Atoms riding Rayleigh waves

Abstract

Under special kinematic conditions helium atoms impinging upon a crystal surface can be inelastically trapped into a surface bound state and ride the created Rayleigh wave. This special case of phonon-assisted selective adsorption, leading to an atom-phonon bound state (atomic polaron), can explain previously unassigned resonant features observed in published helium atom scattering distributions.

Resonance-enhanced inelastic He-atom scattering from subsurface optical phonons of Bi(111)

“…In practice the theoretical SV3 branch, once it is slightly softened so as to best fit the corresponding set of experimental points, would run very close to the resonance curve along a good portion of the SBZ and would therefore benefit from the resonance enhancement. As anticipated in the Introduction, the situation of tangency between a phonon dispersion curve and a resonance curve corresponds to the surfing condition, in which the atom trapped in the bound states and the phonon created in the phonon-assisted selective adsorption process run together with the same group velocity, 10 which ensures a strong enhancement of the iHAS intensity.…”

“…The possibility of tuning the curve representing the (1,0) 2 inelastic resonance condition in the energy-momentum space so as to make it nearly tangent to the optical SV3 dispersion curve allows us to measure it over a large portion of the SBZ. In this special kinematic situation, recently investigated and known as surfing, 10 the atom enters the bound-state channel inelastically by creating an optical SV3 phonon and rides for a while the running charge density wave associated with the SV3 mode, since both the trapped atom and the SV3 phonon travel at the same group velocity along the surface. This exotic form of atomic polaron allows for a strong coupling to the otherwise weak subsurface phonons.…”

Helium-surface interaction potential of Sb(111) from scattering experiments and close-coupling calculations

“…These energies, usually extracted from drift spectra, allow a reliable prediction of the elastic resonance conditions and are used to fit an atom-surface interaction potential. Furthermore, an enhancement of the inelastic background due to the kinematic focusing (KF) effect 24 has also been observed. The obtained atom-surface interaction potential not only is a prerequisite for surface phonon measurements but also can be exploited to enhance certain phonon events which would otherwise be too weak.…”

“…21,22 In a detailed analysis of the intensities on the Bi(111) semimetal surface, a mechanism for the enhancement of inelastic HAS intensities from subsurface optical modes was singled out. 23 This mechanism, which is called the surfing condition, 24 is specific to semimetal surfaces and gives rise to a strong resonance enhancement of the scattered intensities under certain kinematic conditions. 23 It involves the selective adsorption of the incident He atom into a surface-bound state due to the atom-surface interaction potential.…”

Inelastic helium atom scattering from Sb2Te3(111): phonon dispersion, focusing effects and surfing

“…In addition to phonon resonance enhancement a variety of other resonance and focusing phenomena can arise when scattering from a corrugated surface 51 , as listed below. These processes can be described by looking at the equations for the SC and the resonance conditions as well as the lowest modes of the surface phonon dispersion.…”