Interactions between particles in quantum many-body systems can lead to collective behavior described by hydrodynamics. One such system is the electron-hole plasma in graphene near the charge-neutrality point, which can form a strongly coupled Dirac fluid.This charge-neutral plasma of quasi-relativistic fermions is expected to exhibit a substantial enhancement of the thermal conductivity, thanks to decoupling of charge and heat currents within hydrodynamics. Employing high-sensitivity Johnson noise thermometry, we report an order of magnitude increase in the thermal conductivity and the breakdown of the Wiedemann-Franz law in the thermally populated charge-neutral plasma in graphene.This result is a signature of the Dirac fluid and constitutes direct evidence of collective motion in a quantum electronic fluid.
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We thank M. Foster, D. Efetov, and G.-H. Lee for helpful discussions. The major experimental work at Harvard University is supported by the U.S. Department of Energy (grant DE-SC0012260) and at Raytheon BBN Technologies by Internal Research and Development. J.C. acknowledges support from the Function Accelerated nanoMaterial Engineering (FAME) Center, sponsored by Semiconductor Research Corporation MARCO and Defense Advanced Research Projects Agency. K.W. is supported by Army Research Office (ARO) Multidisciplinary University Research Initiative (MURI) (grant W911NF-14-1-0247). J.K.S. is supported by ARO (grant W911NF-14-1-0638) and the Agency for Science, Technology and Research (A*STAR). P.K. acknowledges partial support from the Gordon and Betty Moore Foundation''s EPiQS Initiative (grant GBMF4543) and the Nano Material Technology Development Program through the National Research Foundation of Korea (grant 2012M3A7B4049966). A.L. and S.S. are supported by the NSF under grant DMR-1360789, the Templeton Foundation, and MURI grant W911NF-14-1-0003 from ARO. Research at the Perimeter Institute for Theoretical Physics is supported by the Government of Canada through Industry Canada and by the Province of Ontario through the Ministry of Research and Innovation. K.W. and T.T. acknowledge support from the Elemental Strategy Initiative conducted by the Ministry of Education, Culture, Sports, Science and Technology, Japan. T.T. acknowledges support from a Grant-in-Aid for Scientific Research (grant 262480621) and a grant on Innovative Areas "Nano Informatics" (grant 25106006) from the Japan Society for the Promotion of Science. T.A.O. and K.C.F. acknowledge support from Raytheon BBN Technologies. This work was performed, in part, at the Center for Nanoscale Systems (CNS), a member of the National Nanotechnology Infrastructure Network, which is supported by the NSF under award no. ECS-0335765. CNS is part of Harvard University.
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