MEASUREMENTS OF THE FLUCTUATION-INDUCED MAGNETOCONDUCTIVITY IN THE A-DIRECTION OF AN UNTWINNED Y1BA2CU3O7-DELTA SINGLE-CRYSTAL IN THE WEAK MAGNETIC-FIELD LIMIT

Measurements of the fluctuation-induced magnetoconductivity in the a-d irection of an untwinned Y1Ba2Cu3O7-delta single crystal, not affected by the presence of CuO chains, are presented. The measurements were p erformed with the applied magnetic field, H, perpendicular to the ab-p lanes, and with H less-than-or-equal-to 1 T, which corresponds to the weak magnetic field limit even for reduced temperatures, epsilon, of t he order of 10(-2). In the mean-field region, i.e., for 10(-2) less th an or similar to epsilon less than or similar to 10(-1), these data ma y be explained in terms of direct thermal order-parameter fluctuations , in agreement with our previous paraconductivity and fluctuation-indu ced diamagnetism measurements in the same sample. These results sugges t that the pair-breaking parameter, 6, is less than 5 X 10(-2) at T=10 0 K. This upper limit not only leads to the absence of appreciable Mak i-Thompson effects in copper oxide superconductors (HTSC's), but also has implications in other central open HTSC problems. By using the BCS -like expressions to relate delta and the Cooper-pair relaxation time, tau(phi), we obtain, at T=100 K, tau(phi) less than or similar to 10( -15) s in the clean limit and tau(phi) less than or similar to 10(-14) s in the dirty limit. This last value is comparable with the expected electronic-scattering time.