$\mathrm{La}{}_2\mathrm{O}{}_3\mathrm{Fe}{}_2\mathrm{Se}{}_2$: A Mott insulator on the brink of orbital-selective metallization

We show that the insulating character of the iron selenide $\mathrm{La}{}_2\mathrm{O}{}_3\mathrm{Fe}{}_2\mathrm{Se}{}_2$ can be explained in terms of Mott localization in sharp contrast with the metallic behavior of FeSe and other parent compounds of iron superconductors. We demonstrate that the key ingredient that makes $\mathrm{La}{}_2\mathrm{O}{}_3\mathrm{Fe}{}_2\mathrm{Se}{}_2$ a Mott insulator, rather than a correlated metal dominated by the Hund's coupling, is the enhanced crystal-field splitting, accompanied by a smaller orbital-resolved kinetic energy. The strong deviation from orbital degeneracy introduced by the crystal-field splitting also pushes this material close to an orbital-selective Mott transition. We predict that either doping or uniaxial external pressure can drive the material into an orbital-selective Mott state, where only one or a few orbitals are metallized while the others remain insulating.

Figure 1

Density of states of $\mathrm{La}{}_2\mathrm{O}{}_3\mathrm{Fe}{}_2\mathrm{Se}{}_2$ calculated within PBE and projected on the different atomic species. The zero energy is set at the Fermi level.

Figure 2

Orbital-resolved density of states for $\mathrm{La}{}_2\mathrm{O}{}_3\mathrm{Fe}{}_2\mathrm{Se}{}_2$ (bottom panel) and FeSe (top panel). The zero energy is set at the Fermi level.

Figure 3

Orbital-resolved density $\left(n_m\right)$ and quasiparticle weights $\left(Z_m\right)$ calculated within DFT+SSMF for $\mathrm{La}{}_2\mathrm{O}{}_3\mathrm{Fe}{}_2\mathrm{Se}{}_2$ at ratio $J_h/U=0.20$. The inset shows $Z_m$ for a filling of $d^{5.5}$ electrons per iron (one hole every two iron sites).

Figure 4

(a) Orbital-resolved imaginary part of local Green's function (G) as a function of the Matsubara imaginary frequency. (b) Spectral density (A) as a function of the real frequency computed within DFT+DMFT for $\mathrm{La}{}_2\mathrm{O}{}_3\mathrm{Fe}{}_2\mathrm{Se}{}_2$.