  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.

Strongly Correlated Electrons