# Anisotropy of electronic stopping power in graphite

## TL;DR

This study uses first-principles simulations to investigate how the anisotropic structure of graphite affects electronic stopping power, revealing significant directional dependence and velocity-related behaviors in energy transfer from ions to electrons.

## Contribution

It provides the first detailed analysis of anisotropic effects on electronic stopping power in graphite using time-dependent density functional theory simulations.

## Key findings

- Anisotropic crystal structure causes up to 25% variation in stopping power.
- Velocity dependence shows linear behavior with a threshold for perpendicular trajectories.
- Metallic behavior observed along graphitic planes at certain velocities.

## Abstract

The rate of energy transfer from ion projectiles onto the electrons of a solid target is hard to determine experimentally in the velocity regime between the adiabatic limit and the Bragg peak. First-principles simulations have lately offered relevant new insights and quantitative information for prototypical homogeneous materials. Here we study the influence of structural anisotropy on electronic stopping power with time-dependent density functional theory simulations of a hydrogen projectile in graphite. The projectile traveled at a range of angles and impact parameters for velocities between 0.1 and 1.4 a.u., and the electronic stopping power was calculated for each simulation. After validation with average experimental data, the anisotropic crystal structure was found to have a strong influence on the stopping power, with a difference between simulations parallel and perpendicular to the graphite plane of up to 25%, more anisotropic than expected based on previous work. The velocity dependence at low velocity displays clear linear behavior in general, except for projectiles traveling perpendicular to graphitic layers, for which a threshold-like behavior is obtained. For projectiles traveling along graphitic planes metallic behaviour is observed with a change of slope when the projectile velocity reaches the Fermi velocity of the electrons.