Quadrupole Susceptibility of HoB2C2

Abstract

We have investigated the elastic constants of the ternary rare earth compound HoB 2 C 2 with a tetragonal structure by means of the ultrasonic method. The elastic constants of ( C 11 - C 12 )/2, C 44 and C 66 , corresponding to the transverse ultrasonic modes exhibit the softening proportional to reciprocal temperature below about 40 K. This softening is commonly enhanced in the ordered phase IV below T C 1 =5.9 K. An abrupt increase of elastic constants is found in the ordered phase III below T C 2 =5.0 K characterized by the simultaneous antiferro-type ordering of electric quadrupole and magnetic dipole moments. The quadrupole susceptibility based on a model consisting of a ground state E-doublet and an excited singlet with a low energy in HoB 2 C 2 is employed for the analysis of the softening above T C 1 =5.9 K. The enhancement of the elastic softening in the phase IV of HoB 2 C 2 resembles to the soft C 44 mode in the phase IV in Ce 0.75 La 0.25 B 6 .