In-plane paraconductivity inLa2−xSrxCuO

Abstract

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.

Diamagnetism above the superconducting transition in underdoped La1.9Sr0.1CuO4Mosqueira1, Dancausa2, Vidal3 2011Phys. Rev. BSelf Cite167290View full textAdd to dashboardCite

“…26 Complementarily, the agreement with the GGL approach was extended to high reduced temperatures, for ε 0.1, by using a total-energy cutoff of the order of ε c ∼ 0.55, this last corresponding to the limit imposed by the uncertainty principle to the shrinkage of the superconducting wave function when the temperature increases. 36 This result further confirms our earlier conclusions, obtained through measurements of both the diamagnetism above T c , 3,13-16 and of the in-plane paraconductivity, 38 that the onset temperature, T c , for the superconducting fluctuations in underdoped cuprates is not affected by the opening of a pseudogap in their normal state. Indirectly, these last results also support recent proposals that the large Nernst signal observed in the normal state in LSCO is not associated with superconducting fluctuations.…”

Structural andTcinhomogeneities inherent to doping inLa2−xSrxMosqueira1, Cabo2, Vidal3 2009Phys. Rev. BSelf Cite2811670View full textAdd to dashboardCite

“…8,9 The anomalies observed in some cases, in particular under low magnetic fields, could be easily explained in terms of T c -inhomogeneities with long characteristic lengths [much larger than ξ ab (0)], which do not directly affect the superconducting transition own nature. 7,10 Measurements of the in-plane paraconductivity 11 and of the heat capacity around T c 12 in these underdoped cuprates, together with analyses of the effects of T c -inhomogeneities with long characteristic lengths and of the background choice, 7,13,14 fully agree with these conclusions. In particular, they confirm that above a reduced temperature of the order of ε c = 0.5 the superconducting fluctuation effects vanish, as predicted by taking into account the limits imposed by the uncertainty principle to the shrinkage of the superconducting wavefunction.…”

Paraconductivity in layered cuprates behaves as if due to pairing of nearly free quasiparticles

“…4 If this were the case, however, the most anisotropic cuprates ͓e.g., Bi 2 Sr 2 CaCu 2 O 8+␦ ͑BSCCO͔͒ should display an exponential temperature dependence in the enhancement of conductivity due to SC fluctuations at temperatures T Ͼ T c ͑the so-called paraconductivity͒ associated with vortical fluctuations, typical of a Kosterlitz-Thouless transition in two dimensions. 5 Instead, it is well documented [6][7][8][9][10][11][12][13][14][15][16][17] that paraconductivity in all the families of cuprates is fully accounted for by the standard AslamazovLarkin ͑AL͒ theory 18,19 based on Gaussian SC fluctuations, with the real and imaginary parts of the SC order parameter ⌬ fluctuating around zero. While YBa 2 Cu 3 O 7−x is less anisotropic and displays the AL behavior characteristic of threedimensional systems, all other compounds, which have a more anisotropic structure, display the standard AL behavior for two-dimensional ͑2D͒ systems.…”