title: Incommensurate orbital modulation behind ferroelectricity in CuFeO2.
abstract: 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 L(2,3) absorption edges and moreover that the rotation of the x-ray polarization such as from σ to π or from π to σ is allowed at this reflection. These findings definitely provide direct evidence that the 3d t(2g↓) orbital state of Fe ions has a long-range order in the ferroelectric state. The spin-orbit interaction in Fe ions plays a crucial role to the ferroelectricity in CuFeO(2), coupling two nontrivial spin and orbital orders, both of which break the crystal symmetry.
authors.affiliation: RIKEN SPring Center, Sayo, Hyogo, Japan.
authors.forename: Yoshikazu
authors.initials: Y
authors.lastname: Tanaka
authors.forename: Noriki
authors.initials: N
authors.lastname: Terada
authors.forename: Taro
authors.initials: T
authors.lastname: Nakajima
authors.forename: Munetaka
authors.initials: M
authors.lastname: Taguchi
authors.forename: Taro
authors.initials: T
authors.lastname: Kojima
authors.forename: Yasutaka
authors.initials: Y
authors.lastname: Takata
authors.forename: Setsuo
authors.initials: S
authors.lastname: Mitsuda
authors.forename: Masaki
authors.initials: M
authors.lastname: Oura
authors.forename: Yasunori
authors.initials: Y
authors.lastname: Senba
authors.forename: Haruhiko
authors.initials: H
authors.lastname: Ohashi
authors.forename: Shik
authors.initials: S
authors.lastname: Shin