Abstract
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 τ=τ 0 exp ( E / k B T ) with low activation energy, E = 500 mK, and slow attempt time, τ 0 =8.5×10 -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 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.
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