Roles of critical valence fluctuations in Ce- and Yb-based heavy fermion metals

Abstract

The roles of critical valence fluctuations of Ce and Yb are discussed as a key origin of severalanomalies observed in Ce- and Yb-based heavy fermion systems. Recent development of thetheory has revealed that a magnetic field is an efficient control parameter to induce thecritical end point of the first-order valence transition. Metamagnetism and non-Fermi liquidbehavior caused by this mechanism are discussed by comparing favorably withCeIrIn5,YbAgCu4 andYbIr2Zn20. The interplay of the magnetic order and valence fluctuations offers a key concept forunderstanding Ce- and Yb-based systems. It is shown that suppression of themagnetic order by enhanced valence fluctuations gives rise to the coincidence of themagnetic-transition point and valence crossover point at absolute zero as a function ofpressure or magnetic field. The interplay is shown to resolve the outstanding puzzle inCeRhIn5 in a unified way. The broader applicability of this newly clarifiedmechanism is discussed by surveying promising materials such asYbAuCu4,β-YbAlB4 andYbRh2Si2.