Surface vibrational Raman modes of In: Si(111)(4 x 1) and (8 x 2) nanowires

High-resolution Raman spectroscopy at low frequencies (< 100 cm(-1)) is used to reinvestigate and identify the surface phonons localized in the first atomic layers of In: Si(111)(4 x 1) and (8 x 2). The frequency and symmetry of low-energy surface phonons are strongly related to surface structure. The measured phonons are assigned to characteristic modes of the quasi-one-dimensional indium nanowires on Si(111) by means of density-functional theory calculations and symmetry considerations. It is found that the low-temperature (8 x 2) and the room-temperature (4 x 1) phases of the In: Si(111) surface are characterized by distinct sets of phonon modes. Strong modifications in Raman spectra upon the phase transition are, on the one hand, related to backfolding induced by symmetric quadruplication of the surface unit cell and, on the other hand, to structural changes within the surface unit cell. These are indications for two distinct structural phases, supporting the first-order transition scenario. The characteristic structural changes are thus verified in detail.