Non-Fermi-Liquid Behavior in CePt1+xSi1-x and CePtSi1-yGey

The heavy-fermion system CePtSi is located close to a magnetic instability. In order to study the influence of composition on the physical properties of CePtSi we prepared slightly off-stoichiometric CePt1+xSi1-x polycrystals. CePt1+xSi1-x with x=-0.04 exhibits typical non-Fermi-liquid behavior (C/T(alpha) lnT, (chi)(alpha) T^-0.21, (rho)(alpha) T^1.5). A scaling analysis of the thermodynamic quantities at magnetic fields up to 10T and temperatures up to 6K points to a quantum phase transition described by some unknown non-Gaussian fixed point. In contrast substitution of Si by Ge leads to a quantum phase transition involving weak-coupling spin fluctuations for CePtSi0.89Ge0.11, which can be described within the framework of the self-consistent renormalization theory.