Excitonic effects on the optical spectra and the electron energy loss spectra of Mg2C using linear response theory

The optical and electron energy loss spectra of Mg2C are studied in the framework of time-dependent density functional theory. On top of the ground state of the antifluorite crystal constructed using the full-potential linearized augmented plane wave method, the linear response theory is applied. Optical functions are deduced using the bootstrap and long-range corrected exchange correlation kernels and the random phase approximation. The scheme proposed in our recent work to find material parameter α required in the long-range corrected kernel is now applied to the antifluorite crystals. Signature of the excitonic and the local field effects is noticed on the optical functions. The E2ex peak in the ε2 spectra is largely influenced by the excitonic effects. Increment of 11.05% and 30.40% in the intensity of this exciton peak is noticed in the optical function ε2 calculated using the bootstrap and the long-range corrected kernels with respect to random phase approximation, respectively. In the absence of experimental data for Mg2C, the calculations on isostructural compound Mg2Si are undertaken. The results of Mg2C obtained by considering α = 0.573 from the scheme proposed by Botti et al and α = 0.491 from our scheme based on the pseudopotential form factors are in mutual agreement. The prescription to find α from form factors works very well for antifluorite crystals also. The peaks due to interband transitions and plasmonic oscillations in the electron energy loss spectra in all calculations are well characterized notwithstanding the inclusion of local field effects.