MEASUREMENTS OF THE PARACONDUCTIVITY IN THE A-DIRECTION OF UNTWINNED Y1BA2CU3O7-DELTA SINGLE-CRYSTALS

We report detailed experimental results of the paraconductivity in the a-direction, DELTAsigma(a), of two high-quality Y1Ba2Cu3O7-delta sing le crystals having a large (approximately 0.3 mm2) untwinned region in their centers, and with transition temperatures of 90.0 K and 90.8 K. The exceptional sharpness of their resistive transitions (their upper half widths are less than 0.15 K) allows us to obtain quantitative in formation of DELTAsigma(a)(epsilon), which is not affected by the CuO chains, up to reduced temperatures, epsilon, of the order of 10(-3). T he general scenario compatible with our experiments consists of the as sumption of (1): A mean field-like region (MFR) up to epsilon almost-e qual-to 2 x 10(-2), where DELTAsigma(a)(epsilon) may be explained in t erms of the Lawrence-Doniach-like (LD) approach by taking into account the existence of two Josephson-coupled CuO2 superconducting planes pe r unit cell. The characteristic superconducting length amplitudes in t his MFR are xi(ab)(0) = (1.1 +/- 0.2) nm, and xi(c)(0) = (0.12 +/- 0.0 2) nm. (2) In agreement with our earlier qualitative results, below ep silon almost-equal-to 2 X 10(-2) the measured rho(a)(T) separates from the mean-field behavior. Although so close to the transition we canno t exclude the influence of small stoichiometric inhomogeneities, these DELTAsigma(a) (epsilon) data for epsilon less than or similar to 2 x 10(-2) may be explained, also on a quantitative level, on the grounds of the 3D-XY model, with a critical exponent of x= -1/3 for DELTAsigma (a)(epsilon), and x= -2/3 for fluctuation-induced diamagnetism. This s cenario corresponds to one complex component order parameter, i.e., co nventional 1s0-wave pairing or one complex component unconventional pa iring.