Observation of Moiré Excitons in WSe2/WS2 Heterostructure Superlattices

Abstract: Moir\'e superlattices provide a powerful tool to engineer novel quantum phenomena in two-dimensional (2D) heterostructures, where the interactions between the atomically thin layers qualitatively change the electronic band structure of the superlattice. For example, mini-Dirac points, tunable Mott insulator states, and the Hofstadter butterfly can emerge in different types of graphene/boron nitride moir\'e superlattices, while correlated insulating states and superconductivity have been reported in twisted bilayer graphene moir\'e superlattices. In addition to their dramatic effects on the single particle states, moir\'e superlattices were recently predicted to host novel excited states, such as moir\'e exciton bands. Here we report the first observation of moir\'e superlattice exciton states in nearly aligned WSe2/WS2 heterostructures. These moir\'e exciton states manifest as multiple emergent peaks around the original WSe2 A exciton resonance in the absorption spectra, and they exhibit gate dependences that are distinctly different from that of the A exciton in WSe2 monolayers and in large-twist-angle WSe2/WS2 heterostructures. The observed phenomena can be described by a theoretical model where the periodic moir\'e potential is much stronger than the exciton kinetic energy and creates multiple flat exciton minibands. The moir\'e exciton bands provide an attractive platform to explore and control novel excited state of matter, such as topological excitons and a correlated exciton Hubbard model, in transition metal dichalcogenides.

• The paper reports the first experimental observation of moiré excitons in nearly aligned WSe2/WS2 heterostructures, demonstrating how periodic moiré potentials induce exciton resonance splitting.
• The researchers used TEM, SHG, and 10K optical spectroscopy to confirm superlattice formation and observe gate-dependent exciton peak shifts.
• Theoretical modeling predicts multiple flat exciton minibands with strong localization, paving the way for innovative quantum device applications.

Overview of Moiré Excitons in WSe2/WS2 Heterostructure Superlattices

The study "Observation of Moiré Excitons in WSe2/WS2 Heterostructure Superlattices" presents a significant advancement in our understanding of emergent excitonic phenomena within moiré superlattices. This paper documents the first experimental observation of moiré excitons in nearly aligned WSe2/WS2 heterostructures, providing insight into the role of periodic moiré potentials in engineering novel quantum states in two-dimensional (2D) semiconducting transition metal dichalcogenides (TMDCs).

Moiré Superlattice Formation

The moiré superlattice is formed when two layers of atomically thin materials, such as WSe2 and WS2, with slight lattice mismatches and nearly zero inter-layer twist angles, interface with each other. The resulting superlattice has a periodicity dictated by the twist angle and inherent lattice constants, approximately 8 nm, allowing for new quantum phenomenon to arise. Experimental confirmation of the superlattice formation was achieved using transmission electron microscopy (TEM) and second harmonic generation (SHG), evidencing significant lattice reconstruction and strong inter-layer interactions.

Experimental Results and Observations

Utilizing optical spectroscopy at 10 Kelvin, the paper identifies that the presence of the moiré superlattice results in the WSe2 A exciton resonance splitting into multiple peaks in the absorption spectrum. These peaks, corresponding to distinct exciton states, demonstrate unique dependence on electrical gating and doping, especially notable upon electron doping, indicative of a strong coupling regime.

The reflections and photoluminescence excitation (PLE) spectra reveal that these exciton states emerge only in nearly aligned heterostructures, in stark contrast to large-twist-angle configurations. The gate-dependent behavior of the exciton peaks, notably their shifts and evolutions in response to electron concentration, surpasses explanations provided by traditional exciton interactions, pointing towards a new domain of electron-exciton interplay facilitated by the moiré potential.

Theoretical Interpretation

The theoretical framework applied involves modeling the excitonic states under a periodic moiré potential which is considerably greater than the exciton kinetic energy (~250 meV vs. ~8 meV within the first mini-Brillouin zone). The observed experimental phenomena are explained by predicting the formation of multiple flat exciton minibands, with strong localization of exciton states at particular points in the superlattice. This suggests a pronounced effect of the potential dispersion on both exciton energy states and their spatial distribution.

Implications and Future Directions

These findings not only provide compelling evidence of moiré excitonic states but also offer a platform for further exploration of complex phenomena such as topological exciton bands and strongly correlated phases described by an exciton Hubbard model. The ability to manipulate the band structure of excitons presents avenues for new experimental probes and control methodologies in 2D material-based devices and quantum technology applications.

In summary, the paper advances fundamental knowledge in the design and control of quantum states in TMDCs through moiré engineering. It sets the stage for future experimental and theoretical exploration into the complex interactions present in low-dimensional systems, offering the potential for novel quantum device functionalities.