Direct probing of the stacking order and electronic spectrum of rhombohedral trilayer graphene with scanning tunneling microscopy

Recently, rhombohedral trilayer graphene (r-TLG) has attracted much attention because of its low-energy flat bands, which are predicted to result in many strongly correlated phenomena. However, there has been a need for more experimental evidence for these flat bands in the r-TLG, since the supporting substrates usually have strong destructive effects on the low-energy band structure of graphene systems. Here, we demonstrate that it is possible to directly probe the stacking order and electronic spectrum of the r-TLG on a graphite surface with scanning tunneling microscopy around a monoatomic step edge of the top graphene layer. The tunneling spectra of the r-TLG exhibit four adjacent peaks, which are generated by the low-energy flat bands, flanking the charge neutrality point. Based on these spectra, the true energy gap and the energy gap at the K point of the r-TLG are determined as about 9 and 23 meV, respectively. The observed features are well reproduced by a low-energy effective Hamiltonian.