Prediction of exotic magnetic states in the alkali-metal quasi-one-dimensional iron selenide compound Na2FeSe2

The magnetic and electronic phase diagram of a model for the quasi-one-dimensional alkali-metal iron selenide compound Na2FeSe2 is presented. The novelty of this material is that the valence of iron is Fe2+, contrary to most other iron-chain compounds with valence Fe3+. Using first-principles techniques, we developed a three-orbital tight-binding model that reproduces the ab initio band structure near the Fermi level. Including Hubbard and Hund couplings and studying the model via the density-matrix renormalization group and Lanczos methods, we constructed the ground-state phase diagram. A robust region where the block state up arrow up arrow down arrow down arrow up arrow up arrow down arrow down arrow is stabilized was unveiled. The analog state in iron ladders, employing 2 x 2 ferromagnetic blocks, is by now well established, but in chains a block magnetic order has not been observed yet in real materials. The phase diagram also contains a large region of canonical staggered spin order up arrow down arrow up arrow down arrow up arrow down arrow up arrow at very large Hubbard repulsion. At the block-to-staggered transition region, an exotic phase is stabilized with a mixture of both states: an inhomogeneous orbital-selective charge density wave with the exotic spin configuration up arrow up arrow down arrow up arrow down arrow down arrow up arrow down arrow. Our predictions for Na2FeSe2 may guide crystal growers and neutron-scattering experimentalists towards the realization of block states in one-dimensional iron selenide chain materials.