Mott state and quantum critical points in rare-earth oxypnictides RO1-xFxFeAs (R = La, Sm, Nd, Pr, Ce)

We investigate the magnetic phase diagram of the newly discovered iron-based high temperature oxypnictide superconductors of the type RO1-xFx FeAs, with rare earths R = La, Sm, Nd, Pr and Ce by means of ab initio, SGGA and SGGA + U density functional computations. We find undoped LaOFeAs to be a Mott insulator when incorporating electronic correlations via SGGA + U for any physically relevant value of U. The doped compounds are according to SGGA conductors with a transition from an antiferromagnetic to a nonmagnetic state at a hole doping of concentration x(c) = 0.075 for R = Nd, Pr and at electron doping x(c) = 0.25 for Ce and 0.6 for Sm. Superconductivity in these rare-earth oxypnictides thus appears in the vicinity of a magnetic quantum critical point where electronic correlations are expected to play an important role because of the vicinity of a Mott insulating state at zero doping. (C) 2008 Elsevier B.V. All rights reserved.