X-ray resonant scattering as a direct probe of orbital ordering in transition-metal oxides

X-ray resonant scattering at the K-edge of transition-metal oxides is shown to measure the orbital order parameter, supposed to accompany magnetic ordering in some cases. Virtual transitions to the 3d orbitals are quadrupolar in general. In cases with no inversion symmetry, such as V2O3, treated in detail here, a dipole component enhances the resonance. Hence, we argue that the detailed structure of orbital order in V2O3 is experimentally accessible.