Superconductivity and non-fermi-liquid behavior in the heavy-fermion compound CeCo_1-xNi_xIn₅

The effect of off-plane impurity on superconductivity and non-Fermi-liquid (NFL) behavior in the layered heavy-fermion compound CeCo_1-xNi_xIn₅ is investigated by specific heat, magnetization, and electrical resistivity measurements. These measurements reveal that the superconducting (SC) transition temperature T_c monotonically decreases from 2.3 K (x = 0) to 0.8 K (x = 0.20) with increasing x, and then the SC order disappears above x = 0.25. At the same time, the Ni substitution yields the NFL behavior at zero field for x = 0.25, characterized by the -ln T divergence in specific heat divided by temperature, C_p/T, and magnetic susceptibility, M/B. The NFL behavior in magnetic fields for x = 0.25 is quite similar to that seen at around the SC upper critical field in pure CeCoIn₅, suggesting that both compounds are governed by the same antiferromagnetic quantum criticality. The resemblance of the doping effect on the SC order among Ni-, Sn-, and Pt-substituted CeCoIn₅ supports the argument that the doped carriers are primarily responsible for the breakdown of the SC order. The present investigation further reveals the quantitative differences in the trends of the suppression of superconductivity between Ce(Co,Ni)In₅ and the other alloys, such as the rates of decrease in T_c, dT_c/dx, and specific heat jump at T_c, d(ΔC_p/T_c)/dx. We suggest that the occupied positions of the doped ions play an important role in the origin of these differences.