Breakdown by a magnetic field of the superconducting fluctuations in the normal state in Pb_(1-x)In_x alloys

The effects induced on the magnetization by coherent fluctuating Cooper pairs in the normal state have been measured in Pb_{1-x}In_x alloys up to high magnetic fields, of amplitudes above Hc2(0), the upper critical field extrapolated to T=0K. Our results show that in dirty alloys these superconducting fluctuation effects are, in the entire H-T phase diagram above Hc2(T), independent of the amount of impurities and that they vanish when H~1.1Hc2(0). These striking results are consistent with a phenomenological estimate that takes into account the limits imposed by the uncertainty principle to the shrinkage, when H increases, of the superconducting wave function.