Dissipation in Non-Kramers Doublet of PrMg3

The low-temperature ultrasonic dispersion in the transverse $(C_{11}-C_{12})/2$ mode of PrMg 3 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 $\tau=\tau_{0}\exp(E/k_{\text{B}}T)$ with low activation energy, $E = 500$ mK, and slow attempt time, $\tau_{0}=8.5\times 10^{-10}$ 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 PrMg 3 releases the entropy of $k_{\text{B}}\ln 2$ 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.