Owing to a linear and gapless band structure and a tunability of the chargecarrier type, graphene offers a unique system to investigate transport of DiracFermions at p-n junctions (PNJs). In a magnetic field, combination of quantumHall physics and the characteristic transport across PNJs leads to afractionally quantized conductance associated with the mixing of electron-likeand hole-like modes and their subsequent partitioning. The mixing andpartitioning suggest that a PNJ could be used as an electronic beam-splitter.Here we report the shot noise study of the mode mixing process and demonstratethe crucial role of the PNJ length. For short PNJs, the amplitude of the noiseis consistent with an electronic beam-splitter behavior, whereas, for longerPNJs, it is reduced by the energy relaxation. Remarkably, the relaxation lengthis much larger than typical size of mesoscopic devices, encouraging usinggraphene for electron quantum optics and quantum information processing.
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