Meissner Effect in Superconductors
The Meissner effect is the expulsion of magnetic flux from a superconductor when its state is below the threshold curve of the magnetic field-temperature phase diagram. The material thereby exhibits perfect diamagnetism. Superconductivity is destroyed when either the applied field or the temperature is increased to take it out of this region. The threshold curve is shown in black for type-I superconductors. The intercepts are designated and . For type-II superconductors, complete expulsion of magnetic flux occurs below the red curve, with intercept designated . The upper intercept is then designated . In the intermediate region between the two curves, known as a mixed state, the magnetic field can still penetrate the superconductor in a series of flux tubes, in which the conducting electrons form quantized vortices.[more]
When you drag the locator to a position on the phase diagram, the graphic shows an idealized representation of the corresponding state of the conductor and the magnetic field lines, shown in blue. Stronger magnetic field intensity is indicated by darker field lines.[less]
Some examples of superconductors:
Type I: lead with K, T; mercury with K, T; aluminum with K, T.
Type II: usually alloys or transition metals, also high-temperature superconductors. Example: niobium-tin with K, T, T.
The flux tubes in type-II superconductors exhibit flux quantization. The magnetic flux through each vortex is quantized in integral multiples of
 Wikipedia. "Meissner Effect." (Apr 22, 2020) en.wikipedia.org/wiki/Meissner_effect.
 Wikipedia. "Superconductivity." (Apr 22, 2020) en.wikipedia.org/wiki/Superconductivity.
 Wikipedia. "Type-II Superconductor." (Apr 22, 2020) en.wikipedia.org/wiki/Type-II_superconductor.
 C. Kittel, Chapter 12, Introduction to Solid State Physics, 6th ed., New York: Wiley, 1986.