Dipartimento di Fisica, Università degli Studi della Calabria, 87036 Rende (Cs), Italy.
Materials Science Division, Lawrence Berkeley National Laboratories, Berkeley, California 94720, USA.
Phys Rev Lett. 2015 Aug 14;115(7):075504. doi: 10.1103/PhysRevLett.115.075504.
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~2k_{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 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.
在中性石墨烯中，在 q=Γ（LO 支）和 q=K（TO 支）处的最高光学声子的声子色散中发现了两个显著的尖峰，称为 Kohn 异常，反映了对未掺杂狄拉克电子的显著电子-声子耦合（EPC）。在这项工作中，使用高分辨率电子能量损失谱测量了在 Pt(111)上外延生长的准自由-standing 石墨烯中Γ点附近的声子色散。在从 Γ 到有限动量 q~2k_{F}处观察到 LO 声子的 Kohn 异常，其形状与理论非常吻合，与已知的 EPC 和费米能级值一致。更引人注目的是，我们还在相同动量处观察到了面外光学声子（ZO）支的 Kohn 异常。这一观察结果首次直接证明了 ZO 模式与狄拉克电子的耦合，对于自由-standing 石墨烯来说，这种耦合是被禁止的，但在存在衬底的情况下，这种耦合是被允许的。此外，我们估计 EPC 甚至比 LO 模式更大，使 Pt(111)上的石墨烯成为探索这种新耦合对电子性质影响的最佳体系。
