D. L. Mafra

Group-theory analysis of electrons and phonons in N-layer graphene systems

In this work we study the symmetry properties of electrons and phonons in graphene systems as a function of the number of layers. We derive the selection rules for the electron-radiation interactions and for the electron-phonon interactions at all points in the Brillouin zone. By considering these selection rules, we address the double-resonance Raman-scattering process. The monolayer and bilayer graphenes in the presence of an applied electric field are also discussed.

Phonon self-energy corrections to nonzero wave-vector phonon modes in single-layer graphene.

Phonon self-energy corrections have mostly been studied theoretically and experimentally for phonon modes with zone-center (q=0) wave vectors. Here, gate-modulated Raman scattering is used to study phonons of a single layer of graphene originating from a double-resonant Raman process with q≠0. The observed phonon renormalization effects are different from what is observed for the zone-center q=0 case. To explain our experimental findings, we explored the phonon self-energy for the phonons with nonzero wave vectors (q≠0) in single-layer graphene in which the frequencies and decay widths are expected to behave oppositely to the behavior observed in the corresponding zone-center q=0 processes. Within this framework, we resolve the identification of the phonon modes contributing to the G(⋆) Raman feature at 2450 cm(-1) to include the iTO+LA combination modes with q≠0 and also the 2iTO overtone modes with q=0, showing both to be associated with wave vectors near the high symmetry point K in the Brillouin zone.

A study of inner process double-resonance Raman scattering in bilayer graphene

The dispersion of phonons and the electronic structure of graphene systems can be obtained experimentally from the double-resonance (DR) Raman features by varying the excitation laser energy. In a previous resonance Raman investigation of graphene, the electronic structure was analyzed in the framework of the Slonczewski-Weiss-McClure (SWM) model, considering the outer DR process. In this work we analyze the data considering the inner DR process, and obtain SWM parameters that are in better agreement with those obtained from other experimental techniques. This result possibly shows that there is still a fundamental open question concerning the double resonance process in graphene systems.

Mass-related inversion symmetry breaking and phonon self-energy renormalization in isotopically labeled AB-stacked bilayer graphene

A mass-related symmetry breaking in isotopically labeled bilayer graphene (2LG) was investigated during in-situ electrochemical charging of AB stacked (AB-2LG) and turbostratic (t-2LG) layers. The overlap of the two approaches, isotopic labeling and electronic doping, is powerful tool and allows to tailor, independently and distinctly, the thermal-related and transport-related phenomena in materials, since one can impose different symmetries for electrons and phonons in these systems. Variations in the systems phonon self-energy renormalizations due to the charge distribution and doping changes could be analyzed separately for each individual layer. Symmetry arguments together with first-order Raman spectra show that the single layer graphene (1LG), which is directly contacted to the electrode, has a higher concentration of charge carriers than the second graphene layer, which is not contacted by the electrode. These different charge distributions are reflected and demonstrated by different phonon self-energy renormalizations of the G modes for AB-2LG and for t-2LG.

Observation of the Kohn anomaly near the K point of bilayer graphene

The dispersion of electrons and phonons near the

Unraveling the interlayer-related phonon self-energy renormalization in bilayer graphene

K

Characterizing intrinsic charges in top gated bilayer graphene device by Raman spectroscopy

point of bilayer graphene was investigated in a resonant Raman study using different laser excitation energies in the near-infrared and visible range. The electronic structure was analyzed within the tight-binding approximation, and the Slonczewski-Weiss-McClure parameters were obtained from the analysis of the dispersive behavior of the Raman features. A softening of the phonon branches was observed near the

Determination of LA and TO phonon dispersion relations of graphene near the Dirac point by double resonance Raman scattering

K

Resonance Raman scattering in graphene: Probing phonons and electrons

point and results evidence the Kohn anomaly and the importance of considering electron-phonon and electron-electron interactions to correctly describe the phonon dispersion in graphene systems, confirming the theoretical predictions by Lazzeri et al.

Electronic properties of bilayer graphene probed by Resonance Raman Scattering

In this letter, we present a step towards understanding the bilayer graphene (2LG) interlayer (IL)-related phonon combination modes and overtones as well as their phonon self-energy renormalizations by using both gate-modulated and laser-energy dependent inelastic scattering spectroscopy. We show that although the IL interactions are weak, their respective phonon renormalization response is significant. Particularly special, the IL interactions are mediated by Van der Waals forces and are fundamental for understanding low-energy phenomena such as transport and infrared optics. Our approach opens up a new route to understanding fundamental properties of IL interactions which can be extended to any graphene-like material, such as MoS2, WSe2, oxides and hydroxides. Furthermore, we report a previously elusive crossing between IL-related phonon combination modes in 2LG, which might have important technological applications.