Orbital reformation with vanadium trimerization in d2 triangular lattice LiVO2 revealed by 51V NMR

LiVO2 is a model system of a valence bond solid (VBS) in a 3d(2) triangular lattice. The origin of VBS formation has remained controversial. We investigate the microscopic mechanism by elucidating the d orbital character via on-site V-51 NMR measurements in a single crystal up to 550 K across a structural transition temperature T-c. The Knight shift, K, and nuclear quadrupole frequency, delta nu, show that the 3d orbitals with local trigonal symmetry are reconstructed into a d(yz)d(zx) orbital order below T-c. Together with the NMR spectra with threefold rotational symmetry, we confirm a vanadium trimerization with d-d sigma bonds. The Knight shift extracts the large Van Vleck orbital susceptibility, chi(VV) = 3.6 x 10(-4), in a paramagnetic state above T-c, which is comparable to the spin susceptibility. The results suggest that the orbitally induced Peierls transition in the proximity of the frustrated itinerant state is the dominant driving force of the trimerization transition. DOI: 10.1103/PhysRevB.87.075135