We discuss the ground state of the spin-orbital model for spin-one ions with partially filled t2g 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 Li2RuO3 compound at low temperatures.
@article{arxiv.0802.2692,
title = {Classical dimers and dimerized superstructure in orbitally degenerate honeycomb antiferromagnet},
author = {G. Jackeli and D. I. Khomskii},
journal= {arXiv preprint arXiv:0802.2692},
year = {2008}
}