Phonons at the surface of the nearly-free-electron metal Al(111): realisation of an ideal surface

Surface phonons of the nearly-free-electron metal Al(111) have been measured by high-resolution He-atom time-of-flight experiments for four different directions. This is the first surface which shows no evidence for an anomalous resonant mode found on the noble metals and Pt and attributed to a strong (≊50%) reduction of surface lateral force constants. By fitting the time-of-flight spectra via a lattice-dynamical simulation the surface interlayer force constant is found to agree to within ±3% with the bulk value and the lateral force constant reduction is less than 20%.