Aluminum bulk-plasmon dispersion and its anisotropy

We present a high-resolution electron-energy-loss spectroscopy (EELS) study on the bulk-plasmon dispersion with respect to absolute value and orientation of the transferred momentum vector ħq in an aluminum single crystal. The magnitude q ranges from q=0 A${̊}^{\mathrm{-}1}$ to q=1.5 A${̊}^{\mathrm{-}1}$. Orientations are parallel to [100] and [110] crystal directions. The dispersion has been observed to be biquadratic in q, with unique parameters over the entire q range up to the cutoff wave number, in contrast to earlier studies. Substantial deviations from predictions within the random-phase approximation have been found. Furthermore, the anisotropy of the plasmon dispersion, now measured with high resolution, is considerably smaller than previously reported. The results are discussed in terms of exchange and correlation and band-structure effects.