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Interest in the use of graphene in electronic devices has motivated an explosion in the study of this remarkable material. The simple, linear, Dirac cone band structure offers a unique possibility to investigate its finer details by angle-resolved photoelectron spectroscopy (ARPES). Indeed, ARPES has been performed on graphene grown on metal substrates but electronic applications require an insulating substrate. Epitaxial graphene grown by the thermal decomposition of silicon carbide (SiC) is an ideal candidate for this due to the large scale, uniform, graphene layers produced. The experimental spectral function of epitaxial graphene on SiC has been extensively studied. However, until now the cause of an anisotropy in the spectral width of the Fermi surface has not been determined. In the current work we show, by comparison of the spectral function to a semi-empirical model, that the anisotropy is due to small scale rotational disorder (~± 0.15°) of graphene domains in graphene grown on SiC(0001) samples. The complicated shape described by the line-width is accurately reproduced by the semi-empirical model only when rotational disorder is included. While spectra from rare regions of the sample containing only one or two rotational domains is also presented. In addition to the direct benefit in the understanding of graphene's electronic structure this work suggests a mechanism to explain similar variations in related ARPES data.
