Prediction of exotic magnetic states in the alkali-metal quasi-one-dimensional iron selenide compound <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:mrow><mml:msub><mml:mi>Na</mml:mi><mml:mn>2</mml:mn></mml:msub><mml:msub><mml:mi>FeSe</mml:mi><mml:mn>2</mml:mn></mml:msub></mml:mrow></mml:math>
Bradraj Pandey, Ling-Fang Lin, Rahul Soni, Nitin Kaushal, J. Herbrych, Gonzalo Álvarez, Elbio Dagotto
Abstract
The magnetic and electronic phase diagram of a model for the quasi-one-dimensional alkali-metal iron selenide compound ${\mathrm{Na}}_{2}{\mathrm{FeSe}}_{2}$ is presented. The novelty of this material is that the valence of iron is ${\mathrm{Fe}}^{2+}$, contrary to most other iron-chain compounds with valence ${\mathrm{Fe}}^{3+}$. 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 $\ensuremath{\uparrow}\ensuremath{\uparrow}\ensuremath{\downarrow}\ensuremath{\downarrow}\ensuremath{\uparrow}\ensuremath{\uparrow}\ensuremath{\downarrow}\ensuremath{\downarrow}$ is stabilized was unveiled. The analog state in iron ladders, employing $2\ifmmode\times\else\texttimes\fi{}2$ 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 $\ensuremath{\uparrow}\ensuremath{\downarrow}\ensuremath{\uparrow}\ensuremath{\downarrow}\ensuremath{\uparrow}\ensuremath{\downarrow}\ensuremath{\uparrow}$ 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 $\ensuremath{\uparrow}\ensuremath{\uparrow}\ensuremath{\downarrow}\ensuremath{\uparrow}\ensuremath{\downarrow}\ensuremath{\downarrow}\ensuremath{\uparrow}\ensuremath{\downarrow}$. Our predictions for ${\mathrm{Na}}_{2}{\mathrm{FeSe}}_{2}$ may guide crystal growers and neutron-scattering experimentalists towards the realization of block states in one-dimensional iron selenide chain materials.