Low-Temperature Elastic Properties of Non-Kramers Doublet Compound PrMg₃

We have investigated low-temperature elastic properties of PrMg₃ with a non-Kramers Γ₃ doublet ground state by using ultrasonic measurements under high field up to 18 T down to 20 mK. A softening of the elastic constant (C₁₁−C₁₂)/2 has been observed below 8 K due to the non-magnetic Γ₃ doublet possessing electric quadrupole O_u and O_v with Γ₃ symmetry and magnetic octupole T_xyz with Γ₂. This result appreciably is deviated from a theoretical fitting of the quadrupole susceptibility based on the crystal-electric-field levels below 800 mK. Furthermore, the (C₁₁−C₁₂)/2 shows strange magnetic dependence up to 18 T below 1.7 K, which are inconsistent with the theoretical results