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Observation of the Dirac fluid and the breakdown of the Wiedemann-Franz law in graphene
Crossno, Jesse,Shi, Jing K.,Wang, Ke,Liu, Xiaomeng,Harzheim, Achim,Lucas, Andrew,Sachdev, Subir,Kim, Philip,Taniguchi, Takashi,Watanabe, Kenji,Ohki, Thomas A.,Fong, Kin Chung American Association for the Advancement of Scienc 2016 Science Vol.351 No.6277
<P><B>Electrons that flow like a fluid</B></P><P>Electrons inside a conductor are often described as flowing in response to an electric field. This flow rarely resembles anything like the familiar flow of water through a pipe, but three groups describe counterexamples (see the Perspective by Zaanen). Moll <I>et al.</I> found that the viscosity of the electron fluid in thin wires of PdCoO<SUB>2</SUB> had a major effect on the flow, much like what happens in regular fluids. Bandurin <I>et al.</I> found evidence in graphene of electron whirlpools similar to those formed by viscous fluid flowing through a small opening. Finally, Crossno <I>et al.</I> observed a huge increase of thermal transport in graphene, a signature of so-called Dirac fluids.</P><P><I>Science</I>, this issue p. 1061, 1055, 1058; see also p. 1026</P><P>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.</P>