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

The changes in the electronic structure of oxides and other correlated compounds caused by electronic reconstructions at their surface and interfaces has attracted much attention recently. CrN shows a magnetostructural phase transition as a function of temperature and controversial electronic properties. It has been argued recently that, with the onset of antiferromagnetic order, CrN as a bulk is always semiconducting, but very close to a metal-insulator transition [1]. In order to check if a small perturbation in the system could drastically change its conduction properties, we have performed electronic structure calculations for CrN in a thin film geometry within the LDA+U method. For thin films with increasing thickness (4-10 layers) starting with a critical thickness of 10 (cubic symmetry) or 6 layers (orthorhombic) the gap closes and conducting states appear. The appearence of metallic states is connected with a structural relaxation at the surface, where Cr (N) atoms buckle inside (outside) forming an effective surface dipole moment. Being CrN a low-gap system, these electric dipoles at the surface are able to shift the bands around the Fermi level significantly enough to drive those thin films metallic. \\[4pt] [1] A. S. Botana et al. Phys. Rev. B 85, 235118 (2012)