Variational Monte Carlo study of a two-orbital Hubbard model for iron-based superconductors

The low-temperature properties of Iron-based superconductors are mainly determined by multi-orbital fluctuations in the iron 3d shell, under the effect of local Hubbard-Kanamori interactions, with a particular emphasis on Hund's coupling. Here, we consider a two-orbital Hubbard model, as the minimal description for these materials, and study its ground-state properties using variational wave functions and the Quantum Monte Carlo method. These allow us to calculate superconductive correlations in different cluster sizes, which are the main sign of the presence of superconductivity, as well as orbital occupations. We investigate three different regimes: half-filling, electron-doping and hole-doping. In all these regimes, we observe that the two orbitals are equally occupied; also there is no sign of superconductivity, pointing out that without orbital selectivity the system does not become superconductive at all. Our results suggest that a more complex description of the system, based at least on a three-orbital Hubbard model, is needed.