Resistance anomalies accompanying crossover from heavy-fermion regime to intermediate-valence regime: A study of Cu-Ni substitution and pressure effects on CeCu₂Si₂

We present detailed investigations on the effects of Ni substitution (x) on Ce(Cu_1-xNi_x)₂Si₂ with x < 0.3, and a pressure (p) study for x = 0.04(2), 0.05(2), and 0.09(2) by resistivity measurements. The Cu-Ni substitution suppresses both antiferromagnetic fluctuations and superconductivity in CeCu₂Si₂, and leads to a dramatic crossover from the heavy-fermion (HF) regime to the intermediate-valence (IV) regime with progressive Ni doping. The crossover can be characterized by a marked increase in Kondo temperature evaluated from resistivity, specific heat, and susceptibility measurements. In high-pressure resistivity measurements, the Ni-doped system exhibits similar anomalies to those observed in CeCu₂Si₂, namely, T-linear dependence, an enhancement of residual resistivity, and a marked decrease in the A coefficient of the T²-resistivity. Furthermore, we observed an unexpected upturn anomaly in resistivity in the Ni-doped system under high pressure. By constructing the x-p phase diagram of Ce(Cu_1-xNi_x)₂Si₂, we demonstrate that the crossover from the HF regime to the IV regime can be associated with these characteristic anomalies in resistivity.