Critical Magnetic FieldThe superconducting state cannot exist in the presence of a magnetic field greater than a critical value, even at absolute zero. This critical magnetic field is strongly correlated with the critical temperature for the superconductor, which is in turn correlated with the bandgap. Type II superconductors show two critical magnetic field values, one at the onset of a mixed superconducting and normal state and one where superconductivity ceases. It is the nature of superconductors to exclude magnetic fields (Meissner effect) so long as the applied field does not exceed their critical magnetic field. This critical magnetic field is tabulated for 0K and decreases from that magnitude with increasing temperature, reaching zero at the critical temperature for superconductivity. The critical magnetic field at any temperature below the critical temperature is given by the relationship
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Critical magnetic fields for superconductors
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Phase Diagram ExamplesThe Type II superconductors have much higher critical magnetic fields than Type I, but for most of that field range they are mixtures of normal and superconducting.
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Vortex State for SuperconductorsType 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.
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Vortex State for SuperconductorsAt 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.
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