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 $(1.21\pm 0.03)\times {10}^6\mathrm{m}/\mathrm{s}$ and $(1.02\pm 0.03)\times {10}^6\mathrm{m}/\mathrm{s}$, 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 $(0.86\pm 0.02)\times {10}^6\mathrm{m}/\mathrm{s}$ and $(1.14\pm 0.03)\times {10}^6\mathrm{m}/\mathrm{s}$, 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.

• Received 1 July 2015

DOI:https://doi.org/10.1103/PhysRevB.92.121405

Source: http://link.aps.org/doi/10.1103/PhysRevB.92.121405