Magneto-optical observation of twisted vortices in type-II superconductors

Macroscopic vortex droplets in Bi2Sr2CaCu2O8 crystals pumped by an ac magnetic field

The formation of droplets (or breathers) of vortex matter in Bi(2)Sr(2)CaCu(2)O(8) crystals pumped by the low-frequency ac magnetic field is observed by magneto-optic imaging. The droplets nucleate at the points of crossing the planar defects. The magnetic flux in the droplet and the droplet size increase with time. When the flux in the droplet exceeds a critical value, the droplet shifts along the defect to the sample bulk and a new droplet with the opposite flux direction starts to nucleate at the same point. As a result, a beadlike structure of the droplets arises along the defect. A possible explanation of the phenomenon is proposed.

Spatiotemporal vortex matter oscillations in Bi2Sr2CaCu2O8+delta crystals

We observed an oscillatory behavior, both in space and time, of the induction in Bi2Sr2CaCu2O8+delta crystals exposed to a steady magnetic field. This new "flux waves" phenomenon appears near the order-disorder vortex phase transition, under specific conditions of temperature and induction gradient. A theoretical description of this effect is based on two coupled equations: the Landau-Khalatnikov dynamic equation for the order parameter of the vortex phase transition and the diffusion equation for the time evolution of the magnetic induction. A linear stability analysis of these equations predicts an oscillatory instability characterized by a period and wavelength in accordance with the experimental results.

Experimentally realizable devices for controlling the motion of magnetic flux quanta in anisotropic superconductors

A new generation of microscopic ratchet systems is currently being developed for controlling the motion of electrons and fluxons, as well as for particle separation and electrophoresis. Virtually all of these use static spatially asymmetric potential energies to control transport properties. Here we propose completely new types of ratchet-like systems that do not require fixed spatially asymmetric potentials in the samples. As specific examples of this novel general class of ratchets, we propose devices that control the motion of flux quanta in superconductors and could address a central problem in many superconducting devices; namely, the removal of trapped magnetic flux that produces noise. In layered superconductors there are two interpenetrating perpendicular vortex lattices consisting of Josephson vortices (JVs) and pancake vortices (PVs). We show that, owing to the JV-PV mutual interaction and asymmetric driving, the a.c. motion of JVs and/or PVs can provide a net d.c. vortex current. This controllable vortex motion can be used for making pumps, diodes and lenses of quantized magnetic flux. These proposed devices sculpt the microscopic magnetic flux profile by simply modifying the time dependence of the a.c. drive, without the need for samples with static pinning--for example, without lithography or irradiation.