Suppression Of Waves Research Articles

Enhancement of superconductivity at a quantum critical point in (Ca x Sr1−x )3Rh4Sn13

We report microscopic studies by muon spin spectroscopy of the superconducting properties as a function of chemical and hydrostatic pressure in the cubic ternary intermetallic (Ca x Sr1−x )3Rh4Sn13 compounds. We find evidence of a quantum critical point at a critical pressure p c in the superconducting phase, where the superfluid density increases by a factor of two and the superconducting pairing strength displays a pronounced maximum. The enhancement of superconductivity is related to the structural phase transition at p c , which is accompanied by profound changes of the Fermi surface associated to the suppression of a charge density wave (CDW). The quantum critical point separates a superconducting phase coexisting with CDW from a pure superconducting phase, while in both phases superconductivity has a strong-coupling phonon-mediated BCS-like s-wave character. Together with the related isoelectronic compound (Ca x Sr1−x )3Ir4Sn13, this system shows that conventional BCS superconductors in the presence of competing orders may display behaviour and characteristics of unconventional superconductors.

Numerical Simulation of Plane and Cellular Detonation Wave Suppression in Hydrogen-Air Mixture by Inert Porous Filters

ABSTRACT Calculations of the interaction of plane (one-dimensional) and cellular (two-dimensional) detonation waves in hydrogen-air mixture with inert filters were carried out on the basis of the proposed physical and mathematical models, based on the detailed and reduced kinetics, describing such processes. The realized detonation regimes of attenuation and suppression of detonation were revealed. Comparison of results obtained by detailed and reduced kinetics showed that reduced kinetics gives overestimated detonation velocities compared to the detailed kinetics. But the obtained concentration limits of detonation practically equal to each other for both kinetics models. Comparison of processes of attenuation and suppression of plane and cellular detonation showed that the suppression of cellular detonation is more difficult to achieve compared to a plane detonation wave. Detonation failure criterion that shows that at the increasing the filter particles diameter, it is necessary to increase the volume concentration proportionally in order to successfully suppress detonation both in the case of a plane detonation wave and in the case of cellular detonation, was obtained.

Dark energy with a triplet of classical U(1) fields

We present a new mechanism for cosmic acceleration consisting of a scalar field coupled to a triplet of classical U(1) gauge fields. The gauge fields are arranged in a homogeneous, isotropic configuration, with both electric- and magnetic-like vacuum expectation values. The gauge fields provide a mass-like term via a Chern-Simons interaction that suspends the scalar away from its potential minimum, thereby enabling potential-dominated evolution. We show this mechanism can drive a brief period of acceleration, such as dark energy, without the need for fine tunings. We obtain simple analytic results for the dark energy equation of state and dependence on model parameters. In this model, the presence of the gauge field generically leads to a suppression of long-wavelength gravitational waves, with implications for the experimental search for cosmic microwave background B-modes and direct detection of a stochastic gravitational wave background.

Achieving high-efficiency and broad bandwidth with low filler loading for hierarchical Fe3O4/Co-MOF absorbers

Metal-organic frameworks (MOFs) with intrinsic porosity and high specific surface area have aroused broad interests as a promising electromagnetic (EM) absorbing material. However, the introduction of magnetic components in MOFs has remained as a formidable obstacle for subsequent development. Herein, Fe3O4/Co-MOF composites were fabricated via a facile and scalable co-precipitation process. The microstructure and morphology of the composites were characterized and the combination of hollow Fe3O4 and Co-MOFs was found to contribute towards the dissipation of incident waves due to the magnetic-dielectric synergistic effect. Impressive microwave absorption ability was obtained with a minimum reflection loss of –56.1 dB over a qualified bandwidth (7.9 GHz) at 2.0 mm. Moreover, the radar cross section simulation further confirmed the effective suppression of radar wave scattering using Fe3O4/Co-MOF composites. This study provides a versatile strategy for designing magnetic-dielectric composites based on MOFs as effective and broadband microwave absorbers.

Ultrafast Dynamics of the Topological Semimetal GdSbxTe2–x–δ in the Presence and Absence of a Charge Density Wave

Time-resolved dynamics in charge-density-wave materials have revealed interesting out-of-equilibrium electronic responses. However, these are typically only performed in a single material possessing a CDW. As such, it is challenging to separate subtle effects originating from the CDW. Here, we report on the ultrafast dynamics of the GdSbxTe2–x–δ series of materials where EF can be tuned, resulting in a change from an undistorted tetraganal phase to a CDW with a wavevector that depends on x. Using mid-infrared, near-infrared, and visible excitation, we find the dynamics are sensitive to both EF and the presence of the CDW. Specifically, as the Sb content of the compounds increases, transient spectral features shift to higher probe energies. In addition, we observe an enhanced lifetime and change in the sign of the transient signal upon removing the CDW with high Sb concentrations. Finally, we reveal fluence- and temperature-dependent photoinduced responses of the differential reflectivity, which provide evidence of transient charge density wave suppression in related telluride materials. Taken together our results provide a blueprint for future ultrafast studies of CDW systems.

The waveless model: clinical and research applications of follicular wave suppression in cattle

The waveless model: clinical and research applications of follicular wave suppression in cattle

Computational analysis of speed-accuracy tradeoff

Speed-accuracy tradeoff (SAT) in the decision making of humans and animals is a well-documented phenomenon, but its underlying neuronal mechanism remains unclear. Modeling approaches have conceptualized SAT through the threshold hypothesis as adjustments to the decision threshold. However, the leading neurophysiological view is the gain modulation hypothesis. This hypothesis postulates that the SAT mechanism is implemented through changes in the dynamics of the choice circuit, which increase the baseline firing rate and the speed of neuronal integration. In this paper, I investigated alternative computational mechanisms of SAT and showed that the threshold hypothesis was qualitatively consistent with the behavioral data, but the gain modulation hypothesis was not. In order to reconcile the threshold hypothesis with the neurophysiological evidence, I considered the interference of alpha oscillations with the decision process and showed that alpha oscillations could increase the discriminatory power of the decision system, although they slowed down the decision process. This suggests that the magnitude of alpha waves suppression during the event related desynchronization (ERD) of alpha oscillations depends on a SAT condition and the amplitude of alpha oscillations is lower in the speed condition. I also showed that the lower amplitude of alpha oscillations resulted in an increase in the baseline firing rate and the speed of neuronal intergration. Thus, the interference of the event related desynchronization of alpha oscillations with a SAT condition explains why an increase in the baseline firing rate and the speed of neuronal integration accompany the speed condition.

Surface Wave Suppression for Improved Isolation and Bore-Sight Gain Based on Dipole Moment Conversion

This article proposes a novel approach to surface wave suppression based on dipole moment conversion for isolation and bore-sight gain improvement. To transform the power associated with the surface wave into the source of the radiating wave, an inverted-L shape composed of a thin strip with a via post is adopted. Unlike the conventional mushroom structure, the via post is placed at the leading edge of the strip, so that the longitudinal edge of the strip is aligned with the propagation direction of the surface wave. This inverted-L structure plays a key role in converting the vertical electric dipole moment to the horizontal component, and the conversion efficiency can be further improved by arranging the structure with a narrower interval of <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$0.057\lambda $ </tex-math></inline-formula> in the axis perpendicular to the propagation direction. For validation, multipole moments are calculated from induced currents, and the proposed metasurface is fabricated for two array configurations. Scattering parameters and near- and far-fields are measured to demonstrate the proposed approach experimentally. The fabricated metasurface shows an operating frequency band from 9.74 to 10.62 GHz, and its average isolation and bore-sight gain are improved by 6.3 and 1.6 dB, respectively, without any severe distortion on impedance matching.

Design of cross-linked baffle and analysis of its wave suppression characteristics

Design of cross-linked baffle and analysis of its wave suppression characteristics

A method for improving wave suppression ability of acoustic black hole plate in low-frequency range

The acoustic black holes (ABH) with the power-law profile are applied in many fields, such as vibration control, energy collection, etc. It is widely known that the ABH only works above the cut-on frequency. However, few studies improve the vibration suppression performance of acoustic black hole structures below the cut-on frequency. This paper proposes shunt damping to improve the wave suppression ability of an ABH plate in the low-frequency range. The ABH plate with shunt damping obeys the Kirchhoff–Love plate theory. Based on the Hamiltonian principle, the modified Fourier series is selected as the shape function, and the natural frequency of the ABH plate is calculated by the energy method. Compared with the finite element method results, the correctness of this method is verified. Further, comparing the vibration characteristics of the ABH plate with those of the uniform thickness plate, it can be obtained that the ABH plate has excellent vibration damping performance in high frequency but a poor effect in low frequency. Therefore, a shunt damping with blocking circuits is pasted on one side of the central area of the ABH and successfully suppresses four resonate formants in the low-frequency band. Finally, the high-frequency vibration characteristics of the ABH plate with shunt damping are improved by increasing the thickness of the ordinary damping layer. This paper proposes a shunt damping ABH plate composite structure with excellent vibration suppression performance in both the high-frequency and low-frequency range, providing a new idea for the application of two-dimensional ABH plate structures in vibration control.

Influence of African Easterly Wave Suppression on Atlantic Tropical Cyclone Activity in a Convection‐Permitting Model

AbstractAfrican easterly waves (AEWs) are strongly linked to Atlantic tropical cyclones (TCs) on the synoptic timescale by serving as seedling disturbances for TC genesis. However, it is unclear whether climatological TC frequency is limited by AEWs. We investigated the impact of suppressing AEWs using a 3‐member ensemble of convection‐permitting regional model simulations, in which AEWs were either retained or removed through the lateral boundary conditions. Suppressing AEWs did not substantially change seasonal TC number, but did influence TC intensity, genesis time and location. Suppressing AEWs produced stronger TCs, shifted peak TC genesis from September to August, and reduced (increased) TC genesis in the eastern Atlantic (Gulf of Mexico). Without AEWs, TCs generated under more favorable large‐scale atmospheric conditions. These results indicate that AEWs may not be reliable predictors of basin‐wide seasonal TC frequency. However, simulations provide evidence that AEWs could influence the large‐scale environment that is important for TCs.

Bandgap characteristics of the two-dimensional missing rib lattice structure

In this paper, the bandgap characteristics of a missing rib lattice structure composed of beam elements are investigated by using the Floquet-Bloch theorem. The tuning of the width and position of the bandgap is achieved by changing the local structural parameters, i.e., the rotation angle, the short beam length, and the beam thickness. In order to expand the regulation of the bandgap, the influence of the material parameters of the crossed long beams inside the structure on the bandgap is analyzed. The results show that the mass density and stiffness of the structure have significant effects on the bandgap, while Poisson’s ratio has no effect on the bandgap. By analyzing the first ten bands of the reference unit cell, it can be found that the missing rib lattice structure generates multiple local resonance bandgaps for vibration reduction, and these bandgap widths are wider. The modal analysis reveals that the formation of the bandgap is due to the dipole resonance of the lattice structure, and this dipole resonance originates from the coupling of the bending deformation of the beam elements. In the band structure, the vibrational mode of the 9th band with a negative slope corresponds to a rotational resonance, which is different from that with the conventional negative slope formed by the coupling of two resonance modes. This study can provide a theoretical reference for the design of simple and lightweight elastic metamaterials, as well as for the regulation of bandgaps and the suppression of elastic waves.

Understanding the role of resonances and anti-resonances in shaping surface-wave bandgaps for metasurfaces

An array of surface-mounted prismatic resonators in the path of Rayleigh wave propagation generates two distinct types of surface-wave bandgaps: longitudinal and flexural-resonance bandgaps, resulting from the hybridization of the Rayleigh wave with the longitudinal and flexural resonances of the resonators, respectively. Longitudinal-resonance bandgaps are broad with asymmetric transmission drops, whereas flexural-resonance bandgaps are narrow with nearly symmetric transmission drops. In this paper, we illuminate these observations by investigating the resonances and anti-resonances of the resonator. With an understanding of how the Rayleigh wave interacts with different boundary conditions, we investigate the clamping conditions imposed by prismatic resonators due to the resonator’s resonances and anti-resonances and interpret the resulting transmission spectra. We demonstrate that, in the case of a single resonator, only the resonator’s longitudinal and flexural resonances are responsible for suppressing Rayleigh waves. In contrast, for a resonator array, both the resonances and the anti-resonances of the resonators contribute to the formation of the longitudinal-resonance bandgaps, unlike the flexural-resonance bandgaps where only the flexural resonances play a role. We also provide an explanation for the observed asymmetry in the transmission drop within the longitudinal-resonance bandgaps by assessing the clamping conditions imposed by the resonators. Finally, we evaluate the transmission characteristics of resonator arrays at the anti-resonance frequencies by varying a few key geometric parameters of the unit cell. These findings provide the conceptual understanding required to design optimized resonators based on matching anti-resonance frequencies with the incident Rayleigh wave frequency in order to achieve enhanced Rayleigh wave suppression.

Ultralow-frequency flexural wave attenuation in a beam with periodically attached quasi-zero-stiffness resonators

Ultralow-frequency band gaps are realized to suppress the ultralow-frequency flexural wave propagation in the beams with periodically attached quasi-zero-stiffness (QZS) resonators, which are designed by inserting quasi-zero-stiffness systems into the mass-in-mass structures. The band structure and the transfer matrix of the QZS locally resonant beam are derived by the transfer matrix method to quantify the wave attenuation performance of band gaps. Then, the effect of the stiffness ratio on the bandgap characteristic is studied. It is shown that, thanks to the introduction of the QZS system, the band gaps can be easily transferred to lower frequency or even ultralow frequency without weakening the static stiffness of the resonators. Finally, the flexural wave propagation in locally resonant beam consisting of multiple periodic arrays of QZS resonators is investigated. The result shows that differential design of the bandgap frequencies can be easily realized by adjusting the negative stiffness coefficient of the QZS resonators, so as to obtain broadband flexural wave suppression performance.

The effect of vanadium substitution on the structural and magnetic properties of (Fe[formula omitted]V[formula omitted])[formula omitted]Ga[formula omitted

Fe3Ga4 displays a complex magnetic phase diagram that is sensitive and tunable with both electronic and crystallographic structure changes. In order to explore this tunability, vanadium-doped (Fe1−xVx)3Ga4 has been synthesized and characterized. High-resolution synchrotron X-ray diffraction and Rietveld refinement show that samples up to 20% V-doping remain isostructural to Fe3Ga4 and display a linear increase in unit cell volume as doping is increased. Magnetic measurements reveal a suppression of the antiferromagnetic helical spin-density wave (SDW) with V-doping, revealed by changes in both the low-temperature ferromagnetic–antiferromagnetic (FM–AFM) transition (T1) and high-temperature AFM–FM transition (T2). At 7.5% V-doping, the metamagnetic behavior of the helical AFM SDW phase is no longer observed. These results offer an avenue to effective tuning of the magnetic order in Fe3Ga4 for devices, as well as increased understanding of the magnetism in this system.

Wave patterns of stationary gravity–capillary waves from a moving obstacle in a magnetic fluid

The influence of a magnetic field on the pattern of stationary waves formed on the surface of a magnetic fluid (ferrofluid) when an obstacle moves has been studied both theoretically and experimentally. It is found that a vertical magnetic field narrows the cone of stationary waves and increases their amplitude. In the wake region, the peaks of the Rosensweig instability appear in a magnetic field that is smaller than the critical field that determines this instability occurrence. A horizontal magnetic field parallel to the obstacle velocity expands the cone of waves but reduces their amplitude up to the suppression of stationary waves. A horizontal field perpendicular to the obstacle velocity also expands the cone of waves and stabilizes their amplitude.

The first International Conference on Mechanical System Dynamics grandly convened in Nanjing, China

The first International Conference on Mechanical System Dynamics grandly convened in Nanjing, China

Fast hyperparameter-free spectral approach for 2D seismic data reconstruction

Reconstruction of missing seismic data is a critical procedure for subsequent applications like multiple wave suppression, wave-equation migration imaging and so on. In this paper, a fast, hyperparameter-free and sparse iterative spectral estimation approach is proposed for the reconstruction of two-dimensional seismic data of randomly missing traces. The proposed approach is based on the harmonic structure of the frequency slice of seismic data and the weighted covariance fitting criterion. Specifically, the method first iteratively estimates the spectrum of the frequency slice by solving a weighted covariance fitting problem. Then, the missing data is reconstructed by using the estimated spectrum and a linear minimum mean-squared error estimator. However, the spectral estimation depends on matrix-vector multiplications for each iteration, which has a high computational cost when the data increase to a large size. To solve this problem, a fast iterative technology is proposed by using an inverse fast Fourier transform, which fully exploits the Hermitian–Toeplitz structure of the covariance matrix and the exponential form of the steering vector and it significantly reduces the computational complexity. The proposed algorithm is hyperparameter-free, can provide super spectral resolution, and thus obtain better reconstruction performance. The experimental results of synthetic and real seismic data show that the proposed algorithm has higher reconstruction accuracy and lower computational complexity compared to other commonly used reconstruction algorithms.

Frequency-dependent surface wave suppression at the Dirac point of an acoustic graphene analog

Dirac points in the band structure of acoustic systems are essential features affording classical analogs of quantum condensed matter states. We show that measured dispersion curves near and at the Dirac point of an acoustic graphene analog can be suppressed by strong variations in the impedance boundary between free field and surface wave regimes under certain conditions. Increased Rayleigh scattering and diffractive excitation are shown to increase the dispersed surface wave pressure amplitude, circumventing the impedance-based wave suppression. The improved excitation and scattering conditions for observing acoustic Dirac points for two samples with two distinct operational frequency ranges are reported.

Selective blockade of rat brain T-type calcium channels provides insights on neurophysiological basis of arousal dependent resting state functional magnetic resonance imaging signals.

A number of studies point to slow (0.1–2 Hz) brain rhythms as the basis for the resting-state functional magnetic resonance imaging (rsfMRI) signal. Slow waves exist in the absence of stimulation, propagate across the cortex, and are strongly modulated by vigilance similar to large portions of the rsfMRI signal. However, it is not clear if slow rhythms serve as the basis of all neural activity reflected in rsfMRI signals, or just the vigilance-dependent components. The rsfMRI data exhibit quasi-periodic patterns (QPPs) that appear to increase in strength with decreasing vigilance and propagate across the brain similar to slow rhythms. These QPPs can complicate the estimation of functional connectivity (FC) via rsfMRI, either by existing as unmodeled signal or by inducing additional wide-spread correlation between voxel-time courses of functionally connected brain regions. In this study, we examined the relationship between cortical slow rhythms and the rsfMRI signal, using a well-established pharmacological model of slow wave suppression. Suppression of cortical slow rhythms led to significant reduction in the amplitude of QPPs but increased rsfMRI measures of intrinsic FC in rats. The results suggest that cortical slow rhythms serve as the basis of only the vigilance-dependent components (e.g., QPPs) of rsfMRI signals. Further attenuation of these non-specific signals enhances delineation of brain functional networks.