Metal-insulator transition in a spin-orbital-lattice coupled Mott system:K2V8O16

We have explored the underlying mechanism of the metal-insulator transition (MIT) in hollandite-type vanadate, ${\mathrm{K}}₂{\mathrm{V}}₈{\mathrm{O}}₁₆$, which has a quasi-one-dimensional chain structure and undergoes the MIT and Peierls-like structural transition upon cooling. For this purpose, we have investigated its electronic and magnetic properties in comparison to those of ${\mathrm{Rb}}₂{\mathrm{V}}₈{\mathrm{O}}₁₆$ that also undergoes the MIT but without the Peierls-like structural distortion. We have found that ${\mathrm{K}}₂{\mathrm{V}}₈{\mathrm{O}}₁₆$ is a spin-orbital-lattice coupled Mott system and manifests the orbital-selective Mott transition. The interplay of on-site Coulomb interaction, the magnetic-exchange interaction, and the Jahn-Teller-type tetragonal distortion plays an essential role in driving the MIT of ${\mathrm{K}}₂{\mathrm{V}}₈{\mathrm{O}}₁₆$, inducing the the charge ordering (CO) and orbital ordering of V ${t}_2g$ bands. The CO of $\mathrm{V}³⁺$ and $\mathrm{V}⁴⁺$ occurs in separate chains, preserving the inversion symmetry of the crystal. The ${d}_xy$ orbitals form the spin-singlet state along the chain direction. The Peierls-like distortion does not play an essential role in the MIT.