Phonons at the Fe(110) Surface

The in-plane density of phonon states of clean Fe(110) surface was measured separately for the first, second, and further atomic monolayers using nuclear inelastic scattering of synchrotron radiation. The results show that atoms of the first layer vibrate with frequencies significantly lower and amplitudes much larger than those in the bulk, and that vibrational spectra along two perpendicular in-surface directions are different. The vibrations of the second layer are already very close to those of the bulk. The good agreement of the experimental results and the first-principles calculations allows for detailed understanding of the observed phenomena.

Figure 1

Fitted NRS time spectra for the Fe(110) surface ($S$), subsurface ($S-1$), and deep ($D$) layers measured with the x-ray beam along $[1\overline{1}0]$ direction.

Figure 2

(a) Measured phonon DOS for the surface ($S$), subsurface ($S-1$), and deep ($D$) layers near the Fe(110) surface projected to $[1\overline{1}0]$ (solid symbols) and to [001] (open symbols). The solid lines in ($S$) depict results of DFT calculations. The phonon DOS of an $\alpha \mathrm{\text{−}}{}^{57}\mathrm{Fe}$ foil is shown for comparison in each panel as a continuous line. (b) Double logarithmic plot of the phonon DOS collection from (a).

Figure 3

(a) Dispersion curves and (b) surface DOS along $[1\overline{1}0]$ (solid squares) and [001] (open circles) directions calculated for 29 layers Fe slab. High-symmetry points correspond to two dimensional Brillouin zone of the plane reciprocal lattice (Ref. 2). In (a), the lowest dispersion curve is the Rayleigh mode while the open squares depict surface modes with the highest in-plane intensities.