GITAM, Department of Engineering Physics
Type I Superconductors
The pure metals which exhibit zero resistivity at low temperatures and have the property of excluding magnetic fields from the interior of the superconductor (Meissner effect). The identifying characteristics are zero electrical resistivity below a critical temperature, zero internal magnetic field (Meissner effect), and a critical magnetic field above which superconductivity ceases.
The superconductivity in Type I superconductors is modeled well by the BCS theory. Remarkably, the best conductors at room temperature (gold, silver, and copper) do not become superconducting at all. They have the smallest lattice vibrations, so their behavior correlates well with the BCS Theory.
While instructive for understanding superconductivity, the Type I superconductors have been of limited practical usefulness because the critical magnetic fields are so small and the superconducting state disappears suddenly at that temperature. Type I superconductors are sometimes called "soft" superconductors while the Type II are "hard", maintaining the superconducting state to higher temperatures and magnetic fields.
Type II Superconductors
Superconductors made from alloys are called Type II superconductors. Besides being mechanically harder than Type I superconductors, they exhibit much higher critical magnetic fields. Type II superconductors such as niobium-titanium (NbTi) are used in the construction of high field superconducting magnets.
Type-II superconductors usually exist in a mixed state of normal and superconducting regions. This is sometimes called a vortex state, because vortices of superconducting currents surround filaments or cores of normal material.
 |
Type II superconductors show two critical magnetic field values, one at the onset of a mixed superconducting and normal state, Bc1, and one where superconductivity ceases, Bc2. |
Mixed-State Meissner Effect
 |
In Type II superconductors the magnetic field is not excluded completely, but is constrained in filaments within the material. These filaments are in the normal state, surrounded by supercurrents in what is called a vortex state. Such materials can be subjected to much higher external magnetic fields and remain 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.
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. 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.