Jadidi M Mehdi, Suess Ryan J, Tan Cheng, Cai Xinghan, Watanabe Kenji, Taniguchi Takashi, Sushkov Andrei B, Mittendorff Martin, Hone James, Drew H Dennis, Fuhrer Michael S, Murphy Thomas E
Institute for Research in Electronics & Applied Physics, University of Maryland, College Park, Maryland 20742, USA.
Department of Mechanical Engineering, Columbia University, New York, New York 10027, USA.
Phys Rev Lett. 2016 Dec 16;117(25):257401. doi: 10.1103/PhysRevLett.117.257401. Epub 2016 Dec 13.
Hot electron effects in graphene are significant because of graphene's small electronic heat capacity and weak electron-phonon coupling, yet the dynamics and cooling mechanisms of hot electrons in graphene are not completely understood. We describe a novel photocurrent spectroscopy method that uses the mixing of continuous-wave lasers in a graphene photothermal detector to measure the frequency dependence and nonlinearity of hot-electron cooling in graphene as a function of the carrier concentration and temperature. The method offers unparalleled sensitivity to the nonlinearity, and probes the ultrafast cooling of hot carriers with an optical fluence that is orders of magnitude smaller than in conventional time-domain methods, allowing for accurate characterization of electron-phonon cooling near charge neutrality. Our measurements reveal that near the charge neutral point the nonlinear power dependence of the electron cooling is dominated by disorder-assisted collisions, while at higher carrier concentrations conventional momentum-conserving cooling prevails in the nonlinear dependence. The relative contribution of these competing mechanisms can be electrostatically tuned through the application of a gate voltage-an effect that is unique to graphene.
由于石墨烯的电子热容小且电子-声子耦合弱，石墨烯中的热电子效应很显著，但石墨烯中热电子的动力学和冷却机制尚未完全被理解。我们描述了一种新颖的光电流光谱方法，该方法利用连续波激光在石墨烯光热探测器中的混合来测量石墨烯中热电子冷却的频率依赖性和非线性，作为载流子浓度和温度的函数。该方法对非线性具有无与伦比的灵敏度，并用比传统时域方法小几个数量级的光通量探测热载流子的超快冷却，从而能够精确表征电荷中性附近的电子-声子冷却。我们的测量结果表明，在电荷中性点附近，电子冷却的非线性功率依赖性主要由无序辅助碰撞主导，而在较高载流子浓度下，传统的动量守恒冷却在非线性依赖性中占主导。这些竞争机制的相对贡献可以通过施加栅极电压进行静电调节——这是石墨烯特有的效应。
