Phase Diagram Mapping of a Moiré System Using Surface Acoustic Waves.

Taking advantage of interfacial electron-phonon interactions, surface acoustic waves (SAWs) have emerged as a sensitive and contact-free technique for investigating low-dimensional materials. This noninvasive technique proves especially valuable for probing low-conductivity states and provides crucial radio-frequency conductivity information. In this work, for the first time, we extend SAW measurements to moiré systems to explore the correlated quantum phases. By fabricating twisted bilayer graphene samples within Fabry-Perot SAW resonators on Y-cut LiNbO substrates, we demonstrate that SAWs effectively probe correlated insulating states, Landau level series, and Chern insulator phases. Our results highlight the superior sensitivity of SAWs, uncovering incompressible states inaccessible to traditional electrical transport measurements. Additionally, we observe a significant reduction in the alternating-current conductivity at half-fillings, indicative of frequency-dependent conductivity within correlated states. These findings establish SAWs as a powerful, contactless approach for probing the complex phenomena in moiré systems, including incompressible states and frequency-dependent electronic properties.