Magnetically confined graphene quantum dots

Semiconductor technology is currently based on the manipulation of electronic charge. However, there are additional degrees of freedom such as spin and valley suitable to transfer and process information. They promise a dream ticket: substantial increase in processing speed with a simultaneous reduction of waste heat, a substantial problem in modern supercomputers, and the use of quantum entanglement to perform complicated calculations in a fraction of the time of a conventional classical computer. In a recent joint experimental and theoretical effort, we have realized confined Dirac fermions in graphene with a well- defined valley degree of freedom. These results form a scaffold for future valleytronic device designs that exploit the unique properties of layered heterostructures, enabling the next generation of information technology. We achieve smooth confinement of Dirac fermions in graphene by combining magnetic and electric fields. The resulting quantum dots appear to be much more controlled in terms of filling sequences by orbital, valley and spin levels than etched graphene quantum dots, which typically suffer from disorder at their rough edges. Indeed, our confinement proved smooth enough to observe robust orbital and valley splittings, in excellent agreement with our theoretical models. The distance between graphene and the underlying hexagonal boron nitride substrate varies periodically due to the intricate interactions between the two layers. Therefore, changing the lateral position of the quantum dot with respect to the substrate additionally allows for accurate tuning of the different energy scales. Exploiting these unique properties of low-dimensional materials is key to obtain unprecedented control on the valley degree of freedom. Scalable designs replacing the external magnetic field by, e.g., a ferromagnetic substrate and the STM tip by suitable gate electrodes seem readily possible. Since anti-crossings at the valley degeneracy positions are likely due to strain mixing, a passage through the crossing points can be used to entangle the spin and valley degree of freedoms. These remarkable results are so far restricted to the tip of a scanning tunneling microscope which simultaneously charges and probes the quantum dot. This project aims to develop multiple contact geometries including back gates and additional side contacts to provide additional tuning options with precise control. Side gates will allow for electrical tuning of valley splitting by shifting the quantum dot position, without the need to physically move the STM tip. Finally, time-dependent STM measurements of the quantum dot electrons down to the sub-ns time scale will target the exploitation of the theoretically predicted favorable relaxation and coherence properties of graphene quantum dots. A comprehensive theoretical model to understand the dot dynamics at different time and length scales will be developed and applied to provide additional insight into our measurements. Our results will provide a keystone for future nanoelectronic devices that exploit the unique properties of two- dimensional materials.

As part of the FWF project Magneticically Confined Graphene Quantum Dots, we have investigated quantum states in two-dimensional crystals. Experimental groups have developed techniques to synthesize high-quality two-dimensional crystals such as graphene in the last few years. The resulting nanostructures have outstanding physical properties that enable applications in nanoelectronics, solar cells, touch displays or surface coatings. Defects play a crucial role in being able to individually change and adapt the electronic, optical or mechanical properties of these structures. Within this project, we modeled quantum states in various two-dimensional crystals and developed theoretical models to describe changes in the electronic structure, lattice vibrations, optical and mechanical properties. In cooperation with experimental groups in the US (University of Texas in Austin), in Germany (RWTH Aachen) and at the TU Wien, we have successfully tested our models and thus contributed to the understanding of these new materials.