Question

London penetration depth for Pb at 3k and 7.1k are 39.6nm and 173nm۔ calculate the depth of 0k

Answer 1

Explanation:

The characteristic length, λ, associated with the decay of the magnetic field at the surface of a superconductor is known as the penetration depth, and it depends on the number density ns of superconducting electrons.

We can estimate a value for λ by assuming that all of the free electrons are superconducting. If we set ns = 1029 m−3, a typical free electron density in a metal, then we find that

The small size of λ indicates that the magnetic field is effectively excluded from the interior of macroscopic specimens of superconductors, in agreement with the experimentally observed Meissner effect.

The small scale of the field penetration means that carefully-designed experiments are needed to measure the value of λ. Many experiments have been done with samples that have a large surface to volume ratio to make the penetration effect of the field appreciable. Thin films, thin wires and colloidal particles of superconductors have all been used for this purpose. But it is also possible to use large specimens if the measurement is sensitive to the amount of magnetic flux passing through the superconductor's surface, and not to the ratio of flux excluded by the superconductor to flux through the normal material, which is close to unity.