Moiré Excitons in Van der Waals Heterostructures (1807.03771v1)

Abstract: In van der Waals (vdW) heterostructures formed by stacking two monolayer semiconductors, lattice mismatch or rotational misalignment introduces an in-plane moir\'e superlattice. While it is widely recognized that a moir\'e superlattice can modulate the electronic band structure and lead to novel transport properties including unconventional superconductivity and insulating behavior driven by correlations, its influence on optical properties has not been investigated experimentally. We present spectroscopic evidence that interlayer excitons are confined by the moir\'e potential in a high-quality MoSe2/WSe2 heterobilayer with small rotational twist. A series of interlayer exciton resonances with either positive or negative circularly polarized emission is observed in photoluminescence, consistent with multiple exciton states confined within the moir\'e potential. The recombination dynamics and temperature dependence of these interlayer exciton resonances are consistent with this interpretation. These results demonstrate the feasibility of engineering artificial excitonic crystals using vdW heterostructures for nanophotonics and quantum information applications.

• The paper presents photoluminescence evidence showing moiré-induced confinement of interlayer excitons in hBN-encapsulated MoSe₂/WSe₂ heterobilayers.
• Quantum calculations and optical measurements reveal alternating circular polarization states arising from the modulation of local atomic configurations.
• The study highlights the potential for engineering excitonic crystals and single-photon emitters in future optoelectronic and quantum applications.

Moiré Excitons in Van der Waals Heterostructures

The paper presented in this paper explores the phenomenon of moiré excitons in van der Waals (vdW) heterostructures, specifically focusing on the optical properties induced by moiré superlattices in a MoSe₂/WSe₂ heterobilayer (hBL). This research bridges a gap in the understanding of how moiré patterns affect optical behaviors, adding insight into the already known effects on electronic band structures manifested in unconventional superconductivity and other electronic transport properties.

Overview and Findings

The paper presents spectroscopic evidence that interlayer excitons are effectively confined by moiré potential in high-quality hBN-encapsulated MoSe₂/WSe₂ heterobilayers with minimal rotational misalignment. The team identified several interlayer exciton resonances through photoluminescence (PL) experiments, characterized by alternating circular polarization states. Such patterns are explained by quantum calculations and the spatial variation in optical selection rules attributed to the moiré potential's modulation of local atomic configurations.

The authors conducted a sequence of optical measurements under controlled conditions to resolve the interlayer exciton resonances at low temperatures. Particularly noteworthy is the ability of the paper to attribute these observations directly to the moiré pattern's lateral confinement. The observations are consistent with theoretical expectations of multiple IX states being confined in a quantum dot-like potential with significant lateral confinement energies (~100–200 meV).

Implications

The findings underscore the possibility of engineering artificial excitonic crystals using vdW heterostructures. Such configurations could play a pivotal role in future photonic and quantum information applications, offering meticulous control over exciton properties. The realization of a quantum dot array defined by the moiré pattern introduces opportunities for designing single-photon emitters, exploring exciton crystals, and studying emergent quantum phases of excitons.

More specifically, the spatially varying optical selection rules provide an additional degree of freedom for manipulating excitonic interactions in vdW heterostructures. The novel insights into exciton dynamics suggest potential applications in precision optoelectronic devices, potentially impacting the development of new light-harvesting systems or next-generation display technologies.

Future Directions

This work opens several pathways for future research. Improved sample quality and reduced inhomogeneous broadening are essential for refining control of IX properties via external fields such as electric or magnetic fields. An elaborate paper of the twist angle's influence could fine-tune the moiré period and the depth of moiré potential in a predictable manner, thus controlling the energy spacing of IX resonances. These advances would enhance the applicability of vdW heterostructures in quantum information processing and photonic technologies.

Continued exploration into the interaction of excitons within superlattices, including the potential for more complex excitonic phases and the influence of different material compositions, may also yield fruitful results. Assessing such interactions under varying environmental conditions, including temperature fluctuations and external field strengths, could reveal robust strategies for practical device implementations.

Conclusion

The research provides clear evidence of moiré-induced confinement of interlayer excitons, offering a substantial contribution to the understanding of optical properties in vdW heterostructures. This paper builds foundational knowledge necessary for advancing nanophotonics and quantum computing technologies. The ability to manipulate excitons with moiré superlattices marks a significant step towards engineering more sophisticated quantum and optoelectronic devices.