Iron-based layered compound LaFeAsO is an antiferromagnetic semimetal

LaFeAsO is a parent compound of iron-based F-doped superconductors. An understanding of its ground phase and electronic structure is a precondition to understanding the underlying superconductivity mechanism. Our study shows that LaFeAsO is a quasi-two-dimensional antiferromagnetic semimetal with most carriers being electrons and with a magnetic moment of 2.3 mu(B) located around each Fe atom on the Fe-Fe square lattice. Physically, this is a commensurate antiferromagnetic spin-density wave due to the Fermi-surface nesting. The observed superconduction after the F doping happens on the Fe-Fe layer suggesting a new superconductivity mechanism mediated by spin fluctuations.