Fragile superheavy Fermi liquid in ${\mathrm{YbCo}}_2{\mathrm{Zn}}_{20}$

The cubic Kondo lattice ${\mathrm{YbCo}}_2{\mathrm{Zn}}_{20}$ is one of the heaviest known Fermi liquids. We have measured the low-temperature electrical resistivity $\rho \left(T\right)$, magnetic susceptibility $\chi \left(T\right)$, and heat capacity $C\left(T\right)$ in single crystals of $\mathrm{Yb}{\left({\mathrm{Co}}_{1-x}{\mathrm{Ni}}_x\right)}_2{\mathrm{Zn}}_{20}$ ($x\le 0.126$) and $\mathrm{Yb}{\left({\mathrm{Co}}_{1-x}{\mathrm{Fe}}_y\right)}_2{\mathrm{Zn}}_{20}$ ($y\le 0.07$). While pure ${\mathrm{YbCo}}_2{\mathrm{Zn}}_{20}$ displays maxima in $\rho \left(T\right)$ and $\chi \left(T\right)$, ascribed to an enhanced crystal electric field (CEF) degeneracy, the maxima are suppressed by small amounts ($\approx 6$ at.%) of substitutions of the Co atoms. This goes along with a suppression of superheavy Fermi liquid behavior as manifested in the divergence of $C/T$. We ascribe the observations to local distortions at the Yb site due to the chemical disorder in the environment, rather than to a chemical pressure effect. This fragileness to the local distortion indicates that superheavy Fermion behavior in ${\mathrm{YbCo}}_2{\mathrm{Zn}}_{20}$ is closely linked to an enhanced CEF degeneracy.

Figure 1

The inverse DC magnetic susceptibility ${\chi }_{\mathrm{DC}}^{-1}$ for $B\left|\right|$ [110] in $\mathrm{Yb}{\left({\mathrm{Co}}_{1-x}{\mathrm{Ni}}_x\right)}_2{\mathrm{Zn}}_{20}$ and $\mathrm{Yb}{\left({\mathrm{Co}}_{1-y}{\mathrm{Fe}}_y\right)}_2{\mathrm{Zn}}_{20}$. The inset shows the $x$ dependence of the paramagnetic Curie temperatures and the effective magnetic moments.

Figure 2

Temperature dependence of the AC magnetic susceptibility ${\chi }_{\mathrm{AC}}\left(T\right)$ in $\mathrm{Yb}{\left({\mathrm{Co}}_{1-x}{\mathrm{Ni}}_x\right)}_2{\mathrm{Zn}}_{20}$ and $\mathrm{Yb}{\left({\mathrm{Co}}_{1-y}{\mathrm{Fe}}_y\right)}_2{\mathrm{Zn}}_{20}$ ($y=0.070$) for $B\left|\right|$ [110] $=$ 0.2 T at temperatures down to 0.024 K. All the data are vertically shifted for clarify. The inset shows $T_{\chi \max }$ in the AC magnetic susceptibility (left) and $T_{\rho \max }$ in the electrical resistivity (right) obtained from Fig. 3 as a function of $x$.

Figure 3

Temperature dependence of the resistivity $\rho \left(T\right)$ of $\mathrm{Yb}{\left({\mathrm{Co}}_{1-x}{\mathrm{Ni}}_x\right)}_2{\mathrm{Zn}}_{20}$ and $\mathrm{Yb}{\left({\mathrm{Co}}_{1-y}{\mathrm{Fe}}_y\right)}_2{\mathrm{Zn}}_{20}$ ($y=0.070$) for the current $I\left|\right|$ [110] from 300 to 0.04 K. The inset represents the low-temperature data of $\rho \left(T\right)$ for $x=0.056$ and $y=0.070$ showing a maximum at 0.12 and 0.2 K, respectively.

Figure 4

(a) Temperature dependence of the specific heat divided by temperature, $C/T$, for $\mathrm{Yb}{\left({\mathrm{Co}}_{1-x}{\mathrm{Ni}}_x\right)}_2{\mathrm{Zn}}_{20}$ with $x=$ 0, 0.056, 0.126 and $\mathrm{Yb}{\left({\mathrm{Co}}_{1-y}{\mathrm{Fe}}_y\right)}_2{\mathrm{Zn}}_{20}$ ($y=0.070$) in zero field. The solid and the broken lines show the data of ${\mathrm{YbCo}}_2{\mathrm{Zn}}_{20}$ and ${\mathrm{LuCo}}_2{\mathrm{Zn}}_{20}$ referred from Ref. [14], respectively. (b) Field dependence of the magnetic entropy of $4f$ electrons $S_{4\mathrm{f}}\left(B\right)$ at 0.5 K for $B\left|\right|$ [100]. (c) Temperature dependence of the magnetic entropy of $4f$ electrons $S_{4\mathrm{f}}\left(T\right)$ at zero field.