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Cross correlations in disordered, four-terminal graphene-ribbon conductor: Hanbury-Brown and Twiss exchange as a sign of non-universality of noise

We have investigated current-current correlations in a cross-shaped conductor made of graphene ribbons. We measured auto and cross correlations and compared them with the theoretical predictions for ideal diffusive conductors. Our data deviate from these predictions and agreement can be obtained only by adding contributions from occupation-number noise in the central region connecting the arms of the cross. Furthermore, we have determined Hanbury -- Brown and Twiss (HBT) exchange correlations in this system. Contrary to expectations for a cross-shaped diffusive system, we find finite HBT exchange effects due to the occupation-number noise at the crossing. The strength of these HBT exchange correlations is found to vary with gate voltage, and very a distinct HBT effect with large fluctuations is observed near the Dirac point.