Three-dimensional XY critical fluctuations of the dc electrical conductivity in Bi2Sr2CaCu2O8+delta single crystals

A method is presented to analyze the dc critical fluctuation conductivity D elta sigma of Bi2Sr2CaCu2O8+delta single crystals in zero magnetic field. T wo crossovers are obtained upon approaching T-c; first the "Ginzburg" cross over from the mean-held region to the initial critical regime, where the cr itical exponent a of Delta sigma changes from -1/2 to -2/3, and then, close r to T-c, a crossover of alpha form -2/3 to 1/3 in the critical region. Goo d agreement is found with the expectations from the three-dimensional XY mo del.