The Phenomenon of Superconductivity

Abstract

A superconductor is not only a perfect conductor (ρ = 0) but also a perfect diamagnet (B = 0) below T _c. Meissner and Ochsenfeld discovered in 1933 that the magnetic field is expelled out of the body of the superconductor. Field penetrates the material only a small distance, called London’s penetration depth, λ which is of the order of 30–60 nm in metal superconductors. The transition to superconducting phase has been found to be of the second order as confirmed by the absence of a latent heat and by the appearance of a peak in the specific heat at T _c. These materials also exhibit flux quantization in so far as the field entering a superconducting ring or a cylinder has to be an integral multiple of a flux quantum Φ₀ = h/2π (= 2 × 10^–15 T m²). The strong evidence of the role of phonons in the occurrence of superconductivity came from the isotope effect which shows that T _c is inversely proportional to the square root of the atomic mass. Pippard introduced the concept of long range coherence among the super electrons and defined a characteristic length, the coherence length ξ over which the order parameter changes in a superconductor. This parameter is of the order of 1,000 nm much larger than the parameter λ for these metal superconductors. Optical experiments strongly hinted at the existence of an energy gap in the energy spectrum of these materials. All these experimental facts led the three physicists, Bardeen, Cooper and Schrieffer, to formulate the first successful microscopic theory, the BCS theory of superconductivity. The chapter ends with a short discussion on dc and ac Josephson effect. The design of SQUID, an ultra low magnetic field/voltage measuring device, based upon the Josephson junction behavior, has also been discussed. A large number of SQUIDs are mounted on a helmet shaped cryostat and used for mapping feeble magnetic field inside the brain. This technique is called “magneto-encephalography”.