Interfacial proximity effects in two-dimensional van der Waals heterostructures

Van der Waals heterostructures offer a promising platform for exploring new electronic and optical phenomena in atomically thin materials. The combination of graphene with other van der Waals materials, such as transition metal dichalcogenides or topological insulators, has attracted considerable interest due to their potential for spintronic applications. While pristine graphene exhibits high carrier mobility, low spin-orbit coupling strength, and gate-tunability, it lacks the ability to generate and manipulate spin in a controllable manner, which is essential for many spintronic applications. A promising strategy to overcome this limitation is the introduction of a neighboring material that significantly alters the electronic properties of graphene through proximity effects, resulting in, e.g., substantial spin-orbit coupling, spin splitting, and electronic anisotropy. In this thesis, we explore the potential of graphene/WTe2 and graphene/Bi2Te2Se heterostructures for electrical control of proximity-induced spin phenomena, by combining optical spectroscopy with transport experiments.
We review recent developments in the field of light-driven currents, with special attention to their application in layered van der Waals materials. Particularly, we highlight the relevance of helicity- and light-driven currents in topological and spin-orbit dominated van der Waals materials for various applications, including ultrafast detectors and on-chip current generators. We also show how photon helicity can be used to address chiral and nontrivial surface states in topological systems as well as the valley degree of freedom in two-dimensional van der Waals materials.
To explore the local charge and spin current distribution in proximitized graphene heterostructures for their potential use in spintronic applications, we focus on current-controlled graphene/WTe2 heterostructures and discuss our experimental and theoretical findings on gate- and bias-tunable local spin polarizations. Using magneto-optical Kerr rotation microscopy, we observe signatures of a substantial out-of-plane spin accumulation induced by a corresponding out-of-plane current flow, even for a nominal in-plane transport. Our theoretical model sheds light on the gate- and bias-dependent occurrence and spatial distribution of the Kerr rotation signal, which may arise due to a nonlinear anomalous Hall effect in the heterostructure enabled by its reduced point-group symmetry.
Furthermore, we study epitaxially grown interfaces between graphene and the lattice-matched topological insulator Bi2Te2Se with the aim to access the spin texture at the heterointerface. Using polarization-resolved second-harmonic generation, Raman spectroscopy, as well as time-resolved Kerr rotation microscopy, we are able to detect the alignment of the atomically sharp interfaces. Our polarization-resolved photocurrent measurements reveal a circular photogalvanic effect that is significantly enhanced at the Dirac point of the proximitized graphene. This particular gate-tunability is attributed to the proximity-induced interface spin structure, which could be exploited for spin filters and other spintronic applications.