Optical band-edge shift of anatase Ti 1-x Co x O 2-δ

We report on the optical properties of magnetic cobalt-doped anatase phase titanium dioxide ${\mathrm{Ti}}_{1\ensuremath{-}x}{\mathrm{Co}}_{x}{\mathrm{O}}_{2\ensuremath{-}\ensuremath{\delta}}$ films for low doping concentrations, $0l~xl~0.02,$ in the spectral range $0.2l~\ensuremath{\Elzxh}\ensuremath{\omega}l5\mathrm{eV}.$ For well oxygenated films $(\ensuremath{\delta}\ensuremath{\ll}1)$ the optical conductivity is characterized by an absence of optical absorption below an onset of interband transitions at 3.6 eV and a blueshift of the optical band edge with increasing Co concentration. The absence of below band gap absorption is inconsistent with theoretical models which contain midgap magnetic impurity bands and suggests that strong on-site Coulomb interactions shift the O band to Co-level optical transitions to energies above the gap.