  Landau level broadening mechanisms in electrically neutral and quasineutral
graphene were investigated through micro-magneto-Raman experiments in three
different samples, namely, a natural single-layer graphene flake and a
back-gated single-layer device, both deposited over Si/SiO2 substrates, and a
multilayer epitaxial graphene employed as a reference sample. Interband Landau
level transition widths were estimated through a quantitative analysis of the
magnetophonon resonances associated with optically active Landau level
transitions crossing the energy of the E_2g Raman-active phonon. Contrary to
multilayer graphene, the single-layer graphene samples show a strong damping of
the low-field resonances, consistent with an additional broadening contribution
of the Landau level energies arising from a random strain field. This extra
contribution is properly quantified in terms of a pseudomagnetic field
distribution Delta_B = 1.0-1.7 T in our single-layer samples.
