Critical Magnetic Field

The 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

Critical magnetic fields for superconductors

Phase Diagram Examples

The 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.

Vortex State for Superconductors

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.

Vortex State for Superconductors

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.