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Crystals 2016, 6(4), 44; doi:10.3390/cryst6040044

One-Dimensional Mass-Spring Chains Supporting Elastic Waves with Non-Conventional Topology

Department of Materials Science and Engineering, University of Arizona, Tucson, AZ 85721, USA
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Author to whom correspondence should be addressed.
Academic Editors: Victor J. Sanchez-Morcillo, Vicent Romero-Garcia and Luis M. Garcia-Raffi
Received: 26 February 2016 / Revised: 4 April 2016 / Accepted: 13 April 2016 / Published: 16 April 2016
(This article belongs to the Special Issue Phononic Crystals)
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Abstract

There are two classes of phononic structures that can support elastic waves with non-conventional topology, namely intrinsic and extrinsic systems. The non-conventional topology of elastic wave results from breaking time reversal symmetry (T-symmetry) of wave propagation. In extrinsic systems, energy is injected into the phononic structure to break T-symmetry. In intrinsic systems symmetry is broken through the medium microstructure that may lead to internal resonances. Mass-spring composite structures are introduced as metaphors for more complex phononic crystals with non-conventional topology. The elastic wave equation of motion of an intrinsic phononic structure composed of two coupled one-dimensional (1D) harmonic chains can be factored into a Dirac-like equation, leading to antisymmetric modes that have spinor character and therefore non-conventional topology in wave number space. The topology of the elastic waves can be further modified by subjecting phononic structures to externally-induced spatio-temporal modulation of their elastic properties. Such modulations can be actuated through photo-elastic effects, magneto-elastic effects, piezo-electric effects or external mechanical effects. We also uncover an analogy between a combined intrinsic-extrinsic systems composed of a simple one-dimensional harmonic chain coupled to a rigid substrate subjected to a spatio-temporal modulation of the side spring stiffness and the Dirac equation in the presence of an electromagnetic field. The modulation is shown to be able to tune the spinor part of the elastic wave function and therefore its topology. This analogy between classical mechanics and quantum phenomena offers new modalities for developing more complex functions of phononic crystals and acoustic metamaterials. View Full-Text
Keywords: phononic structures; topological elastic waves; time-reversal symmetry breaking phononic structures; topological elastic waves; time-reversal symmetry breaking
This is an open access article distributed under the Creative Commons Attribution License which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. (CC BY 4.0).

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Deymier, P.; Runge, K. One-Dimensional Mass-Spring Chains Supporting Elastic Waves with Non-Conventional Topology. Crystals 2016, 6, 44.

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