Dilatometric measurements and multipole ordering in DyB2C2 and HoB2C2
Tatsuya Yanagisawa, Tatsuya Moriwaki, Terutaka Goto, Yuichi Nemoto, Shingo Miyata, Ryuta Watanuki, and Kazuya Suzuki
We have performed dilatometric measurements by the capacitance method on tetragonal rare earth compounds DyB2C2 and HoB2C2 that show antiferro-quadrupole (AFQ) ordering. The tetragonal structure is invariant across the paramagnetic phase to the AFQ phase at TQ=24.7 K in DyB2C2. In the case of intermediate phase IV of HoB2C2, between TC1=5.9 K and TC2=5.0 K, the lattice length along the  direction reveals an abrupt increase, while the lengths along the  and  directions shrink slightly. This lattice distortion is described in terms of triclinic strain εyz = εzx≠0 indicating the spontaneous ferro-quadrupole moment <Oyz>= <Ozx>≠0. A plausible model, based on an octupole ordering <Tx>=<Ty>=<Tz>≠0, in phase IV of HoB2C2, is argued.
Figures: Relative change of thermal expansion ΔL=L as a function of temperature along the , ,  and  directions of DyB2C2 and HoB2C2, normalized to their values at 35 K and 10 K, respectively.
(*This research was performed at Niigata University in 2004.)