Critical dynamics associated with dynamic disordering near the depinning transition in different vortex phases

Memory formation and evolution of the vortex configuration associated with random organization

“…To realize random organization, initially a highly disordered vortex configuration was prepared for each measurement. This has been accomplished by driving the vortices with a small dc current yielding 100 μV, which corresponds to plastic flow, for a sufficiently long time to reach the steady state, and then the vortex configuration was frozen by abruptly switching off the dc current [27,38]. The thus-prepared highly disordered initial vortex assembly was periodically driven by square ac currents giving rise to displacement amplitudes in the range d=0.6-20 μm (i.e.…”

“…For this purpose, we have carried out two-step measurements. First, in what we call an input (or training [25]) experiment, the vortex distribution with a highly disordered configuration was prepared by a dc current [27], and then the vortices were periodically driven by an ac current I ac . The displacement amplitude d d inp º ( )of the ac drive was fixed to be d 3 inp = μm, which is slightly larger than d 0.4 c = μm, by adjusting the frequency f of I ac .…”

“…The a-Mo x Ge 1−x film with a thickness of 350 nm was prepared by rf sputtering on a silicon substrate mounted on a water cooled rotating copper stage [13,19,23,24,27,28]. The superconducting transition temperature in zero field (B=0) at which the resistivity vanishes is 6.3 K. The width of the sample is 0.3 mm and the vortices are driven in the direction of the sample width.…”

“…For this purpose, we have conducted two-step measurements [25,26] for a stripshaped a-Mo x Ge 1−x film [5,23] where the reordering of the vortex distributions is exclusively determined by the local shear due to the random pinning potential and ac drive. In the first input (or training) experiment, we initially prepared the most disordered vortex distribution [27], which was subsequently subjected to an ac drive with a moderately small displacement amplitude d inp slightly larger than d c . After being subjected to the periodic shear of n cycles with d inp , the vortex configuration was frozen by abruptly switching off the driving current [22].…”

Competition between dynamic ordering and disordering for vortices driven by superimposed ac and dc forces

“…To examine how random organization proceeds with increasing the number of shear cycles n, we have recently performed [30] two-step measurements [6,7] for the vortex system in the strip-shaped a-Mo x Ge 1−x film [5,22]. In the first experiment, which we name an 'input' experiment, a highly disordered initial vortex configuration [31] was subjected to ac shear with a small displacement d inp just above the critical one d c for RIT. Applying the periodic shear for n cycles, we froze the distribution of vortices by abruptly turning off the current that drove the vortices [31][32][33][34].…”