Change of the Ground State upon Hole Doping Unveiled by Ni Impurity in High-Tc Cuprates

The electronic ground state in high-T-c cuprates where the superconducting state is suppressed by Ni Substitution has been investigated in La2-xSrxCu1-yNiyO4 from the specific heat and muon spill relaxation measurements. It has been found that the ground state changes from a magnetically ordered state with the strong hole-trapping by Ni to a metallic state with the Kondo effect of Ni with increasing hole-concentration. Moreover, the analysis of the results has revealed that a phase separation into the magnetically ordered phase and the metallic phase occurs around the boundary of two phases.