Vortex Dynamics: Plastic Flow, Voltage Bursts,
and Avalanches in Superconductors
Franco Nori
University of Michigan, Ann Arbor
Abstract:
Vortices in superconductors exhibit a variety of equilibrium phases,
including liquid, lattice, and glassy states. In addition, vortices
might be driven, producing several dynamical phases with steady
states of plastic and elastic motion. After an introduction to
static vortex physics, we will describe these dynamical phases
and several related open questions of current interest.
We will characterize, and illustrate with several color videos,
dynamical instabilities (i.e., flux avalanches or cascades producing
voltage bursts), as well as the evolution of the topological order and
vortex flow paths ("vortex streets" surrounded by regions of pinned flux).
Computer simulations can be a valuable tool for the analysis of the
microscopic spatio-temporal dynamics of individual flux-lines in
superconductors, lending insight to commonly measured bulk macroscopic
quantities such as magnetization and critical currents.
We have performed extensive simulations [1] of flux-gradient-driven flux
lines (i.e., there is no artificial uniform external force on the vortices)
as an external field H(t) is quasi-statically ramped up and down.
We explore a wide variety of relevant parameters which are difficult
to continuously tune experimentally, such as the density, strength,
radius, and location of the pinning sites. We find a rich variety
of behavior in which all these parameters play an important role.
Our predictions (e.g., magnetization hysteresis loops) can be
directly compared with commonly-measured experimental quantities.
We analyze both global (e.g., M(H), J_c(H)) and local (e.g.,
B(x,y,H(t)), M(x,y,H(t)), J_c(x,y,B)) measurable quantities.
Our results elucidate the topological order dynamics of a driven
plastic lattice interacting with a rigid disordered substrate,
a problem that has recently attracted considerable attention [2].
[1] C. Reichhardt, et al, Phys. Rev. B 52, 10441 (1995); 53, R8898 (1996);
54, in press (1996); J. Groth et al, Phys. Rev. Lett. 77, 3625 (1996).
[2] F. Nori, Science 271, 1373 (1996).
Short video clips can be found at http://www-personal.engin.umich.edu/~nori