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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Using the Green's function of the 3D heat equation, we develop an analytical account of the thermal behaviour of superconducting films subjected to electrical currents larger than their critical current in the absence of an applied magnetic field. Our model assumes homogeneity of films and current density, and besides thermal coefficients employs parameters obtained by fitting to experimental electrical field -current density characteristics at constant bath temperature. We derive both a tractable dynamic equation for the real temperature of the film up to the supercritical current density J * (the lowest current density inducing transition to the normal state), and a thermal stability criterion that allows prediction of J * . For two typical YBCO films, J * predictions agree with observations to within 5%. These findings strongly support the hypothesis that a currentinduced thermal instability is generally the origin of the breakdown of superconductivity under high electrical current densities, at least at temperatures not too far from Tc.

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