DIMENSIONAL CROSSOVER OF QUANTUM COLLECTIVE CREEP IN BI2SR2CACU2O8+Y

We studied magnetic field dependence of magnetization relaxation in Bi 2Sr2CaCu2O8+y single crystals down to 500 mK. In the quantum collectiv e creep (QCC) regime at low temperatures, the normalized relaxation ra te S as a function of field shows a characteristic dip at low fields. We ascribe the origin of this dip to the 2D-3D dimensional crossover i n the vortex system. At higher temperatures, the field dependence of S is more complicated. We discuss the field dependence of S based on tw o competing mechanisms due to QCC and thermally activated motion of vo rtices.