Another important characteristic feature of the superconducting state is that when the
current density through the sample exceeds a critical value 7C
, it is found that superconductivity disappears. Thi s i s not surprising since the current through the superconductor will itself generate a magnetici eld and at sufi ciently high current densities,
the magnetic field at the surface of the sample will exceed the critical field and extinguish superconductivity. Thi s plausible direct relation between Bc and Jc i s only true
for Type I superconductors, whereas in Type II superconductors, Jc depends in a complicated way on the interaction between the current and thel ux vortices. New high-rc
superconductors have exceedingly high critical fields, as apparent in Table 8.7, that do
not seem to necessarily translate to high critical current densities. The critical current
density in Type II superconductors depends not only on the temperature and the
applied magnetic field but also on the preparation and hence the microstructure (e.g. ,
polycrystallinity) of the superconductor material . Critical current densities in new
high-rc superconductors vary widely with preparation conditions. For example, in
Y-Ba-Cu-O
,
Jc may be greater than 1 07 A cm-2 in some carefully prepared thini lms
and single crystal s but around 1 03-1 06 A cm-2 in some of the polycrystalline bulk
material (e.g. , sintered bulk samples). In NbsSn, used in superconducting solenoid
magnets, on the other hand, Jc i s close to 1 07 A cm-2 at near 0 K.