A variety of new and interesting electronic properties have been predicted in graphene monolayer doped to Van Hove singularities (VHSs) of its density-of-state. However, tuning the Fermi energy to reach a VHS of graphene by either gating or chemical doping is prohibitively difficult, owning to their large energy distance (~ 3 eV). This difficulty can be easily overcome in twisted bilayer graphene (TBG). By introducing a small twist angle between two adjacent graphene sheets, we are able to

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... ate two low-energy VHSs arbitrarily approaching the Fermi energy. Here, we report experimental studies of electronic properties around the VHSs of a slightly TBG through scanning tunneling microscopy measurements. The split of the VHSs is observed and the spatial symmetry breaking of electronic states around the VHSs are directly visualized. These exotic results provide motivation for further theoretical and experimental studies of graphene systems around the VHSs. 2 I. INTRODUCTION In the past few years, twisted bilayer graphene (TBG), a system with a twist angle between two adjacent graphene sheets, has been intensively studied [1] [2] [3] [4][5][6][7][8][9][10] [11] [12] [13] [14] [15] [16] [17] [18] . It exhibits many novel electronic properties that distinct from the electronic properties of graphene monolayer and Bernal bilayer. For example, the TBG has angle dependent low-energy Van Hove singularities (VHSs) [1,3-6] and Fermi velocity renormalization [15,16], and it can exhibit energy dependent chiral tunneling [9] and exotic quantum Hall effect [2,17,18]. Among all these unique electronic properties, the low-energy VHSs, which originate from the two low-energy saddle points in the band structures, have attracted much attention over the years in the studies of the TBG. Previously, it was predicted that graphene could exhibit novel correlated states when the Fermi level is close to the VHSs of graphene monolayer [19-25]. Unfortunately, the VHSs of graphene monolayer are too far (~ 3 eV) to be reached in experiment [26]. Therefore, an experimental study of electronic properties at the VHSs of graphene is still lacking up to now. The TBG systems with small twist angles open up new prospects in this direction because their low-energy VHSs are fully accessible with the present doping and gating techniques. Here we show that by reducing the twist angle, it is possible to tune the VHSs of the TBG arbitrarily approaching the Fermi energy. When the VHSs is tuned close to the Fermi level, we observe the split of the VHSs and the distortion of electronic states around the VHSs. Such a result indicates that the TBG can exhibit novel electronic properties around the VHSs. 3 II. EXPERIMENT The TBG was grown on a 25 micron thin Rh foil via a traditional ambient pressure chemical vapor deposition (CVD) method [4] and the growth processes are shown in Supplementary Fig. 1 [27] . The Rh foil was firstly heated from room temperature to 1000 ºC in 30 min under an Ar flow of 850 sccm and a hydrogen gas flow of 100 sccm. Then the furnace was held at the same gas environment for 60 min at 1000 ºC. Finally, CH 4 gas was introduced with a flow ratio of 5 sccm, and the growth time is varied from 3 to 15 min for controlling the thickness of graphene. The sample was then slowly cooled down to room temperature. The scanning tunneling microscopy (STM) system was an ultrahigh vacuum single-probe scanning probe microscope (USM-1300) from UNISOKU with magnetic fields up to 15 T. All STM and scanning tunneling spectroscopy (STS) measurements were performed at liquid-helium temperature (~4.2 K) and the images were taken in a constant-current scanning mode. The STM tips were obtained by chemical etching from a wire of Pt(80%) Ir(20%) alloys. Lateral dimensions observed in the STM images were calibrated using a standard graphene lattice as well as a Si (111)-(7×7) lattice. The STS spectra were calibrated using an Ag (111) surface. The STS spectrum (the dI/dV-V curve) was carried out with a standard lock-in technique by applying alternating current modulation of the bias voltage (871 Hz) to the tunneling bias. The energy resolution of our experiment is limited by both the temperature (~3k B T ≈ 1 meV, where k B is the Boltzmann constant and T~4.2 K) and the oscillation of bias voltage added in quadrature. The STS spectra of low spectroscopic resolution were carried out