La2O3Fe2Se2: a Mott insulator on the brink of orbital-selective metallization

We show that the insulating character of the iron selenide La2O3Fe2Se2 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 La2O3Fe2Se2 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.