The low-frequency dynamics of Van Vleck compounds and the thermal relaxation of Pr¹⁴¹ in PrNi₅

The analytical expression of Becker, Fulde and Keller (BFK) for the low-frequency dynamical susceptibility is generalized for an arbitrary RE ion level scheme case. The NMR linewidth of RE nucleus is calculated and it is shown that the electron-hole part of this susceptibility does not contribute to the width for T≫T_c. The coupling parameter j ρ for PrNi₅ is estimated from the available experimental data (j ρ∼8.10^-3) and it is found that the electronic susceptibility is much enhanced in PrNi₅, (1 - α)^-1 ≃ 10. It is argued that the single ion-conduction electron process cannot explain the thermal relaxation of Pr¹⁴¹ in PrNi₅ and the importance of incorporating the RKKY exchange mechanism in the frame of the first principle theory of BFK is discussed.