OSCILLATORY DYNAMICS OF THE DRIVEN VORTEX SOLID IN YBA2CU3O7-DELTA SINGLE-CRYSTALS

Investigations have been carried out on the dynamics of the vortex sol id in clean detwinned YBa2Cu3O7-delta single crystals at temperatures and fields just below the thermodynamic melting line. It has been obse rved that the dynamics of this system is crucially dependent upon the modulation form of the driving current. Very different responses were obtained in the case of double-polarity square-wave and single-polarit y square-wave currents, and these responses were attributed to relativ ely ''ordered'' and ''disordered'' dynamic states, respectively. In pa rticular, we present investigations on a type of dynamic vortex state that manifested itself as a large amplitude oscillation in the voltage response amplitude (V-0 varied by as much as a factor of 3). This osc illatory state arose when a periodically modulated drive current (f(m) similar to 100 Hz) was applied, which was asymmetric either in terms o f the positive and negative going parts of its period or in terms of i ts amplitude. We have found that the frequency of these V-0(t) oscilla tions (typically in the range 0.01-0.1 Hz) did not depend upon the fre quency of the drive but that it was proportional to the asymmetry of t he current modulation. In this paper we explore the range of fields, t emperatures, applied current amplitudes, and asymmetries over which th e oscillatory response was observed. We have shown that the period of the oscillations corresponds approximately to the time that it would t ake for the vortex system to drift coherently twice across the width o f the sample. Using this and other features of our observations, we pr opose an interpretation of the oscillatory effects based upon the idea of defect superstructure that drifts across the sample with the same velocity as that of individual vortices. [S0163-1829(98)03625-X].