Ferroquadrupolar lattice instability in an antiferromagnetically ordered state in single-crystalline DyCuGe

We report on the specific heat, magnetic properties, and ultrasonic measurements of a single-crystalline DyCuGe. We reveal an antiferromagnetic ordering in the (001) plane at $T_N=7.4K$, which is higher than the reported transition temperature of polycrystalline samples. The transverse elastic moduli show a pronounced softening down to $T_N$ due to interlevel quadrupole-mediated interactions between the ground and excited Kramers doublets. Below $T_N$, we further detected a huge softening accompanied by ultrasound attenuation in the transverse $C_{44}$ mode, suggesting a lattice instability. Our experimental and calculated results manifest that ferroquadrupolar interactions give rise to the lattice instability even in an antiferromagnetically ordered state.