Dissipation in Non-Kramers Doublet of PrMg₃

The low-temperature ultrasonic dispersion in the transverse (C11 - C12)/2 mode of PrMg₃ with the non-Kramers-doublet ground state has been investigated in temperatures down to 20 mK. The dispersion is caused by a thermally activated motions characterized by an Arrhenius-type relaxation time of τ = τ₀ exp(E/k_BT) with low activation energy, E = 500 mK, and slow attempt time, τ₀ = 8.5 x 10^-10) s. The coupling of the electric quadrupoles of the non-Kramers-doublet ground state to transverse lattice vibrations leads to a vibronic ground state with dissipation. The vibronic state in PrMg₃ releases the entropy of k_B ln 2 with lowering temperature across the activation energy. A Kondo-like singlet state due to the binding of the non-Kramers doublet to the lattice vibrations appears at low temperatures far below the activation energy.