When a superconducting sample is in the Meissner state, it acts like a diamagnetic material: an externally applied magnetic field is prevented from entering the sample by a current (the Meissner current), which runs in a thin boundary layer near the surface. If the strength of the applied magnetic field be increased, the sample transits from the Meissner state to the vortex state. This transition can be studied in detail by solving the time-dependent Ginzburg-Landau equations of superconductivity.

In simulations done on Argonne's SP System, a superconducting crystal in the form of a rectangular prism was considered. The crystal is placed in a magnetic field which is parallel to the prism axis (into the plane of the figure shown above), and is subject to a transport current. The resulting Lorentz force acts to the right in the plane of the figure.

A first series of simulations considered a crystal with a cross section measuring 32 times 48 penetration lengths, with a planar defect in the center plane. After establishing the Meissner state, the strength of the applied magnetic field was increased; at the same time, a weak transport current was introduced. The figure above illustrates a stage during the transition from Meissner to vortex state. The color blue represents the Meissner state, red the state where the system behaves like a normal metal; all other colors correspond to an intermediate state. (Click here for an animation of the complete transition process.)