Evidence of longitudinal resonance and optical subsurface phonons in Al(001)

A calculation of the surface phonon dispersion curves of Al(001) based on density functional perturbation theory confirms the intrinsic nature of the controversial longitudinal surface phonon resonance, which has been reported to occur in most metal surfaces. The results support previous density response pseudopotential and semi-empirical Born-von-Karman calculations by Franchini, Bortolani, et al, proving the extended nature of the surface perturbation. The latter implies the existence of a Lucas pair of optical surface modes localized in the second layer.