Quantized Magnetic Flux in Superconductors

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Abstract
The observation that the magnetic flux in a hollow superconducting cylinder is quantized in units of hc/2e has confirmed Fritz London's prediction that superconductivity is a macroscopic quantum phenomenon and that the superconducting state is a single- valued quantum state. It has also conclusively demonstrated that the pair interaction of the Bardeen-Cooper- Schrieffer theory of superconductivity is the important one, at least for the few superconductors studied. A superconducting ring or hollow cylinder must "decide" at the transition temperature what final quantum state it will be in when the temperature is lowered. This "thinking process" has been observed in experiments which demonstrate that the free energy of a hollow superconducting cylinder at the transition temperature is periodic in the magnetic flux. Very recently quantum-mechanical interference effects have been observed in the Josephson tunneling characteristics of multiply linked superconducting circuits. These experiments complement the other experiments on quantized flux and provide perhaps the most elegant proof of long-range order in the superconducting state.