Vortices are represented by circles that are colored according to the instantaneous vortex velocity. Blue indicates vortices that are stationary inside pinning sites. Yellow vortices are moving at a moderate rate, and red vortices are moving quickly. An external field is being very slowly increased by adding a single vortex to the left edge of the sample each time the sample reaches mechanical equilibrium. All vortex motion is a result of vortex-vortex interactions and the gradient in vortex density; no uniform current is applied. Avalanche disturbances propagate from the dense left edge of the sample to the relatively less dense right edge. Vortices that reach the right edge of the sample are removed in order to maintain a constant average flux profile.
In most avalanches, chains of vortices move from pinning site to pinning site, with each vortex moving into the site vacated by a vortex to its right. The winding chains extend down the flux gradient and appear at different locations in different avalanches. Chain size varies from sample-spanning to just three or four vortices. Note that the avalanche disturbances propagate neither instantly nor at constant speed through the sample, but that they pulse through the sample, with most of the vortices involved stopping exactly one pinning site to the right of their initial positions. Events with longer lifetimes consist of more than one pulse of motion.
This sample has a high density of weak randomly placed pinning sites, with a pinning density of 5.93/lambda^2, and pinning strengths uniformly distributed from fp= 0.2 to 1.0. The coloring of instantaneous vortex velocities has been scaled according to the maximum vortex speed attained during the movie; vortex velocities in this sample are typically lower than velocities in the sample with strong pinning sites shown in the other movie. Since the pinning is weak, avalanche motion tends to be broad and vortices around a moving vortex chain are likely to also move.
Created by: Cynthia Olson and Jared Groth
Last Modified: 12/18/96