  Twist, as a simple and unique degree of freedom, could lead to enormous novel
quantum phenomena in bilayer graphene. A small rotation angle introduces
low-energy van Hove singularities (VHSs) approaching the Fermi level, which
result in unusual correlated states in the bilayer graphene. It is reasonable
to expect that the twist could also affect the electronic properties of
few-layer graphene dramatically. However, such an issue has remained
experimentally elusive. Here, by using scanning tunneling
microscopy/spectroscopy (STM/STS), we systematically studied a twisted trilayer
graphene (TTG) with two different small twist angles between adjacent layers.
Two sets of VHSs originating from the two twist angles were observed in the
TTG, indicating that the TTG could be simply regarded as a combination of two
different twisted bilayer graphene. By using high-resolution STS, we observed
split of the VHSs and directly imaged spatial symmetry breaking of electronic
states around the VHSs. These results suggest that electron-electron
interactions play an important role in affecting the electronic properties of
graphene systems with low-energy VHSs.
