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-

Abstract

We show theoretically that the nuclear spin-lattice relaxation rate (T_1T)^{-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 (T_1T)^{-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 YbAuCu_4 and also gives a systematic explanation for YbXCu_4 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.