Giant red shift of the absorption spectra due to nonstoichiometry in GdCoO3–δ

Abstract

The GdCoO_3–δ perovskite is a semiconductor with the energy gap E_g ≈ 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 ε₀ = 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–δ. We have found that oxygen vacancies result in the in-gap states inside the charge-transfer energy gap of the GdCoO₃. It is a multielectron effect due to strong electron correlations forming the electronic structure of the GdCoO_3–δ. These in-gap states decrease the transparency window and result in a narrow absorption minimum. The predicted temperature dependence of the absorption spectra has been confirmed by our measurements.