Crossover between Aslamazov-Larkin and short-wavelength fluctuation regimes in high-temperature-superconductor conductivity experiments

We present paraconductivity measurements in three different high-temperature superconductors: a melt-textured YBa2CU3O7 sample, a Bi2Sr2CaCU2O8 epitaxial thick film, and a highly textured Bi2Sr2Ca2Cu3O10 tape. The crossovers between different temperature regimes in excess conductivity have been analyzed. The Lawrence-Doniach (LD) crossover, which separates the two-dimensional and three-dimensional regimes, shifts from lower to higher temperatures as the compound anisotropy decreases. Once the LD crossover is overcome, the fluctuation conductivity of the three compounds shows the same universal behavior: for epsilon = lnT/T-c > 0.23 all the curves bend down according to the 1/epsilon(3) law. This asymptotic behavior was theoretically predicted previously for the high-temperature region where the short-wavelength fluctuations become important.