FERMI SURFACE

in solid-state physics, abstract boundary or interface useful for characterizing and predicting the thermal, electrical, magnetic, and optical properties of metals, semimetals, and semiconductors. It is closely related to lattice periodicity, the underlying feature of all crystalline solids, and to the occupation of electron energy bands in such materials. According to the band theory, every electron possesses some level of energy that places it in a valence band or a conduction band. The electrons in any given band may be thought of as occupying a volume in momentum-space, a conception that physicists believe provides the most suitable characterization of electron energy levels since the energies are related to momentum in the classical manner. Physicists, however, have found it convenient to use a modification of momentum called the wave vector k. For such simple metals as sodium and lithium, the volume of occupied states in k space is a sphere, known as the Fermi sphere. The Fermi sphere has a surface of constant energythe Fermi surface. At absolute zero (-273.15 C), the Fermi surface separates the occupied electron states in k space from the empty states outside the Fermi sphere. Many of the more complex metals, however, do not have an occupied group of levels in k space. Correspondingly, the Fermi surfaces of such metals often have bumps and depressions of rather large magnitude, though these deviations from the spherical configuration must have the same basic symmetry as the appropriate crystal. In effect, the particular shape of the Fermi surface in a metal reflects the electronic structure of the metal. Moreover, the changes of energy of electrons at or near the Fermi surface greatly affect the metal's ability to conduct electricity, its magnetic effects, and certain other properties.