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  1. Free, publicly-accessible full text available March 31, 2025
  2. Free, publicly-accessible full text available February 1, 2025
  3. Abstract

    Topological insulators hold promises to realize exotic quantum phenomena in electronic, photonic, and phononic systems. Conventionally, topological indices, such as winding numbers, have been used to predict the number of topologically protected domain-wall states (TPDWSs) in topological insulators, a signature of the topological phenomenon called bulk-edge correspondence. Here, we demonstrate theoretically and experimentally that the number of TPDWSs in a mechanical Su-Schrieffer-Heeger (SSH) model can be higher than the winding number depending on the strengths of beyond-nearest-neighbor interactions, revealing the breakdown of the winding number prediction. Alternatively, we resort to the Berry connection to accurately characterize the number and spatial features of TPDWSs in SSH systems, further confirmed by the Jackiw-Rebbi theory proving that the multiple TPDWSs correspond to the bulk Dirac cones. Our findings deepen the understanding of complex network dynamics and offer a generalized paradigm for precise TPDWS prediction in potential applications involving localized vibrations, such as drug delivery and quantum computing.

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  4. In the present study, thermal stability of α-Ga2O3 under vacuum and ambient pressure conditions was investigated in situ by x-ray diffraction and transmission electron microscopy (TEM). It was observed that the thermal stability of α-Ga2O3 increased by 200 °C when pressure was lowered from an atmospheric to a vacuum level. This finding can be explained by oxygen diffusion under different oxygen partial pressures. In addition, in situ TEM imaging revealed that, once past the decomposition temperature, the onset of phase change propagates from the top crystal surface and accumulates strain, eventually resulting in a fractural film. The mechanism of α-Ga2O3 to β-Ga2O3 transition is evaluated through experiments and is discussed in this manuscript.

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    Free, publicly-accessible full text available July 1, 2024
  5. Free, publicly-accessible full text available April 1, 2024
  6. Free, publicly-accessible full text available March 28, 2024
  7. Periodic networks on the verge of mechanical instability, called Maxwell lattices, are known to exhibit zero-frequency modes localized to their boundaries. Topologically polarized Maxwell lattices, in particular, focus these zero modes to one of their boundaries in a manner that is protected against disorder by the reciprocal-space topology of the lattice’s band structure. Here, we introduce a class of mechanical bilayers as a model system for designing topologically protected edge modes that couple in-plane dilational and shearing modes to out-of-plane flexural modes, a paradigm that we refer to as “omnimodal polarization.” While these structures exhibit a high-dimensional design space that makes it difficult to predict the topological polarization of generic geometries, we are able to identify a family of mirror-symmetric bilayers that inherit the in-plane modal localization of their constitutive monolayers, whose topological polarization can be determined analytically. Importantly, the coupling between the layers results in the emergence of omnimodal polarization, whereby in-plane and out-of-plane edge modes localize on the same edge. We demonstrate these theoretical results by fabricating a mirror-symmetric, topologically polarized kagome bilayer consisting of a network of elastic beams via additive manufacturing and confirm this finite-frequency polarization via finite element analysis and laser-vibrometry experiments.

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