PLASMON DISPERSION AND DYNAMIC EXCHANGE-CORRELATION POTENTIALS FROM 2-PAIR EXCITATIONS IN DEGENERATE PLASMAS

Electron energy-loss experiments have shown a rapid softening of the b ulk plasmon dispersion across the series of the alkali metals. Motivat ed by these observations, we reconsider the evaluation of the dynamic, long-wavelength exchange-correlation potential f(xc)(omega) in the el ectron fluid, which is of interest for applications in time-dependent density functional theory. The value of Re[f(xc)(omega(pl))] at the pl asma frequency omega(pl) determines the exchange-correlation contribut ion to the leading plasmon dispersion coefficient in the homogeneous e lectron fluid. Whereas an interpolation scheme originally proposed by Gross and Kohn assumes a monotonic increase of Re[f(xc)(omega) - f(xc) (0)] across the plasma frequency, we examine the possibility of strong ly non-monotonic behaviour arising from a resonance process between pl asmons and two-pair excitations. This process is evaluated with the he lp of sum rules and selfconsistency requirements within a single-pole approximation for the dielectric function. The cases of a fermion plas ma and of a boson plasma are treated in parallel and the reliability o f the results for the fermion plasma at low coupling is tested by calc ulations within a random-phase approximation for the dielectric functi on. In all cases it is found that the resonance process accumulates os cillator strength in the neighbourhood of 2 omega(pl), thus decreasing the value of Re[f(xc)(omega pl)] below the static value f(xc)(0) fixe d by the compressibility sum rule. Although this lowering does not suf fice to account by itself for the measured plasmon dispersion coeffici ent in the low-density alkali metals, our results provide useful input for combined band-structure and exchange-correlation calculations.