Unconventional Dispersion Research Articles

Artificially engineered metaconcrete with wide bandgap for seismic surface wave manipulation

The energy of ground-born vibration is transported mostly by surface waves. Therefore, engineered structures which absorb externally applied energy and improve structural performance against explosive dynamic loading are highly desired. In this paper, we demonstrate the application of designed metamaterial barriers with engineered metaconcrete for reflecting and redirecting surface Rayleigh waves to avoid disastrous consequences. The metabarrier is constructed by embedding metaconcrete cubes in a semi-infinite space. The local resonance type metaconcrete cell is composed of a heavy lead ball coated by a soft rubber layer and mortar. One significant limitation of metaconcrete is the narrow bandwidth. Here, we apply an array of metaconcretes where the radii of the lead ball and soft coating increase or decrease simultaneously. Via transmission ratio analysis, we show that the metaconcrete-based metabarrier is able to achieve broadband wave attenuation. More interestingly, the elastic wave trapping and surface wave mode conversion can be realized and controlled by the metabarrier. The mechanism of trapping and conversion is fully explained with an unconventional dispersion relation. A time history analysis based on several actual earthquake records obtained from public database is also presented. The result in this paper highlights new applications of metaconcretes and it can be applied to novel seismic shields for protecting civil infrastructures against seismic destruction.

Reconstructed Gaussian basis to characterize flexural wave collimation in plates with periodic arrays of annular acoustic black holes

While embedding acoustic black holes (ABHs) in plates has proved to be a lightweight and useful solution for noise and vibration reduction, their potential for wave manipulation is only in its beginnings. Recent studies seem to indicate that amazing properties can be attained with them. A plate with ABH indentations can be viewed as a single-phase metamaterial that can alter wave propagation direction thanks to the ABH power-law thickness profile. Some typical unconventional dispersion properties of multi-phase metamaterials, like collimation or negative refraction, can be recovered in a simple way. This work focuses on the collimation of flexural waves in plates by means of ABH arrangements. The first goal of the paper is to propose a predictive semi-analytical method to describe such phenomenon in an efficient manner. The method is also expected to allow one to perform quick parametric analyses for different ABH configurations. To that purpose, the Gaussian expansion method (GEM) is extended to deal with ABH phononic crystals on infinite plates. The Lagrangian for a single cell is built and a new basis of Gaussian functions is constructed satisfying the periodic boundary conditions of the problem. The Rayleigh-Ritz method is then adopted to compute the displacement field of annular and circular ABH arrays. The accuracy of the reconstructed GEM approach is validated by comparing dispersion curves and modal shapes with those obtained from finite element simulations. Equi-frequency contours are then used to determine the frequency bands prone to collimation. The focus is placed on annular ABH configurations which allow one to achieve collimation at lower frequencies than the circular ones. The second main contribution of this work therefore consists of a thorough analysis to characterize the influence of geometric parameters on the performance of ABH arrays. Moreover, new designs of annular ABH arrangements on plates are proposed for wave conduction, including curvature, and energy focusing in the long wavelength limit.

Elastic wave propagation in moving phononic crystals and correlations with stationary spatiotemporally modulated systems

This work presents a generalized physical interpretation of unconventional dispersion asymmetries associated with moving phononic crystals (PCs). By shifting the notion from systems with time-variant material fields to physically traveling materials, the newly adopted paradigm provides a novel approach to the elastic dispersion problem and, in the process, highlights discrepancies between moving PCs and stationary ones with dynamic material fields. Equations governing the motion of an elastic rod with a prescribed moving velocity observed from a stationary reference frame are used to predict propagation patterns and asymmetries in wave velocities obtained as a result of the induced linear momentum bias. Three distinct scenarios are presented corresponding to a moving rod with a constant modulus, a spatially varying one, and one that varies in space and time. These cases are utilized to extract and interpret correlations pertaining to directional velocities, dispersion patterns, as well as nature of band gaps between moving periodic media and their stationary counterparts with time-traveling material properties. A linear vertical shear transformation is then derived and utilized to neutralize the effect of the moving velocity on the resultant band diagrams. Finally, dispersion contours associated with the transient response of a finite moving medium are used to validate the entirety of the presented framework.

Dynamics of periodic ribbed plates with inner resonance: Analytical homogenized model and dispersion features

The dynamic behavior of a periodic ribbed plate with local resonance is investigated. The behavior of the cell made of a beam clamped along a plate edge analyzed through multi-scale asymptotic method enables to derive the governing equations of the effective mechanical behavior. This approach allows obtaining a full homogenized analytical model that provides a relevant representation of the flexural and torsional mechanisms at both global and local scales. The complex dynamic behavior is shown to encompass several mechanisms associated with enriched kinematics. Two types of flexural and torsional waves are evidenced and governed by two distinct differential equations that describes (i) waves where both beam and plate moves, and (ii) guided waves where the plate only is set in motion. The inner resonance of the plate induces unconventional dispersion features, with singularities associated either with the symmetric eigenmodes for the bending waves or the antisymmetric modes for the torsional waves. The guided waves are alternatively related to the symmetric and antisymmetric modes of the bended plate and are propagative above the corresponding eigenfrequencies. The predictions of the homogenized model are successfully compared to numerical calculations conducted using Wave Finite Element based methods, for two realistic examples of ribbed plates. The study provides design rules to tailor ribbed plate panels having specific atypical features in a given frequency range.

Ferromagnet/Superconductor Hybridization for Magnonic Applications

AbstractIn this work, a new hybridization of superconducting and ferromagnetic orders is demonstrated, promising for magnonics. By measuring the ferromagnetic and spin wave resonance absorption spectra of a magnetostatically coupled permalloy/niobium bilayer at different temperatures, magnetostatic spin wave resonances with unconventional dispersion are observed. The mechanism behind the modified dispersion, confirmed with micromagnetic simulations, implies screening of the alternating magnetostatic stray fields of precessing magnetic moments in the ferromagnetic layer by the superconducting surface in the Meissner state.

Thinking outside the box: placing hydrophilic particles in an oil phase for the formation and stabilization of Pickering emulsions.

Pickering emulsions, also known as particle stabilized emulsions, are one kind of extremely important emulsion for both fundamental research and practical applications. Many colloidal particles have been utilized as emulsifiers to stabilize Pickering emulsions. However, the most challenging issue is preparing Pickering emulsions with highly hydrophilic particles, because their adsorption onto oil-water interfaces is either thermodynamically or kinetically unfavorable. Although several strategies have been developed to overcome the poor ability of the hydrophilic particles to stabilize the emulsions, surface modification and functionalization of the hydrophilic particles or a change in solvent (i.e. water phase) conditions such as pH and ionic strength is required. Herein, we present an effective and not yet explored strategy to stabilize Pickering emulsions with unmodified highly hydrophilic particles, strikingly, without changing the solvent conditions. The innovative aspect of the strategy presented here is the unconventional dispersion of hydrophilic particles in an oil phase before emulsification, while the results experimentally demonstrate the theoretical calculations predicted more than a decade ago. This study will promote the diversity of Pickering emulsions and expand their real-world applications.

Two-dimensional structure-embedded acoustic lenses based on periodic acoustic black holes

Recent studies have introduced a new class of two-dimensional acoustic metamaterials whose dispersion and propagation properties result from the use of geometric inhomogeneities in the form of Acoustic Black Holes (ABH). The ABH is an element able to smoothly bend acoustic rays and slow down elastic bending waves, therefore providing a variety of unconventional dispersion and propagation properties that are typically observed in more complex multi-material and locally resonant designs. Our approach enables full integration of acoustic lenses in thin-walled structural elements while preserving the structural character. The lenses are capable of different high-level functionalities such as focusing, collimation, and negative refraction. Numerical and experimental results show that such structures exhibit broadband operating conditions that span both the metamaterial and the phononic range.

Insight into the collective vibrational modes driving ultralow thermal conductivity of perovskite solar cells

The past few years have witnessed a rapid evolution of hybrid organic-inorganic perovskite solar cells as an unprecedented photovoltaic technology with both relatively low cost and high-power conversion. The fascinating physical and chemical properties of perovskites are benefited from their unique crystal structures represented by the general chemical formula $AM{X}_{3}$, where the $A$ cations occupy the hollows formed by the $M{X}_{3}$ octahedra and thus balance the charge of the entire network. Despite a vast amount of theoretical and experimental investigations have been dedicated to the structural stability, electrical, and optical properties of hybrid halide perovskite materials in relation to their applications in solar cells, the thermal transport property, another critical parameter to the design and optimization of relevant solar cell modules, receives less attention. In this paper, we evaluate the lattice thermal conductivity of a representative methylammonium lead triiodide perovskite $({\mathrm{CH}}_{3}{\mathrm{NH}}_{3}{\mathrm{PbI}}_{3})$ with direct nonequilibrium ab initio molecular dynamics simulation. Resorting to full first-principles calculations, we illustrate the details of the mysterious vibration of the methylammonium cluster $({\mathrm{CH}}_{3}{\mathrm{NH}}_{3}^{+})$ and present an unambiguous picture of how the organic cluster interacting with the inorganic cage and how the collective motions of the organic cluster drags the thermal transport, which provide fundamental understanding of the ultralow thermal conductivity of ${\mathrm{CH}}_{3}{\mathrm{NH}}_{3}{\mathrm{PbI}}_{3}$. We also reveal the strongly localized phonons associated with the internal motions of the ${\mathrm{CH}}_{3}{\mathrm{NH}}_{3}^{+}$ cluster, which contribute little to the total thermal conductivity. The importance of the ${\mathrm{CH}}_{3}{\mathrm{NH}}_{3}^{+}$ cluster to the structural instability is also discussed in terms of the unconventional dispersion curves by freezing the partial freedoms of the organic cluster. These results provide more quantitative description of organic-inorganic interaction and coupling dynamics from accurate first-principles calculations, which are expected to underpin the development of emerging photovoltaic devices.

Compton-like polariton scattering in hyperbolic metamaterials.

We study the scattering of polaritons by free electrons in hyperbolic photonic media and demonstrate that the unconventional dispersion and high local density of states of electromagnetic modes in composite media with hyperbolic dispersion can lead to a giant Compton-like shift and dramatic enhancement of the scattering cross section. We develop a universal approach to study multiphoton processes in nanostructured media and derive the intensity spectrum of the scattered radiation for realistic metamaterial structures.

Relativistic quasiparticle self-consistent electronic structure of hybrid halide perovskite photovoltaic absorbers

Solar cells based on a light absorbing layer of the organometal halide perovskite CH$_3$NH$_3$PbI$_3$ have recently reached 15% conversion efficiency, though how these materials work remains largely unknown. We analyse the electronic structure and optical properties within the quasiparticle self-consistent GW approximation. While this compound bears some similarity to conventional sp semiconductors, it also displays unique features. Quasiparticle self-consistency is essential for an accurate description of the band structure: bandgaps are much larger than what is predicted by the local density approximation (LDA) or GW based on the LDA. Valence band dispersions are modified in a very unusual manner. In addition, spin orbit coupling strongly modifies the band structure and gives rise to unconventional dispersion relations and a Dresselhaus splitting at the band edges. The average hole mass is small, which accounts for the long diffusion lengths recently observed. The surface ionisation potential (workfunction) is calculated to be 5.7 eV with respect to the vacuum level, explaining efficient carrier transfer to TiO$_2$ and Au electrical contacts.

Composite right/left‐handed metamaterial transmission lines with unconventional dispersion and applications

AbstractIn this article, novel metamaterial transmission lines exhibiting right‐handed wave propagation at low frequencies and left‐handed wave transmission at high frequencies are presented. The unconventional dispersion characteristics of these composite right/left‐handed transmission lines is obtained by parallel connecting left‐handed and right‐handed unit cells, both implemented by using complementary split‐ring resonators. Thanks to the diplexer action of the structure, either the right‐handed (tuned at the lower frequency band) or the left‐handed (tuned at the upper frequency band) unit cell transmits the injected power, thus achieving the unconventional dispersion of the structure. It is also shown in this article that these artificial lines are of interest for the design of certain microwave components exhibiting dual‐band characteristics. Examples are provided including design and characterization. The measurements are indicative of good device performance at the two operating bands and wide bandwidths. © 2010 Wiley Periodicals, Inc. Microwave Opt Technol Lett 52:904–909, 2010; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/mop.25077