On the basis of obeying meissner effect,semiconductors are divided in two different classes as given below.
Type I Superconductors(strictly follows M.E.):
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(smoothly follows M.E.):
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.
Differences between Type I and Type II Superconductors:
Soft Superconductors (Type I) :
1. Soft superconductors are those which can tolerate impurities without affecting the superconducting properties.
2. They have low critical field.
3. Show complete Meissner effect.
4. The current flows through the surface only.
5. Eg. Tin, Aluminium
Hard Superconductors (Type II) :
1. Hard superconductors are those which cannot tolerate impurities, i.e., the impurity affects the superconducting property.
2. They have high critical field.
3. Hard super conductors trap magnetic flux and hence Meissner effect is not complete.
4. It is found that current flows throughout the material.
5. Eg. Tantalum, Neobium