Spin gap and superconducting fluctuations in Bi2Sr2CaCu2O8+delta probed byCu-63 NMR

We report temperature dependence of the Gaussian component of spin-echo dec ay rate (1/T-2G) in under, optimally and overdoped single crystals Bi2Sr2Ca Cu2O8+delta (Bi2212) with the superconducting (SC) transition temperatures T-c = 79, 86, and 77.3 K, respectively. For all the samples, 1/T-2G was fou nd to decrease below a temperature T-c* lower than T* but higher than T-c. Here T* is the temperature below which the "pseudogap" emerges. TI undergoe s a similar variation with T-c as function of doping level. This result is corroborated by those results of nuclear spin-lattice relaxation rate 1/T-1 and the Knight shift reported previously. These anomalies below T-c* are s uggested to be precursive to the SC transition. Based on the NMR results, t hree crossover temperatures, T-mK, T* and T-c* are marked in the phase diag ram on Bi2212. (C) 2000 Elsevier Science Ltd. All rights reserved.