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

Abstract

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

19 pages, 6 figures

References in corpus (3)

• Dynamical polarization of graphene at finite doping
• Collective modes of the massless Dirac plasma
• π-Plasmon Dispersion in Free-Standing Graphene by Momentum-Resolved Electron Energy-Loss Spectroscopy

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• MoTe2 as a natural hyperbolic material across the visible and the ultraviolet region
• Tunable Interband Transitions in Twisted h-BN/Graphene Heterostructures
• Optical activity and transport in twisted bilayer graphene: the essence of spatial dispersion effects
• Electron energy-loss spectroscopy of quasi-two-dimensional crystals: Beyond the energy-loss functions formalism
• Electron energy-loss spectrum and exciton band structure of ${\mathrm{WSe}}_2$ monolayer studied by ab initio Bethe-Salpeter equation calculations
• Ab-initio Van der Waals electrodynamics: polaritons and electron scattering from plasmons and phonons in BN-capped graphene
• Ab initio study of angle-resolved electron reflection spectroscopy of few-layer graphene