Elastic constants of antiferro-quadrupole ordering system DyB2C2

Elastic constants of tetragonal system DyB2C2 with antiferro-quadrupole (AFQ) ordering have been measured by using ultrasonic method. Remarkable elastic softenings of the C44 and (C11-C12)/2 have been observed towards the AFQ transition at TQ=25 K. The present results show that a CEF ground state is a pseudo-quartet consisting of two Kramers doublets of E1/2 and E3/2 in C4h site symmetry of Dy3+ ion. The dominant softening of C44 probably leads to the order parameter of Oyz and Ozx in AFQ phase. Magnetic phase diagram applied fields along the [1 0 0] direction determined by field dependence of the C44 is presented