Bulletin of the American Physical Society
APS March Meeting 2021
Volume 66, Number 1
Monday–Friday, March 15–19, 2021; Virtual; Time Zone: Central Daylight Time, USA
Session Y58: Hydrodynamic Electron FlowInvited Live
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Sponsoring Units: DCMP Chair: James Hone, Columbia Univ |
Friday, March 19, 2021 11:30AM - 12:06PM Live |
Y58.00001: Theory for viscous electron transport in two-dimensional electron systems Invited Speaker: Shaffique Adam When the quantum-mechanical carrier-carrier scattering dominates over all other scattering processes, electrons behave like a classical fluid. In this talk, I will discuss the differences in hydrodynamic transport between monolayer graphene, bilayer graphene and GaAs heterostructures. For example, I will show that hydrodynamics in bilayer graphene is more robust than in monolayer graphene, and has a characteristic “v shape” in the temperature-density plane, as opposed to a “lung shape” for monolayer graphene [1]. For unipolar hydrodynamic electrons like in GaAs heterostructures, the viscous behavior only influences the resistance through interactions with the sample boundaries, that can be modified by patterning crenellations into the sample [2]. On the other hand, for ambipolar electron fluids like bilayer graphene, we show that the electron transport decomposes into two components -- a universal Coulomb drag 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 [3]. Finally, I will discuss the universal hydrodynamic insulator state that can be observed in ultra-clean dual-gated bilayer graphene by electrostatically opening a bandgap in an ambipolar electron fluid. |
Friday, March 19, 2021 12:06PM - 12:42PM Live |
Y58.00002: Disorder-enabled hydrodynamics of charge and heat transport in monolayer graphene Invited Speaker: Giovanni Vignale Hydrodynamic behavior in electronic systems is commonly associated with extremely clean samples, such that momentum-conserving collisions between electrons dominate over momentum-non-conserving electron-impurity and electron-phonon collisions. In this talk I show that semimetals like graphene support two distinct hydrodynamics regimes: an ambipolar hydrodynamic regime near the charge neutrality point (i.e., with nearly equal densities of electrons and holes), and a unipolar hydrodynamic regime when a single species of carriers dominates. The Lorenz ratio between thermal and electric conductivity is enhanced in the ambipolar regime (i.e., it is larger than the universal value of the Wiedemann-Franz law), and is suppressed in the unipolar regime. The crossover between the two regimes, as well as the magnitude of the violation of the Wiedemann-Franz law, is sensitive to the amount of disorder. In particular, the ambipolar regime, characterized by vanishing thermal resistivity and finite electric resistivity is observable only for carrier densities lower than a crossover value nc, which tends to zero in the limit of zero disorder, i.e., when momentum-non-conserving collisions are absent. In this sense the ambipolar hydrodynamics can be viewed as a disorder-enabled hydrodynamics. |
Friday, March 19, 2021 12:42PM - 1:18PM Live |
Y58.00003: Transport in bilayer and multilayer graphene Invited Speaker: Steven Simon Using the semiclassical quantum Boltzmann equation (QBE), we |
Friday, March 19, 2021 1:18PM - 1:54PM Live |
Y58.00004: Bilayer graphene as a model hydrodynamic conductor Invited Speaker: James Hone This talk will describe our work establishing bilayer graphene as a model hydrodynamic semiconductor, in which carrier-carrier collisions play a dominant role in determining the conductivity over a wide range of temperature and carrier density. We measure conductivity of ultraclean bilayer graphene encapsulated within hBN, with dual gates providing independent control over carrier density and bandgap. At charge neutrality, the conductivity is temperature-independent over a wide range to above room temperature, with a magnitude predicted for electron hole scattering. This behavior cannot be accounted for by dissipative scattering from impurities or phonons. In the gapped regime, the charge-neutral conductivity shows scaling behavior predicted by electron-hole scattering. Away from charge neutrality, a two-fluid model is required to accurately reflect the interaction between electron between electron-hole and dissipative scattering mechanisms. In bilayer graphene, a simple limit this model provides quantitative agreement with experiments at all densities, temperatures, and band gaps, using a single set of four parameters. The model allows mapping of the phase space for hydrodynamic conductivity for different disorder levels, and is straightforward to extend to new materials. |
Friday, March 19, 2021 1:54PM - 2:30PM Live |
Y58.00005: Geometric control of universal hydrodynamic flow in a two dimensional electron fluid Invited Speaker: Alex Hamilton Electron transport in most solid state systems is dominated by extrinsic factors, such as sample geometry and scattering from impurities, and is essentially independent of the intrinsic properties of the electron system. An exception is the hydrodynamic regime, in which Coulomb interactions transform electron kinematics from independent particles to the collective motion of a viscous “electron fluid”. The viscosity is a universal and intrinsic property of the electron system, independent of the sample details. Experimental signatures of hydrodynamic flow of the electron fluid are revealed through the interaction with the sample boundaries, just as water flow in a pipe is affected by wide the pipe is and how rough the walls are. In contrast to the universal nature of the viscosity, this roughness is specific to each experiment, introducing an arbitrary and unknown fitting parameter when trying to quantitatively compare experiments with theoretical models. |
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