Resonant x-ray diffraction performed at the LII and LIII absorption edges of Ru has been used to investigate the magnetic and orbital ordering in Ca2RuO4 single crystals. A large resonant enhancement due to electric dipole 2p→4d transitions is observed at the wave-vector characteristic of antiferromagnetic ordering. Besides the previously known antiferromagnetic phase transition at TN=110 K, an additional phase transition, between two paramagnetic phases, is observed around 260 K. Based on the polarization and azimuthal angle dependence of the diffraction signal, this transition can be attributed to orbital ordering of the Ru t2g electrons. The propagation vector of the orbital order is inconsistent with some theoretical predictions for the orbital state of Ca2RuO4.
@article{arxiv.cond-mat/0508258,
title = {Orbital ordering transition in Ca$_2$RuO$_4$ observed with resonant x-ray diffraction},
author = {I. Zegkinoglou and J. Strempfer and C. S. Nelson and J. P. Hill and J. Chakhalian and C. Bernhard and J. C. Lang and G. Srajer and H. Fukazawa and S. Nakatsuji and Y. Maeno and B. Keimer},
journal= {arXiv preprint arXiv:cond-mat/0508258},
year = {2009}
}