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.