Scientific recognition for Prof. R. Asgari


Dynamics of electrons in two-dimensional (2D) crystal materials like graphene is a subject of considerable interest, somewhat owing to its relevance to a wide variety of potential electronic and opto-electronic applications.  Optical excitations generate non-equilibrium distribution of carriers and optical spectroscopy provides the best process of determining such distribution functions. The purpose of ultrafast spectroscopy is to explore the carrier relaxation dynamics and transport dynamics after ultra-short pulse excitation. The rise time of non-degenerate electron distribution creation is of the order of a few femtoseconds (fs) therefore, we may say that high temperature non-equilibrium electron distribution has the same rise time as the laser pulse duration. Then over a time scale of a few fs, the non-equilibrium electrons redistribute their energy among themselves. Generally, it takes place through electron-electron, electron-phonon and electron-impurity interactions. Coupling between layers is often extremely important for the electronic and opto-electronic properties of multilayer van der Waals systems, including multilayer graphene structures. Because the coupling between phonon modes localized in different layers is exceptionally weak in these materials, unconventional effects can become important. The new paper to appear in Nature Communications in which Prof. Asgari is a coauthor, presents a detailed experimental and theoretical study of a unique electronic cooling pathway observed in multilayer epitaxial graphene systems. We demonstrate that under many circumstances direct action-at-a-distance Coulomb scattering can lead to important heat transport between remote layers, bypassing the lattice, and in particular that this can be the dominant electron-cooling mechanism in the important case of multilayer epitaxial graphene devices. Basically, we compute the cooling powers of both acoustic phonon cooling and disorder-assisted electron-phonon cooling of a multilayer graphene and it turns out that interlayer Coulombic energy transfer can dominate for a wide range of electron temperature and sample characteristics. This paper addresses the fundamental theory of Coulomb coupling between remote 2D electron systems, and the nature of heat extraction in multilayer systems on the nanoscale. A theory of hot-carrier equilibration is developed which is based on interlayer energy transfer via screened Coulomb interactions. This theory is free of any fitting parameters for calculating thermal equilibration times, compare closely with the experimental relaxation time. Although this paper focuses on multilayer graphene systems, there are significant implications for more general stacks. This work shows, perhaps surprisingly, that the Coulomb scattering mechanism can be important even at quite high temperature, not just the single-Kelvin regime. The model calculations show this could be quite general and not restricted to just graphene multilayers, and indeed could apply to many stacked systems people are currently investigating.


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