E field parallel to the c axis of the crystals at about 3.2 eV, is due to the formation of a Zhang-Rice-like (ZR) exciton. The weak oscillator strength is due to the low mobility of the ZR-like exciton between the edge-sharing CuO4 plaquettes of CuGeO3. The temperature dependence of the ZR-like energy is explained by a renormalization effect involving phonon-electron interactions or changes in spin ordering, while the increase of ZR-like exciton intensity with the Si content is argued to arise from a different hybridization of the Si-O bond with respect to Ge-O.