The isotope effect in superconductivity

Ever since Kamerlingh‐Onnes discovered in 1911 that the electrical resistance of mercury abruptly vanished at a temperature just a few degrees above the absolute zero, the phenomenon of superconductivity has presented an intriguing challenge to physicists. Subsequent experiments by Onnes and others definitely established that the resistivity of a superconductor, if at all finite, must be immeasurably small, and less than ${10}^{\text{−20}}\hspace{0.17em}\mathrm{ohm}\hspace{0.17em}\mathrm{cm}.$ Consequently the superconductor is assumed to have zero resistance. Onnes soon found that the superconducting property was not peculiar to mercury but was characteristic of a sizable group of metals. Twentyone of the metallic elements are known to be superconductors as are innumerable alloys and intermetallic compounds. The known superconductors are exhibited in Fig. 1 and are seen to fall into definite areas of the periodic table. None of the monovalent metals are observed to be superconducting, at least down to temperatures of the order of a few tenths of a degree absolute.