Competition between valence and spin fluctuations in the vicinityof the quantum critical point of the heavy fermion compoundYbAuCu4

Abstract

The quantum criticality of the Yb-based heavy fermion compoundYbAuCu4 with noninteger valence close to unity has been investigated through low-temperature resistivity,magnetization, and nuclear magnetic resonance measurements in several fixed magnetic fieldsH. We found that,with increasing H, YbAuCu4 is driven from the originally antiferromagnetically ordered ground state (Néel temperatureTN∼0.9 K) to a nonmagnetic Fermi liquid one through the field-tuned quantum critical point(QCP) at kOe. The experimental results also provide the first evidence that a crossover valencetransition near the magnetic QCP is stabilized with the application of external field, inmarked contrast to the destabilization of the first-order valence transition. TheT–H phasediagram of YbAuCu4 and the T–X phase diagramestablished for the YbXCu4 (X = Pd,Au, Cu, Ag) series indicate that the evolution of valence fluctuations near the magneticQCP quickly interacts with the critical spin fluctuations. We suggest that the competitionbetween the Kondo temperature and crossover valence transition temperature is central tothe long-standing puzzles of localized–itinerant duality and the extent of degeneracy in thecrystal electric field ground state.