Abstract
We present acoustic signatures of the electric quadrupolar degrees of freedom in the honeycomb-layer compound UNi4B. The transverse ultrasonic mode C-66 shows softening below 30 K both in the paramagnetic phase and antiferromagnetic phases down to similar to 0.33 K.
Furthermore, we traced magnetic field-temperature phase diagrams up to 30 T and observed a highly anisotropic elastic response within the honeycomb layer. These observations strongly suggest that Gamma(6) (E-2g) electric quadrupolar degrees of freedom in localized 5f(2) (J = 4) states are playing an important role in the magnetic toroidal dipole order and magnetic-field-induced phases of UNi4 B, and evidence some of the U ions remain in the paramagnetic state even if the system undergoes magnetic toroidal ordering.