- A serious obstacle that impedes the application of low
and high temperature superconductor (SC) devices is the presence of trapped
flux. Flux lines or vortices are 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 SC or reduce the magnetic field in the immediate vicinity
of the superconducting device. Naturally, the most desirable would be to
remove the vortices from the bulk of the SC. There is no known phenomenon,
however, that could form the basis for such a process. Here we show that the
application of an ac current to a SC that is 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 low and high temperature SCs. As a first step here we
demonstrate that with an appropriate choice of the pinning potential the
ratchet effect can be used to remove vortices from low temperature SCs in the
parameter range required for various applications.
[Fig.1: Patterning with an asymmetric potential of a SC]
[Fig.3b: Removing vortices from a SC using an asymmetric potential]