title: Angle-dependent van Hove singularities in a slightly twisted graphene bilayer.
abstract: Recent studies show that two low-energy van Hove singularities (VHSs) seen as two pronounced peaks in the density of states could be induced in a twisted graphene bilayer. Here, we report angle-dependent VHSs of a slightly twisted graphene bilayer studied by scanning tunneling microscopy and spectroscopy. We show that energy difference of the two VHSs follows ΔE(vhs)∼ℏν(F)ΔK between 1.0° and 3.0° [here ν(F)∼1.1 × 10(6) m/s is the Fermi velocity of monolayer graphene, and ΔK = 2Ksin(θ/2) is the shift between the corresponding Dirac points of the twisted graphene bilayer]. This result indicates that the rotation angle between graphene sheets does not result in a significant reduction of the Fermi velocity, which quite differs from that predicted by band structure calculations. However, around a twisted angle θ∼1.3°, the observed ΔE(vhs)∼0.11 eV is much smaller than the expected value ℏν(F)ΔK∼0.28 eV at 1.3°. The origin of the reduction of ΔE(vhs) at 1.3° is discussed.
authors.affiliation: Department of Physics, Beijing Normal University, People's Republic of China.
authors.forename: Wei
authors.initials: W
authors.lastname: Yan
authors.forename: Mengxi
authors.initials: M
authors.lastname: Liu
authors.forename: Rui-Fen
authors.initials: RF
authors.lastname: Dou
authors.forename: Lan
authors.initials: L
authors.lastname: Meng
authors.forename: Lei
authors.initials: L
authors.lastname: Feng
authors.forename: Zhao-Dong
authors.initials: ZD
authors.lastname: Chu
authors.forename: Yanfeng
authors.initials: Y
authors.lastname: Zhang
authors.forename: Zhongfan
authors.initials: Z
authors.lastname: Liu
authors.forename: Jia-Cai
authors.initials: JC
authors.lastname: Nie
authors.forename: Lin
authors.initials: L
authors.lastname: He