  Experimental evidence of the d-f Coulomb interaction responsible for the
resonant x-ray scattering (RXS) from antiferroquadrupolar order in DyB2C2 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'
but the opposite sign for sigma-sigma' channel. This result, when compared with
the calculated structure factors, means that the quadrupolar moments of the 4f
and 5d electrons have opposite signs to each other. Interference between
nonresonant Thomson scattering from the lattice distortion and the resonant
scatterings is also studied and a direct coupling between 4f and the lattice is
concluded.
