Orbital reformation with vanadium trimerization in $d^2$ triangular lattice LiVO$_2$ revealed by $^{51}$V NMR

TL;DR

This study uses $^{51}$V NMR to reveal that vanadium trimerization in LiVO$_2$ is driven by an orbitally induced Peierls transition involving $d_{yz}d_{zx}$ orbital order, elucidating the microscopic mechanism behind the valence bond solid formation.

Contribution

The paper provides microscopic evidence of orbital reformation and vanadium trimerization in LiVO$_2$, highlighting the role of orbitally induced Peierls transition in a frustrated $3d^2$ triangular lattice.

Findings

Identification of $d_{yz}d_{zx}$ orbital order below $T_c$

Confirmation of vanadium trimerization with $d$-$d$ $\sigma$ bonds

Large Van-Vleck orbital susceptibility in the paramagnetic state

Abstract

LiVO$_2$ is a model system of the valence bond solid (VBS) in $3d^2$ triangular lattice. The origin of the VBS formation has remained controversial. We investigate the microscopic mechanism by elucidating the $d$ orbital character via on-site $^{51}$V 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 orbital with local trigonal symmetry are reconstructed into a $d_{yz}d_{zx}$ orbital order below $T_c$. Together with the NMR spectra with three-fold rotational symmetry, we confirm a vanadium trimerization with $d$-$d$ $\sigma$ bonds. The Knight shift extracts the large Van-Vleck orbital susceptibility, $\chi^{\rm VV} = 3.6 \times 10^{-4}$, in a paramagnetic state above $T_c$, which is comparable to the spin susceptibility. The results suggest that orbitally induced Peierls transition in the proximity of the frustrated itinerant state is the dominant driving force of the trimerization transition.