Quantized Landau level spectrum and its density dependence in graphene

Scanning tunneling microscopy and spectroscopy in a magnetic field was used to study Landau quantization in graphene and its dependence on charge carrier density. Measurements were carried out on exfoliated graphene samples deposited on a chlorinated SiO2 thermal oxide, which allowed for the observation of the Landau level sequences characteristic of single-layer graphene while tuning the density through the Si backgate. Upon changing the carrier density, we find abrupt jumps in the Fermi level after each Landau level is filled. Moreover, at low doping levels, a marked increase in the Fermi velocity is observed, which is consistent with the logarithmic divergence expected due to the onset of many-body effects close to the Dirac point.