Abstract
A periodically driven system with spatial asymmetry can exhibit a directed motion facilitated by thermal or quantum fluctuations1. This so-called ratchet effect2 has fascinating ramifications in engineering and natural sciences3,4,5,6,7,8,9,10,11,12,13,14,15,16,17,18. Graphene19 is nominally a symmetric system. Driven by a periodic electric field, no directed electric current should flow. However, if the graphene has lost its spatial symmetry due to its substrate or adatoms, an electronic ratchet motion can arise. We report an experimental demonstration of such an electronic ratchet in graphene layers, proving the underlying spatial asymmetry. The orbital asymmetry of the Dirac fermions is induced by an in-plane magnetic field, whereas the periodic driving comes from terahertz radiation. The resulting magnetic quantum ratchet transforms the a.c. power into a d.c. current, extracting work from the out-of-equilibrium electrons driven by undirected periodic forces. The observation of ratchet transport in this purest possible two-dimensional system indicates that the orbital effects may appear and be substantial in other two-dimensional crystals such as boron nitride, molybdenum dichalcogenides and related heterostructures. The measurable orbital effects in the presence of an in-plane magnetic field provide strong evidence for the existence of structure inversion asymmetry in graphene.
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Acknowledgements
The authors thank V.V. Bel'kov and E.L. Ivchenko for fruitful discussions. The authors also acknowledge support from the German Research Foundation (DFG) through projects SPP 1459, GRK 1570 and SFB 689, the European Union through ConceptGraphene, a linkage Grant of the International Bureau of the Federal Ministry of Education and Research (BMBF) at the German Aerospace Center (DLR), the Russian Foundation for Basic Research (RFBR), the Russian Federation President Grant MD-2062.2012.2, and the ‘Dynasty’ Foundation.
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S.D.G. and S.A.T. conceived the experiments. C.D., P.O., J.Ka., M.H., F.M. and S.D.G. designed the experimental set-up and performed the measurements. C.D., P.O., S.D.G. and S.A.T. analysed the data. R.Y., S.L-A., S.K., J.Ko., P.M.A., M.W. and R.V. grew, fabricated and characterized samples. S.A.T. developed the microscopic theory. J.F. and M.G. performed the first-principles calculations. S.D.G., S.A.T., J.F., C.D., P.O. and M.G. co-wrote the paper. All authors commented on the manuscript.
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Drexler, C., Tarasenko, S., Olbrich, P. et al. Magnetic quantum ratchet effect in graphene. Nature Nanotech 8, 104–107 (2013). https://doi.org/10.1038/nnano.2012.231
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DOI: https://doi.org/10.1038/nnano.2012.231
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