Large pulsed magnetic fields up to 60 Tesla are used to suppress the
contribution of superconducting fluctuations (SCF) to the ab-plane conductivity
above Tc in a series of YBa2Cu3O(6+x). These experiments allow us to determine
the field H'c(T) and the temperature T'c above which the SCFs are fully
suppressed. A careful investigation near optimal doping shows that T'c is
higher than the pseudogap temperature T*, which is an unambiguous evidence that
the pseudogap cannot be assigned to preformed pairs. Accurate determinations of
the SCF contribution to the conductivity versus temperature and magnetic field
have been achieved. They can be accounted for by thermal fluctuations following
the Ginzburg-Landau scheme for nearly optimally doped samples. A phase
fluctuation contribution might be invoked for the most underdoped samples in a
T range which increases when controlled disorder is introduced by electron
irradiation. Quantitative analysis of the fluctuating magnetoconductance allows
us to determine the critical field Hc2(0) which is found to be be quite similar
to H'c(0) and to increase with hole doping. Studies of the incidence of
disorder on both T'c and T* allow us to propose a three dimensional phase
diagram including a disorder axis, which allows to explain most observations
done in other cuprate families.Comment: Paper presented at the "Eurasia-Pacific Summer School & Conference on
Correlated Electrons", Turunc-Marmaris, Turkey, July 4-14, 201
