Fragile Superheavy Fermi Liquid In Ybco2zn20

The cubic Kondo lattice YbCo2Zn20 is one of the heaviest known Fermi liquids. We have measured the low-temperature electrical resistivity rho(T), magnetic susceptibility chi(T), and heat capacity C(T) in single crystals of Yb (Co1-xNix)(2)Zn-20 (x <= 0.126) and Yb(Co1-xFey)(2)Zn-20 (y <= 0.07). While pure YbCo2Zn20 displays maxima in rho(T) and chi(T), ascribed to an enhanced crystal electric field (CEF) degeneracy, the maxima are suppressed by small amounts (approximate to 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 YbCo2Zn20 is closely linked to an enhanced CEF degeneracy.