13 July

Theoretical study of disorder and proximity effects in three-dimensional models of topological insulators

Friday 13 July 2018, 12:00pm

ICN2 Seminar Hall, ICN2 Building, UAB

PhD: Kenan Song

Directors: Prof. Stephan Roche, Theoretical and Computational Nanoscience Group Leader and Prof. Pablo Ordejón, Director and Theory and Simulation Group Leader at ICN2.

Short Abstract: Owing to their unique spin and charge properties, three-dimensional topological insulators (3D TIs) have attracted significant attention since their initial prediction in 2007. In particular, 3D TIs have surface states that exhibit spin-momentum locking, making them resistant to back scattering and potentially useful for future low-power or spintronic applications. The first part of this PhD project focused on the 3D TI Bi2Se3, and the impact that defects and impurities have on the properties of its surface states. It was found that twin boundaries in the bulk of the TI open an energy gap in the surface states of thin TI films (< 3 nm), while hydrogenation of the TI can actually restore one of the surface states. Meanwhile, tight binding calculations based on the Fu-Kane-Mele model showed that non-magnetic impurities can shift the energy of the surface state but maintain its spin-momentum locking. On the other hand, magnetic impurities break time reversal symmetry and open a gap in the surface state. This effect can be probed with measurements of spin transport anisotropy.

In the second part of this PhD project, Bi2Se3 was combined with graphene in order to study the proximity effect between the two materials. The TI was found to induce a band gap in the graphene, arising from bonding distortion. Additionally, significant spin-orbit coupling (SOC) was imprinted in the graphene bands by the TI, and the nature of this SOC, which depends strongly on the alignment of the graphene/TI interface, can also be probed with spin transport measurements. Overall, the work carried out in this project provides fundamental understanding on how TI films behave in the presence of disorder, and how they impact the electronic and spintronic properties of graphene, and may help in the development of future devices based on these materials.