Low-temperature physics

During Mott's tenure of the Cavendish Chair, the low-temperature physics research carried out in the Mond Laboratory placed it at the forefront of the field internationally. The achievements during the Bragg era were to be reinforced by Pippard's and Shoenberg's ingenious experiments and insights into the nature of the Fermi surface, by Vinen and Hall'sdiscovery of vortex quantisation in liquid helium and by Josephson's discovery of quantum tunnelling of Cooper pairs. The vision was to carry out experiments with relatively simple apparatus but great experimental skill, very much in the Rutherford tradition.

Determination of Fermi surfaces

The Fermi surface is a theoretical three-dimensional boundary in momentum space within which the conduction electrons of a metal are contained at absolute zero. For the freeelectron model this surface is a sphere, but for a real metal the momentum states are determined by band theory, and the Fermi surface often has a remarkably complex shape, having crystal symmetry within the Brillouin zone structure of the atomic lattice, which is important in determining many of the properties of the metal (Box 15.1; see also Section 12.9.2). In the early 1950s there was no experimental determination of the shape of this surface for any metal, and band-structure calculations were not good enough to predict it reliably.

In 1954, however, Pippard showed that, in the extreme anomalous limit of the skin effect, the surface resistance provides a measure of one component of the curvature of the Fermi surface, averaged around the zone on which the electrons are moving parallel to the sample surface, and therefore ‘effective’. He concluded that it might be possible, by making sufficient observations of the surface resistance in different orientations, to determine the geometry of the Fermi surface, at least in the fairly simple case of a metal such as copper.

He carried out this programme during the academic year 1955–56, which he spent on sabbatical leave at the University of Chicago, where Morrel Cohen had arranged for the Institute for the Study of Materials to grow a very large single crystal of copper. The success of the experiment depended on cutting extremely smooth surfaces through the crystal at different angles, and this was expertly performed at the institute.