Anomalous spin polarization and dualistic electronic nature of ${\mathrm{CrO}}_2$

Spin-resolved O $1s$ x-ray absorption measurements on ${\mathrm{CrO}}_2$ reveal that the spin polarization of the states at the Fermi level approaches 100%, providing direct evidence of the half-metallic nature of ${\mathrm{CrO}}_2.$ The measurements also show the existence of an atomiclike Cr $3d$ state not far away from the Fermi level with a spin polarization of only 50%, establishing its Mott-Hubbard character. We conclude that ${\mathrm{CrO}}_2$ has a dualistic electronic nature, in which the states at the Fermi level are bandlike, while those at higher energies are strongly localized of completely different origin and symmetry. The finding of an exceptionally large number of O $2p$ holes suggests their important role in preventing the strongly correlated ${\mathrm{CrO}}_2$ from being an insulator.