Metal-insulator transition in the hollanditeK2V8O16with a frustrated zigzag ladder probed byV51NMR

Abstract

We report the experimental results of $^{51}\mathrm{V}$ NMR measurements on the hollandite ${\mathrm{K}}_{2}$V${}_{8}$O${}_{16}$ consisting of a frustrated zigzag ladder with the orbital degrees of freedom. The metal-insulator transition is found to involve the spin-singlet formation by the $^{51}\mathrm{V}$ Knight shift $K$, the nuclear spin-lattice relaxation rate $1/{T}_{1}$, and the spin-echo decay rate $1/{T}_{2}$ measurements. In the insulating state, the anisotropic electric-field gradient supports the ${d}_{\mathit{xy}}$ orbital order with the spin singlet along the chain. The ${d}_{\mathit{xy}}$ orbital is magnetically most active in the metallic state, as observed by the anisotropic Knight shift, which suggests the strong electron correlation in the ${d}_{\mathit{xy}}$ band. Despite the large enhancement of the spin susceptibility, no apparent spin correlation is developed in the frustrated metallic state. Pressure suppresses the electron correlation continuously, as highlighted in the decrease of the metal-insulator transition and the spin susceptibility keeping the largest ${d}_{\mathit{xy}}$ contribution in the metallic state. A robust spin-singlet insulating phase with the large spin gap and paramagnetic spins appears above 1 GPa, which suggests a competition of the charge-orbital ordering pattern.