Electronic structure, spin state, and magnetism of the square-lattice Mott insulator ${\text{La}}_2{\text{Co}}_2{\text{Se}}_2{\text{O}}_3$ from first principles

Electronic and magnetic structures of the recently synthesized cobalt oxyselenide ${\text{La}}_2{\text{Co}}_2{\text{Se}}_2{\text{O}}_3$ (structurally similar to the superconducting iron pnictides) are studied through density-functional calculations. The obtained results show that this material is a Mott insulator and it has a very stable ${\text{Co}}^{2+}$ high-spin ground state with a $t_{2g}$-like orbital ordering, which is substantiated by the calculated crystal-field excitation energies. The square lattice of the ${\text{Co}}^{2+}$ spins is found to have a strong antiferromagnetic (a weak ferromagnetic) coupling for the second-nearest neighbors via O $\left({\text{Se}}_2\right)$ and an intermediate antiferromagnetic one for the first-nearest neighbors with the strength ratio about 10:1:3. The present results account for the available experimental data and the prediction of a planar frustrated $\left(2\times 2\right)$ antiferromagnetic structure would motivate a new experiment.

Figure 1

(Color online) (a) Crystal structure of ${\text{La}}_2{\text{Co}}_2{\text{Se}}_2{\text{O}}_3$ having the ${\text{Co}}_2{\text{Se}}_2\text{O}$ layers. For each constituent ${\text{CoSe}}_4{\text{O}}_2$ octahedron the corresponding local coordinates are used, e.g., the $xyz$ for the body-centered Co site and the $y$ and $z$ interchanged for the $c$-axis edge ${\text{Co}}^{2+}$. (b) Orbitally resolved $\text{Co}3d$ DOS for the nonmagnetic state by LDA. The lower-lying $t_{2g}$-like orbitals ($x^2-y^2$, $yz$, and $xz$) and the higher $e_g$ ($xy$ and $3z^2-r^2$) all split. (c) DOS of the HS FM half-metallic state by LSDA. The solid (dashed) curves refer to the majority (minority) spin. Fermi level is set at zero energy.

Figure 2

DOS of the HS (left panel) and LS (right panel) solutions in the checkerboard AF state by the LSDA-Fock-0.25 hybrid functional. The ${\text{Co}}^{2+}$ ions in the HS ground state have an $xz$-type ($t_{2g}$ hole) orbital ordering. The much less stable LS state has the $t_{2g}^6+e_g^1\left(:xy\right)$ configuration. See Fig. 1 for other notations.

Figure 3

(Color online) The ground-state $\left(2\times 2\right)$ planar magnetic structure of the HS ${\text{Co}}^{2+}$ square lattice with the 2nn AF exchange $J_1$ via O (red bold lines), the 1nn AF $J_2$ (blue solid lines), and the 2nn FM exchange $J_3$ via ${\text{Se}}_2$ (cyan thin lines). See also Fig. 1a. The $J_2$ couplings are frustrated. Only the spin-up and spin-down ${\text{Co}}^{2+}$ sites are shown for clarity. The listed exchange constants (in unit of meV) are calculated using the $\text{LSDA}+U$ with effective $U=4$, 3, or 2 eV.