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In neutral graphene, two prominent cusps known as Kohn anomalies are found in the phonon dispersion of the highest optical phonon at q= Γ (LO branch) and q= K (TO branch), reflecting a significant electron-phonon coupling (EPC) to undoped Dirac electrons. In this work, high-resolution electron energy loss spectroscopy is used to measure the phonon dispersion around the Γ point in quasifreestanding graphene epitaxially grown on Pt (111). The Kohn anomaly for the LO phonon is observed at finite momentum q∼ 2 k F from Γ, 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 …
American Physical Society
Publication date: 
14 Aug 2015

Antonio Politano, Fernando de Juan, Gennaro Chiarello, Herbert A Fertig

Biblio References: 
Volume: 115 Issue: 7 Pages: 075504
Physical review letters