Hole trapping by Ni, Kondo effect, and electronic phase diagram in nonsuperconducting Ni-substituted La_2-xSr_xCu _1-yNi_yO₄

In order to investigate the electronic state in the normal state of high- T_c cuprates in a wide range of temperature and hole concentration, specific-heat, electrical-resistivity, magnetization, and muon-spin-relaxation measurements have been performed in nonsuperconducting Ni-substituted La _2-xSr_xCu_1-yNi_yO₄ where the superconductivity is suppressed through the partial substitution of Ni for Cu without disturbing the Cu-spin correlation in the CuO₂ plane so much. In the underdoped regime, it has been found that there exist both weakly localized holes around Ni and itinerant holes at high temperatures. With decreasing temperature, all holes tend to be localized, followed by the occurrence of variable-range hopping conduction at low temperatures. Finally, in the ground state, it has been found that each Ni²⁺ ion traps a hole strongly and that a magnetically ordered state appears. In the overdoped regime, on the other hand, it has been found that a Kondo-type state is formed around each Ni²⁺ spin at low temperatures. In conclusion, the ground state of nonsuperconducting La_2-xSr_xCu_1-yNi _yO₄ changes upon hole doping from a magnetically ordered state with the strong hole trapping by Ni²⁺ to a metallic state with Kondo-type behavior due to Ni²⁺ spins, and the quantum phase transition is crossoverlike due to the phase separation into short-range magnetically ordered and metallic regions.