Interaction of surface acoustic waves with epitaxial graphene

本项目研究了表面声波(SAW)与碳化硅外延石墨烯(EG)层的相互作用。锯片产生可调谐的应变场和压电场，从而调节石墨烯的能带结构，并与载流子发生强烈的相互作用。这些场的运动特征对于捕获载体和以明确的速度传输它们特别有趣。我们将探索这些在石墨烯层中控制载流子和自旋传输的功能。这就需要更好地了解石墨烯中锯片到载流子之间的动量传递过程，以及声表面波引起的应变和压电场对电子能带结构和自旋分裂的影响。研究将分别在单层石墨烯和多层石墨烯上进行，分别生长在碳化硅的硅面和C面上。在所有情况下，用于声表面波应用的样品的处理和电声电流的测量将直接在EG/SiC结构上进行。研究声表面波压电场与石墨烯中载流子之间的相互作用将集中在实现非线性区域，在该区域中，强压电场导致石墨烯电荷密度的高度调制。为了实现这种非线性输运机制，我们将探索沿EG/SiC结构传播的声表面波压电场的强度增强，以及通过费米能级向Dirac点移动来降低石墨烯中的平均载流子密度。在这些条件下，载流子将被严格限制在最小的压电能下，它们将以定义好的声表面波速度移动。石墨烯也是一种很有前途的自旋信息传输候选材料。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。由于其低衬底诱导散射和低自旋轨道耦合，多层EG有望获得较长的自旋弛豫时间。这一点，再加上碳化硅中快速而明确的声表面波速度，应该允许声自旋传输沿数百微米的距离进行。除了压电场，与声表面波一起传播的应变场也是石墨烯中载流子控制的候选对象。应变引起的传输预计在多层膜中尤其重要，在多层膜中，锯片的压电场被强烈屏蔽在第一层之外。当声表面波波长与石墨烯中载流子平均自由程的量级相同时，短周期应变波有望沿声表面波传播方向调制石墨烯的能带结构。

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.