J400_22b 337..340

A serious obstacle impeding the application of lowand hightemperature superconductor devices is the presence of trapped magnetic flux: 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, limiting the operation of numerous superconducting devices. Methods used to overcome this difficulty include the pinning of vortices by the incorporation of impurities and defects, the construction of flux ‘dams’, slots and holes, and magnetic shields 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 we 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. The mechanism responsible for this phenomenon is the so-called ‘ratchet effect’, and its working principle applies to both lowand high-temperature superconductors. We demonstrate theoretically that, with an appropriate choice of pinning potential, the ratchet effect can be used to remove vortices from low-temperature superconductors in the parameter range required for various applications. We consider a type II superconductor film of the geometry shown in Fig. 1a, placed in an external magnetic field H. The superconductor is patterned with a pinning potential Uðx; yÞ 1⁄4 UðxÞ which is periodic with period l along the x direction, has an asymmetric shape within one period, and is translationally invariant along the y direction of the sample. The simplest example of an asymmetric periodic potential, obtained for example by varying the sample thickness, is the asymmetric sawtooth potential (Fig. 1b). In the presence of a current with density J flowing along the y axis the vortices move with the velocity