A flux quantum (fluxoid) is approximately equal to \(2 \times 10^{-7} gauss-cm^2\ \). A type II superconductor is placed in a small magnetic field, which is then slowly increased until the field starts penetrating the superconductor. The strength of the field at this point is \(4\pi \times 10^4\ \) gauss.
The applied field is further increased until superconductivity is completely destroyed. The strength of the field is now \( 12\pi \times 10^4\ \) gauss. The correlation length of the superconductor is:
1
\(40 \text{Å}\ \)
2
\(400 \text{Å}\ \)
3
\(1256 \text{Å} \ \)
4
\(2900 \text{Å} \ \)