given by an exponential decay,
B(x) = *"eXPH)
where A. is a "characteristic length" of penetration, called the penetration depth, and
depends on the temperature and Tc (or the material). At the critical temperature, the
penetration length is infinite and any magnetic field can penetrate the sample and destroy
the superconducting state. Near absolute zero of temperature, however, typical
penetration depths are 10-100 nm. Figure 8.48 shows the Bc versus T behavior for
three example superconductors, tin, mercury, and lead.
Superconductors are classified into two types, called Type I and Type II, based on
their diamagnetic properties. In Type I superconductors, as the applied magnetic
field B increases, so does the opposing magnetizationMuntil the field reaches the critical
field Bc, whereupon the superconductivity disappears. At that point, the perfect
diamagnetic behavior, the Meissner effect, is lost, as illustrated in Figure 8.49. A Type
I superconductor below Bc is in the Meissner state, where it excludes all the magnetic