PhD ceremony Mr. T. Maassen: Electron spin transport in graphene-based devices

PhD ceremony: Mr. T. Maassen, 14.30 uur, Academiegebouw, Broerstraat 5, Groningen

Dissertation: Electron spin transport in graphene-based devices

Promotor(s): prof. B.J. van Wees

Faculty: Mathematics and Natural Sciences

Computers using the electron spin instead of charge to process information are a promising concept for the future. Therefore we need to find a material that can be used to transport spins without altering their spin orientation.

Graphene, the one atom thick honeycomb lattice of carbon atoms, has emerged as a possible candidate for this purpose due to its high charge carrier mobilities and expected low spin scattering at room temperature. However, the results of spin transport experiments stay well below the expected spin relaxation lengths and times, the typical parameters describing spin scattering.

This thesis focuses on determining limiting factors for spin transport in graphene. Therefore, we examine the influence of ferromagnetic contacts, spurious electrical potentials in the environment, and the effect of the substrate on the graphene.

We show that low resistive contacts can lead to strongly underestimated spin transport properties of the graphene channel and that the effect of spurious potentials can be screened by inducing additional charges in the system. These charges can be electrically excited or can originate from the change in band structure between a single graphene layer and a stack of few layer graphene. We also show that a change of the substrate can strongly influence spin transport in a graphene layer and discuss the surprising effect of localized states coupled to the transport channel. Finally, we comment on the ongoing discussion on the dominant spin relaxation mechanism in graphene.