Coulomb correlations in a germanide iron-based superconductor: The example of YFe2Ge2

Abstract

We report results of a detailed theoretical study of the effect of Coulomb correlations on the structural, electronic, and magnetic properties of the non-pnictide iron-based superconductor ${\mathrm{YFe}}_{2}{\mathrm{Ge}}_{2}$. In particular, by employing the density functional theory plus dynamical mean-field theory method ($\mathrm{DFT}+\mathrm{DMFT}$) we perform a structural optimization of the unit-cell volume and the internal coordinate ${z}_{\mathrm{Ge}}$ of Ge. Our results for the lattice parameters obtained by $\mathrm{DFT}+\mathrm{DMFT}$ show a much better agreement with experiment as compared to those calculated by DFT implying the importance of correlation effects. We show that ${\mathrm{YFe}}_{2}{\mathrm{Ge}}_{2}$ is a moderately correlated system with electron-electron correlations being strong enough to renormalize the low-energy band structure but not sufficient to form Hubbard bands, in close analogy to the iron-based pnictides and chalcogenides. The analysis of the local spin-spin correlation function and momentum-dependent spin susceptibility $\ensuremath{\chi}(\mathbf{q})$ indicates the correlation induced formation of short-lived local moments in the Fe ${t}_{2g}$ states and the presence of competing spin fluctuations, in agreement with experiment.