Spectroscopic study of the electronic states of single-crystalCuGeO3

The electronic states of ${\mathrm{CuGeO}}_3$ that exhibit a spin-Peierls transition at 14 K were investigated by optical reflectivity and photoemission measurements. A quantitative analysis of the charge-transfer excitation has revealed that the p-d hybridization energy between Cu and O for ${\mathrm{CuGeO}}_3$ is about half of that for ${\mathrm{La}}_2$ ${\mathrm{CuO}}_4$. Thus ${\mathrm{CuGeO}}_3$ has a much narrower pdσ band than high-${\mathit{T}}_{\mathit{c}}$ cuprates, which makes carrier doping difficult in this system. The photoemission measurement also suggests the small p-d transfer energy.