Person:

Andersen, Trond

Two-dimensional (2D) materials offer a promising platform for exploring condensed matter phenomena and developing technological applications. However, the reduction of material dimensions to the atomic scale poses a challenge for traditional measurement and interfacing techniques that typically couple to macroscopic observables. We demonstrate a method for probing the properties of 2D materials via nanometer-scale nuclear quadrupole resonance (NQR) spectroscopy using individual atom-like impurities in diamond. Coherent manipulation of shallow nitrogen-vacancy (NV) color centers enables the probing of nanoscale ensembles down to ∼30 nuclear spins in atomically thin hexagonal boron nitride (h-BN). The characterization of low-dimensional nanoscale materials could enable the development of new quantum hybrid systems, combining atom-like systems coherently coupled with individual atoms in 2D materials.

Moiré superlattices in twisted van der Waals (vdW) materials have recently emerged as a promising platform for engineering electronic and optical properties. A major obstacle to fully understanding these systems and harnessing their potential is the limited ability to correlate direct imaging of the moiré structure with optical and electronic properties. Here, we develop a secondary electron microscope technique to directly image stacking domains in fully functional vdW heterostructure devices. After demonstrating imaging of AB/BA and ABA/ABC domains in multi-layer graphene, we employ this technique to investigate reconstructed moiré patterns in twisted bilayer WSe2/WSe2 and directly correlate increasing moiré periodicity with the emergence of two distinct exciton species in photoluminescence measurements. These states can be tuned individually through electrostatic gating and feature different valley coherence properties. Our observations can be understood as resulting from the formation of an array of two intralayer exciton species residing in alternating locations in the superlattice, and open up new avenues for realizing tunable exciton arrays in twisted vdW heterostructures, with applications in quantum optoelectronics and explorations of novel many body systems.

Van der Waals heterostructures obtained via stacking and twisting have been used to create moiré superlattices, enabling new optical and electronic properties in solid-state systems. Moiré lattices in twisted bilayers of transition metal dichalcogenides (TMDs) result in exciton trapping, host Mott insulating and superconducting states, and act as unique Hubbard systems whose correlated electronic states can be detected and manipulated optically. Structurally, these twisted heterostructures feature atomic reconstruction and domain formation. However, due to the nanoscale sizes of moiré domains, the effects of atomic reconstruction on the electronic and excitonic properties could not be systematically investigated. Here, we use near 0o twist angle MoSe2/MoSe2 bilayers with large rhombohedral AB/BA domains to directly probe excitonic properties of individual domains with far-field optics. We show that this system features broken mirror/inversion symmetry, with the AB and BA domains supporting interlayer excitons with out-of-plane electric dipole moments in opposite directions. The dipole orientation of ground-state Γ-K interlayer excitons can be flipped with electric fields, while higher-energy K-K interlayer excitons undergo field-asymmetric hybridization with intralayer K-K excitons. Our study reveals the impact of crystal symmetry on TMD excitons and points to new avenues for realizing topologically nontrivial systems, exotic metasurfaces, collective excitonic phases, and quantum emitter arrays via domain-pattern engineering.