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 supercon-ducting 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. 1 Vortex removal from superconductors A serious obstacle impeding the application of low-and high-temperature superconductor devices is the presence of trapped magnetic flux [1–4]. Flux lines or vortices can be induced by fields as small as the Earth's magnetic field. Once present, vortices dissipate energy and generate internal noise. Methods used to overcome these difficulties include the pinning of vortices by the incorporation of impurities and defects [5], the construction of flux dams [6], slots and holes [7], the application of high-frequency magnetic fields [8], and the application of magnetic shields [2–4], which block the penetration of new flux lines in the bulk of the superconductor or reduce the magnetic field in the immediate vicinity of the superconducting device. The most desirable method would be to remove the vortices from the bulk of the superconductor, but there was hitherto no known phenomenon that could form the basis for such a process. Here I show that the application of an alternating current to a superconductor patterned with an asymmetric pinning potential can induce vortex motion whose direction is determined only by the asymmetry of the pattern [9]. The mechanism responsible for this phenomenon is the so-called ratchet effect [10–18], and its working principle applies to both low-and high-temperature supercon-ductors. I demonstrate theoretically that, …