Emergence of a ZO Kohn anomaly in quasi-freestanding epitaxial graphene

Abstract

In neutral graphene, two prominent cusps known as Kohn anomalies are found in the phonon dispersion of the highest optical phonon at $q=\Gamma$ (LO branch) and $q=K$ (TO branch), reflecting a significant electron-phonon coupling to undoped Dirac electrons. In this work, high-resolution electron energy loss spectroscopy is used to measure the phonon dispersion around the $\Gamma$ point in quasi-freestanding graphene epitaxially grown on Pt(111). The Kohn anomaly for the LO phonon is observed at finite momentum $q\sim 2k_F$ from $\Gamma$, with a shape in excellent agreement with the theory and consistent with known values of the EPC and the Fermi level. More strikingly, we also observe a Kohn anomaly at the same momentum for the out-of-plane optical phonon (ZO) branch. This observation is the first direct evidence of the coupling of the ZO mode with Dirac electrons, which is forbidden for freestanding graphene but becomes allowed in the presence of a substrate. Moreover, we estimate the EPC to be even greater than that of the LO mode, making graphene on Pt(111) an optimal system to explore the effects of this new coupling in the electronic properties.

Final version with journal reference. Significantly more experimental data was added to the main text and the supplemental material. 5 pages of main text + 6 of supplement

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• Emergent Chern-Simons excitations due to electron--phonon interaction
• Symmetry-breaking supercollisions in Landau-quantized graphene
• Electron-phonon Coupling and Kohn Anomaly due to the Floating 2D Electronic Bands on the Surface of ZrSiS
• In-plane optical phonon modes of current-carrying graphene
• Friedel oscillation near a van Hove singularity in two-dimensional Dirac materials
• Coexistence of spontaneous polarization and superconductivity in hole-doped oxyhydrides ATiO_2H (A=K, Rb, Cs): first-principles study