Critical points in the cuprate phase diagram

We discuss how the electronic ground state evolves with doping x in the hole-doped high-Tc cuprates based on the recent spectroscopic experiments which have given us detailed information in the real and momentum space. Focuses are on the lightly doped regime starting from x=0, the insulator–superconductor transition at x∼0.07, and near the optimal doping x∼0.18. We propose that x∼0 is a critical point where a Mott-insulator-to-‘nodal-metal’ transition takes place upon doping into the parent antiferromagnetic insulator. With further doping high-Tc superconductivity is adruptly switched on at x∼0.07 which seems to be associated with the flat band near the antinodal region crossing the Fermi level. According to the recent STM study, the superconducting ‘phase’ is microscopically phase separated. We address the question whether or not this phase separation is related with a proposed quantum critical point at x∼0.18.