Changes In Phase Velocity Research Articles

Unconventional acoustic wave propagation transitions induced by resonant scatterers in the high-density limit

Experiments on ultrasound propagation through a gel doped with resonant encapsulated microbubbles provided evidence for a discontinuous transition between wave propagation regimes at a critical excitation frequency. Such behavior is unlike that observed for soft materials doped with non-resonant air or through liquid foams, and disagrees with a simple mixture model for the effective sound speed. Here, we study the discontinuous transition by measuring the transition as a function of encapsulated microbubble volume fraction. The results show the transition always occurs in the strong-scattering limit (l/λ < 1, l and λ are the mean free path and wavelength, respectively), that at the critical frequency the effective phase velocity changes discontinuously to a constant value with increasing microbubble volume fraction, and the measured critical frequency shows a power law dependence on microbubble volume fraction. The results cannot be explained by multiple scattering theory, viscous effects, mode decoupling, or a critical density of states. It is hypothesized the transition depends upon the microbubble on-resonance effective properties, and we discuss the results within the context of percolation theory. The results shed light on the discontinuous transition’s physics, and suggest soft materials can be engineered in this manner to achieve a broad range of physical properties with potential application in ultrasonic actuators and switches.

Monitoring early age concrete hydration through time-dependent wave dispersion

This paper explores into the intricate realm of wave dispersion behavior within the early stages of fresh concrete, with a specific focus on the dynamic interplay between micro-structure evolution and rheological properties in the quasi-solid state. Based on the characteristic of wave dispersion is factored by impulse frequency, the range used for the experiment is from 40 kHz to 100 kHz, where the wavelength is enough to be influenced by the medium. The time-dependent analytical solution and FE simulation show more linear changes in phase velocity at different times during the quasi-solid state. On the other hand, experimental data exhibits more dispersion behavior, likely due to the effect of viscoplasticity. To simulate the viscoplasticity effect, the FE simulation is compared between cases with applied viscoplasticity and without viscoplasticity. The phase velocity in the viscoplasticity case decreases by approximately 30% compared to the non-viscoplasticity case. This demonstrates that the effect of viscoplasticity cannot be overlooked in the quasi-solid state of concrete. The wave dispersion behavior in the early stages is predominantly influenced by particles and viscoplasticity, causing changes in phase velocity over time and impulse frequency. The highest phase velocity is observed between 50 kHz to 60 kHz. As the concrete undergoes hydration and hardening, both group velocity and phase velocity generally increase with the same frequency impulse. This study aims to unravel the complexities inherent in these phenomena, shedding light on the fundamental mechanisms governing wave dispersion in quasi-solid materials.

Phononic-Crystal-Based SAW Magnetic-Field Sensors.

In this theoretical study, we explore the enhancement of sensing capabilities in surface acoustic wave (SAW)-based magnetic field sensors through the integration of engineered phononic crystals (PnCs). We particularly focus on amplifying the interaction between the SAW and magnetostrictive materials within the PnC structure. Through comprehensive simulations, we demonstrate the synchronization between the SAWs generated by IDTs and the resonant modes of PnCs, thereby leading to an enhancement in sensitivity. Furthermore, we investigate the ΔE effect, highlighting the sensor's responsiveness to changes in external magnetic fields, and quantify its magnetic sensitivity through observable changes in the SAW phase velocity leading to phase shifts at the end of the delay line. Notably, our approach yields a magnetic field sensitivity of approximately S~138 °mT for a delay line length of only 77 µm in homogeneous magnetic fields. Our findings underline the potential of PnCs to advance magnetic field sensing. This research offers insights into the integration of engineered materials for improved sensor performance, paving the way for more effective and accurate magnetic field detection solutions.

Experimental signatures of the transition from acoustic plasmon to electronic sound in graphene.

Fermi liquids respond differently to perturbations depending on whether their frequency is higher (collisionless regime) or lower (hydrodynamic regime) than the interparticle collision rate. This results in a different phase velocity between the collisionless zero sound and the hydrodynamic first sound. We performed terahertz photocurrent nanoscopy measurements on graphene devices, with a metallic gate close to the graphene layer, to probe the dispersion of propagating acoustic plasmons, the counterpart of sound modes in electronic Fermi liquids. We report the observation of a change in the plasmon phase velocity when the excitation frequency approaches the electron-electron collision rate that is compatible with the transition between the zero and the first sound mode.

Effects of corrugated boundaries on Rayleigh waves in a piezoelectric semiconductor substrate covered with a metal layer

In the present work, analytical solutions for Rayleigh waves propagating in a metal covered layer over a piezoelectric semiconductor (PSC) substrate with corrugated surface and corrugated interface are obtained. The solutions of the various physical fields can be expressed as a summation of infinite harmonic waves by using the Bloch–Floquet theorem due to the existence of the periodic corrugated surface and corrugated interface. The effects of the height and contour shape of the corrugated surface and corrugated interface, the doping density of the PSC substrate and the thickness of the metal covered layer on the dispersion and attenuation curves and mode shapes of Rayleigh waves are discussed via numerical example. The results show that the existence of the corrugated surface will lead to the decrease of the phase velocity and non-monotonous change with wavenumber, besides, and significantly increases the attenuation near k0=π/Λ(k0 and Λ are the wavenumber and period, respectively). In addition, the effects of periodic corrugated surface are much greater than those of periodic corrugated interface and are closely related with the ratio of the wavelength to the structure period. Moreover, the sinusoidal corrugated surface can be used to better control the propagation characteristics of Rayleigh waves by comparison.

Passive guided wave tomography for monitoring corrosion in pipes

The use of guided waves enables to monitor wide areas of structures with a limited number of sensors and/or a limited acquisition time. In particular, guided wave tomography algorithms give a quantitative image of the residual thickness of the monitored structured, for example a corroded pipe. The idea is to use the dispersion curves to link a change in phase or group velocity to a change in thickness. The method developed at CEA-List includes an auto-calibration phase, eliminating the need of a baseline to work, therefore enabling its use in presence of varying environmental and operational conditions. One drawback of tomography techniques is that they need many sensors compared to more conventional algorithms such as RAPID or Delay and Sum. We propose here a passive version of tomography to limit its intrusiveness by using data obtained by post-processing the ambient elastic noise in the structure. This enables the use of Fiber Bragg Gratings (FBG) on optical fibers as ultrasonic sensors. The use of FBGs drastically reduces the burden of the system, as several gratings can be multiplexed on a single optical fiber. Furthermore, these sensors are suited for many applications because of their resistance to harsh conditions (extreme temperatures, explosive atmosphere, radiations…).

Temperature Performance Study of SAW Sensors Based on AlN and AlScN.

In this paper, the temperature performance of AlN-SAW resonators and AlScN-SAW resonators is studied. They are simulated by COMSOL Multiphysics, and their modes and the S11 curve are analyzed. The two devices were fabricated using MEMS technology and tested using VNA, and the test results were consistent with the simulation results. Temperature experiments were carried out with temperature control equipment. With the change in temperature, the changes in S11 parameters, TCF coefficient, phase velocity, and quality factor Q were analyzed. The results show that the temperature performance of the AlN-SAW resonator and the AlScN-SAW resonator is very good, and both have good linearity. At the same time, the sensitivity of the AlScN-SAW resonator is greater by 9.5%, the linearity is greater by 15%, and the TCF coefficient is greater by 11.1%. The temperature performance is excellent, and it is more suitable as a temperature sensor.

Study on the Stage Characteristics of Noise Dispersion in the Deformation Evolution of Red Sandstone

The uniaxial compression experiment of red sandstone is observed simultaneously by using acoustic emission and digital speckle correlation methods. The deformation evolution of red sandstone is divided into microfracture random expansion stage, deformation localization stage, subinstability stage, and instability failure stage. Green’s function and dispersion curve of each stage are obtained from the noise data picked up by acoustic emission equipment, and the dispersion characteristics of each evolution stage are analyzed. The results show the following: (1) In the stage of random propagation of microcracks, the noise in the low frequency range passes through at a higher phase velocity, the phase velocity changes periodically, the correlation coefficient is high at the initial time, and the variation trend of frequency dispersion curve is relatively consistent. (2) In the deformation localization stage, the frequency range without zero phase velocity moves to the high frequency range, and the phase velocity changes periodically. (3) In the subinstability stage, dense phase velocity zeros appear on the dispersion curve image, the dispersion curve tends to be disordered, various indicators change obviously, and the correlation coefficient decreases rapidly. (4) In the unstable failure stage, the fracture evolution is completed, and the variation trend of each index of the frequency dispersion curve is consistent. (5) The size and difference of the sensitive kernel function of the two layers are related to the evolution region of the fracture. The sensitivity kernel value of the medium layer where the fracture evolves is high, and the longer the evolution time, the greater the difference.

Relation between Mass Sensitivity and Complex Power Flow in Love Wave Sensors.

In this paper, we investigate the connection between average power flows in Love wave waveguides with the mass sensitivity of Love wave sensors. In fact, loading with a Newtonian liquid gives rise to two extra power flows, in the transverse direction towards the loading Newtonian liquid. The first is an active power flow feeding viscous losses in the Newtonian liquid and the second is a reactive power flow that is responsible for the phase delay of the Love wave and consequently for the changes in phase velocity of the Love wave. Since loading with a lossless mass also leads to changes in the phase velocity, we assert that mass sensitivity of Love wave sensors is connected to the average reactive power flow, in the transverse direction , bouncing back and forth, between the interior of the waveguide and the loading Newtonian liquid. Subsequently, we found the thickness of the effective surface layer of mass that is equivalent to loading with a semi-infinite Newtonian liquid. The analytical formulas developed in this paper are illustrated by the results of numerical calculations performed for an exemplary Love wave waveguide composed of a PMMA surface layer deposited on an ST-Quartz substrate.

Comparison of the Group and Phase Velocity of the Polarized Wave and the Equatorial Anomaly of the Ionosphere

The behavior of phase and group velocities of a left-polarized wave in the F-region of the ionosphere is studied in this paper. Despite the fact that the magnitudes of a left polarized wave's phase and group velocities in the F-region of the ionosphere are almost identical at low latitudes, they are schematically asymmetric under acceptable conditions. Under the same conditions, the group velocity changes in the same way as the electron density in this region; however, the phase velocity changes in the opposite direction. According to the findings, the left-polarized wave's group velocity and electron density have a linear connection. The phase velocity, on the other hand, cannot be said to be the same.&#x0D; Keywords: Ionosphere, The polarized wave, Group-phase velocity, Equatorial anomaly

Electromagnetic Waves in Maxwell’s Theory

The description of a plane traveling electromagnetic wave existing in the physical literature by identical solutions of wave equations for the strengths of electric and magnetic fields is physically incorrect, since such solutions contradict the physical meaning of Maxwell’s equations and violate the energy conservation law. The paper gives a physically correct description of electromagnetic waves in the framework of Maxwell’s theory. New solutions of Maxwell’s wave equations for traveling electromagnetic wave are proposed, in which the strength of its electric and magnetic components change in time with shifts of a quarter of the period and a quarter of the wavelength along coordinate. The solutions describe a traveling electromagnetic wave, in which the energy of the electrical component is sequentially converted into the energy of the magnetic component and vice versa; the total energy density of the lossless wave remains constant in space at any time; the mutual orientation of the intensity vectors of the electric, magnetic fields and phase velocity changes from a left-handed three to a right-handed three every quarter of the wavelength; the energy flux density of the traveling wave is described by the Umov vector. It is shown that the formation of a standing electromagnetic wave does not require the loss of half a wave of one of the components of the wave reflected at the interface between the media. In a standing wave, the total energy density remains constant in time, but it is a function of coordinates: there are points in space where the total energy density of the wave at any time is zero – these are nodes, and there are points where it has a maximum value – these are antinodes. Due to the inhomogeneity of the distribution of the total energy density of the wave in space, a standing electromagnetic wave cannot be considered as a harmonic oscillator, but a lossless traveling electromagnetic wave can.

Nonlinear adiabatic electron plasma waves: I. General theory and nonlinear frequency shift

This paper provides a complete self-consistent nonlinear theory for electron plasma waves, within the framework of the adiabatic approximation. The theory applies whatever the variations of the wave amplitude provided that they are slow enough, and it is also valid when the plasma is inhomogeneous and non-stationary. Moreover, it accounts for: (i) the geometrical jump in action resulting from separatrix crossing; (ii) the continuous change in phase velocity making the wave frame non-inertial; (iii) the harmonic content of the scalar potential; (iv) a non-zero vector potential; (v) the transition probabilities from one region of phase space to the other when an orbit crosses the separatrix; and (vi) the possible change in direction of the wavenumber. The relative importance of each of the aforementioned effects is discussed in detail, based on the derivation of the nonlinear frequency shift. This allows us to specify how the general formalism may be simplified, depending on the value of the wavenumber normalized to the Debye length. Specific applications of our theory are reported in Paper II.

Phase and frequency linear response theory for hyperbolic chaotic oscillators.

We formulate a linear phase and frequency response theory for hyperbolic flows, which generalizes phase response theory for autonomous limit cycle oscillators to hyperbolic chaotic dynamics. The theory is based on a shadowing conjecture, stating the existence of a perturbed trajectory shadowing every unperturbed trajectory on the system attractor for any small enough perturbation of arbitrary duration and a corresponding unique time isomorphism, which we identify as phase such that phase shifts between the unperturbed trajectory and its perturbed shadow are well defined. The phase sensitivity function is the solution of an adjoint linear equation and can be used to estimate the average change of phase velocity to small time dependent or independent perturbations. These changes in frequency are experimentally accessible, giving a convenient way to define and measure phase response curves for chaotic oscillators. The shadowing trajectory and the phase can be constructed explicitly in the tangent space of an unperturbed trajectory using co-variant Lyapunov vectors. It can also be used to identify the limits of the regime of linear response.

Reconfigurable quantum phononic circuits via piezo-acoustomechanical interactions

We show that piezoelectric strain actuation of acoustomechanical interactions can produce large phase velocity changes in an existing quantum phononic platform: aluminum nitride on suspended silicon. Using finite element analysis, we demonstrate a piezo-acoustomechanical phase shifter waveguide capable of producing ±π phase shifts for GHz frequency phonons in 10s of μm with 10s of volts applied. Then, using the phase shifter as a building block, we demonstrate several phononic integrated circuit elements useful for quantum information processing. In particular, we show how to construct programmable multi-mode interferometers for linear phononic processing and a dynamically reconfigurable phononic memory that can switch between an ultra-long-lifetime state and a state strongly coupled to its bus waveguide. From the master equation for the full open quantum system of the reconfigurable phononic memory, we show that it is possible to perform read and write operations with over 90% quantum state transfer fidelity for an exponentially decaying pulse.

Antivertiginous Effect of Caloric Stimulation for Acute Peripheral Vertigo.

To clarify therapeutic effect of caloric stimulation for acute peripheral vertigo. Two patients with acute peripheral vertigo accompanied by spontaneous nystagmus. Therapeutic. Changes in the maximum slow phase velocity of spontaneous nystagmus and subjective vertigo. Reduction in the maximum slow phase velocity of spontaneous nystagmus and mitigation of subjective vertigo was observed in both patients. Caloric stimulation could be one potential option as a suppressant for acute peripheral vertigo.

Investigation of High Interaction Efficiency of Sheet Beam TWT Based on All-Period Optimization and Beam Bunching Analysis

This article explores an extremely high beam&#x2013;wave interaction (BWI) efficiency technology of sheet beam traveling wave tubes (SBTWTs). An all-period phase velocity tapering optimization method is proposed to improve BWI efficiency of slow wave structure (SWS). The effects of the special electron bunching state and synchronous phase velocity change on the improvement of BWI efficiency generated by this method are analyzed. To solve the computational burden caused by dozens of parameters, self-developed 1-D nonlinear BWI code is used to calculate the BWI. For the best optimization effect, particle swarm optimization (PSO) algorithm selected from a variety of algorithms is utilized to search for the global optimal solution. The PIC simulation verification results show that by the all-period optimization method, the 80-period staggered double vane (SDV) SWS achieves a stable output power of 18 kW with 31 kV voltage 0.9-A current sheet beam at 30 GHz. The BWI efficiency of all-period phase velocity tapering SWS has reached 64.5&#x0025; at center frequency, the efficiency between 28 and 35 GHz exceeds 40&#x0025;.

Rayleigh-wave attenuation across the conterminous United States in the microseism frequency band

Mapping variations in the attenuation of seismic energy is important for understanding dissipative mechanisms in the lithosphere, and for modeling ground shaking associated with earthquakes. We cross-correlate ambient seismic signal recorded across the EarthScope Transportable Array in the 3–15 s period range. We apply to the resulting cross correlations a new method to estimate lateral variations in Rayleigh-wave attenuation, as a function of period, beneath North America. Between 3 and 6 s, our maps are dominated by a strong eastward decrease in attenuation. This pattern vanishes at longer periods, confirming early observations based on regional earthquakes. Attenuation maps and phase-velocity maps are anti-correlated at periods between 3 and 6 s, but the anti-correlation is also largely lost at longer periods. This corresponds to the attenuation coefficient decreasing with period more rapidly in the west than in the east, while the change in phase velocity with period is more uniform across the continent. Our results point to a transition in the properties of upper-crustal materials with depth, probably related to the closure of fluid-filled cracks and pores, and imply that measures of attenuation from seismic noise carry significant information on crustal rheology.

Cold Airflow for the Semicircular Canals Decreases Heart Rate

Background Arterial pressure (AP) and heart rate (HR) near or after the end of cold caloric test or the test of semicircular canal function were not changed, but those were not measured from the beginning of the test continuously. This study was aimed to evaluate the changes in AP and HR continuously during caloric test and 2) relationship between those changes and the semicircular canal function. Methods the Basic Principles of the Declaration of Helsinki and their expansion in the regulations governing research supported by the U.S. Government. This study was approved by the Institutional Review Boards, and written informed consent was obtained from all subjects. Airflow at 15°C was applied to the left ear (Ear15), and the left tragus (Tragus15); another airflow at 37°C was applied to the left ear (Ear37) for one minute each in healthy subjects. AP, HR were recorded and analyzed in time series. Furthermore, the cold airflow was applied to the right and left ear for one minute, and maximum changes in AP, HR and slow phase velocity (SPV) of nystagmus were measured in outpatients. Results AP remained constant or decreased during the Ear15, whereas their HR immediately decreased at the onset of the airflow uniformly. Thus, the decrease in HR was unlikely induced by the baroreflex. Neither AP nor HR changed during the Ear37 and Tragus15 stimulation. Thus, the decreases in HR were not induced by the mechanical stimulation and stimulation for the skin. Degree of nystagmus was significantly correlated with the changes in HR but in AP in outpatients. Conclusions Cold airflow for the ears immediately decreased HR, and the semicircular canals might be related the change.

Theoretical and numerical studies of the phase velocity of wakefields in plasma driven by self-modulated proton beams with electron beam seeding

Significant progress has been made in the studies of wakefield excitation in plasma by a self-modulated high energy proton beam in the past decade. The electron beams accelerated up to 2 GeV by using such a wakefield were demonstrated in the AWAKE experiment at CERN in 2018. Aiming at the application of high energy particle accelerators, new ideas have been investigated in recent years, such as seeding the proton beam self-modulation with an electron beam in order to enhance the strength and stability of the wakefield or adding a density transition in the plasma distribution to enhance the phase velocity and the strength of the wakefield. Here in this work, we investigate the effects of electron beam seeding on the phase velocity of the wakefield generated by the modulated proton beam in plasma. The physical mechanisms responsible for the phase velocity change and the roles played by the electron beam seeding are discussed. The theoretical analysis and two-dimensional particle-in-cell simulations show that both the growth rate and the phase velocity of the wakefield generated by the modulated proton beam can be enhanced by the electron beam seeding. The higher the charge density of the electron beam, the more significant the enhancement effects. The effects of electron beam energy and proton beam longitudinal profiles on the increase of phase velocity are also studied. It is shown that the evolution of the electron beam distribution has a significant effect on the seeding self-modulation process, and thus affecting the phase velocity. A self-focusing electron seeding beam can increase the phase velocity of the wakefield even to superluminal while an expanding seeding beam can reduce the phase velocity and destroy the stability of the whole process. This work may benefit the proton beam seeding self-modulation acceleration and its applications.

Proton Bunch Self-Modulation in Plasma with Density Gradient.

We study experimentally the effect of linear plasma density gradients on the self-modulation of a 400GeV proton bunch. Results show that a positive or negative gradient increases or decreases the number of microbunches and the relative charge per microbunch observed after 10m of plasma. The measured modulation frequency also increases or decreases. With the largest positive gradient we observe two frequencies in the modulation power spectrum. Results are consistent with changes in wakefields' phase velocity due to plasma density gradients adding to the slow wakefields' phase velocity during self-modulation growth predicted by linear theory.