Enhancement of Nuclear Spin-Lattice Relaxation Rate and Spin Susceptibility due to Valence Fluctuations -Origin of Anomalously Enhanced Wilson Ratio in Ce and Yb Systems-

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We show theoretically that the nuclear spin-lattice relaxation rate (T₁T)⁻¹ and the spin susceptibility exhibit divergent behaviors toward zero temperature at the quantum critical point (QCP) of the first-order valence transition. Remarkable enhancement in (T₁T)⁻¹ and the spin susceptibility is induced by valence fluctuations even at the valence-crossover temperature far away from the QCP. This mechanism well explains peculiar behaviors observed recently in YbAuCu₄ and also gives a systematic explanation for YbXCu₄ for X=In, Au, Ag, Tl, and Pd from the viewpoint of the closeness to the QCP. This also explains anomalously enhanced Wilson ratio observed in the paramagnetic Ce and Yb based compounds. This offers a new concept that spin fluctuations are induced via relative charge fluctuations, which can be generally applied to the systems with valence instabilities.