Topological insulators (TIs) are of special interest due to their distinct transport properties, i.e., a linear band dispersion and a helical spin momentum locking. Especially two-dimensional structures, where the one-dimensional Dirac-edge states give rise to the quantum spin Hall (QSH) effect, turned out to be an exceptional template for quantized and spin-polarized transport phenomena which carry great potential for spintronic applications.
In this project we plan to investigate the controlled interaction of topological surface states which are brought into close proximity to each other in two-dimensional (2D) as well as in three-dimensional (3D) TI systems. HgTe is the preferred TI material, because it is currently the only TI material where topological surface states are accessible in transport experiments without being obscured by bulk contributions. It exhibits superior transport properties due to the molecular beam epitaxial crystal growth and allows the fabrication of customized high quality two- and three-dimensional layer systems. Additionally, advanced lithographic techniques allow for the fabrication of high quality micro- and nano-scale devices for advanced transport investigations.
Apart from intense transport characterizations of specially designed devices this project will put strong efforts on the optimization of the lithographic fabrication processes for HgTe-based micro-and nano-structures. Special focus will be on the development of low thermal budget processes and new gating techniques which are expected to improve the transport properties of the devices.
One of our main goals will be the fabrication of devices where transport can be controlled and modified by tunneling processes between helical edge channels. This goal requires well defined nanometer scale tunnel junctions as well as precisely positioned gate structures on high quality TI sample material. In fact, the possibility of exploiting spin-flip and spin-preserving tunnel processes will be crucial for the intriguing perspective of utilizing topological edge or surface states in spintronic applications. The availability of coupled bilayer structures consisting of either two quantum wells or a pair of three-dimensional TI layers will open a new perspective of investigating further exotic correlation effects of topological surface states, as for example the formation of interlayer excitonic states in coupled Dirac systems.
 P. Sternativo and F. Dolcini, Phys. Rev. B 89, 035415 (2014).