Evolution of quantum criticality in CeNi₉₋ₓCuₓGe₄

Abstract

Crystal structure, specific heat, thermal expansion, magnetic susceptibility and electrical resistivity studies of the heavy fermion system CeNi9−xCuxGe4 (0≤x≤1) reveal a continuous tuning of the ground state by Ni/Cu substitution from an effectively fourfold-degenerate non-magnetic Kondo ground state of CeNi9Ge4 (with pronounced non-Fermi-liquid features) towards a magnetically ordered, effectively twofold-degenerate ground state in CeNi8CuGe4 with TN = 175 ± 5 mK. Quantum critical behavior, C/T\propto \chi \propto {-}\ln {T} , is observed for x\cong 0.4 . Hitherto, CeNi9−xCuxGe4 represents the first system where a substitution-driven quantum phase transition is connected not only with changes of the relative strength of the Kondo effect and RKKY interaction, but also with a reduction of the effective crystal field ground state degeneracy.