Department of Physics , University of Seoul , Seoul 02504 , Republic of Korea.
Research Group of Food Safety , Korea Food Research Institute , Wanju 55365 , Republic of Korea.
Nano Lett. 2018 Mar 14;18(3):1575-1581. doi: 10.1021/acs.nanolett.7b03566. Epub 2018 Feb 12.
We observe that carriers in graphene can be accelerated to the Fermi velocity without heating the lattice. At large Fermi energy | E| > 110 meV, electrons excited by a high-power terahertz pulse E relax by emitting optical phonons, resulting in heating of the graphene lattice and optical-phonon generation. This is owing to enhanced electron-phonon scattering at large Fermi energy, at which the large phase space is available for hot electrons. The emitted optical phonons cause carrier scattering, reducing the drift velocity or carrier mobility. However, for | E| ≤ 110 meV, electron-phonon scattering rate is suppressed owing to the diminishing density of states near the Dirac point. Therefore, E continues to accelerate carriers without them losing energy to optical phonons, allowing the carriers to travel at the Fermi velocity. The exotic carrier dynamics does not result from the massless nature, but the electron-optical-phonon scattering rate depends on Fermi level in the graphene. Our observations provide insight into the application of graphene for high-speed electronics without degrading carrier mobility.
我们观察到，在不加热晶格的情况下，石墨烯中的载流子可以被加速到费米速度。在较大的费米能 | E | > 110 meV 下，由高功率太赫兹脉冲 E 激发的电子通过发射光学声子来弛豫，这导致石墨烯晶格的加热和光学声子的产生。这是由于在大费米能下增强了电子-声子散射，在这种情况下，热电子有很大的相空间可供利用。发射的光学声子引起载流子散射，降低了漂移速度或载流子迁移率。然而，对于 | E | ≤ 110 meV，由于在狄拉克点附近的态密度减小，电子-声子散射率受到抑制。因此，E 继续加速载流子而不会使其能量损失给光学声子，从而使载流子以费米速度运动。这种奇特的载流子动力学不是来自于无质量的性质，而是电子与光学声子的散射率取决于石墨烯中的费米能级。我们的观察结果为在不降低载流子迁移率的情况下将石墨烯应用于高速电子学提供了深入的了解。
