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

We have investigated low-temperature elastic properties of PrMg₃ with a non-Kramers Gamma₃ doublet ground state using ultrasonic measurements. Although the Gamma₃ doublet possesses electric quadrupoles O_u and O_v with Gamma₃-symmetry and magnetic octupole T_xyz with Gamma₂, PrMg₃ reveals no sign of symmetry breaking long-range ordering down to 20 mK. [1] The elastic constant of (C₁₁-C₁₂)/2 in PrMg₃ exhibits remarkable softening below 8 K, which is well described in terms of a Curie-type quadrupole susceptibility for the Gamma₃ doublet. An appreciable upturn in (C₁₁-C₁₂)/2 below 800 mK in PrMg₃ may indicate an transition into an exotic ground state due to the quadrupole Kondo state being screened by conduction electrons. Frequency dependence in (C₁₁-C₁₂)/2 and its attenuation coefficients at low temperature in PrMg₃ suggests quadrupole fluctuations possessing a characteristic relaxation time tau = tau₀exp(E/kT) with tau₀ = 6x10^-11 sec and an activation energy E = 410 mK. The low-temperature behavior in (C₁₁-C₁₂)/2 of PrMg3 under magnetic fields is also presented.