Using x-ray absorption spectroscopy at the Ru-L2,3 edge we reveal that the Ru4+ ions remain in the S=1 spin state across the rare 4d-orbital ordering transition and spin-gap formation. We find using local spin density approximation + Hubbard U (LSDA+U) band structure calculations that the crystal fields in the low temperature phase are not strong enough to stabilize the S=0 state. Instead, we identify a distinct orbital ordering with a significant anisotropy of the antiferromagnetic exchange couplings. We conclude that La4Ru2O10 appears to be a novel material in which the orbital physics drives the formation of spin-singlet dimers in a quasi 2-dimensional S=1 system.
@article{arxiv.cond-mat/0606445,
title = {Orbitally driven spin-singlet dimerization in $S$=1 La$_{4}$Ru$_{2}$O$_{10}$},
author = {Hua Wu and Z. Hu and T. Burnus and J. D. Denlinger and P. G. Khalifah and D. Mandrus and L. -Y. Jang and H. H. Hsieh and A. Tanaka and K. S. Liang and J. W. Allen and R. J. Cava and D. I. Khomskii and L. H. Tjeng},
journal= {arXiv preprint arXiv:cond-mat/0606445},
year = {2007}
}