Collective flux pinning

Article By:

Vicent, José Luis Departmento de Física de Materiales, Facultad de Ciencias Físicas, Universidad Complutense de Madrid, Madrid, Spain.

Last reviewed:2005

DOI:https://doi.org/10.1036/1097-8542.YB051950

Type II superconductors exhibit two superconducting phases in applied magnetic fields. The first is a state of perfect diamagnetism (Meissner phase), the same state as is found in type I superconductors, with complete exclusion of the magnetic flux. However, if the value of the applied magnetic field is increased, a transition to a second state develops, in which the magnetic field threads inside the superconductor and is associated with a regular array of supercurrent vortices, a vortex lattice, with each vortex surrounding one quantum of magnetic flux (Φ₀ = 2.07 × 10⁻⁷ gauss cm² = 2.07 × 10⁻¹⁵ weber). This state is called the mixed, vortex, or Abrikosov state. In the core of these vortices the superconducting state vanishes and the current carriers are electrons; outside the vortices the sample remains superconducting and the carriers are Cooper pairs. Therefore, if the vortices are pushed and they move, electrical resistance and dissipation will develop in the sample, and superconductivity will be lost. This will, indeed, occur in the presence of a very small current if the vortices are not anchored by some mechanism. Thus, one of the most relevant topics in applied superconductivity is to look for pinning mechanisms that will anchor or pin the vortices and preserve the superconducting state, although the sample is in the mixed state. Ordinarily, pinning is provided through randomly distributed defects in the crystal lattice of the superconductor. New fabrication techniques in the realm of nanotechnology allow the fabrication of nanostructured superconductors with periodic arrays of pinning centers. In the presence of such arrays, the phenomenon of collective flux pinning can occur: The whole vortex lattice can be pinned collectively. This phenomenon offers a promising approach to tailor pinning mechanisms at will.

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