Quantum Phenomena in Two-Dimensional Materials Driven by Atomic Scale Modifications
The extreme surface sensitivity of two-dimensional (2D) materials provides an unprecedented opportunity to engineer the physical properties of these materials via changes to their surroundings, including a substrate, adsorbates, defects, etc. In particular, the decoration of the 2Dmaterial with adatoms can be utilized to tailor material properties and induce novel quantum phenomena. In this context, first I will discuss the case of 2D semiconducting transition metal dichalcogenides (TMDs), wherein new electronic phenomena such as tunable bandgaps and strongly bound excitons and trions emerge from strong many-body effects, beyond the spin and valley degrees of freedom induced by spin-orbit coupling and by lattice symmetry. Combining single-layer TMDs with other 2D materials in van der Waals heterostructures offers an intriguing means of controlling the electronic properties through these many-body effects, by means of engineered interlayer interactions. We utilized state-of-the-art micro-focused angle-resolved photoemission spectroscopy (microARPES) and in-situ surface doping to manipulate the electronic structure of single-layer tungsten disulfide (WS2) on hexagonal boron nitride (WS2/h-BN). Upon surface doping, we observe an unexpected giant renormalization of the spin-orbit splitting of the single-layer WS2 valence band in addition to a bandgap reduction, which is attributed to the formation of trionic quasiparticles. These findings suggest that the electronic, spintronic, and excitonic properties are widely tunable in 2D TMD/h-BN heterostructures, as these are intimately linked to the quasiparticle dynamics of the materials.
In another example, in-situ low-temperature transport measurements are performed in an ultrahigh vacuum to systematically investigate resonant scattering by hydrogen adatoms on bilayer graphene. Resonant scatterers produce strong momentum scattering and can generate much sought after spin-galvanic effect in graphene. I will present the observation of two distinct resonant scattering peaks in the electric field dependent bilayer graphene sheet resistance, which evolves as a function of atomic hydrogen dosage. Theoretical calculations show that the observed peaks are due to graphene sublattice-dependent resonances, and analysis of the gate-dependent resistance curves show that hydrogen atoms preferentially adsorb to the non-dimer site over the dimer site. Using this newly developed capability for sublattice-resolved transport spectroscopy, we investigate the thermally-induced diffusion and desorption of hydrogen adatoms on bilayer graphene and find that with increasing temperature, the hydrogen vacates the dimer site before the non-dimer site. This is crucial for magnetic ordering of localized moments in graphene, which is predicted to become ferromagnetic if the localized moments lie on the same sublattice. These experiments open up the pathway to generate robust spin currents in bilayer graphene via spin-dependent charge carrier scattering and provide an important insight for developing long-range spin ordering in adatom decorated graphene layers.