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

TL;DR

This paper investigates the dual behavior of 4f-electrons in rare-earth oxy-iron pnictides, revealing local moment behavior in some compounds and Kondo screening in others, with implications for superconductivity under pressure.

Contribution

It provides a first-principles analysis of 4f-electron behavior in REOFeAs and CeOFeP, highlighting the conditions for Kondo screening and its impact on superconductivity.

Findings

4f states in Pr and Nd compounds do not interact with Fe 3d bands, showing local moments.

In Ce compounds, strong 3d-4f hybridization leads to observable Kondo screening.

Pressure increases the Kondo temperature, potentially competing with superconductivity.

Abstract

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 E_F, and essentially do not interact with the iron 3d bands located near E_F, 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 T_K around 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 around 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.