Ultrasonic study of antiferro-quadrupole ordering in HoB 2C 2

Abstract

Elastic properties of the ternary rare earth compound HoB 2C 2 with a tetragonal symmetry have been investigated by means of ultrasonic measurements. The elastic constants of C 44, C 66 and ( C 11− C 12)/2 exhibit the characteristic softening below about 40 K . In order to analyze quadrupole susceptibility, we assume a pseudo-degeneracy model consisting of a ground state E-doublet and an excited state A- or B-singlet at energy Δ∼5.0 K . The considerable softening of the elastic constant and ultrasonic attenuation in phase IV between T C1 =5.9 K and T C2 =5.0 K have been found. The slow relaxation rate of 7×10 −9 s suggests that the fluctuation of quadrupole moments is enhanced in phase IV. We also show a H– T phase diagram of HoB 2C 2 under magnetic fields along [1 0 0] .