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Microscopic nature of correlations in multi-orbital AFe2As2 (A=K, Rb, Cs): Hund's coupling versus Coulomb repulsion

We investigate via LDA+DMFT (local density approximation combined with dynamical mean field theory) the manifestation of correlation effects in a wide range of binding energies in the hole-doped family of Fe-pnictides $A$Fe$_2$As$_2$ ($A={\rm K}$, Rb, Cs) as well as the fictitious FrFe$_2$As$_2$ and $a$-axis stretched CsFe$_2$As$_2$. This choice of systems allows for a systematic analysis of the interplay of Hund's coupling $J_H$ and on-site Coulomb repulsion $U$ in multi-orbital Fe-pnictides under negative pressure. With increasing ionic size of the alkali metal, we observe a non-trivial change in the iron $3d$ hoppings, an increase of orbitally-selective correlations and the presence of incoherent weight at high-binding energies that do not show the typical lower Hubbard-band behavior but rather characteristic features of a Hund's metal. This is especially prominent in $a$-stretched CsFe$_2$As$_2$. We also find that the coherent/incoherent electronic behavior of the systems is, apart from temperature, strongly dependent on $J_H$ and we provide estimates of the coherence scale $T^*$. We discuss these results in the framework of reported experimental observations.