A new hydrodynamics for plasmons

Plasma oscillations (plasmons) in solids are collective electron charge-density oscillations. They are observed, e.g., as peaks in the electromagnetic absorption spectra of thin metal films, where several types of plasmon mode occur (bulk, surface, standing, two-dimensional). Metallic plasmons typically have frequencies in the ultraviolet region, but in the epitaxial GaAlAs system, the plasmons lie in the far infrared because of the small electron effective mass and high dielectric constant of the underlying semiconductor. Plasmons are therefore of interest for possible far-IR-active devices. They may also be a limiting factor in the design of ultrasmall (nanosized) transistors and other electronic components. An understanding of the mutual plasmons of separated nanostructures is a key to prediction of van der Waals or dispersion forces, with applications ranging into chemistry and biology. In positively charged metal clusters or metal films one can prove a harmonic potential theorem. This states that, despite the Coulomb interaction between the electrons, there is a mode in which the entire electron gas, including its highly inhomogeneous edge regions, moves rigidly at the bare harmonic-well frequency. This motion has proved to be a crucial test case for approximate theories of time-dependent many body phenomena. In particular, the author has shown that this test is failed both by the usual approximations for the ∇n pressure term in hydrodynamics, and by the most sophisticated expressions for the exchange-correlation kernel fxc in time-dependent density functional theory. To rectify these failures the author has proposed modified theories