Fig. 1: Horizontally driven vortex lattice. A vertical fault line separates two regions of approximately regular structure.
Argonne researchers have been conducting large-scale numerical simulations of the dynamics of magnetic flux vortices in high-temperature superconductors. Such simulations complement laboratory experiments and, in many cases, are required because it is simply infeasible to conduct the physical experiments necessary to analyze the complex three-dimensional, time-dependent phenomena. Essential for these simulations has been access to the massively parallel IBM Scalable POWERparallel SP computer, which was acquired with funding under the Department of Energy HPCC program.
One issue recently examined concerned the motion of a vortex lattice under the influence of an applied current. Experimental observations indicate that, in a significant region of the phase diagram, the motion of a driven vortex lattice is predominantly plastic motion. Earlier explanations relied heavily on the notion that vortex interactions compete with pinning forces associated with random inhomogeneities in the bulk of a superconductor. Numerical simulations of the motion of a driven vortex lattice in a clean finite sample showed that a different mechanism may be responsible for plastic vortex motion. The simulations were based on the time-dependent Ginzburg-Landau model of superconductivity, which gives a field description of the vortex lattice. The structure of the vortex lattice was analyzed by special geometric methods.
The researchers found that a current increases the vortex spacing in the direction of vortex motion and enforces the formation of fault lines to accommodate the resulting strains. The fault lines separate regions of approximately uniform structure and thus define a ``superstructure'' in the driven vortex lattice. The fault lines remain approximately stationary as the vortex lattice moves and serve as a source of plastic deformations.
Because of its great potential for energy technology applications, high-temperature superconductivity is a forefront area of materials science and condensed-matter physics research.
The mechanism recently identified by Argonne researchers provides new insight into the behavior of high-temperature superconductors. Because this mechanism is the result of the intrinsic behavior of the vortex lattice and is independent of bulk pinning, it may be responsible for the plastic motion observed in very clean superconductors.
This work is supported by the Mathematical, Information, and Computational Sciences Division subprogram of the Office of Computational and Technology Research, U.S. Department of Energy.
The project is a collaborative effort of researchers in Argonne's Materials Science Division and the Mathematics and Computer Science Division. The principal investigators are as follows:
For a movie and other illustrations of vortex dynamics, click here.