Negative Effective Density Research Articles

Degradation mechanism of SiC diodes under thermal and irradiation stress

Abstract Silicon carbide (SiC)-based diodes are widely used due to their high temperature resistance. In this paper, breakdown voltage of 4H-SiC JBS diodes and capacitance-voltage of 4H-SiC MOS capacitors with identical interfacial structure were both measured at high temperature to study their interfacial properties. By analyzing the variation of interfacial properties, the correlation between thermal stress and failure mode of 4H-SiC JBS diodes was further established to reveal their degradation mechanism. During initial high temperature storage, interfacial negative effective charge density of 4H-SiC JBS diodes may increase, leading to the decrease of breakdown voltage. As storage time increases, interfacial charge density began reducing, resulting in the increase of breakdown voltage. Avalanche luminescence verifies that the variation of interfacial charge density under thermal stress led to the degradation of 4H-SiC JBS diodes and further affected their breakdown voltage. Finally, degradation mechanism of SiC-JBS, SiC-SBD and SiC-PIN diodes under irradiation stress was investigated. After irradiation, forward current of some SiC diodes increased at a small forward voltage. However, the reverse characteristics deteriorated obviously and even failed.

An efficient multiscale method for subwavelength transient analysis of acoustic metamaterials.

A reduced-order homogenization framework is proposed, providing a macro-scale-enriched continuum model for locally resonant acoustic metamaterials operating in the subwavelength regime, for both time and frequency domain analyses. The homogenized continuum has a non-standard constitutive model, capturing a metamaterial behaviour such as negative effective bulk modulus, negative effective density and Willis coupling. A suitable reduced space is constructed based on the unit cell response in a steady-state regime and the local resonance regime. A frequency domain numerical example demonstrates the efficiency and suitability of the proposed framework.This article is part of the theme issue 'Current developments in elastic and acoustic metamaterials science (Part 2)'.

Study on SH wave propagation in the elastic metamaterial layer

Elastic metamaterial (EM) is a new type of structured material mainly built with repeatedly arranged sub-wavelength unit cells. EMs have shown many surprisingly new dynamic properties and provided us with a new idea for elastic wave/vibration control. Although a large amount of work has been conducted by researchers, wave behaviors in some important waveguides constructed with EM remain for further investigation. In this work, SH (shearing horizontal) wave propagating in the EM layer is studied. The effects of the abnormal properties of effective medium parameters (EMPs) on the SH wave dispersion features are discussed by comparison with a classical elastic layer. Our study shows that the behavior of EMPs can significantly influence the dispersion properties of SH wave propagation within the EM layer. Moreover, no SH surface wave mode could be supported near the stress-free surface of the EM layer, and the SH wave traveling in the EM layer must be composed of bulk wave components, and hence it will be blocked completely in those EM layers with imaginary effective transverse wave velocity which can be realized by single negative effective density or shear modulus. This work may serve as a theoretical reference for relevant metamaterial-based elastic wave/vibration control.

Estimating effective acoustic properties of various configuration of perforated panels

Acoustic metamaterial attains uncommon material properties over natural material such as negative effective mass density, negative effective bulk modulus, or both. To start with, the present research demonstrates and establishes robust method to estimate and measure acoustic metamaterial properties of a Helmholtz resonator analytically using transfer matrix (TM) method and experimentally in detail. The proposed method extracts the reflection and transmission coefficients from corresponding TM to evaluate the effective acoustic metamaterial properties. For a single Helmholtz resonator, the effective bulk modulus has been observed negative; however, the effective density remains positive. In order to attain double negative material properties, the perforated panel (PP) and microperforated panel (MPP) have been introduced in parallel to resonator. The 1D electro-acoustic modeling has been carried out for all configurations to estimate effective properties analytically. Successively, the experiments have been conducted to measure the effective properties from acoustic metamaterial prospective. The analytical and experimental investigations on five different cases reveal that one can attain negative effective compressibility and effective density by cascading two PPs or MPPs with a finite duct. Moreover, by backing a Helmholtz resonator from both sides with two PPs or MPPs in parallel, one can attain double negative properties comparatively higher than earlier configuration in low-frequency zone in broadband. In total, eight different configurations have been investigated analytically and experimentally, where it has been demonstrated that the proposed compact model is more effective than a finite array of same Helmholtz resonators.

Broadband muffler by merging negative density and negative compressibility

We present a broadband muffler using a metamaterial consisting of a membrane structure and a Helmholtz resonator. The membrane structure with ventilation exhibited negative effective density in the 290–435 Hz frequency range. The Helmholtz resonator exhibited negative effective compressibility in the 440–600 Hz range. We connected the two frequency ranges to continue the negativity by matching the end frequency of the negative effective density and the start frequency of the negative effective compressibility. The metamaterial muffler showed a wide range of imaginary components of wavenumbers from 290 to 600 Hz. The muffler exhibits 10 dB transmission loss or 10 % power transmission in the frequency range 275–500 Hz. The realization of a metamaterial muffler with a broad and low-frequency performance range is unprecedented when considering the resonant properties of the metamaterial.

Particle-like solutions in the generalized SU(2) Proca theory

The generalized SU(2) Proca theory is a vector-tensor modified gravity theory where the action is invariant under both diffeomorphisms and global internal transformations of the SU(2) group. This work constitutes the first approach to investigate the physical properties of the theory at astrophysical scales.We have found solutions that naturally generalize the particle-like solutions of the Einstein-Yang-Mills equations, also known as gauge boson stars.Under the requirement that the solutions must be static, asymptotically flat, and globally regular, the t'Hooft-Polyakov magnetic monopole configuration for the vector field rises as one viable possibility. The solutions have been obtained analytically through asymptotic expansions and numerically by solving the boundary value problem. We have found new features in the solutions such as regions with negative effective energy density and imaginary effective charge. We have also obtained a new kind of globally charged solutions for some region in the parameter space of the theory. Furthermore, we have constructed equilibrium sequences and found turning points in some cases. These results hint towards the existence of stable solutions which are absent in the Einstein-Yang-Mills case.

Hyperbolically symmetric static charged cosmological fluid models

ABSTRACT In this work, the study on static fluid distributions under the influence of electromagnetism has been carried out with an emphasis on the hyperbolically symmetric metric. For this purpose, modified gravitational formulations in the presence of charge are used to calculate the effective energy–momentum tensor, which is then further refined by taking into account tetrad field components in the Minkowski coordinate system. Also, we compute the Tolman mass and a suitable formulation of the mass function. It exhibits that the hyperbolically symmetrical source has a negative effective matter density in all stellar formulations. This demonstrates that the quantum processes together with certain excessive constraints are deemed important to explain any physical implementations under the effects of the electromagnetic field. Additionally, we assessed the structure scalars and implemented the orthogonal splitting of the structure scalars and Riemann tensor in this modified gravity. Subsequently, various explicit precise cosmological solutions and their generating functions are developed.

A low-frequency and wideband resonant metamaterial plate with front radial membrane

The various types of metamaterials only have a sound transmission loss (STL) peak at the resonant frequency but are still constrained by the law of mass sound insulation at other frequencies. In this paper, a low-frequency and wideband resonant metamaterial plate with a front radial membrane was designed in order to improve the noise reduction band. Bandgap and STL were calculated by using the finite element method. Studies have shown that in the range of 1 Hz–100 Hz for new metamaterials, the frequency band with STL greater than 30 dB accounts for 75%, and the noise reduction starting frequency is 11 Hz. The mechanisms were investigated by a comprehensive analysis of mode shapes and sound intensity streamlines and then verified by the negative effective density and equivalent mass–spring model. The mechanism analysis shows that there is a wide bridge coupling bandgap between the respective bandgaps of the plate and the membrane. This novel metamaterial not only guarantees the low-frequency and wideband acoustic performance but also alleviates the problem of instability of the noise reduction performance of the membrane material after long-term use, providing a potential application in low-frequency and wideband noise control.

Rayleigh waves in locally resonant metamaterials

The design of metamaterials for surface waves control is an emerging field of research which can impact several technical applications, from electronic devices based on surface acoustic waves (SAW) to wave barriers for seismic isolation. So far, studies on the interaction of surface waves with locally resonant metamaterials have been limited to the context of metasurfaces, i.e., thin resonant interfaces or structures attached to the free surface of a waveguide. In this work we remove this constraint by formulating an original dispersion relation for vertically polarized surface waves of the Rayleigh-type existing in resonant metamaterials. We consider the case of a resonant layer of thickness H coupled to a non-resonant half-space. To easily account for the resonant material while setting the dispersion relation, we propose a mixed dynamic-static homogenization approach, valid in the long wavelength regime. We show the existence of a band gap in the spectrum of the Rayleigh surface waves and relate its width to the region of negative effective density of the resonant metamaterial. We highlight how the thickness of the resonant layer affects the frequency stop-band and the magnitude of wave attenuation. Next, we derive and discuss the limit cases of a fully resonant half-space, i.e., a resonant layer with thickness H>>λ where λ is the wavelength of the Rayleigh wave, and of a metasurface for small values of H<<λ. In the latter case, we prove that our formulation properly model metasurfaces recovering previous formulations. As a case study, we analyse the propagation of seismic Rayleigh waves across a deep barrier of meter-size resonators embedded in the soil. The barrier is modeled using the proposed analytical approach. By means of high fidelity finite element simulations and Bloch analyses, we validate our novel findings including the homogenization approach and the derived dispersion relations. Our work extends the knowledge on mechanical waves in metamaterials providing an analytical framework for the study of vertically polarized surface waves in resonant and partially resonant waveguides.

Enhanced acoustic transmission through an aqueous acoustic metasurface

Acoustic metasurfaces are ultrathin (subwavelength thickness) structures that can demonstrate extreme acoustic properties, such as negative or near-zero values, similar to those seen in acoustic metamaterials. The negligibly small thickness of acoustic metasurfaces make them a more practical alternative to metamaterials for certain applications. In this work, we experimentally demonstrate an aqueous acoustic metamaterial that employs subwavelength, flexural elements that acoustically act in parallel to achieve enhanced acoustic transmission and broadband negative effective density. The metasurface was constructed from a brass plate, which was machined to have a parallel arrangement of circular, flexural elements on the surface, and experimentally tested in water over the nominal range of 50–100 kHz. The reflected and transmitted acoustic data were compared between a brass plate with and without the circular flexural elements. It was observed that the aqueous acoustic metasurface made from a brass plate with circular flexural elements showed improved broadband transmission. A detailed description of the experimental testing and analysis will be discussed. [Work supported by the Office of Naval Research]

Tuning the working frequency of elastic metamaterials by heat

The limitation of passive elastic metamaterials is their fixed and narrow working frequency range. In this work, the manipulation of elastic waves using heat by an elastic metamaterial with resonant microstructures is presented for the first time. The effective density of the elastic metamaterial at different wave frequencies and temperatures is determined using effective continuum theory. We proved that the elastic metamaterial can have not only frequency - but also temperature-dependent negative effective density. By properly selecting materials for the building block, the effective mass density of the elastic metamaterial can be varied by increasing the temperature of the material. Elastic wave attenuation, waveguiding and wave focusing of the elastic metamaterial are then demonstrated by controlling the temperature of the elastic metamaterial. The developed methodology could significantly influence research areas including elastic wave attenuation, waveguiding, wave imaging and numerous other applications.

Practical realisation of an active acoustic metamaterial building block

Acoustic metamaterials (AMMs) have been demonstrated as an alternative approach to achieving high levels of noise control using an array of subwavelength unit cells, exhibiting behaviour not seen in conventional materials. Specifically, active AMMs offer the potential for greater levels of broadband wave manipulation, tunability and adaptability. However, determining the optimal control source strengths that achieve broadband negative effective material properties is not straightforward. This study presents a practical method of designing an active system that directly minimises the effective material properties. The optimal control source strengths required for both single monopole and single dipole control sources to minimise the effective bulk modulus and the effective density respectively have been calculated via an optimisation procedure. The ability of the proposed active AMM building blocks to achieve broadband negative effective material properties is tested experientially through offline simulations using measured frequency responses. Interestingly, the optimised single monopole and optimised single dipole control sources achieve broadband negative effective bulk modulus and density respectively. However, when combining the two optimised control sources, the active control system only achieves bands of double negativity and, thus, only exhibits bands of negative refractive index.

Assembly methods of solving inverse problems as an integral element of additive technologies for interpretations of gravity anomalies

In recent years, the theory of interpretation of gravitational anomalies has been replenished with fundamentally different from familiar, additive technologies for extracting information about the studied geo-density medium. The concept of «additivity» implies a summation in the results of the interpretation of information carried by each of the found admissible solutions to the inverse problem. At the same time, the results of the interpretation themselves are not expressed, as usual, in terms of one of these solutions. In the ore inverse problem of gravity exploration, an effective working tool for such technologies has become the assembly algorithms for constructing valid interpretation options. These algorithms have universal capabilities in the matter of accounting for a priori data and do not have high requirements for the formation of the initial approximation of the field source model, which is crucial from the point of view of their application in additive technologies. At the same time, all hitherto known modifications of the mounting method were designed for formulating inverse problems in which there are bodies with effective densities of the same sign. Due to the integration of the method with the procedure for separating gravitational fields, the problem of different sign densities was solved. The separation of the observed field into two components, due to the influence of sources with positive and negative effective density, is carried out by the approximation method. To approximate the discrete values of gravity, the sets of elementary sources under each point of observation are used. The masses of sources are determined by solving a system of linear algebraic equations. The sources are successively immersed at different depths corresponding to different variants of the selection of the interpreted field components. The article evaluates the current state and prospects for the further development of additive technologies for interpreting gravitational anomalies based on the methods that implement the concept of the mounting approach by V. N. Strakhov. Model of model examples are given illustrating the capabilities of the presented algorithms.

Nonlinear acoustic metamaterials

Acoustic metamaterials (AMM) have become a very active topic for research in numerous domains of engineering and science because of their promise to create materials, structures, and devices that can control acoustic wave propagation in ways that exceed the capabilities of naturally occurring or conventional composite materials. The majority of AMM research has been focused on linear behavior such as negative dynamic effective stiffness and density, cloaking, and negative refraction. One drawback of the focus on linear behavior is the restriction of the effective material properties of interest to narrow frequency bands that cannot be changed with external stimulus. Nonlinearity has been explored as a means of increasing the bandwidth of performance by enabling tunable band gaps and material configurability. Other recent work has investigated nonlinear AMM to access phenomena such as harmonic generation, non-reciprocity, enhanced energy absorption, solitons, mode hopping and conversion, chaos, and intrinsic localized modes. This talk will provide an overview of nonlinear AMM, starting with a background on AMM, then surveying existing research on the topic and its relationship to seminal works in nonlinear acoustics, and finally discussing promising avenues of future research. [Work supported by the National Science Foundation EFRI program and ARL:UT.]

On the emergence of negative effective density and modulus in 2-phase phononic crystals

In this paper we report metamaterial properties including negative and singular effective properties for what would traditionally be considered non locally resonant 2-phase phononic unit cells. The negative effective material properties reported here occur well below the homogenization limit and are, therefore, acceptable descriptions of overall behavior. The material property combinations which make this possible were first revealed by a novel level set based topology optimization process which we describe. The optimization process revealed that a 2-phase unit cell in which one of the phases is simultaneously lighter and stiffer than the other results in dynamic behavior which has all the attendant characteristics of a locally resonant composite including negative effective properties far below the homogenization limit. We investigate this further using the Craig–Bampton decomposition and clarify that these properties emerge through an interplay between the fundamental internal modeshape of the unit cell and a rigid body mode. Through explicit numerical calculations on 1-D, 2-phase unit cells, we show that negative effective properties only appear for the specific material property combination mentioned above. Furthermore, we provide a proof which supports this conclusion. The concept is also shown to hold for 2-D unit cells where we show that an appropriately designed hexagonal unit cell made of 2 material phases exhibits negative effective shear modulus and density in an appropriate frequency regime in which it also exhibits negative refraction. As in the 1-D case, we show that the homogenized description in the 2-D case is rigorous as well. An important conclusion of this paper is that the class of unit cells expected to result in negative properties can be expanded beyond the classic unit cell (three-phase unit cells with an explicit locally resonant phase) to include topologically simpler 2-phase unit cells as well.

Nonlinear metamaterial from piecewise discontinuous acoustic properties

Numerous metamaterials have recently been designed which exhibit either negative effective density and/or bulk modulus leading to extraordinary wave bearing capabilities. This research increases the design space by considering displacement-dependent properties which are discontinuous over a period of oscillation. Such nonlinear behavior has been studied in dynamical systems with intermittent contact. This work applies those analytical results together with numerical methods to provide insight into the harmonic and subharmonic generation in acoustic metamaterials with properties differing from compression to rarefaction. Stability is analyzed providing criteria for the onset of chaotic behavior.

Ultrathin multi-slit metamaterial as excellent sound absorber: Influence of micro-structure

An ultrathin (subwavelength) hierarchy multi-slit metamaterial with simultaneous negative effective density and negative compressibility is proposed to absorb sound over a wide frequency range. Different from conventional acoustic metamaterials having only negative real parts of acoustic parameters, the imaginary parts of effective density and compressibility are both negative for the proposed metamaterial, which result in superior viscous and thermal dissipation of sound energy. By combining the slit theory of sound absorption with the double porosity theory for porous media, a theoretical model is developed to investigate the sound absorption performance of the metamaterial. To verify the model, a finite element model is established to calculate the effective density, compressibility, and sound absorption of the metamaterial. It is theoretically and numerically confirmed that, upon introducing micro-slits into the meso-slits matrix, the multi-slit metamaterial possesses indeed negative imaginary parts of effective density and compressibility. The influence of micro-slits on the acoustical performance of the metamaterial is analyzed in the context of its specific surface area and static flow resistivity. This work shows great potential of multi-slit metamaterials in noise control applications that require both small volume and small weight of sound-absorbing materials.

Reciprocity, passivity, and causality in media with coupled strain-momentum constitutive relations

Metamaterials are heterogeneous materials and structures which, under certain circumstances, may be interpreted as a homogeneous material displaying extreme physical properties such as negative effective density or modulus. Acoustic metamaterials (AMM) therefore expand the parameter space for acoustic device design and are thus of interest for a wide range of applications. Most AMM are composed of linear and passive constituent materials. Any physically meaningful approximation of their overall response must therefore obey reciprocity, passivity, and causality. This requirement constrains the effective parameters and delineates the range of possible material properties for an AMM. While restrictions for standard constitutive equations in acoustics and elastodynamics are well known, they have not been explored in detail for media with coupled strain-momentum behavior which is required to fully describe AMM [Norris et al., Proc. R. Soc. A, 467, 1749–1769 (2011)]. This work derives the restrictions on coupled acoustic constitutive equations by requiring the overall response to be reciprocal, passive, and causal. The special cases of lossless and very low-loss materials are then considered and some useful approximate restrictions are also presented. Restrictions for the analogous case of generally anisotropic elastic strain-momentum coupled media will also be reported. [Work supported by ONR.]

Theory of multiresonant metamaterials forA0Lamb waves

We develop an analytical wave approach to describe the physics properties of multiresonant metamaterials for Lamb waves propagating in plates. The metamaterial that we characterize consists of a 10 by 10 uniform, periodic array of long rods attached to the surface of the plate that forms the substrate in which antisymmetric ${A}_{0}$ Lamb waves are excited. We show that the ${A}_{0}$ Lamb wave propagation through the metamaterial can be accurately modeled using a simplified theory that replaces the two-dimensional array with a one-dimensional beam with a linear array of 10 rods. The wave propagation problem is solved rigorously for this one-dimensional system using the scattering matrix for a single rod. The exact eigenvalues of the system are approximated in a long wavelength expansion to determine a simple expression for the effective wave number and dispersion of the metamaterial. The modeled dispersion is compared with an experimental measurement of the dispersion inside the metamaterial with excellent agreement. The multiresonant rods, restricted to longitudinal vibration consistent with ${A}_{0}$ Lamb waves excited in the plate, produce two wide stop bands in the frequency domain from 0 to 10 kHz where the stop or passband boundaries align with the minima and maxima of the rod's impedance. We show that a negative effective density is obtained in the stop band. With the simple yet highly accurate relations given in this paper we have a tool to develop more complex metamaterials with rods and plates of different properties.

On the Einstein-Cartan cosmology vs. Planck data

The first comprehensive analyses of Planck data reveal that the cosmological model with dark energy and cold dark matter can satisfactorily explain the essential physical features of the expanding Universe. However, the inability to simultaneously fit the large and small scale TT power spectrum, the scalar power index smaller than unity, and the observations of the violation of the isotropy found by few statistical indicators of the CMB urge theorists to search for explanations. We show that the model of the Einstein-Cartan cosmology with clustered dark matter halos and their corresponding clustered angular momenta coupled to torsion can account for small-scale-large-scale discrepancy and larger peculiar velocities (bulk flows) for galaxy clusters. The nonvanishing total angular momentum (torsion) of the Universe enters as a negative effective density term in the Einstein-Cartan equations causing partial cancellation of the mass density. The integrated Sachs-Wolfe contribution of the Einstein-Cartan model is negative, and it can therefore provide partial cancellation of the large-scale power of the TT CMB spectrum. The observed violation of the isotropy appears as a natural ingredient of the Einstein-Cartan model caused by the spin densities of light Majorana neutrinos in the early stage of the evolution of the Universe and bound to the lepton CP violation and matter-antimatter asymmetry.