Subsurface-Channeling-Like Energy Loss Structure of the Skipping Motion on an Ionic Crystal

Abstract

The skipping motion of Ne+ ions in grazing scattering from the LiF(001) surface is studied for velocity below 0.1 a.u. with a time-of-flight technique. It is demonstrated that suppression of electronic excitation and dominance of optical phonon excitation in the projectile stopping results in an odd 1,3,5,... progression of the energy loss peaks, a feature usually ascribed to subsurface channeling. The experimental findings are well reproduced by parameter-free model calculations where thermal vibrations are the dominant cause for the ion trapping and detrapping.

Probing the Plasmon-Phonon Hybridization in Supported Graphene by Externally Moving Charged Particles

“…Given that typical ion speeds used in the LEIGS technique [23][24][25][26] are precisely in the subthreshold velocity range, it appears that small variations in the doping density close to the neutrality point of graphene may cause dramatic changes in the mechanism of energy loss due to suppressing or enabling the plasmonphonon hybridization.…”

“…This technique was recently used to study collective modes in epitaxial graphene on a SiC substrate, revealing low-energy features in the REEL spectra [18,19] that were interpreted in terms of the plasmonphonon hybridization [19][20][21][22]. On the other hand, recent measurements of the energy loss spectra and the angular distributions of the Ne + ions with energies in the keV range, which are grazingly scattered with an angle of incidence of about 1°f rom a LiF(001) surface [23,24], showed strong promise as efficient probe for multiple excitations of the FK phonons on a polar insulating surface [25,26]. It is noteworthy that both REELS and the low-energy ion grazing scattering (LEIGS) [23][24][25][26] use charged particles that move at the speeds comparable to the Fermi speed v F ≈c/300 (where c is the speed of light in vacuum) of graphene's π electron bands in the Dirac cone approximation [2].…”

“…On the other hand, recent measurements of the energy loss spectra and the angular distributions of the Ne + ions with energies in the keV range, which are grazingly scattered with an angle of incidence of about 1°f rom a LiF(001) surface [23,24], showed strong promise as efficient probe for multiple excitations of the FK phonons on a polar insulating surface [25,26]. It is noteworthy that both REELS and the low-energy ion grazing scattering (LEIGS) [23][24][25][26] use charged particles that move at the speeds comparable to the Fermi speed v F ≈c/300 (where c is the speed of light in vacuum) of graphene's π electron bands in the Dirac cone approximation [2]. Since the LEIGS technique was never used to study plasmon-phonon hybridization on a surface, it is of interest to investigate dynamic polarization of a largearea monolayer graphene supported by a polar insulator due to external point charge particle that moves (almost) parallel to graphene.…”

“…Since the LEIGS technique was never used to study plasmon-phonon hybridization on a surface, it is of interest to investigate dynamic polarization of a largearea monolayer graphene supported by a polar insulator due to external point charge particle that moves (almost) parallel to graphene. In addition to the energy loss rate, or stopping power of that particle [23,25], the complexity of ion trajectories during skipping motion on the target surface in LEIGS also requires careful analysis of the dynamic image force [24,26].…”

“…We use the dielectric response formalism based on graphene's dielectric function within the random phase approximation (RPA) [34][35][36] to calculate the total electrostatic potential in the graphene plane, as well as the stopping and image forces on the incident charge in a setting of interest for the LEIGS technique [23][24][25][26].…”