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

We consider the role played by the 4f 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 4f states are located well below and above the Fermi level EF, and essentially do not interact with the iron 3d bands located near EF, resulting in local moment behavior. In the Ce compounds, our results reveal a qualitatively different picture, with the 3d-4f hybridization being sufficiently strong to give rise to an observable Kondo screening of the local 4f 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 TK similar or equal to 10 K. At ambient pressure, TK is found to be negligibly small for CeOFeAs. Under applied hydrostatic pressure, we predict an exponential increase of TK which reaches values comparable to the superconducting T-c similar or equal to 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 Tc 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. Copyright (C) EPLA, 2008