Evolution of quantum criticality in the system CeNi₉Ge ₄

The heavy fermion system CeNi₉Ge₄ exhibits a paramagnetic ground state with remarkable features such as: a record value of the electronic specific heat coefficient in systems with a paramagnetic ground state, γ C/T ≃ 5.5 J/molK² at 80 mK, a temperature-dependent Sommerfeld-Wilson ratio, R = χ/γ, below 1 K and an approximate single ion scaling of the 4f-magnetic specific heat and susceptibility. These features are related to a rather small Kondo energy scale of a few Kelvin in combination with a quasi-quartet crystal field ground state. Tuning the system towards long range magnetic order is accomplished by replacing a few at.% of Ni by Cu or Co. Specific heat, susceptibility and resistivity studies reveal T_N ∼ 0.2 K for CeNi₈CuGe₄ and T_N ∼ 1K for CeNi₈CoGe₄. To gain insight whether the transition from the paramagnetic NFL state to the magnetically ordered ground state is connected with a heavy fermion quantum critical point we performed specific heat and ac susceptibility studies and utilized the μSR technique and quasi-elastic neutron scattering.