Superconducting fluctuations, pseudogap and phase diagram in cuprates

We report transport measurements using pulsed magnetic fields to suppress the superconducting fluctuations (SCF) conductivity in a series of YBa 2 Cu 3 O 6+ x samples. These experiments allow us altogether to measure the temperature T c ′ at which SCF disappear, and the pseudogap temperature T * . While the latter are consistent with previous determinations of T * , we find that T c ′ is slightly larger than similar data taken by Nernst measurements. A careful investigation near optimal doping shows that T * becomes smaller than T c ′, which is an unambiguous evidence that the pseudogap cannot be assigned to preformed pairs. Studies of the incidence of disorder on both T c ′ and T * allow us to propose a phase diagram including disorder which explains most observations done in other cuprate families, and to discuss the available knowledge on the pseudogap line in the phase diagram.