The discovery of superconductors

The phenomenon of superconductivity, in which the electrical resistance of certain materials completely
vanishes at low temperatures, is one of the most interesting and sophisticated in condensed matter
physics. It was first discovered by the Dutch physicist Heike Kamerlingh Onnes, who was the first to
liquefy helium (which boils at 4.2 Kelvin at standard pressure). In 1911 Kamerlingh Onnes and one of his
assistants discovered the phenomenon of superconductivity while studying the resistance of metals at low
temperatures. They studied mercury because very pure samples could easily be prepared by distillation.

The historic measurement of superconductivity in mercury is shown in Figure 1. As in many other metals,
the electrical resistance of mercury decreased steadily upon cooling, but dropped suddenly at 4.2 K, and
became undetectably small. Soon after this discovery, many other elemental metals were found to exhibit
zero resistance when their temperatures were lowered below a certain characteristic temperature of the
material, called the critical temperature