Conducting states caused by a surface electric dipole in CrN(001) very thin films

We report a series of electronic-structure calculations for CrN films within the LDA + U method. In the bulk, it was found previously that with the onset of antiferromagnetic order, a gap opens and an insulating state appears. However, for thin films with increasing thickness (4-10 layers), we find that starting with a critical thickness of 10 (cubic symmetry) or 6 layers (orthorhombic) the gap closes and conducting states appear. The appearance of metallic states is connected with a structural relaxation at the surface, where Cr (N) atoms buckle inside (outside), forming an effective dipole moment. With CrN being a low-gap system, the electric dipoles at the surface caused by the Cr atoms displacing inwards shift the bands around the Fermi level significantly enough to drive those thin films metallic. The potential shift due to these surface dipoles is also visible in Cr and O core-level shifts.