  The pseudo-magnetic field generated by mechanical strain in graphene can have
dramatic consequences on the behavior of electrons and holes. Here we show that
pseudo-magnetic field fluctuations present in crumpled graphene can induce
significant intravalley scattering of charge carriers. We detect this by
measuring the confocal Raman spectra of crumpled areas, where we observe an
increase of the D'/D peak intensity ratio by up to a factor of 300. We
reproduce our observations by numerical calculation of the double resonant
Raman spectra and interpret the results as experimental evidence of the phase
shift suffered by Dirac charge carriers in the presence of a pseudo-magnetic
field. This lifts the restriction on complete intravalley backscattering of
Dirac fermions.
