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

TL;DR

This study reveals that strain and charged-defects can cause significant electron-hole asymmetry in graphene monolayer, with measurable differences in Fermi velocities, impacting the understanding of Dirac fermion behavior.

Contribution

The paper demonstrates direct measurement of electron-hole asymmetry in graphene caused by strain and charged defects using Landau level spectroscopy, highlighting the role of next-nearest-neighbor hopping.

Findings

Strain increases electron-hole velocity asymmetry in graphene.

Charged defects induce opposite asymmetry in electron and hole velocities.

Large asymmetry results from enhanced scattering mechanisms.

Abstract

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 tunnelling of massless Dirac fermions. Here we demonstrate that both strain and charged-defect scattering could break this symmetry dramatically in graphene monolayer. In our experiment, the Fermi velocities of electrons and holes are measured directly through Landau level spectroscopy. In strained graphene with lattice deformation and curvature, the and are measured as 1.2 x 106 m/s and 1.02 x106 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 and are measured to be 0.86x 106 m/s and 1.14 x106 m/s, respectively. Such a large asymmetry is attributed to the fact that the massless Dirac fermions in graphene monolayer are scattered more strongly when they are attracted to the charged-defect than when they are repelled from it.