Advances in Moiré Physics Enable Long-Lived Excitons and Novel Lattice Interactions

The complex relationship between electrons and atomic structure is central to modern physics, but the way light and sound interact at the quantum level remains largely unknown. Ranju Dalal, Harsimran Singh, and Rwik Dutta, alongside colleagues from the Indian Institute of Science and the University of Texas at Austin, have investigated this interplay within layered materials called WSe2/WS2 heterostructures. Their research reveals unexpectedly long-lived excitons , quasiparticles formed when light excites electrons , confined by the material’s unique moiré pattern. This work not only identifies these ‘moiré intralayer biexcitons’ and their strong coupling to atomic vibrations known as phasons, but also demonstrates the potential to engineer new quantum phenomena and ultra-fast optoelectronic devices operating at gigahertz frequencies.

By carefully suppressing unwanted electrical charges within the heterostructure, the team were able to observe the dynamics of these moiré intralayer excitons, finding they persist for over a picosecond , a remarkably long time. They then characterised the formation of moiré intralayer intervalley biexcitons, revealing a binding energy of approximately 16 meV, also sustained by the moiré confinement. Crucially, the researchers detected clear evidence of interaction between these excitons and low-energy phasons, manifesting as oscillations in the exciton dynamics that depend on the angle between the layers.

These twist-angle-dependent gigahertz oscillations provide direct, time-domain signatures of strong coupling between the moiré excitons and phasons. The findings establish moiré superlattices as hybrid systems where excitonic and phononic quasiparticles strongly interact, opening up new avenues for materials science. This research, involving contributions from Kenji Watanabe and Takashi Taniguchi at the National Institute for Materials Science in Japan, highlights the potential of these structures for both fundamental studies of quantum interactions and the development of advanced technologies.

Moiré Excitons in Twisted WSe2/WS2 Heterostructures The fabrication

This text details supplementary materials from a scientific paper concerning the study of moiré excitons in WSe2/WS2 heterostructures, outlining the methods, theoretical background, and references used in the research.

Moiré Heterostructures Exhibit Long-Lived Excitons

Scientists have uncovered remarkably long lifetimes in moiré intralayer excitons (IALX) within WSe2/WS2 heterostructures, establishing these systems as interacting hybrid platforms. The research team suppressed ultrafast charge-transfer to interlayer excitons, allowing detailed observation of the IALX dynamics and revealing lifetimes exceeding 1000 picoseconds. These extended lifetimes arise from the localized Wannier and in-plane charge-transfer nature of the excitons, a direct consequence of moiré confinement within the superlattice structure. Experiments were conducted on hexagonal boron nitride encapsulated WSe2/WS2 heterostructures on sapphire substrates at 4 Kelvin, utilising transmission geometry to ensure accurate differential-transmission measurements.

Further investigation revealed moiré intralayer intervalley biexcitons, four-particle states exhibiting a binding energy of approximately 16 meV. These biexcitons also demonstrate prolonged stability, with lifetimes around 1000 picoseconds, attributable to the confining effects of the moiré superlattice. The team employed two-colour pump-probe spectroscopy to map the dynamics of these excitons, identifying strong photobleached signals near the resonances of peak-I and peak-II, alongside spectral shifts and induced absorption. Analysis of the transient differential transmission of peak-I and peak-II in one device, and peaks I, II, and III in another, showed distinct relaxation dynamics for each moiré IALX state, with small negative delay signals observed.

Remarkably, the study detected polarization-independent, twist-angle-dependent GHz oscillations in the IALX dynamics. These oscillations, measured in the time domain, provide direct evidence of strong coupling between the moiré-IALX and ultrasoft lattice modes, specifically phasons with energies around 10 micro-eV. Steady-state transmission contrast measurements of devices with twist angles of 1.65° and 0.18° revealed two and three moiré IALX states, respectively, confirming the influence of twist angle on exciton formation. First-principles GW-BSE calculations, utilising a newly developed unit-cell matrix projection method, support the understanding of these dynamics, demonstrating that peak-I corresponds to a Wannier-type exciton localized at an AA stacking site, enhancing stability by minimizing non-radiative recombination.

Long-Lived Excitons and Strong Phason Coupling

This research details the dynamic behaviour of intralayer excitons within moiré superlattices constructed from WSe2/WS2 heterostructures. By carefully controlling excitation rates to minimise charge transfer between layers, scientists have revealed unexpectedly long exciton lifetimes exceeding 1000 picoseconds, attributable to the localized nature of these quasiparticles. Furthermore, the observation of moiré intralayer intervalley biexcitons, with a binding energy of approximately 16 meV and similarly extended lifetimes, suggests potential for creating sources of entangled photons and building blocks for quantum simulation. Crucially, the study presents the first direct evidence of strong coupling between these excitons and low-energy phasons, manifested as gigahertz-scale oscillations in exciton dynamics that are sensitive to the twist angle of the moiré superlattice. The authors acknowledge the experimental challenges posed by the ultralow energy of phason modes, which conventional spectroscopic techniques struggle to detect, and highlight how their time-domain approach overcomes this limitation. Future work could extend this excitation rate engineering to other two-dimensional materials, such as perovskites and quantum dots, and explore a wider range of low-energy collective modes, ultimately establishing moiré materials as uniquely tunable platforms for light-matter interactions and advanced optoelectronic devices.