Thermal expansion and ultrasonic measurements of ferroquadrupole ordering in HoB 6

Abstract

Rare-earth hexaboride HoB 6 shows a ferroquadrupole ordering of O yz , O zx and O xy with Γ 5 symmetry at T Q =6.1 K accompanied by a structural change from cubic to trigonal. In order to examine the order parameter in HoB 6, we have performed ultrasonic and thermal expansion measurements. The lattice length along [0 0 1] expands ΔL/L [0 0 1] =2.5×10 −4 just below T Q, while the length along [1 1 1] shrinks appreciably ΔL/L [1 1 1] = −4.5×10 −3. This result confirms the cubic-trigonal distortion observed by neutron scattering experiments.