CuFeO_2 is one of the multiferroic materials and is the first case that the electric polarization is not explained by the magnetostriction model or the spin-current model. We have studied this material using soft x-ray resonant diffraction and found that superlattice reflection 0, 1 - 2q, 0 appears in the ferroelectric and incommensurate magnetic ordered phase at the Fe L2,3 absorption edges and moreover that the rotation of the x-ray polarization such as from {\sigma} to {\pi} or from {\pi} to {\sigma} is allowed at this reflection. These findings definitely provide direct evidence that the 3d orbital state of Fe ions has a long range order in the ferroelectric state and support the spin-dependent d-p hybridization mechanism.
@article{arxiv.1012.3586,
title = {Incommensurate Orbital Modulation in Multiferroic CuFeO2},
author = {Yoshikazu Tanaka and Noriki Terada and Taro Nakajima and Taro Kojima and Yasutaka Takata and Setsuo Mitsuda and Masaki Oura and Yasunori Senba and Haruhiko Ohashi and Shik Shin},
journal= {arXiv preprint arXiv:1012.3586},
year = {2010}
}