Local spin susceptibility in the orbital-frustrated vanadium oxide BaV ₁₀O₁₅

BaV₁₀O₁₅ having a bilayer triangular lattice exhibits a semiconductor-insulator transition accompanied by the structural phase transition at T_c ∼130 K. We show the microscopic properties of the phase transition investigated by ⁵¹V NMR measurements on a single crystal. We have observed ⁵¹V NMR spectra from two of the three inequivalent V sites at high temperatures. The Knight shift tensors are determined at 300 K for the two V sites. The local spin susceptibility strongly depends on the V sites and temperature despite the nearly independent magnetic susceptibility. Each spectrum is split into two below 200 K far above T _c, suggesting a charge/order ordering transition. The local spin susceptibilities of the three sites gradually vanish with decreasing temperature, consistent with the V trimer formation. The microscopic evidence for the spin singlet state is clearly observed below T_c.