Are the surfaces of CrO2 metallic?

Previous photoelectron spectroscopy studies of CrO2 have found either no density of states or a very low density of states at the Fermi level, suggesting that CrO2 is a semiconductor or a semi-metal. This is in contradiction to calculations that predict that CrO2 should be a half-metallic ferromagnet. Recently, techniques have been developed to grow high-quality epitaxial films of CrO2 on TiO2 substrates by chemical vapour deposition. We present photoelectron spectroscopy measurements of epitaxial CrO2(110)/TiO2(110) and CrO2(100)/TiO2(100) grown using a CrO3 precursor. In addition, measurements of epitaxial Cr2O3(0001)/Pt(111) films grown by thermal evaporation of Cr in an oxygen atmosphere are presented as a reference for reduced CrO2 films. The measurements of the CrO2 surfaces show no emission at the Fermi level after sputtering and annealing the surfaces in oxygen, even though our soft core photoemission data and low-energy electron diffraction measurements provide evidence that stoichiometric CrO2 is present. The consequence of this is that neither surface of CrO2 is metallic. This behaviour could result from a metal to semiconductor transition at the (110) and (100) surfaces.