3RD-FREQUENCY-MOMENT SUM-RULE AND LONG-WAVELENGTH PLASMON DISPERSION IN LAYERED ELECTRON LIQUIDS

We derive the third-frequency-moment (omega-3) sum rule and exact high-frequency dielectric response function for the layered electron liquid in zero magnetic field. The latter is examined in the kd --> 0 and kd --> infinity limits characteristic of bulk behavior and two-dimensional (2D) single-layer behavior, respectively (k is the 2D wave number, d is the distance between any two adjacent layers). We incorporate the correlational part of the omega-3 sum rule into a mean-field-theory description of the long-wavelength dispersion of the in-phase and out-of-phase plasmon modes in the presence of very strong correlations.