7.2 Meissner Effect

Meissner effect essentially describes the response of a superconducting material when placed in a magnetic field. It was discovered by W. Meissner and R. Ochsenfeld in Germany in 1933.

When a superconductor is placed in an external magnetic field H (H < H_c) and cooled belowT_C, the magnetic field does not penetrate into the material completely. The field penetrates up to a very small depth decaying exponentially and is of the order of 100 nm or so, and is called as London penetration depth, l.

The Meissner effect is often confused with the diamagnetism as predicted by Lenz’s law which states that application of a changing magnetic field to a conductor induced a magnetic moment which opposes the applied field magnetic field. In a perfect conductor, an arbitrarily large current can be induced, and the resulting magnetization exactly cancels the applied field giving rise to χ = -1.

In contrast, Meissner effect implies spontaneous expulsion of magnetic flux lines which occurs during transition to superconductivity. So when the material is in normal state at T >T_C, the flux lines would penetrate but when it reaches superconducting state at T <T_C, the magnetic flux lines would be abruptly expelled which is NOT Lenz’s law. It is not a diamagnetic effect which, in a normal material, is caused by opposite moment developed due to orbiting electrons.