NOVEL VORTEX DYNAMICS IN UNTWINNED YBA2CU3O7-DELTA SINGLE-CRYSTALS

Magnetotransport measurements on a clean, untwinned YBa2Cu3O7-delta si ngle crystal show that the vortex dynamics at temperatures just below the vortex lattice melting transition am highly dependent on the type of modulation of the probing current. While in the case of DC current the flux flow is disordered, the ''shaking'' of the vortex lattice by a square-wave current leads to a more uniform vortex motion. A small a symmetry (similar to 10%) in the durations of the positive and negativ e parts of the square-wave period induces periodical oscillations of t he voltage response amplitude. The period of oscillations (similar to 100 s) is the same order of magnitude as the time needed for vortices to cross the sample (transit time). We relate the observed voltage osc illations to a periodically ordered vortex motion.