Contrast of LiFeAs with isostructural, isoelectronic, and non-superconducting MgFeGe

TL;DR

This study compares the electronic structures of superconducting LiFeAs and non-superconducting MgFeGe using first principles calculations, finding minimal differences in orbital occupations but potential significance in band redistribution above the Fermi level.

Contribution

It provides a detailed first-principles analysis of electronic differences between LiFeAs and MgFeGe, including the effects of advanced exchange-correlation potentials.

Findings

Fe 3d occupations are identical in both compounds

Band redistribution above the Fermi level may influence superconductivity

mBJ potential does not improve agreement with experimental data for LiFeAs

Abstract

Stoichiometric LiFeAs at ambient pressure is an 18 K superconductor while isoelectronic MgFeGe is not, despite their extremely similar electronic structures. To investigate possible sources of this distinctively different superconducting behavior, we quantify the differences using first principles density functional theory, establishing first that the Fe total 3d occupations are identical in the two compounds. Individual 3d orbital occupations also differ very little ($\sim 0.01$). The differences in Fermi surfaces (FSs) do not seem significant; however a redistribution of bands just above the Fermi level does represent a possibly significant distinction. Because the bands and FSs of LiFeAs are less in agreement with experiment than for other iron-pnictides, we study the effects of additional exchange-correlations effects beyond GGA (the generalized gradient approximation) by applying the modified Becke-Johnson potential (mBJ) exchange potential, which gives much improved bandgaps in insulators compared to GGA and might be useful for semimetals such as the Fe-based superconductors. Overall, we conclude that the mBJ corrections do not improve the description of LiFeAs as compared to experiment.