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The interplay between structural phase and electronic correlations has beenan intriguing topic of research. An prominent example is the pressure-induceduncollapsed to collapsed tetragonal phase transition observed inCaFe$_2$As$_2$, which is accompanied with the emergence of superconductivity inthe collapsed phase. Recently, a very similar structural phase transition wasdiscovered in LaFe$_2$As$_2$, but in contrast to CaFe$_2$As$_2$,superconductivity was only observed in the uncollapsed phase, not the collapsedphase. Previous studies have attributed this puzzling observation to thedifferences in the two materials' band coherence, orbital occupation, and Fermisurface topology. Here, we present a comparative study of LaFe$_2$As$_2$ andCaFe$_2$As$_2$ using the DFT+DMFT method. Surprisingly, we find that althoughLa appears to have a valence higher than Ca, the doped one electron actuallyprimarily resides on the La site. This leads to almost the same total Fe-$3d$occupancy and electronic correlation strength as well as similar Lifshiftztransition in the Fermi surface topology for the two materials. In addition, weshow that the two materials in both structural phases belong to the category ofHund's metals. Our results indicate that the electronic structures ofLaFe$_2$As$_2$ and CaFe$_2$As$_2$ are not too different, which further suggestthat superconductivity might also be induced in the collapsed phase ofLaFe$_2$As$_2$ under similar non-hydrostatic conditions as for CaFe$_2$As$_2$.

Electronic Correlations and Absence of Superconductivity in the Collapsed Phase of LaFe$_2$As$_2$