Tuning the Kondo effect in Yb(Fe$_{1-x}$Co$_{x}$)$_{2}$Zn$_{20}$

TL;DR

This study investigates how the Kondo effect evolves in Yb(Fe$_{1-x}$Co$_{x}$)$_{2}$Zn$_{20}$ compounds, revealing changes in ground state degeneracy, Kondo temperature, and coherence behavior with varying Co substitution.

Contribution

It provides a detailed experimental analysis of the Kondo effect tuning via Co substitution in heavy fermion compounds, highlighting changes in degeneracy and coherence temperature.

Findings

YbFe$_2$Zn$_{20}$ has a Kondo ground state with N=8 and T_K~30 K.

YbCo$_2$Zn$_{20}$ exhibits a Kondo state with N=2 and T_K~2 K.

Kondo coherence is suppressed at intermediate Co levels and re-emerges at high Co concentrations.

Abstract

We study the evolution of the Kondo effect in heavy fermion compounds, Yb(Fe$_{1-x}$Co$_{x}$)$_{2}$Zn$_{20}$ (0$\leqslant$ x $\leqslant$ 1), by means of temperature-dependent electric resistivity and specific heat. The ground state of YbFe$_2$Zn$_{20}$ can be well described by a Kondo model with degeneracy $N$ = 8 and a $T_K\sim$30 K. In the presence of a very similar total CEF splitting with YbFe$_2$Zn$_{20}$, the ground state of YbCo$_2$Zn$_{20}$ is close to a Kondo state with degeneracy $N$ = 2 and a much lower $T_K\sim$ 2 K. Upon Co substitution, the coherence temperature of YbFe$_2$Zn$_{20}$ is suppressed, accompanied by an emerging Schottky-like feature in specific heat associated with the thermal depopulation of CEF levels upon cooling. For 0.4$\lesssim$ x $\lesssim$ 0.9, the ground state remains roughly the same which can be qualitatively understood by Kondo effect in the presence of CEF splitting. There is no clear indication of Kondo coherence observable in resistivity within this substitution range down to 500 mK. The coherence re-appears at around x$\gtrsim$ 0.9 and the coherence temperature increases with higher Co concentration levels.