T3: Excited States in Moiré Superlattices

When two atomically thin van der Waals (vdW) layers are vertically stacked together, the atomic alignment between the layers exhibits periodical variations, leading to a new type of in-plane superlattices known as the moiré superlattices. In moiré superlattices formed by transition metal dichalcogenide (TMD) monolayers, optical properties are dominated by tightly bound excitons that are stable at room temperature and relevant for optoelectronic devices. The fundamental properties of excitons are modified by the moiré potential including their optical selection rules, spin-valley correspondence, mobility, and quantum dynamics. When additional holes or electrons are introduced, the doped moiré superlattices can host correlated electronic states such as the Mott and Chern insulating states, strip phases, and exciton condensates. The lectures will provide a basic introduction to excitons, phonons, and correlated states probed via exciton resonances in TMD moiré superlattices with a discussion of open questions and challenges.

Topics covered:

• Basic concepts: Excitons and trions in two-dimensional systems and monolayers, spin-valley locking, excitons in moiré superlattices, resonant energy, lifetimes, and transport; exciton condensate, exciton insulators, probing correlated states via exciton resonances.

• Experimental techniques: Linear optical spectroscopy, light scattering techniques, time-resolved spectroscopy methods, coherent nonlinear spectroscopy, time-resolved angle-resolved photoemission spectroscopy.

• Applications: Correlated phases and optoelectronic devices.