NEGATIVE SURFACE-PLASMON DISPERSION COEFFICIENT - A PHYSICALLY ILLUSTRATIVE, EXACT FORMULA

It is shown that in the nonretarded regime, the frequency of the long-wavelength surface plasmon of a free-electron metal is directly related to the overlap of the metals unperturbed electron density and the normalized, fluctuating potential of the surface plasmon. This result directly expresses the idea that the surface plasmons frequency depends on the efficiency with which its electric potential energy can be interchanged with electron kinetic energy. It shows that a negative surface-plasmon dispersion coefficient can be interpreted in terms of decreasing overlap of the fluctuating potential and the unperturbed electron density, as the plasmon wave vector increases. © 1990 The American Physical Society.