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We have studied in fields up to 60T the variation of the transverse magnetoresistance (MR) of underdoped YBCO_{6.6} crystals either pure or with T_ reduced down to 3.5K by electron irradiation [1]. We show that the normal state MR is restored above a threshold field H_ì(T), which is found to vanish at T_ì>>T_. This allows us to evidence a (H,T) range where superconductivity survives at least as fluctuations. In the pure YBCO_{6.6} sample a 50 Tesla field is already required to completely suppress the superconducting fluctuations at T_. When T_ is decreased by disorder, we found that the fluctuation range expands significantly as T_ì is only slightly depressed. This T_ì behaves similarly versus defect content as the onset temperature T_{í} of the Nernst signals measured on the same samples [2] which indicates that the T_ decrease is partly due to the loss of the phase coherence. Moreover we found that T_ì, H_ì(T) and T_{í} which can be related to pair formation are depressed, although moderatly, by the introduction of defects in contrast to.the pseudogap temperature which is known to be insensitive to disorder, showing that these energy scales are not related. It will also be shown, using high fields to suppress superconductivity, that the resistivity upturns which are often interpreted as signatures of a Metal Insulator Crossover appear with introduction of controlled disorder [3], and are in low Tc cuprate families, associated with inherent specific defects responsible for their low optimal T_ values. Altogether these studies allow us to demonstrate that the so called "generic" (T,n_) phase diagram of cuprates has to be represented in 3D with disorder as a third axis parameter. [1] F. Rullier-Albenque et al, Phys.rev. Lett. 99, 027003 (2007). [2] F. Rullier-Albenque et al., Phys. Rev. Lett. 96, 067002 (2006). [3] F. Rullier-Albenque et al., Europhys. Lett. 81, 37008 (2008). |
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