Plasmon spectrum in low-dimensional electron systems over insulating cryogenic films: Screening, quantum degeneracy, and multisubband effects

Abstract

Plasmon modes in low-dimensional (quasi-one- and quasi-two-) electron systems floating over a cryogenic (liquid helium among others) film covering a solid substrate are calculated. Screening effects on the electron-electron interaction due to the substrate are taken into account and the plasmon spectrum is evaluated in the random-phase approximation. The spectrum consists of longitudinal and transverse branches whose frequencies depend on the film thickness and the dielectric constant of the substrate. For a metal substrate the gapless longitudinal mode has a dispersion quite different from that for the bulk case. The transverse plasma mode is optical like with the gap close to the spectroscopic intersubband frequency.