Evidence for nonuniversal behavior of paraconductivity caused by predominant short-wavelength Gaussian fluctuations inYBa2Cu

Abstract

We report on in-plane paraconductivity measurements in thin YBa2Cu306 9 films. Our analysis of the data shows that the temperature dependence of paraconductivity is affected by lattice disorder and deviates at all temperatures from the universal power laws predicted by both scaling and mean-field theories.This gives evidence for the absence of critical fluctuations and for the failure of the Aslamazov-Larkin universal relation between critical exponent and dimensionality of the spectrum of Gaussian fluctuations. We account quantitatively for the data within the experimental error by introducing a shortwavelength cutoff into this spectrum. This implies that three-dimensional short-wavelength Gaussian fluctuations dominate in YBa2Cu306 9 and suggests a rapid attenuation of these fluctuations with decreasing wavelength in short-coherence-length systems as compared to the case of the conventional Ginzburg-Landau theory.

Critical fluctuations and pseudogap observed in the microwave conductivity ofBi2Sr2CaCuPeligrad1, Mehring2, Dulčić3 2004Phys. Rev. B122100View full textAdd to dashboardCite

“…While classical low temperature superconductors exhibited only Gaussian type fluctuations, 3 it was estimated that high-T c superconductors could give experimental access to a study of critical fluctuations 4 . Yet, early studies of the dc fluctuation conductivity 5,6 were interpreted within the Gaussian behavior, except for a narrow region (fraction of a degree) around T c which was left out as possibly critical. Later on, the penetration depth measurements in Y Ba 2 Cu 3 O 7−δ revealed critical behavior as wide as ±10K from T c , 7 belonging to the universality class 3D XY.…”

“…More recent measurements of thermal expansivity 8,9 and two-coil inductive measurements 10 have confirmed this wide range of critical fluctuations, while dc fluctuatuation conductivity measurements still claimed very narrow critical regions 11,12,13,14,15,16 . A major problem in the analysis of a dc fluctuation conductivity is to account for the short wavelength cutoff, 5,6,11,15 and energy cutoff 17,18 at higher temperatures. These effects are so strong that other inherent properties of the fluctuation conductivity can be obscured.…”

Fluctuation conductivity in Y–Ba–Cu–O films with artificially produced defects

“…This kind of FC behavior has been shown to be typical for badly structured high-T c oxides [2]. However, in most of the previous papers devoted to the problem, FC fitted by the LD model alone is reported [4][5][6][7][8][9], suggesting the presenñå of different structural defects in the samples studied. The conclusion is corroborated by measurements of the critical current densities j c of thin epitaxial YBCO films, which are known to be at least an order of magnitude larger than that for high-quality YBCO single crystals [18].…”

“…Measurement of the fluctuation conductivity (FC) is known to be a powerful method of getting reliable information about the normal charge scattering and superconducting coupling mechanisms in HTSC just in the PG temperature region [4][5][6][7][8][9][10][11]. Using FC analysis, such microscopic parameters as the coherence length along the c axis, x c ( ) 0 , and the phase relaxation time of fluctuating pairs, t j (100 K), can be determined.…”

“…As T is lowered, r xx T ( ) deviates downward from the T-linear dependence at some representative temperature T *0 >> T c , giving rise to the excess conductivity which is usually considered as a pseudogap [1,15]. However, in the temperature interval T c < T < T ñ0 the excess conductivity is well described by the conventional fluctuating theories [12][13][14] and is usually treated as s¢( ) T [4][5][6][7][8][9][10]16]. T c0 = (100 ± 5) K is the temperature which determines the pair-breaking parameter d th = = -( ) T T /T c c c 0 [12,13].…”