II. Exploring the role of the Crystal Electric Field in the vicinity of a Quantum Critical Point

Abstract

Very low temperature thermdynamic properties of the Yb$T_4M$ family of compounds are analyzed in a broad range of behavior including a quantum critical point QCP between magnetic and Fermi-liquid ground states GS. Doniach-Lavagna phase diagram limitations are improved by taking into account crystal electric field CEF splittings. The studied alloys: Yb$T_{5-x}M_x$ (with $T$= Ni, Cu, and $M$= Cd, Mg, Pd, Au, Zn, Ag), allow to gain insight into the evolution of the GS behavior undergoing the QCP region as a function of chemical doping as control parameter. Three types of behaviors are recognized in this system: i) a magnetic one, with weak interactions at $T_m\leq 1$\,K and very low Kondo temperature of their respective doublets GS. Between $T_m$ and $T_q\approx 0.3$\,K, quantum fluctuations start to dominate the scenario with the specific heat $C_{4f}/T(T\geq T_q)$ showing $T$ power law dependencies and a very heavy-fermion {\it plateau} below $T_q$. ii) beyond the QCP the typical non-fermi-liquid logarithmic $T$ dependence: $C_{4f}/T \propto \ln(T/T_0)$ and iii) at the non-magnetic limit, the alloys behave as valence-fluctuation systems with $T_K$ overcoming the CEF splitting. With this experimental information, a more realistic phase diagram can be drawn around the QCP where the scenario is dominated by low lying quantum fluctuations, without $C_{4f}/T|_{Lim T\to 0}$ divergences but a clear drop of its value.