Total suppression of superconductivity by high magnetic fields in YBa2Cu3O6.6

We have studied the variation of transverse magnetoresistance of underdoped YBCO6.6 crystals, either pure or with reduced T-c down to 3.5 K by electron irradiation, in fields up to 60 T. We find evidence that the superconducting fluctuation contribution to the conductivity is suppressed only above a threshold field H-c'(T), which is found to vanish at T-c' > T-c. In the pure YBCO6.6 sample, H-c' is already 50 T at T-c. We find that increasing disorder weakly depresses H-c'(0), T-c', and T-nu, the onset of the Nernst signal. Thus, these energy scales appear more characteristic of the 2D local pairing than the pseudogap temperature which is not modified by disorder.