Quadrupole and lattice effects of orbitally degenerate 4f-electron systems

Abstract

The quadrupole orderings and lattice effects in 4f-electron systems withorbital degeneracy in rare earth compounds have been investigated byultrasonic and thermal expansion measurements. A ferroquadrupole (FQ)Oxy-typeordering associated with a trigonal distortion in HoB6 witha Γ5ground state is presented. The trigonal distortion in orderedphase IV CexLa1−xB6 (x = 0.75 and 0.70) with aΓ8 ground stateindicates Oxy-typeFQ ordering in phase IV. In both systems, HoB6and CexLa1−xB6,C44anomalies are observed. In tetragonal HoB2C2,an ordered phase IV appears near a tetra-critical point, where the antiferroquadrupolephase II and antiferromagnetic phase III compete with each other. Noticeableultrasonic attenuation and softening of elastic constants in phase IV of HoB2C2 dueto quadrupole fluctuation with a slow relaxation time τ ∼ 7 × 10−9 sare found.