Local moment vs . Kondo behavior of the 4f-electrons in rare-earth iron oxypnictides

We consider the role played by the 4 f states in the rare-earth oxyarsenides REOFeAs (RE=Ce, Pr, Nd) and the oxyphosphate CeOFeP, using a first-principles technique that combines the local density approximation and dynamical mean-field theory (LDA+DMFT). In the Pr and Nd compounds, the 4 f states are located well below and above the Fermi level E F , and essentially do not interact with the iron 3 d bands located near E F , resulting in local moment behavior. In the Ce compounds, our results reveal a qualitatively different picture, with the 3 d -4 f hybridization being sufficiently strong to give rise to an observable Kondo screening of the local 4 f moment. Our LDA+DMFT electronic structure calculations allow us to estimate the Kondo temperature T K for both CeOFeP and CeOFeAs. For the phosphate, the order of magnitude of our estimate is consistent with the experimental observation of T K ≃10 K. At ambient pressure, T K is found to be negligibly small for CeOFeAs. Under applied hydrostatic pressure, we predict an exponential increase of T K which reaches values comparable to the superconducting T c ≃40 K at pressures above 10 GPa. We conjecture that the competition between the Kondo effect and superconductivity may be at the origin of the monotonous decrease of T c observed in CeOFeAs under pressure. We argue that the quantitative aspects of this competition are inconsistent with a weak-coupling BCS description of the superconductivity in the oxyarsenides.