Meissner effect

In condensed-matter physics, the Meissner effect (or Meissner–Ochsenfeld effect) is the expulsion of a magnetic field from a superconductor during its transition to the superconducting state when it is cooled below the critical temperature. This expulsion occurs because the external magnetic field induces lossless, persistent screening currents near the material's surface, which generate an opposing magnetic field that exactly cancels the external field within the bulk. As a result, the superconductor acts as a perfect diamagnet and repels nearby magnets. The magnetic field is not completely excluded from the superconductor: it penetrates a short distance from the surface, characterized by the London penetration depth, before decaying exponentially into the interior. The German physicists Walther Meissner and Robert Ochsenfeld discovered this phenomenon in 1933 by measuring the magnetic field distribution outside superconducting tin and lead samples. The samples, in the presence of an applied magnetic field, were cooled below their superconducting transition temperature, whereupon the samples cancelled nearly all interior magnetic fields. They detected this effect only indirectly because the magnetic flux is conserved by a superconductor: when the interior field decreases, the exterior field increases. The experiment demonstrated for the first time that superconductors were more than just perfect conductors and provided a uniquely defining property of the superconductor state.