In 1911 the Danish physicist Kamerlingh Onnes found that the electrical resistivity of mercury appeared to vanish completely below 4.2 K: a steady current flowed in a ring without need of a sustaining electric field. The discovery followed from his success in 1908 of liquifying helium, thereby making temperatures down to about 1 K accessible to experiment. This phenomenon of superconductivity was subsequently found in many other metals and alloys, but until recently the known critical temperatures Te at which the transition to the superconducting state occurs had not exceeded 24 K; all superconducting technology depended on the availability of (expensive) liquid helium. In 1986–7 new classes of ‘high-Te’ superconductors were discovered having critical temperatures which exceed the boiling point at atmospheric pressure of (cheap) liquid nitrogen, 77.4 K. These new superconductors are complex compounds, such as YBa2Cu3O7-δ; their properties and possible technological applications are being intensively studied.

The Meissner effect

Materials which become superconducting have the remarkable property of being ‘perfectly diamagnetic’ in their superconducting state. In a static magnetic field, up to a certain critical magnitude, magnetic flux is completely expelled from the inner regions of a large sample when the sample is cooled below its transition temperature (Fig. 14.1). An electric current is generated whose field exactly cancels the applied field in the interior of the sample. From the Maxwell equation ∇ × B = µ0J, the current flow is confined to a region close to the surface of the sample, since ∇ × B = 0 in its interior.