Disorder, metal-insulator crossover and phase diagram in high-T c cuprates

We have studied the influence of disorder induced by electron irradiation on the normal-state resistivities ρ( T ) of optimally and underdoped YBa 2 Cu 3 O x single crystals, using pulsed magnetic fields up to 60 T to completely restore the normal state. We evidence that point defect disorder induces low- T upturns of ρ( T ) which saturate in some cases at low T in large applied fields as would be expected for a Kondo-like magnetic response. Moreover, the magnitude of the upturns is related to the residual resistivity, that is to the concentration of defects and/or their nanoscale morphology. These upturns are found quantitatively identical to those reported in lower- T c cuprates, which establishes the importance of disorder in these supposedly pure compounds. We therefore propose a realistic phase diagram of the cuprates, including disorder, in which the superconducting state might reach the antiferromagnetic phase in the clean limit.