BULK AND SURFACE-PLASMONS IN SOLIDS

In three-dimensional systems the dispersion of bulk plasmons in the electrostatic limit depends upon nonlocal effects in the conductivity or dielectric function. In addition to nonlocal effects, a non-abrupt surface electron density profile is necessary for the existence of higher multipole surface plasma modes. A spatially periodic bulk electron density gives rise to a band structure in the collective charge density excitation spectrum. For two-dimensional electron systems, like electrons confined to a narrow quantum well, the bulk 2D plasmon frequency is inversely proportional to the square root of the wavelength even in a simple local theory. Both regular and higher multipole edge modes can occur in the absence of nonlocal effects. For a periodic array of 2D electron layers, as in a superlattice, the collective charge density excitations can be classified as intrasubband and intersubband plasma modes. Each of these bands can support a surface plasma mode if the array of layers ends at a medium with different background dielectric constant than that containing the 2D electron layers. The basic concepts underlying the existence of all of these modes will be reviewed using the simplest models that contain the essential physics of the problem.