Anharmonicities in phonon combinations and overtones in bilayered graphene: A temperature-dependent approach

This paper studies phonon anharmonicities related to the phonon combination LOZO′ and phonon overtone 2ZO in a AB-stacked bilayer graphene (2LG). The results explain in detail the rule of the ZO′ layer breathing mode in the 2LG electron and phonon relaxations, especially at temperatures above 543 K, where anomalous behaviors are observed for the LOZO′ frequencies, linewidths (and therefore, lifetimes), and integrated areas. Surprisingly, the 2ZO frequencies and linewidths do not show any dependence with temperature (ZO is the out-of-phase vibration of the layers). This result is explained via nonsymmetric lattice distortions and via the almost null Grüneisen parameter associated to the ZO mode. Recently, the correct assignments for the phonon combination and overtone modes studied here have been put in debate once again in a theoretical work by Popov [Carbon 91, 436 (2015)]. This work shows how temperature-dependent Raman spectroscopy is used to propose a solution for these recent assignment problems. Finally, although 2LG is the system used here, the measurements and discussions to approach electron and phonon relaxations have the potential to be extended to any other multilayered structure that presents ZO′- and ZO-like phonon modes.