MEASUREMENTS OF FLUCTUATION-INDUCED DIAMAGNETISM ABOVE THE SUPERCONDUCTING TRANSITION IN YBA2CU3O7-DELTA SINGLE-CRYSTALS IN LOW MAGNETIC-FIELDS - COMPARISON WITH PARACONDUCTIVITY

We report detailed experimental results on the magnetic susceptibility of high-quality YBa2Cu3O7-delta single crystals for the applied magne tic field H parallel, chi(ab), and perpendicular, chi(c), to the c dir ection. The data were obtained from 5 to 300 K, and with mu0H less-tha n-or-equal-to 0.6 T, for which chi(ab) and chi(c), above the supercond ucting transition correspond to the so-called weak magnetic-field limi t, even for reduced temperatures, epsilon=(T - T(CO))/T(CO), of the or der of epsilon=2X10(-3), where T(CO) is the mean-field-like normal-sup erconducting transition temperature. For T > T(CO), chi(c), is found t o be temperature independent and its average value for the samples mea sured here is (2.7+/-0.5)X10(-5). In contrast, X(ab)(T) is found to be temperature independent, and equal to (5.2+/-1)X10(-5), only above T - T(CO) almost-equal-to 25 K and up to the highest temperature studied here (300 K). Between T(CO) and T - T(CO) almost-equal-to 25 K, chi(a b)(T) is appreciably rounded. The excess diamagnetism above but near T (CO), for H parallel to the c axis, DELTAchi(ab), is analyzed in terms of the existing approaches for independent Gaussian fluctuations of t he superconducting-order-parameter amplitude in layered materials and also compared with the paraconductivity measured in the same samples. The so-called mean-field-like region will correspond to, approximately , 10(-2) less than or similar epsilon less than or similar 0.15. Beyon d the lower epsilon limit, closer to T(CO), both the paraconductivity and DELTAchi(ab)(epsilon) Separate from the mean-field region behavior . Although so close to the transition we cannot exclude the influence of small stoichiometric inhomogeneities, these data for epsilon less t han or similar 10(-2) may be explained on the grounds of the three-dim ensional XY model, with a critical exponent of x = -1/3 for the paraco nductivity and x = -2/3 for the fluctuation-induced diamagnetism. A sc enario compatible with the experimental results is characterized by th e values of the correlation length amplitude in the ab plane and in th e c direction of, respectively, xi(ab) (0) = (1.1+/-0.2) nm and xi(c) (0) = (0.12+/-0.02) nm, two Josephson coupled superconducting planes i n the unit-cell length, and a complex order parameter with two real co mponents, i.e., conventional 1s0-wave pairing or one-complex-component unconventional pairing.