Abstract
Ballistic transport occurs whenever electrons propagate without collisions deflecting their trajectory. It is normally observed in conductors with a negligible concentration of impurities, at low temperature, to avoid electron–phonon scattering. Here, we use suspended bilayer graphene devices to reveal a new regime, in which ballistic transport is not limited by scattering with phonons or impurities, but by electron–hole collisions. The phenomenon manifests itself in a negative four-terminal resistance that becomes visible when the density of holes (electrons) is suppressed by gate-shifting the Fermi level in the conduction (valence) band, above the thermal energy. For smaller densities, transport is diffusive, and the measured conductivity is reproduced quantitatively, with no fitting parameters, by including electron–hole scattering as the only process causing velocity relaxation. Experiments on a trilayer device show that the phenomenon is robust and that transport at charge neutrality is governed by the same physics. Our results provide a textbook illustration of a transport regime that had not been observed previously and clarify the nature of conduction through charge-neutral graphene under conditions in which carrier density inhomogeneity is immaterial. They also demonstrate that transport can be limited by a fully electronic mechanism, originating from the same microscopic processes that govern the physics of Dirac-like plasmas.
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Acknowledgements
We gratefully acknowledge A. Ferreira for continued technical support of the experiments. We are also grateful to D. Abanin, V. Fal’ko, T. Giamarchi, L. S. Levitov, M. Müller, M. Polini, J. Song, D. Valentinis, D. van der Marel and J. Wallbank for very helpful discussions. Financial support from the Swiss National Science Foundation, the NCCR QSIT, and the EU Graphene Flagship Project are also gratefully acknowledged.
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Y.N., D.-K.K. and D.S.-D. fabricated devices and Y.N. and D.-K.K. performed measurements. A.F.M. derived the expression for the conductivity and supervised the analysis of the data done by Y.N. Y.N., D.-K.K. and A.F.M. wrote the manuscript. All authors discussed the results and contributed to their interpretation.
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Nam, Y., Ki, DK., Soler-Delgado, D. et al. Electron–hole collision limited transport in charge-neutral bilayer graphene. Nature Phys 13, 1207–1214 (2017). https://doi.org/10.1038/nphys4218
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DOI: https://doi.org/10.1038/nphys4218
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