The Fermi surface of a crystalline solid is the boundary between states that are occupied by electrons at absolute zero temperature and states that are empty at this temperature. It is usually measured in momentum space, rather than real space, by techniques such as angle-resolved photoemission spectroscopy. Now Martin Wenderoth, of the University of Göttingen, and colleagues have shown that it is possible to image the Fermi surface of a solid in real space with a scanning tunnelling microscope (STM; Science 323, 1190–1193; 2009).

In a typical STM measurement an electron from the tip of the microscope tunnels into a surface and becomes a bulk electron wave with amplitude that decays with distance. If a defect atom is present beneath the surface, the electron wave can be reflected, interfering with the incoming wave to form a standing-wave pattern. When the Fermi surface is spherical, a weak interference pattern is observed at the surface. However, when the Fermi surface is not spherical, electrons can be focused along certain directions, resulting in pronounced interference patterns at the surface. These patterns reflect information about the flow of electrons in the bulk of the material, and hence the shape of the Fermi surface.

The STM image on the left is 9 nm across and shows four cobalt atoms below a copper(111) surface; the image on the right is 3.5 nm across and shows one cobalt atom below a copper(100) surface. The insets are 4 nm across and show the local electron density of states derived from the images (left insets), and from theory (right insets).