  The relativistic nature of Dirac electrons and holes in graphene profoundly
affects the way they interact with impurities. Signatures of the relativistic
behavior have been observed recently in scanning tunneling measurements on
individual impurities, but the conductance measurements in this regime are
typically dominated by electron and hole puddles. Here we present measurements
of quantum interference noise and magnetoresistance in graphene pn junctions.
Unlike the conductance, the quantum interference noise can provide access to
the scattering at the Dirac point:it is sensitive to the motion of a single
impurity, it depends strongly on the fundamental symmetries that describe the
system and it is determined by the phase-coherent phenomena which are not
necessarily obscured by the puddles. The temperature and the carrier density
dependence of resistance fluctuations and magnetoresistance in graphene p-n
junctions at low temperatures suggest that the noise is dominated by the
quantum interference due to scattering on impurities and that the noise minimum
could be used to determine the point where the average carrier density is zero.
At larger carrier densities, the amplitude of the noise depends strongly on the
sign of the impurity charge, reflecting the fact that the electrons and the
holes are scattered by the impurity potential in an asymmetric manner.
