Evidence for the Stopping of Slow Ions by Excitations of Optical Phonons in Insulators

Abstract

The energy loss of Ne 1 ions with keV energies scattered under grazing incidence from a LiF(001) surface is studied with a time-of-flight technique. Since charge exchange in front of the wide-bandgap insulator is widely suppressed, the energy loss of slow ions moving in front of the solid can be investigated under specific interaction conditions. From the theoretical analysis of data we find evidence for an energy loss mechanism based on the excitations of optical phonons in the insulator. PACS numbers: 79.20.Rf, 34.50.Bw When atomic particles collide with solid matter, excitations in projectile and target give rise to stopping phenomena. For slow ions electronic excitations, charge exchange, and binary collisions with target atoms ("nuclear stopping") clearly dominate the dissipation of energy in ion-solid collisions [1]. Whereas nuclear stopping can be described in a straightforward manner, the treatment of electronic processes is a more intricate subject. For metal targets, considerable progress in the theoretical description of electronic stopping has been achieved over about the last decade [2]. Excitation of electron-hole pairs is the primary mechanism for electronic stopping of slow ions, i.e., projectiles with velocities y ø y 0 (y 0 Bohr velocity 1 atomic unit 1 a.u.). In the framework of corresponding theories, characteristic features for the stopping of slow ions by metal targets, e.g., a linear dependence of the stopping power 2dE͞dx on the projectile velocity y or oscillations of 2dE͞dx with atomic number of the projectiles Z 1 ("Z 1 oscillations"), can be described on a quantitative level [3].An interesting aspect concerning the stopping of slow ions is the role of collective excitations. Whereas at higher projectile velocities excitations of plasmons (typical energieshv ഠ 10 eV) result in substantial contributions to energy loss [2,4], these excitations will have small effects on the stopping of slow ions [5]. From a theoretical analysis Echenique and Howie [6] concluded that at low velocities only collective excitations at low frequencies, as, e.g., optical phonons in ionic insulators (typical energieshv ph ഠ some 10 meV [7]) will contribute. In order to observe this mode of stopping, other sources for dissipation of energy of slow ions (single excitations, charge exchange, nuclear stopping) has to be reduced. To our knowledge, corresponding experimental work has not been reported so far.In this Letter, we report on joint experimental and theoretical studies where unequivocal evidence for the stopping of slow ions in front of an ionic insulator by excitation of optical phonons is obtained. We will demonstrate that under specific experimental conditions for collisions of ions with solid matter, the excitations of phonons clearly dominate the stopping process. Our experiments are performed with Ne 1 ions at keV energies ͑y # 0.1 a.u.͒ scattered under grazing angles of incidence (typically F in ഠ 1 ± ) from a clean and flat LiF(001) surface. Under these scattering conditions ("planar surface chann...

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].…”