Magnetic state of K_(0.8)Fe_(1.6)Se_2 from a five-orbital Hubbard model in the Hartree-Fock approximation

摘要

Motivated by the recent discovery of Fe-based superconductors close to an antiferromagnetic insulator in the experimental phase diagram, here the five-orbital Hubbard model (without lattice distortions) is studied using the real-space Hartree-Fock approximation, employing a 10 x 10 Fe cluster with Fe vacancies in a √5 × √3 pattern. Varying the Hubbard and Hund couplings, and at electronic density n = 6.0, the phase diagram contains an insulating state with the same spin pattern as observed experimentally, involving 2×2 ferromagnetic plaquettes coupled with one another antiferromagnetically. The presence of local ferromagnetic tendencies is in qualitative agreement with Lanczos results for the three-orbital model also reported here. The magnetic moment ~3μB/Fe is in good agreement with experiments. Several other phases are also stabilized in the phase diagram, in agreement with recent calculations using phenomenological models.