Observation of the antiferroquadrupolar order in DyB2C2 by resonant   x-ray scattering
  We have investigated the antiferroquadrupolar(AFQ) order in DyB$_2$C$_2$ by resonant x-ray scattering. X-rays with energies near the $L_3$ absorption edge of Dy were employed. Superlattice peaks that correspond to three kinds of propagation vectors of (1 0 0), (1 0 1/2) and (0 0 1/2) were investigated in detail with polarization analyses. The experimental results are analyzed using a formalism on resonant x-ray scattering and a model of the AFQ order. The magnetic and quadrupolar scatterings are explained by the model satisfactorily. A detailed investigation on the critical behavior of the AFQ ordering is also reported. The critical exponent $\beta$ is deduced to be 0.35, not far from the three-dimensional Heisenberg system. We have also succeeded in detecting the diffuse scattering above $T_{Q}$. 
T. Matsumura, N. Oumi, K. Hirota, H. Nakao, Y. Murakami, Y.   Wakabayashi, T. Arima, S. Ishihara, Y. Endoh