Application of the ratchet effect to improve material quality (reducing vortex density in superconductors and smoothing surfaces)

Although Brownian ratchets have been conceived to describe the operation of molecular motor proteins, their basic principles are also applicable to a wide range of different physical systems. In this paper I line up two such possible applications in condensed-matter physics. The first one is the removal of vortices from superconductors. Magnetic fields frequently penetrate superconducting materials in the form of vortices, and once present, they dissipate energy and generate internal noise, limiting the operation of numerous superconducting devices. We demonstrate theoretically that the application of an alternating current to a superconductor patterned with an appropriate ratchet-like pinning potential induces an outward vortex motion. The second application is based on the fact that the Schwoebel barrier induces an asymmetry in the lattice potential of nearly flat solid surfaces. During epitaxial growth this asymmetry leads to a fast and unwanted increase in the surface roughness. We show, however, that one can take advantage of the asymmetry by applying an alternating electric field parallel to the surface, which induces a net electromigrational flow of the surface atoms from the peaks towards the wells, and thus results in a smoother surface. Received: 20 November 2001 / Accepted: 14 January 2002 / Published online: 22 April 2002