Magnetic States of the Two-Leg Ladder Iron Selenides

Neutron scattering experiments have unveiled a dominant spin arrangement in the two-leg ladder selenide compound BaFe$_2$Se$_3$, involving ferromagnetically ordered 2$\times$2 iron-superblocks, that are antiferromagnetically coupled among them (the ``block-AFM'' state). Our numerical study of the electronic five-orbital Hubbard model, within the Hartree-Fock approximation and using first principles techniques for the hopping amplitudes, has shown that the exotic block-AFM state is indeed stable at realistic electronic densities $n \sim 6.0$. Another state with wavevector $(\pi,0)$ becomes stable in other portions of the phase diagrams, including $n \sim 5.5$, as found experimentally in KFe$_2$Se$_3$. In addition, our study unveils several competing magnetic phases that could be experimentally stabilized varying either $n$ chemically or the electronic bandwidth by pressure. Similar results were obtained using two-orbital models, studied here via Lanczos and DMRG techniques [1]. [1] Qinlong Luo, et al, arXiv: 1205.3239, and references therein.