Graphene has emerged as an electronic material that is promising for deviceapplications and for studying two-dimensional electron gases with relativisticdispersion near two Dirac points. Nonetheless, deviations from Dirac-likespectroscopy have been widely reported with varying interpretations. Here weshow evidence for strain-induced spatial modulations in the local conductanceof single-layer graphene on SiO2 substrates from scanning tunneling microscopic(STM) studies. We find that strained graphene exhibits parabolic, U-shapedconductance vs. bias voltage spectra rather than the V-shaped spectra expectedfor Dirac fermions, whereas V-shaped spectra are recovered in regions ofrelaxed graphene. Strain maps derived from the STM studies further revealdirect correlation with the local tunneling conductance. These results areattributed to a strain-induced frequency increase in the out-of-plane phononmode that mediates the low-energy inelastic charge tunneling into graphene.
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