posted 9 Mar 2021, 06:47 by Peter Boggild
James Hone (2021 - March 8 - 15:00 CET, GMT+1) Columbia University, Dept. of Mechanical Engineering New York NY USA jh2228@columbia.edu
Hydrodynamic electronic transport occurs when carrier-carrier collisions constitute the dominant scattering mechanism. This talk will present two recent studies of hydrodynamic behaviour in monolayer and bilayer graphene. I will first describe our work establishing bilayer graphene a model hydrodynamic semiconductor, in which carrier-carrier collisions play a dominant role over a wide range of temperature and carrier density. Remarkably, a simple model captures the complex interplay between carrier-carrier scattering and conventional dissipative scattering. This model consists of a universal Coulomb drag contribution that dominates at charge neutrality and decays with increasing density, and a non-universal dissipative contribution corresponding to collective motion of the electron-hole plasma. We compare this model to electrical transport measurements of ultraclean bilayer graphene encapsulated within hBN, with dual gates providing independent control over carrier density and bandgap. At charge neutrality, these samples show electron-hole limited conductivity over a wide temperature range. A single set of fit parameters provides quantitative agreement with experiments at all densities, temperatures, and gaps measured, allowing for separate extraction of the electron-hole and dissipative contributions. Our work provides the first complete description of electronic transport in bilayer graphene and provides an intuitive understanding for electron-hole limited transport across a wide range of parameters. In the second part of the talk, I will describe a new approach to determine the viscosity of electrons in graphene using geometrical magnetoresistance (MR) in a Corbino disk. In the degenerate limit where Fermi energy EF is much higher than thermal energy kBT, the shear viscosity scales unexpectedly with 1/T. As kBT approaches EF, we for the first time observe a crossover to a T2 dependence in monolayer graphene, signifying a quantum critical Dirac fluid. Remarkably, viscosity in the entire EF −T phase space is described by a universal expression considering both finite-momentum and zero-momentum modes. These results provide valuable insight into electron hydrodynamics, and the new probe of viscosity developed here can be directly employed in other systems of interest such as twisted bilayer graphene.
James Hone is currently Wang Fong-Jen Professor of Mechanical Engineering at Columbia University, and director of PAS3, Columbia’s Materials Science Research and Engineering Center (MRSEC). He received his BS in physics from Yale in 1990, and PhD in experimental condensed matter physics from UC Berkeley in 1998, and did postdoctoral work at the University of Pennsylvania and Caltech, where he was a Millikan Fellow. He joined the Columbia faculty in 2003.
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