Observation of phonon peaks and electron-phonon bound states in graphene

Theoretically, coupling of phonons with quasiparticles in the Landau levels (LLs) of graphene was predicted to generate a new sequence of discrete states in the density of states, i.e., phonon peaks. However, it was believed that these phonon peaks are extremely weak due to weak electron-phonon coupling in graphene. Here, we report the experimental observation of giant phonon peaks in a graphene monolayer induced by enhanced electron-phonon coupling. We demonstrate that emission or absorption of phonons of quasiparticles in the LLs of graphene generates the phonon peaks, which even exhibits fine structures corresponding to bound states of an electron and of a phonon. We also show that it is possible to tune the relative strength of the phonon peaks at nanoscale by controlling interactions between graphene and the supporting substrate. The local strain in graphene may play a vital role in enhancing the electron-phonon coupling, which results in the observed giant phonon peaks. Our experiment indicates that it is possible to explore interesting emergent phenomena, such as superconductivity, induced by electron-phonon coupling in graphene.