Abstract: Transport in systems with many particles experiencing frequent mutual collisions (such as gases or liquids) has been studied for more than two centuries and is accurately described by the theory of hydrodynamics. It has been argued theoretically for a long time that the collective behaviour of charge carriers in solids can also be treated by the hydrodynamic approach. However, despite attempts, until recently very little evidence of hydrodynamic electron transport has been found.
Graphene encapsulated between hexagonal boron nitride (hBN) offers an ideal platform to study electron hydrodynamics as it hosts an ultra-clean electronic system with electron-electron collisions being the dominant scattering source above liquid nitrogen temperatures. In the first part of my talk we will discuss why electron hydrodynamics has not been observed before and how it manifests itself in graphene. It will be shown that electrons in graphene can behave as a very viscous fluid forming vortices of applied electron current [1,2]. In the second part, we will discuss methods which can be applied to measure electron viscosity and talk about superballistic flow of viscous electron fluids through graphene point contacts [3]. Then we will talk about the behaviour of electron fluids in the presence of magnetic field where I will report the experimental measurements of the odd (Hall) viscosity in two dimensions [4]. This dissipationless transport coefficient has been widely discussed in theoretical literature on fluid mechanics, plasma physics and condensed matter physics, yet, until now, any experimental evidence has been lacking, making the phenomenon truly a unicorn. Last but not least, we will discuss how electron hydrodynamics can motivate the development of resonant terahertz detectors and I will report some recent progress in this direction [5].
[1] Negative Local Resistance Caused by Viscous Electron Backflow in Graphene, D. A. Bandurin, A. Principi, G.H. Auton, E. Khestanova, K.S. Novoselov, I. V Grigorieva, L.A. Ponomarenko, A.K. Geim, and M. Polini, Science 351, 1055 (2016).
[2] Fluidity Onset in Graphene, D. A. Bandurin, A. Shytov, L. S. Levitov, R. Krishna Kumar, A. I. Berdyugin, M. Ben Shalom, I. V. Grigorieva. A. K. Geim and G. Falkovich, Nat. Comm. 9, 4533 (2018).
[3] Superballistic Flow of Viscous Electron Fluid through Graphene Constrictions, R. Krishna Kumar, D.A. Bandurin, F.M.D. Pellegrino, Y. Cao, A. Principi, H. Guo, G.H. Auton, M. Ben Shalom, L.A. Ponomarenko, G. Falkovich, I. V. Grigorieva, L.S. Levitov, M. Polini, and A.K. Geim, Nat. Phys. 13, 1182 (2017).
[4] Measuring Hall viscosity of Graphene’s Electron Fluid, I. Berdyugin, S. G. Xu, F. M. D. Pellegrino, R. Krishna Kumar, A. Principi, I. Torre, M. Ben Shalom, T. Taniguchi, K. Watanabe, I. V. Grigorieva, M. Polini, A. K. Geim and D. A. Bandurin, Science 364, 6436, 162-165 (2019).
[5] Resonant Terahertz Detection Using Graphene Plasmons, D. A. Bandurin, D. Svintsov, I. Gayduchenko, S. G. Xu, A. Principi, M. Moskotin, I. Tretyakov, D. Yagodkin, S. Zhukov, T. Taniguchi, K. Watanabe, I. V. Grigorieva, M. Polini, G. Goltsman, A. K. Geim and G. Fedorov, Nat. Comm. 9, 5392 (2018).