Quadrupole and hexadecapole ordering in DyBC; Direct observation with resonant X-ray diffraction

Tanaka, Yoshikazu*; Inami, Toshiya; Lovesey, S. W.*; Knight, K. S.*; Yakhou, F.*; Mannix, D.*; Kokubun, Jun*; Kanazawa, Masayuki*; Ishida, Kotaro*; Nanao, Susumu*; Nakamura, Tetsuya*; Yamauchi, Hiroki*   ; Onodera, Hideya*; Oyama, Kenji*; Yamaguchi, Yasuo*

Direct evidence of the spatial ordering of Dy 4 quadrupole and hexadecapole moments in DyBC is demonstrated by resonant X-ray diffraction enhanced by an electric quadrupole event (2 resonance) at the Dy L absorption edge. The diffraction data show that the structural phase transition at = 24.7 K is accompanied by a reduction in the symmetry of the Dy site to 2/, from 4/, and the spatial ordering of the time-even Dy multipoles with A character. Below the crystal structure is described by the space group 4/ and Dy ions occupy sites (4). The distortion at T involves the lattice occupied by B and C ions, and it amounts to a buckling of B-C planes that are normal to the two-fold rotation axis of 2/. An immediately plausible model of low-energy Dy states correlates data on the specific heat, our X-ray diffraction signals, and magnetic ordering below 15.3 K which has been observed in magnetic neutron diffraction.