Detecting giant electron-hole asymmetry in a graphene monolayer generated by strain and charged-defect scattering via Landau level spectroscopy

The electron-hole symmetry in graphene monolayer, which is analogous to the inherent symmetric structure between electrons and positrons of the Universe, plays a crucial role in the chirality and chiral tunneling of massless Dirac fermions. Here we demonstrate that both strain and charged-defect scattering could break this symmetry dramatically in a graphene monolayer. In our experiment, the Fermi velocities of electrons v(F)(e) and holes v(F)(h) are measured directly through Landau level spectroscopy. In strained graphene with lattice deformation and curvature, the v(F)(e) and v(F)(h) are measured as (1.21 +/- 0.03) x 10(6) m/s and (1.02 +/- 0.03) x 10(6) m/s, respectively. This giant asymmetry originates from enhanced next-nearest-neighbor hopping in the strained region. Around positively charged defect, we observe opposite electron-hole asymmetry, and the v(F)(e) and v(F)(h) are measured to be (0.86 +/- 0.02) x 10(6) m/s and (1.14 +/- 0.03) x 10(6) m/s, respectively. Such a large asymmetry is attributed to the fact that the massless Dirac fermions in a graphene monolayer are scattered more strongly when they are attracted to the charged defect than when they are repelled from it.