Damping of Landau levels in neutral graphene at low magnetic fields: A   phonon Raman scattering study
  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. 
F. M. Ardito, T. G. Mendes-de-Sa, A. R. Cadore, P. F. Gomes, D. L.   Mafra, I. D. Barcelos, R. G. Lacerda, F. Iikawa, and E....
cond-mat.mtrl-sci cond-mat.mes-hall