Electronic excitations in quasi-2D crystals: What theoretical quantities are relevant to experiment?

摘要: The ab initio theory of electronic excitations in atomically thin [quasi-two-dimensional (Q2D)] crystals presents extra challenges comparison to both the bulk and purely 2D cases. We argue that conventionally used energy-loss function $-$Im $1/\epsilon({\bf q},\omega)$ (where $\epsilon$, ${\bf q}$, $\omega$ are dielectric function, momentum, energy transfer, respectively) is not, generally speaking, suitable quantity for interpretation electron-energy loss spectroscopy (EELS) Q2D case, we construct different functions pertinent EELS experiments on crystals. Secondly, emphasize importance develop a convenient procedure elimination spurious inter-layer interaction inherent use 3D super-cell method calculation Thirdly, resolve existing controversy so-called $\pi$ $\pi+\sigma$ monolayer graphene by demonstrating dispersive collective (plasmons) non-dispersive single-particle (inter-band) transitions fall same ranges, where they strongly influence each other.