Giant red shift of the absorption spectra due to nonstoichiometry in GdCoO$_{3-\delta}$

The GdCoO$_{3-\delta}$ perovskite is a semiconductor with the energy gap $E_g \approx 0.5$ eV from electrical transport measurements. It reveals unusual optical absorption spectra without transparency window expected for semiconductors. Instead we have measured the narrow transmittance peak at the photon energy ${\varepsilon_0} = 0.087$ eV. To reconcile the transport and optical data we have studied the effect of oxygen vacancies on the electronic structure of the GdCoO$_{3-\delta}$. We have found that oxygen vacancies result in the in-gap states inside the charge- transfer energy gap of the GdCoO$_3$. It is a multielectron effect due to strong electron correlations forming the electronic structure of the GdCoO$_{3-\delta}$. These in-gap states decrease the transparency window and result in a narrow absorption minimum. Predicted temperature dependence of the absorption spectra has been confirmed by our measurements.