Interaction of surface acoustic waves with epitaxial graphene

• Applicant: Dr. Alberto Hernández-Mínguez
• Subject Area: Experimental Condensed Matter Physics
• Term: from 2013 to 2017
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 242778186

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.

• DFG Programme: Priority Programmes
• Subproject of: SPP 1459: Graphen