When the Fermi level is aligned with the Dirac point of graphene, reduced charge screening greatly enhances electron–electron scattering1,2,3,4,5. In an optically excited system, the kinematics of electron–electron scattering in Dirac fermions is predicted to give rise to novel optoelectronic phenomena6,7,8,9,10,11. In this paper, we report on the observation of an intrinsic photocurrent in graphene, which occurs in a different parameter regime from all the previously observed photothermoelectric or photovoltaic photocurrents in graphene12,13,14,15,16,17,18,19,20: the photocurrent emerges exclusively at the charge neutrality point, requiring no finite doping. Unlike other photocurrent types that are enhanced near p–n or contact junctions, the photocurrent observed in our work arises near the edges/corners. By systematic data analyses, we show that the phenomenon stems from the unique electron–electron scattering kinematics in charge-neutral graphene. Our results not only highlight the intriguing electron dynamics in the optoelectronic response of Dirac fermions, but also offer a new scheme for photodetection and energy harvesting applications based on intrinsic, charge-neutral Dirac fermions.
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We thank L. Levitov, V. Phong, F. Koppens, K.-J. Tielrooij, M. Lundeberg, A. Woessner and O. Shtanko for discussions. We also thank Y. Sun and B. Han for help with device fabrication. Work in the P.J.-H. group was partly supported by the Center for Excitonics, an Energy Frontier Research Center funded by the US Department of Energy (DOE), Office of Science, Office of Basic Energy Sciences (BES) under award number DESC0001088 (fabrication and measurement) and partly through AFOSR grant number FA9550-16-1-0382 (data analysis), as well as the Gordon and Betty Moore Foundation’s EPiQS Initiative through grant number GBMF4541 to P.J.-H. This work made use of the MRSEC Shared Experimental Facilities at MIT, supported by the National Science Foundation (NSF) under award number DMR-14-19807 and of Harvard CNS, supported by NSF ECCS under award number 1541959. Y.C., Y.L., T.P., W.F., J.K., S.-Y.X. and N.G. acknowledge funding support by the STC Center for Integrated Quantum Materials, NSF grant number DMR-1231319. Y.L. and T.P. also acknowledge the US Army Research Office through the MIT Institute for Soldier Nanotechnologies, under award number W911NF-18-2-0048. K.W. and T.T. acknowledge support from the Elemental Strategy Initiative conducted by the MEXT, Japan, JSPS KAKENHI grant numbers JP18K19136 and the CREST (JPMJCR15F3), JST. N.M.G. is supported by the Air Force Office of Scientific Research Young Investigator Program (YIP) award no. FA9550-16-1-0216 and by the NSF Division of Materials Research CAREER award no. 1651247. N.M.G. also acknowledges support through a Cottrell Scholar Award, and through the Canadian Institute for Advanced Research (CIFAR) Azrieli Global Scholar Award. J.C.W.S. acknowledges support from the Singapore National Research Foundation (NRF) under NRF fellowship award NRF-NRFF2016-05 and a Nanyang Technological University (NTU) start-up grant (NTU-SUG).