石墨烯中电子自旋弛豫动力学的飞秒时间分辨磁光谱研究

石墨烯有弱自旋-轨道耦合与超精细作用，被认为是理想的自旋电子学材料。理论预测其自旋弛豫时间为微秒至毫秒量级。然而，至今报道的最长测量自旋弛豫时间仅12ns，比预测值小3-6个数量级。理论预测与实验值的巨大差异的起因至今无共识。我们认为原因在于实验测量对石墨烯施加了强的影响，极大地缩短了石墨烯的电子自旋弛豫时间，导致测量值与预测值不具有可比性。实验测量使用一对自旋阀。我们认为正是自旋阀结构中的磁电极极大地影响了石墨烯的自旋弛豫，导致不能测量到本征石墨烯的自旋弛豫时间。本项目提出非磁半导体/石墨烯异质结，实现全光学自旋极化注入和自旋弛豫动力学时间分辨磁光探测，首次在时域上演示纯洁石墨烯的自旋弛豫动力学及其随磁掺杂浓度的变化。同时，研究EY和DP自旋弛豫机制在石墨烯中的有效性，从而弄清目前报道的测量和理论预测自旋弛豫时间的巨大差异的起因。本项目发展全新的实验技术，具有显著的创新性。

Graphene is regarded as an ideal spintronic material and predicted to have a long spin relaxation time in the range of us to ms due to its weak spin-orbit coupling and hyperfine interaction. However, a short spin relaxation time of only 12 ns was measured so far experimentally, being 3-6 orders of magnitudes shorter than one predicted theoretically. The origin of such a big discrepancy between the measured and predicted values is incompletely understood so far. We think that the origin is from very strong effects of experimental measurements on spin relaxations in graphene, which extremely shortens the spin relaxation time of graphene so that the measured spin relaxation time and predicted one of pristine graphene is incomparable. A pair of spin valves was used in experimental measurement of spin relaxation. We think that it was the magnetic electrodes in spin valves that extremely affected spin relaxation behaviors in graphene, led to that the intrinsic spin relaxation time of graphene could not be measured. In this project, a nonmagnetic semiconductor/graphene heterostructure is proposed to realize all-optical injection of electron spin polarization into graphene, while spin relaxation dynamics is all-optical measured by time-resolved magneto-optical spectroscopy. Spin relaxation dynamics in pristine graphene and their magnetic doping density dependence will be demonstrated directly in time domain for the first time worldwide. Meanwhile, the effectiveness of EY and DP spin relaxation mechanisms will be tested in graphene so that the origin of the big discrepancy between measured and predicted spin relaxation times can be understood completely. New experimental techniques will be developed in this project so that there are significant innovations.