Wake effect induced by ions traversing single-layer graphene

A swift ion penetrating a solid polarizes the target electrons, which rearrange to screen its charge. When the ion velocity exceeds the characteristic velocity of these electrons, the screening cannot keep pace with the ion, resulting in an oscillatory perturbation in the induced electron density trailing the projectile, known as the wake [1]. Vager and Gemmell observed this wake using fast HeH⁺ ions transmitted through a carbon foil (85 Å) [2]. In their study, the wake potential established inside the solid by the higher-Z He ion captures H and draws it behind He, causing the subsequent Coulomb explosion to shift the kinetic energy of the H⁺ fragment toward deceleration. However, it remains unresolved whether such a wake effect occurs in the ultimate limit of a single atomic layer. The present study addresses this question by employing a free-standing single-layer graphene as the target. By systematically varying the target thickness to 2, 3–5, and 6–8 layers, the temporal evolution of the single-layer wake effect is traced to determine whether the response established within a single atomic plane is already identical to that in bulk solids or represents an incomplete stage that continues to develop with increasing interaction time.

Experiments were performed using the 4-MV Van de Graaff accelerator at Kyoto University. HeH⁺ ions with energies of 0.47, 0.75, and 1.0 MeV were directed at normal incidence onto self-supporting graphene targets consisting of single, 2, 3–5, and 6–8 layers. The incident beam was collimated before the target, and H⁺ fragments emitted in the beam direction (0°) were energy-analyzed and detected. The resulting position spectra were stored in a multichannel analyzer and converted into the kinetic-energy distribution of the H⁺ fragments emitted at 0°.

The deceleration and acceleration yields are determined by integrating the counts in the corresponding regions of the kinetic-energy spectrum of H⁺ fragments emitted at 0°. This analysis examines the wake effect in the thinnest possible target and how it evolves with increasing target thickness and interaction time. The results indicate that the wake effect in a single graphene layer is already present, while the magnitudes of deceleration and acceleration vary with target thickness. Quantitative analysis and detailed interpretation of these results will be presented and discussed at the conference.

References.

[1] N. Bohr, Mat.-Fys. Medd. K. Dan. Vidensk. Selsk. 18, No. 8 (1948).

[2] Z. Vager and D. S. Gemmell, Phys. Rev. Lett. 37, 1352 (1976).