## Tuesday, December 6, 2011

### Magnetic Susceptibility (Establishment of a New Theory)

The initial set-back as discussed earlier, however didn't demoralize the researchers and many of them were struggling to solve this puzzle. However, one of the remarkable achievements towards understanding the superconducting state of matter was the discovery of Meissner Effect after about twenty-two years from the discovery of superconductors. In 1933, Meissner and Ochsenfeld experimentally verified that the internal field flux (B$_0$) is essentially zero in case of pure metals. They did an experiment in which he cooled down a superconductor with a background field. Also, a test coil was wrapped over the superconducting specimen. The specimen was then cooled down with a field surrounding it. As the normal to superconducting transition occurred in the specimen, all the flux was expelled out of the specimen. Hence, from the susceptibility equation,

H + 4\pi \chi H = 0

Thus, the susceptibility is strongly negative at - 1/4$\pi$ showing a perfectly dia-magnetic behavior of the specimen in its superconducting state. The appearance of magnetic flux in the specimen core can also conclude the destruction of superconducting state of the matter. The transition curves are also reversible for a pure sample of lead. However, this is not universal. The reversibility in the transition curve can only be obtained in a well constrained experiment. One of the most important factors is that the sample is metallurgically pure and topologically single. Also, if the core of the ring is vacuum or non-superconducting, some hysteresis is seen. This is because of the induced field in the ring which opposes the decay. In practice, the inherent impurity in the specimen also causes flux trapping and hysteresis.