Vortex State |
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| Type II superconductors usually exist in a vortex state with normal cores surrounded by superconducting regions. This allows magnetic field penetration. As their critical temperatures are approached, the normal cores are more closely packed and eventually overlap as the superconducting state is lost. | |
| At the lower of the two critical
magnetic fields in a Type II superconductor, magnetic
fields begin to penetrate through cores of normal
material surrounded by superconducting current vortices.
As long as these vortices are stationary (pinned), the
magnetic fields can penetrate while still maintaining
zero electric resistivity paths through the material. A
size of about 300 nm is typical for the normal cores.
While the Meissner effect is modified to allow magnetic
fields through the normal cores, magnetic fields are
still excluded from the superconducting regions. As the
temperature or the external magnetic field is increased,
the normal regions are packed closer together. The
vortices feel a force when current flows, and if they
move, the superconducting state is lost. Microscopic
defects can act to pin the vortices and maintain the
superconducting state to a higher temperature. So the
microscopic structure and fabrication techniques
influence their properties greatly. Magnetic fields do penetrate the Type-II superconductors through the normal cores in a mixed-state Meissner effect. The vortices can actually be imaged by magneto-optical techniques. The magnetic field associated with the vortices can rotate the plane of polarization of incoming linearly polarized light by the Faraday effect. With a ferrite type detector and crossed polarizers, the vortices are seen as bright spots and can be observed in real time. |
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