Crossover of energy deposition and semi-stable chemical bonding in the nonadiabatic dynamics of GaN under proton irradiation.

Understanding the ion-solid interactions of charged particles in materials facilitates the development of ion beam irradiation techniques. Combining Ehrenfest dynamics and time-dependent density-functional theory (ED-TDDFT), we investigated the electronic stopping power (ESP) of an energetic proton in GaN crystal and studied the ultrafast dynamic interaction between the proton and target atoms during the nonadiabatic process. We found a crossover phenomenon of ESP at 0.36 a.u. along the <100> and <110> channels, which is interpreted by the charge transfer between the host material and the projectile and the stopping force exerted on the proton. At velocities of 0.2 and 1.7 a.u., we demonstrated that the reversal of the average number of charge transfer and the average axial force resulted in the reversed energy deposition rate and ESP in the corresponding channel. Further analysis of the evolution of non-adiabatic electronic states revealed the existence of the transient and semi-stable N-H chemical bonding during irradiation process, which is introduced by the electron clouds overlap of N sp3 hybridization and the s orbitals of the proton. These results provide meaningful information for the interactions between energetic ions with matter.