From the Meissner Effect to the Isotope Effect: Precursors to the Microscopic Theory of Superconductivity

After the discovery by Kamerlingh Onnes in 1911 of the low temperature disappearance of resistance in mercury to a state of perfect conductivity, there was a long period of more than two decades before there was a major experimental advance. In 1933, Meissner and Ochsenfeld discovered that a superconductor is not only a perfect conductor but in addition it is a perfect diamagnet. In 1935 F. and H. London presented a phenomenological understanding of the electromagnetic properties of the superconducting state, which included the London penetration depth for applied magnetic fields and later introduced the concept of a ``stiffness'' of the superconducting wave function. In 1950, Ginzburg and Landau developed a phenomenological theory for the superconducting state using general thermodynamic arguments.In the same year, Maxwell, and Serin et.al discovered the Isotope Effect which indicated that the electron-phonon~interaction would play an important role in the theory of superconductivity.