Graphene spin circuits and spin-orbit phenomena in van der Waals heterostructures with topological insulators

Doctoral thesis, 2021

Electronics has become ubiquitous in many aspects of our society, where microcontrollers, computers, and artificial intelligence are widely used for automation, home appliances, medicine, transportation, navigation, communications, and entertainment. The remarkable pace of the technological advancement in the electronics industry over the last 70 years, driven by the ever-increasing demands for faster processing rates, improved power efficiency, and larger information storage capacity, was made possible by the developments in condensed matter physics and the progress in semiconductor manufacturing technology. It was noticed that the number of transistors per unit area on a chip doubles approximately every 2 years, an empirical trend known as Moore's law. This exponential growth was maintained by a continuous shrinking of the transistor dimensions over the past decades. However, it is believed that in the near future this trend could no longer be sustained as the minimum feature size of nanoelectronic components approaches the fundamental atomic limit. In addition, in such small devices, quantum mechanical effects lead to increased leakage currents that pose heat dissipation and power consumption issues. To cope with these issues and continue the development of electronics beyond Moore's law, alternative routes and prospective architectures for future computing electronic devices are being investigated.

Spintronics offers an alternative approach to conventional charge-based information processing by using the electron spin, i.e. its magnetic properties, to record, store, transfer and manipulate information. Due to the smaller amount of energy needed to control electron spin in comparison with the energy required to move electrical charges in common transistors, spin-based devices may offer decreased power consumption and increased operation speed.

In this thesis, we investigate spin-based phenomena in the recently emerged family of two-dimensional (2D) materials. We demonstrate the large-area graphene as a robust spin interconnect capable of preserving and transporting spin polarization over long distances. Furthermore, a spin summation operation in multiterminal graphene devices is realized and employed to construct a prototype spin majority logic gate operating with pure spin currents. A great advantage of 2D materials is the possibility to stack various layers on top of each other to form van der Waals heterostructures, in which the interactions between constituent materials can change their properties and induce novel physical effects. In this thesis, we study the emerging proximity-induced spin-orbit phenomena in the van der Waals heterostructures of graphene and topological insulators (TIs). We show that the strong spin interactions in such heterostructures allow to perform tunable spin-charge conversion directly in graphene, which is not possible in pristine material. The obtained findings demonstrate the robust performance of graphene as a spin interconnect for emerging spin-logic architectures and present all-electrical and gate-tunable spintronic devices based on graphene-TI heterostructures, paving the way for next-generation spin-based computing.