Vibrational dynamics of a c (2×2 ) phase induced by nitrogen adsorption on Cu(001)

Helium-atom scattering and density-functional perturbation theory (DFPT) calculations invoking the linear-response approximation and the pseudopotential approach have been used to study the vibrational dynamics of c (2×2) -like phases produced by nitrogen-ion implantation and subsequent annealing of Cu(001) surfaces. We find that, while the c (2×2) phonon dispersion relations are different from those of clean Cu(001), neither the acoustic nor the optical surface phonon mode energies measured along both [100] and [1 1̄ 0] directions are dependent on N coverage once the c (2×2) pattern is formed. We show that the dispersion of the surface phonon modes is well reproduced with an analysis of the DFPT calculations of a nonstress relieved c (2×2) structure. A marked softening of a zone center optical mode is very apparent both experimentally and in the calculations. We show this softening arises largely because of interplanar Cu surface relaxations induced by N adsorption.