Direct spectroscopic observation of the energy gap formation in the spin density wave phase transition at the Cr(110) surface

The spin density wave phase transition in chromium has been studied by angl e-resolved photoemission on a (110) surface. The electronic band structure is shown to exhibit backfolding in the antiferromagnetic state. Direct spec troscopic evidence is obtained on the gradual opening of an energy gap at t he Fermi level. The gap extends almost isotropically around the Gamma point , its magnitude reaching similar to 200 meV at room temperature. Density-fu nctional band structure calculations confirm the location of the energy gap in reciprocal space and yield enhanced magnetic moments towards the surfac e. The transition probed in the near-surface layers exhibits a critical tem perature of similar to 440 K.