Interaction of surface acoustic waves with epitaxial graphene

本项目研究了表面声波（saw）与SiC上外延石墨烯（EG）层的相互作用。saw产生可调谐的应变场和压电场，可调节石墨烯带结构并与载流子强相互作用。这些场的运动特性对于捕获载流子并以明确的速度传输它们特别有趣。我们将探索石墨烯层中载流子和自旋的控制输运的这些特征。反过来，这需要更好地了解石墨烯中从saw到载流子的动量传递过程，以及saw诱导的应变和压电场对电子能带结构和自旋分裂的影响。用于声学调制实验的外延石墨烯层将采用表面石墨化方法制备。研究将分别在SiC的Si面和C面上生长单层和多层石墨烯。在所有情况下，用于saw应用的样品的处理和电声电流的测量将直接在EG/SiC结构上完成。石墨烯中SAW压电场与载流子之间相互作用的研究将集中在实现非线性机制上，其中强压电场诱导石墨烯电荷密度的高调制。为了实现这种非线性传输机制，将探索沿EG/SiC结构传播的SAW压电场的强度增强，以及通过将费米能级位移到狄拉克点来降低石墨烯中的平均载流子密度。在这些条件下，载流子将被强烈地限制在压电能量的最小值处，在那里它们将以明确的SAW速度移动。石墨烯也是一种很有前途的自旋信息输运材料。使用铁磁源和漏极触点将允许自旋注入和提取到石墨烯通道中，以实现其声学传输。由于多层EG的低衬底诱导散射和低自旋-轨道耦合，期望具有较长的自旋弛豫时间。这一点，再加上SiC中快速、明确的声表面波速度，应该允许沿着数百微米的距离进行声自旋传输。除了压电场之外，随SAW传播的应变场也是石墨烯载流子控制的候选。应变诱导输运预计在多层中尤其重要，其中saw的压电场在第一层之外被强烈屏蔽。最后，当SAW波长与石墨烯中的载流子平均自由程相同数量级时，预计短周期应变波将沿SAW传播方向调制石墨烯带结构。

This project investigates the interaction of surface acoustic waves (SAWs) with epitaxial graphene (EG) layers on SiC. SAWs produce tunable strain and piezoelectric fields, which modulate the graphene band structure and interact strongly with carriers. The moving character of these fields is particularly interesting for capturing carriers and transporting them at a well-defined velocity. We will explore these features for the controlled transport of carriers and spins in graphene layers. This, in turn, requires a better knowledge of the momentum transfer process from SAWs to carriers in graphene, as well as the effect of the SAW-induced strain and piezoelectric fields into the electronic bandstructure and spin-splitting.The epitaxial graphene layers for the acoustic modulation experiments will be produced by the surface graphitization method. The investigations will be carried out on monolayer as well as multilayer graphene grown on the Si- and C- face of SiC, respectively. In all cases, the processing of the samples for the application of SAWs and measurement of electroacoustic currents will be done directly on the EG/SiC structure.The investigation of the interaction between SAW piezoelectric fields and carriers in graphene will be focused in the achievement of the non-linear regime, where a strong piezoelectric field induces a high modulation of the graphene charge density. In order to achieve this non-linear transport regime, intensity enhancement of the SAW piezoelectric field travelling along the EG/SiC structure will be explored, as well as the reduction of the average carrier density in graphene by displacement of the Fermi level towards the Dirac point. Under these conditions, the carriers will be strongly confined at the minimum of the piezoelectric energy, where they will move with the well defined SAW velocity.Graphene is also a promising candidate for spin information transport. Use of ferromagnetic source and drain contacts will allow spin injection and extraction into the graphene channel for its acoustic transport. Long spin relaxation times are expected in multilayer EG due to its low substrate-induced scattering and low spin-orbit coupling. This, together with the fast, well defined SAW velocity in SiC, should allow acoustic spin transport along distances of hundreds of micrometers.In addition to the piezoelectric field, the strain field propagating with the SAW is also a candidate for carrier control in graphene. Strain induced transport is expected to be especially important in multilayers, where the piezoelectric field of SAWs is strongly screened beyond the first layer. Finally, short-period strain waves are expected to modulate the graphene band structure along SAW propagation direction when the SAW wavelength is of the same order of magnitude as the carrier mean free path in graphene.