Twisted Bilayer Sonic Crystal: A New Playground for Twistronics

声波晶体是一种人工工程材料，可以以非常规的方式控制声音，例如伪装声音或将声音限制在所需的位置。虽然过去主要研究了单层声波晶体，但这项研究旨在研究一种全新类型的声波晶体，其中两个声波晶体以小角度错位堆叠。这种双层结构形成了艺术性的莫尔图案，通常在纺织品中发现。该项目将通过赋予声波晶体一套新的操纵声音的能力，广泛推进声学功能材料领域。由此产生的双层声波晶体，预计将促进应用，如能量收集和增强的声发射和传感。研究活动将涉及本科生，以及来自代表性不足群体的学生。创新的外展活动也将得以实现，例如与宾夕法尼亚州立大学的帕尔默艺术博物馆合作举办一个关于双层声波石墨烯的特别展览，主题是在艺术（莫尔图案）和科学之间建立一种意想不到的联系。扭转电子学是研究电子行为的领域，可以通过控制二维材料（如石墨烯）层之间的扭曲来显着改变电子行为。最近在双电子学中的一个重大发现是所谓的魔角，这是两片石墨烯之间的非凡扭曲角，会产生超平坦带，产生莫特绝缘态和非常规超导性。这项研究从双电子学的最新发展中汲取灵感，并试图利用扭曲和层间耦合作为两个新的自由度来设计一个新的声波晶体家族，即，扭曲双层声波晶体分析和计算模型将被开发，以阐明扭曲的双层声波晶体的带结构与广泛的扭曲角和层间耦合强度。本文将建立一个框架来识别扭曲双层声波晶体中的魔角。声学魔角的重要属性，如其相应的本征模，物理边界，和缺陷的鲁棒性，将被揭示。最后，我们将对扭曲双层声波晶体的拓扑特征，以及损耗与其能带的相互作用（无论是有利的还是不利的）获得重要的见解。该奖项反映了NSF的法定使命，并通过使用基金会的知识价值和更广泛的影响审查标准进行评估，被认为值得支持。

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.