In-plane paraconductivity in La2−xSrxCuO4 thin film superconductors at high reduced temperatures: Independence of the normal-state pseudogap

Abstract

The in-plane resistivity has been measured in La2−xSrxCuO4 (LSCO) superconducting thin films of underdoped (x=0.10,0.12), optimally doped (x=0.15), and overdoped (x=0.20,0.25) compositions. These films were grown on (100)SrTiO3 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 Δσab) was extracted from these data in the reduced-temperature range 10−2≲ɛ≡ln(T/Tc)≲1. This Δσab(ɛ) was then analyzed in terms of the mean-field-like Gaussian-Ginzburg-Landau (GGL) approach extended to the high-ɛ 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, ξc(0), is constant for 0.10<~x<~0.15 [ξc(0)≃0.9Å], and decreases with increasing x in the overdoped range [ξc(0)≃0.5Å for x=0.20 and ξc(0)∼0Å for x=0.25]. 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 ɛc remains unchanged well within the experimental uncertainties. Our results strongly suggest that at all temperatures above Tc, 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-ɛ 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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