Experimental observation of the breakdown by a magnetic field of the superconducting fluctuations in the normal state

Abstract

The effects induced on the normal state magnetization by fluctuating Cooper pairs have been measured in ${\mathrm{Pb}}_{1\ensuremath{-}x}{\mathrm{In}}_{x}$ alloys up to magnetic fields above ${H}_{C2}(0)$, the upper critical field extrapolated to $T=0\phantom{\rule{0.3em}{0ex}}\mathrm{K}$. Our results show that in dirty alloys these superconducting fluctuation effects are, in the entire $H\ensuremath{-}T$ phase diagram above ${H}_{C2}(T)$, independent of the amount of impurities and that they vanish when $H\ensuremath{\sim}1.1{H}_{C2}(0)$. These striking results seem to be consistent with the limits imposed by the uncertainty principle to the shrinkage of the superconducting wave function.