Observation of Griffiths phase at an anti-ferromagnetic quantum critical point in Ce(Cu_1−xCo_x)₂Ge₂

A comprehensive study has been carried out on polycrystalline samples of Ce(Cu_1-xCo_x)₂Ge₂ by means of electrical resistivity, magnetic susceptibility, specific heat and thermo electric power measurements. The long-range antiferromagnetic order, at T_N = 4.1 K in CeCu₂Ge₂, is suppressed by non-iso-electronic cobalt (Co) doping at a critical value of the concentration x_c = 0.6, accompanied by non-Fermi-liquid behavior. A power-law dependence of heat capacity and susceptibility i.e. C(T )/T and χ(T ) ∝ T ^-1+λ down to 0.4 K are observed around x_c = 0.6 which is compatible with the quantum Griffiths phase scenario and attributed to an antiferromagnetic quantum critical point. Our results on Ce(Cu_1-xCo_x)₂Ge₂ manifest a complex magnetic phase diagram, where we have observed a continuous decrease of T_N with large negative slope down to T_N = 0.83 K at x = 0.1, and then a comparatively small negative slope takes over. We conclude that the rapid decrease of T_N upon Co-doping is mainly due to carrier concentration change and associated change of T_K and T_RKKY . We have not seen any superconducting phase in the quantum critical regime down to 0.4 K

Reference: Rajesh Tripathi, Debarchan Das, C. Geibel, S. K. Dhar, and Z. Hossain, Phys. Rev. B, 98, 165136 (2018)