Twisted Bilayer Sonic Crystal: A New Playground for Twistronics

声音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。虽然过去主要研究单层声波晶体，但本研究旨在研究一种全新的声波晶体，其中两个声波晶体以小角度错位堆叠。这样的双层结构形成一种艺术波纹图案，通常在纺织品中发现。该项目将通过赋予声音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有望促进诸如能量收集和增强声发射和传感等应用。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帕尔默艺术博物馆合作举办一场关于双层声波石墨烯的特别展览，主题是在艺术（波纹图案）和科学之间建立意想不到的联系。扭曲电子学是一个研究电子行为的领域，它可以通过控制二维材料（如石墨烯）层之间的扭曲来显著改变电子行为。涡旋电子学最近的一个重大发现是所谓的魔角，这是两片石墨烯之间的特殊扭转角，可以产生超平坦的带，从而产生莫特绝缘状态和非常规的超导性。本研究从双扭电子学的最新发展中获得灵感，试图利用扭转和层间耦合作为两个新的自由度来设计一种新的声波晶体，即双扭层声波晶体。将建立分析和计算模型，以揭示具有大范围扭转角和层间耦合强度的扭曲双层声波晶体的能带结构。本文将建立一个框架来识别扭曲双层声波晶体中魔角的声学版本。将揭示声魔角的重要性质，如其相应的特征模态、物理边界和对缺陷的鲁棒性。最后，将获得关于扭曲双层声波晶体的拓扑特征的重要见解，以及损耗如何与它们的能带相互作用，无论是有利的还是不利的。该奖项反映了美国国家科学基金会的法定使命，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.