Classical dimers and dimerized superstructure in orbitally degenerate honeycomb antiferromagnet

TL;DR

This paper investigates how orbital degrees of freedom in a honeycomb lattice induce spin dimerization and form a valence bond crystal, explaining experimental observations in Li₂RuO₃.

Contribution

It introduces a theoretical model showing orbital-driven spin dimerization and a dimerized superstructure in a honeycomb antiferromagnet, including a magnetoelastic coupling mechanism.

Findings

Orbital degrees of freedom induce spin dimerization.

Dimer Jahn-Teller effect lifts degeneracy, forming a valence bond crystal.

Theory explains nonmagnetic superstructure in Li₂RuO₃.

Abstract

We discuss the ground state of the spin-orbital model for spin-one ions with partially filled $t_{2g}$ levels on a honeycomb lattice. We find that the orbital degrees of freedom induce a spontaneous dimerization of spins and drive them into nonmagnetic manifold spanned by hard-core dimer (spin-singlet) coverings of the lattice. The cooperative ``dimer Jahn-Teller'' effect is introduced through a magnetoelastic coupling and is shown to lift the orientational degeneracy of dimers leading to a peculiar valence bond crystal pattern. The present theory provides a theoretical explanation of nonmagnetic dimerized superstructure experimentally seen in Li$_2$RuO$_3$ compound at low temperatures.