Electric Quadrupolar Contributions in the Magnetic Phases of UNi₄B

We present acoustic signatures of the electric quadrupolar degrees of freedom in the honeycomb-layer compound UNi₄B. The transverse ultrasonic mode C₆₆ shows softening below 30 K both in the paramagnetic phase and antiferromagnetic phases down 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 Γ₆ (E_2g) electric quadrupolar degrees of freedom in localized 5f² (J = 4) states are playing an important role in the magnetic toroidal dipole order and magnetic-field-induced phases of UNi₄B, and evidence some of the U ions remain in the paramagnetic state even if the system undergoes magnetic toroidal ordering.