  Antiferroquadrupolar (AFQ) ordering has been conjectured in several
rare-earth compounds to explain their anomalous magnetic properties. No direct
evidences for AFQ ordering, however, have been reported. Using the resonant
x-ray scattering technique near the Dy L_III absorption edge, we have succeeded
in observing the AFQ order parameter in DyB2C2 and analyzing the energy and
polarization dependence. Much weaker coupling between orbital degrees of
freedom and lattice in 4f electron systems than in 3d compounds provides an
ideal platform to study orbital interactions originated from electronic
mechanisms.
