Hole-trapping and Kondo effect in Ni-substituted La_2-xSr _xCu_1-yNi_yO₄ with x = 0.08-0.30

We have measured the specific heat in a wide range of hole-concentration in partially Ni-substituted La_2-xSr_xCu_1-yNi _yO₄, in order to investigate Ni-substitution effects on the electronic state. For 10% Ni-substituted samples of y = 0.10, it has been found that the electronic specific heat divided by temperature, Cel/T, decreases with decreasing temperature at low temperatures for x≤0.20, which is concluded to be due to a decrease in the effective hole-concentration caused by strong hole-trapping by Ni²⁺ ions. For x>0.20, on the other hand, an increase in Cel/T with decreasing temperature has been observed, which is due to the Kondo effect. In fact, both a logarithmic temperature-dependence of the electrical resistivity and a negative magnetoresistance have been observed for x>0.20. It has been concluded that the ground state of non-superconducting La_2-xSr_xCu_1-yNi_yO₄ changes upon hole doping from a magnetically ordered state to a metallic state and that the quantum phase transition is crossover-like due to the phase separation in the overdoped regime.