d-f Coulomb and quadrupole-strain interactions in Dy B 2 C 2 observed by resonant x-ray scattering

Experimental evidence of the d-f Coulomb interaction responsible for the resonant x-ray scattering (RXS) from antiferroquadrupolar order in DyB{sub 2}C{sub 2} is presented. The energy dependences of the RXS intensity with polarization analysis are analyzed by considering the interference between the resonances of dipolar (E1) and quadrupolar (E2) transition processes. It is found that the structure factors for the E1 and E2 processes have the same sign for {sigma}-{pi}{sup '} but the opposite sign for {sigma}-{sigma}{sup '} channel. This result, when compared to the calculated structure factors, means that the quadrupolar moments of the 4f and 5d shells have opposite signs to each other. Interference between nonresonant Thomson scattering from the lattice distortion and resonant scatterings is also studied, revealing the relation between the displacements of the B and C atoms and the charge distributions of the 4f and 5d shells.