Theory of charge-spin conversion phenomena in two-dimensional electronic systems: from graphene heterostructures to Rashba-coupled interfaces

Using the electron’s spin in addition to its charge represents a promising avenue for future solid-state devices. The potential of this field of research, called spintronics, has been propelled by the advent of graphene and related atomically-thin materials, which have enabled unprecedented electric control over spin dynamics and spin-charge conversion effects in layer-by-layer systems. This thesis aims to contribute towards a broader understanding of spin-dependent phenomena in two spintronic platforms of much current interest; honeycomb layers and interfaces hosting two-dimensional electron gases and topologically protected states. These systems are characterized by rich symmetry-breaking spin-orbit coupling effects, which render theoretical descriptions of electronic structure and spin transport highly nontrivial. Therefore, this work aims to develop a unified microscopic treatment that captures, on equal footing, disorder-limited spin dynamics and disorder-enhanced spin- charge conversion effects, two complementary phenomena at the heart of modern spin- tronics. On the first front, we put forward a diagrammatic method that allows the derivation of space and time-dependent kinetic equations for generic 2D electronic systems. Ap- plied to adatom-decorated graphene, it uncovers the interband spin-orbit scattering at the origin of sizable current-induced spin currents. Secondly, we study the possibility of acquiring twist-angle control over spin-charge conversion effects in novel graphene-based heterostructures, where a rotation angle between adjacent layers strongly modifies the spin texture of electronic bands, thus opening the possibility of realizing unconven- tional spin galvanic effects. Our formulation is also applied to studying spin-orbit torques in ferromagnet bilayers. We find that skew scattering from ubiquitous short- range impurities can produce significant damping-like torques, allowing for all-electrical magnetization switching of a nearby micromagnet. Our work highlights the crucial role played by electronic structure modifications at interfaces in the generation of spin-dependent forces experienced by transport electrons and the necessity for an adequate treatment of impurity scattering for describing the behaviour of realistic spintronic materials