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 H C2 (0), the upper critical field extrapolated to T =0 K. Our results show that in dirty alloys these superconducting fluctuation effects are, in the entire H − T phase diagram above H C2 (T ), independent of the amount of impurities and that they vanish when H ∼ 1.1H C2 (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.

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The in-plane resistivity has been measured in La 2Ϫx Sr x CuO 4 ͑LSCO͒ superconducting thin films of underdoped (xϭ0.10,0.12), optimally doped (xϭ0.15), and overdoped (xϭ0.20,0.25) compositions. These films were grown on (100)SrTiO 3 substrates, and have about 150 nm thickness. The in-plane conductivity induced by superconducting fluctuations above the superconducting transition ͑the so-called in-plane paraconductivity ⌬ ab ) was extracted from these data in the reduced-temperature range 10 Ϫ2 Շϵln(T/T c )Շ1. This ⌬ ab () was then analyzed in terms of the mean-field-like Gaussian-Ginzburg-Landau ͑GGL͒ approach extended to the high-region by means of the introduction of a total-energy cutoff, which takes into account both the kinetic energy and the quantum localization energy of each fluctuating mode. The obtained GGL coherence length amplitude in the c direction, c (0), is constant for 0.10рxр0.15 ͓ c (0)Ӎ0.9 Å͔, and decreases with increasing x in the overdoped range ͓ c (0)Ӎ0.5 Å for xϭ0.20 and c (0)ϳ0 Å for xϭ0.25]. These results strongly suggest, therefore, that the superconducting fluctuations in underdoped and overdoped LSCO thin films may still be described, as in the optimally doped cuprates, in terms of the extended GGL approach; the main effect of doping is simply to change the fluctuations' dimensionality by varying the transversal superconducting coherence length amplitude. In contrast, the total-energy cutoff amplitude c remains unchanged well within the experimental uncertainties. Our results strongly suggest that at all temperatures above T c , including the high reduced-temperature region, doping mainly affects the normal-state properties in LSCO thin films and that its influence on the superconducting fluctuations is relatively moderate; even in the high-region, the in-plane paraconductivity is found to be independent of the opening of a pseudogap in the normal state of the underdoped films. We expect this last conclusion to be independent of the structural details of our films, i.e., applicable also to bulk samples.

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