Understanding the design rules to obtain materials that enable a tight control of phonon transport over a broad range of frequencies would aid major developments in thermoelectric energy harvesting, heat management in microelectronics, and information and communication technology. Using atomistic simulations we show that the metamaterials approach relying on localized resonances is very promising to engineer heat transport at the nanoscale. Combining designed resonant structures to alloying can lead to extremely low thermal conductivity in silicon nanowires. The hybridization between resonant phonons and propagating modes greatly reduces the group velocities and the phonon mean free paths in the low frequency acoustic range below 4 THz. Concurrently, alloy scattering hinders the propagation of high frequency thermal phonons. Our calculations establish a rationale between the size, shape, and period of the resonant structures, and the thermal conductivity of the nanowire, and demonstrate that this approach is even effective to block phonon transport in wavelengths much longer than the size and period of the surface resonant structures. A further consequence of using resonant structures is that they are not expected to scatter electrons, which is beneficial for thermoelectric applications.

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Blocking Phonon Transport by Structural Resonances in Alloy-Based Nanophononic Metamaterials Leads to Ultralow Thermal Conductivity

We study the interaction of surface acoustic waves (SAWs) with a contact-based vibrational resonance of $1\text{ }\text{ }\ensuremath{\mu}\mathrm{m}$ silica microspheres forming a two-dimensional granular crystal adhered to a substrate. The laser-induced transient grating technique is used to excite SAWs and measure their dispersion. The measured dispersion curves exhibit ``avoided crossing'' behavior due to the hybridization of the SAWs with the microsphere resonance. We compare the measured dispersion curves with those predicted by our analytical model and find excellent agreement. The approach presented can be used to study the contact mechanics and adhesion of micro- and nanoparticles as well as the dynamics of microscale granular crystals.

Interaction of a contact resonance of microspheres with surface acoustic waves.

We study the interaction of surface acoustic waves (SAWs) with the contact-based, axial vibrational resonance of 1 μm silica microspheres forming a two-dimensional granular crystal adhered to a substrate. The laser-induced transient grating technique is used to excite SAWs and measure their dispersion. The measured dispersion curves exhibit “avoided crossing” behavior due to the hybridization of the SAWs with the microsphere resonance. We compare the measured dispersion curves with those predicted by our analytical model, and find excellent agreement. The approach presented can be used to study the contact mechanics and adhesion of microand nanoparticles as well as the dynamics of microscale granular crystals.

Interaction of a Contact Resonance of Microspheres with Surface Acoustic Waves

We present an acoustic metamaterial for flexural waves with both negative density and elastic modulus. By means of an analytical lumped-element approach, we introduce a combination of resonant elements on the surface of a plate. Following on from experimental demonstrations and theoretical work, negative density is achieved by introducing elements such as stubs that involve normal-force interactions, whereas negative modulus is achieved by introducing new types of resonant element that involve lateral forces and rotational inertia. Our approach therefore opens the way for the realization of double-negative acoustic meta-plates to control flexural waves. Applications include cloaking and filtering.

https://iopscience.iop.org/article/10.1088/1367-2630/16/12/123053/pdf

Double-negative flexural acoustic metamaterial

Contact-based vibrations play an essential role in the dynamics of granular materials. Significant insights into vibrational granular dynamics have previously been obtained with reduced-dimensional systems containing macroscale particles. We study contact-based vibrations of a two-dimensional monolayer of micron-sized spheres on a solid substrate that forms a microscale granular crystal. Measurements of the resonant attenuation of laser-generated surface acoustic waves reveal three collective vibrational modes that involve displacements and rotations of the microspheres, as well as interparticle and particle-substrate interactions. To identify the modes, we tune the interparticle stiffness, which shifts the frequency of the horizontal-rotational resonances while leaving the vertical resonance unaffected. From the measured contact resonance frequencies we determine both particle-substrate and interparticle contact stiffnesses and find that the former is an order of magnitude larger than the latter. This study paves the way for investigating complex contact-based dynamics of microscale granular crystals and yields a new approach to studying micro- to nanoscale contact mechanics in multiparticle networks.

Complex Contact-Based Dynamics of Microsphere Monolayers Revealed by Resonant Attenuation of Surface Acoustic Waves

The attenuation of Rayleigh waves due to their interaction with resonating structures randomly distributed on the surface of a semi-infinite elastic medium is calculated along with the Rayleigh wave frequency. The resonating structures are modeled by single oscillators coupled to the displacement field at the surface of the elastic medium. Using the coherent potential approximation, the dependence of the frequency and damping constant of the Rayleigh waves on wave vectors are determined for various values of the concentration of oscillators on the surface.

E. A. Garova

Papers

Interaction of Rayleigh waves with randomly distributed oscillators on the surface.