Twisted Bilayer Sonic Crystal: A New Playground for Twistronics

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Sonic crystals are artificially engineered materials that can control sound in unconventional ways, such as to camouflage sound or confine sound at desired locations. While single layers of sonic crystals have been primarily studied in the past, this research seeks to study a fundamentally new type of sonic crystals, where two sonic crystals are stacked with a small angle misalignment. Such a bilayer configuration forms an artistic moiré pattern, commonly found in textiles. This project will broadly advance the field of acoustic functional materials by endowing sonic crystals with a new set of capabilities to manipulate sound. The resulting bilayer sonic crystals are expected to facilitate applications such as energy harvesting and enhanced acoustic emission and sensing. The research activities will involve undergraduate students, as well as students from under-represented groups. Innovative outreach activities will also be enabled, such as a partnership with the Palmer Museum of Art at Penn State University for a special exhibition on bilayer sonic graphene, with the theme to forge an unexpected bond between art (moiré pattern) and science. Twistronics is the field that studies electronic behavior that can be dramatically altered by controlling the twist between layers of two-dimensional materials, such as graphene. A recent major discovery in twistronics is the so-called magic angles, which are extraordinary twist angles between two sheets of graphene that give rise to utra-flat bands, creating the Mott insulating state and unconventional superconductivity. This research draws inspiration from the recent development in twistronics and seeks to exploit twist and interlayer coupling as two new degrees of freedom to devise a new family of sonic crystals, i.e., twisted bilayer sonic crystals. Analytical and computational models will be developed to shed light on the band structure of twisted bilayer sonic crystals with a wide range of twist angles and interlayer coupling strength. A framework will be established to identify the acoustic version of magic angles in twisted bilayer sonic crystals. Important properties of acoustic magic angles, such as their corresponding eigenmodes, physical bounds, and robustness to defects, will be revealed. At last, important insights will be gained on the topological features of twisted bilayer sonic crystals, as well as on how loss interacts with their bands, either favorably or adversely.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.