8.9.2 Type I and Type II Superconductors
The superconductivity below the critical temperature has been observed to disappear
in the presence of an applied magnetic field exceeding a critical value denoted by Bc.
This critical field depends on the temperature and is a characteristic of the material.
Figure 8.47 shows the dependence of the critical field on the temperature. The critical
field is maximum, £c(0), when T = 0 K (obtained by extrapolation8). As long as
the applied field is below Bc at that temperature, the material is in the superconducting
state, but when the field exceeds Bc, the material reverts to the normal state. We
know that in the superconducting state, the applied magnetic field lines are expelled
from the sample and the phenomenon is called the Meissner effect. The external
field, in fact, does penetrate the sample from the surface into the bulk, but the magnitude
of this penetrating field decreases exponentially from the surface. If the field
at the surface of the sample is B0, then at a distance x from the surface, the field is