Dynamic structure factor S(k,ω) of beryllium by x-ray inelastic scattering experiments

The dynamic structure factor S(k,{ω}) of beryllium is determined by inelastic x-ray scattering experiments for a wide region of momentum transfer k. The results show that the dispersion curve deviates from random-phase-approximation (RPA) predictions, especially for k>kc, while for k>kF it splits into two branches. The lower branch, corresponding to the plasmonlike component, shows a negative slope for 0.9kF1.5kF, whereas for greater values of k it remains nondispersive. The upper branch shows considerable dispersion and tends to become parallel to the recoil energy curve. The observed deviations from RPA can be attributed to short-range correlation effects and the existence of higher-order excitations according to the theory of Awa et al.