Phonon Signatures of Correlated Phases in WS₂/WSe₂ Superlattice

Two-dimensional moiré superlattices have been reported to host an abundance of electron correlation or topological phases, which present an emergent platform to study the many-body interactions between (quasi-)particles (photons, electrons, excitons, phonons) in the strong correlation regime. Although there have been abundant studies on electron or exciton behaviors at correlated phases, the reports on phonon signatures in such systems are still rare. Here we explored the optical phonon behavior in WS₂/WSe₂ heterosructure using Raman spectrascopy. First, we found the local enhancement of Raman signal at integer/fractional fillings. The critical temperatures of correlated insulating states were extracted by measuring the temperature dependence of Raman enhancement factor, which are consistent with that measured through electrical transport methods or photoluminescence/absorption spectra. Second, we found the crucial role of exciton in determining the Raman signal enhancement through multi-wavelength excitation measurement. The excitonic resonance dependence implies the possible mechanism that the correlated phases lead to a modification of phonons through the exciton-phonon coupling. By measuring Huang-Rhys factor using temperature dependent photoluminescence spectra, we found the experimental evidence for enhanced exciton-phonon coupling at correlated phases. All the reported features give an alternative ‘phonon’ perspective for exploring strong correlation physics. The phonon signatures provide new probes of the correlated phases, and the demonstrated strong exciton-phonon coupling at filling factors provides an invaluable view for understanding many-body interactions of quantum phases in moiré systems.