Electronic Correlations and Absence of Superconductivity in the Collapsed Phase of LaFe2As2

Abstract

The interplay between structural phase and electronic correlations has been an intriguing topic of research. An prominent example is the pressure-induced uncollapsed to collapsed tetragonal phase transition observed in CaFe2As2, which is accompanied with the emergence of superconductivity in the collapsed phase. Recently, a very similar structural phase transition was discovered in LaFe2As2, but in contrast to CaFe2As2, superconductivity was only observed in the uncollapsed phase, not the collapsed phase. Previous studies have attributed this puzzling observation to the differences in the two materials' band coherence, orbital occupation, and Fermi surface topology. Here, we present a comparative study of LaFe2As2 and CaFe2As2 using the DFT+DMFT method. Surprisingly, we find that although La appears to have a valence higher than Ca, the doped one electron actually primarily resides on the La site. This leads to almost the same total Fe-3d occupancy and electronic correlation strength as well as similar Lifshiftz transition in the Fermi surface topology for the two materials. In addition, we show that the two materials in both structural phases belong to the category of Hund's metals. Our results indicate that the electronic structures of LaFe2As2 and CaFe2As2 are not too different, which further suggest that superconductivity might also be induced in the collapsed phase of LaFe2As2 under similar non-hydrostatic conditions as for CaFe2As2.