Fragile superheavy Fermi liquid in YbCo2Zn20

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 $\ensuremath{\rho}(T)$, magnetic susceptibility $\ensuremath{\chi}(T)$, and heat capacity $C(T)$ in single crystals of $\mathrm{Yb}{({\mathrm{Co}}_{1\ensuremath{-}x}{\mathrm{Ni}}_x)}₂{\mathrm{Zn}}₂₀$ ($x\ensuremath{\le}0.126$) and $\mathrm{Yb}{({\mathrm{Co}}_{1\ensuremath{-}x}{\mathrm{Fe}}_y)}₂{\mathrm{Zn}}₂₀$ ($y\ensuremath{\le}0.07$). While pure ${\mathrm{YbCo}}₂{\mathrm{Zn}}₂₀$ displays maxima in $\ensuremath{\rho}(T)$ and $\ensuremath{\chi}(T)$, ascribed to an enhanced crystal electric field (CEF) degeneracy, the maxima are suppressed by small amounts ($\ensuremath{\approx}6$ 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.