Cooperative Electron-Phonon Coupling and Buckled Structure in Germanene on Au(111)

Germanene, a single-atom-thick germanium nanosheet in a honeycomb lattice, was proposed to be a Dirac fermion material beyond graphene. We performed scanning tunneling microscopy and in situ Raman spectroscopy studies combined with first-principles calculations on the atomic structures and the electronic and phonon properties of germanene on Au(111). The low-buckled 1 X 1 germanene honeycomb lattice was determined to exist in an unexpected rectangular root 7 X root 7 superstructure. Through in situ Raman measurements, distinctive vibrational phonon modes were discovered in root 7 X root 7 germanene, revealing the special coupling between the Dirac fermion and lattice vibrations, namely, electron phonon coupling (EPC). The significant enhancement of EPC is correlated with the tensile strain, which is evoked by the singular buckled structure of root 7 X root 7 germanene on the Au(111) substrate. Our results present clear evidence for the existence of epitaxial germanene and elucidate the exotic properties of germanene on Au(111).