Evidence of two-dimensional Josephson strings in high-purity YBa2Cu3O7-delta single crystals

A strange oscillating magnetization appears in high purity YBa2Cu3O7-delta single crystals fur a parallel to the lavers applied field. For these magne tic oscillations few things are yet known. Our results over an extended fie ld and temperature regime show that they are actually evidence of the two-d imensional vortices existing at low enough temperatures. We demonstrate tha t the oscillations are a reliable and accurate tool for extracting crucial parameters in the vortex matter study. And, surprisingly, our data reveal t hat for a parallel field, the temperature dependence of thr magnetization c ontradicts the present theoretical expectations. [S0163-1829(99)13621-X].