Search

The propagation velocities of geomagnetic Pc5 pulsations in the azimuthal and meridional directions were analyzed for a series of events. Two methods were used: based on the phase delays of the signal between stations and the displacement of vortex centers of their equivalent current systems. The analysis showed that the propagation of pulsations and vortices coincides in direction—along the meridian, they predominantly propagate northward. In most cases, the propagation velocity is 5 km/s for pulsations and 2 km/s for vortices. In the azimuthal direction, pulsations and vortices propagate westward, with pulsation propagation velocity of 10 km/s and vortex propagation velocity of 3 km/s. However, the distributions of azimuthal velocities for both pulsations and vortices exhibit comparable maxima corresponding to eastward propagation: pulsations at a velocity of 10 km/s and vortices at 5 km/s. It is concluded that the measured phase velocities of pulsations at the ionospheric level are approximately twice the group velocities of vortices. [ABSTRACT FROM AUTHOR]

We deal with dynamics of the β -Fermi–Pasta–Ulam–Tsingou chain with one free end, subjected to the sudden sinusoidal force. We examine the evolution of the total energy supplied into the chain at large times. In the harmonic case ( β = 0 ), the energy grows in time linearly at the driving frequencies, corresponding to nonzero group velocities. The rate of energy supply is shown to decrease with increasing excitation frequency. Loading with the cut-off frequency, corresponding to zero group velocity, results in the energy growth proportionally to t . Explanation of behavior in time of the energy is proposed by analysis of the obtained approximate closed-form expressions for the particle velocity. In the weak anharmonic case, large-time asymptotic approximation for the total energy is obtained by using the renormalized dispersion relation. The approximation allows one to estimate rate of the energy supply at the driving frequencies, which belong both in the pass-band and in the stop-band of the harmonic chain. Consistency of the asymptotic estimates with the results of numerical simulations is discussed. [ABSTRACT FROM AUTHOR]

Debye–Callaway model in combination with the Murnaghan and Clapeyron equations was used to calculate the hydrostatic pressure effects on lattice thermal conductivity (LTC) of wurtzite gallium nitride. The calculations are for the longitudinal and transverse phonon modes. The results are efficiently fitted with the whole temperature (1–400) of the experimental data. The peak value of LTC declines with the applied pressure from 0 to 14 GPa. This result is due to the decreasing Debye temperature, group velocity and lattice volume. Furthermore, pressure affected the number of dislocations, sample size and Gruneisen parameter (longitudinal and transverse) modes. Consequently, the values of above parameters at zero GPa are 2.5 × 10 13 m - 2 , 1.8 mm , 0.93 and 0.52 , whilst the values at 14 GPa are 15 × 10 15 m - 2 , 1.68 mm , 0.818 and 0.469 , respectively. The results show that hydrostatic pressure does not affect the number of impurities and electron concentrations. [ABSTRACT FROM AUTHOR]

In this article, the design and simulation of an optical gyroscope based on a hybrid ring resonator using the Sagnac effect are presented. In this design, by using a transfer matrix, numerical analysis, and tight-binding methods, as well as managing control factors in the sensor, such as the transfer coefficient, ring radius of each resonator, operating wavelength, and coupling coefficient, the sensor output parameters are investigated. The output parameters include sensitivity, angular random walk (ARW), minimum angular rotation, minimum group velocity, and the phase shift resulting from the power ratio changes. In this gyroscope, the maximum group velocity in terms of angular rotation with the increase of the number of rings is an exponential function in the range of 750–2500 m/s. The output phase changes as a linear function with an upward slope as the radius of the rigs increases in the range of 2130–3430 nm. Also, with the increase in the radius of the rings, the phase change relative to the operating wavelength decreases. By increasing the imaginary part of the coupling and transfer coefficients in this sensor, the changes in output power decrease compared to the output phase. The transmission peaks in the gyroscope output increase due to the rise in the number of rings. The sensitivity lowers with an increasing number of rings. The temperature affects the gyroscope output, and an increase in temperature causes a rise in ARW and a decrease in the signal-to-noise ratio. Also, the signal-to-noise ratio increases with the rise of the output power due to the transfer and coupling coefficients. [ABSTRACT FROM AUTHOR]

In a four-level quantum dot system connected with a Sagnac interferometer (Mach–Zehnder-type), photon drag (PD) under the influence of tunnelling-dependent cross-Kerr nonlinearity is explored. The PD, electric susceptibility, group index, group relativistic velocities are studied as functions of the tunnelling frequency. Additionally, in the presence of tunnelling-based Kerr effect in the system, improved positive group index and minimum group relativistic velocities are attained. Large drag angles are consequently observed in the system. The highest reached values for the group index are n g = 1.60 × 10 6 and 6 × 10 6 . Likewise, the highest values for PD are reported as θ d = ± 0.080 , ± 0.112 and ± 0.120 radians. The outcomes shown here will advance quantum information, image coding/design, and rotation sensing technology. [ABSTRACT FROM AUTHOR]

Background: The elastic modulus of polyethylene terephthalate (PET) sheets is typically measured through destructive tests that require specific sample preparation and time-consuming testing procedures. Objective: To improve the efficiency of measuring the elastic modulus of PET sheets, research on a non-destructive measurement approach using guided Lamb waves was conducted. Methods: In this approach, the group velocity of the zero-order symmetric Lamb wave mode (S0 mode) at a single frequency is first measured from PET sheets. The semi-analytical finite element method (SAFEM) is used as the forward model to calculate the corresponding numerical group velocity. Particle swarm optimisation (PSO) is used to update the elastic modulus in the SAFEM model until the numerical group velocity from the model matches the experimental results. Results: The results show that measuring the group velocity data at a single frequency is sufficient for elastic modulus measurement while the material thickness can be assumed as a constant, which improves the efficiency of the measurement. The identified modulus differs from the tensile modulus of the material due to the frequency dependence of the elastic modulus. However, this discrepancy could be eliminated by using a linear regression model. Conclusions: The method mentioned above can achieve non-destructive and efficient measurement of the elastic modulus of PET sheets, which can potentially be applied for in-line quality inspection in PET bottle production processes. [ABSTRACT FROM AUTHOR]

This paper presents an air-coupled impact echo (IE) technique that relies on the phase spectrum of the collected data to find the frequencies corresponding to the reflections from delaminations. The proposed technique takes advantage of the fact that the IE compression wave is not a propagating wave, but it is the 1st order symmetrical (S1) mode Lamb wave at zero group velocity (S1-ZGV). Therefore, it searches the phase spectra of the data collected by multiple sensors to locate the frequency corresponding to the lowest phase difference. As a result, the technique reduces the effect of propagating waves, including the direct acoustic wave and ambient noise. It is named the Constant Phase IE (CPIE). The performance of the CPIE is experimentally compared with the regular amplitude spectrum-based IE technique and two other multisensor IE techniques. The CPIE shows a performance advantage, especially in a noisy environment. [ABSTRACT FROM AUTHOR]

In this study, Γ-shaped rectangular resonators, which are created in a metal insulator metal (MIM) structure, have been studied analytically and numerically. The metal is silver and rectangular resonators are filled by Si and the peak of transparency profile is tuned in the communication band with proper geometrical parameters. By employing 2-D finite difference time domain method (FDTD), simulations show a plasmonic induced transparency (PIT) window in the transmission diagram. The PIT is created by instructive and destructive interference between bright and dark resonator modes. The presented structure demonstrates tunable transparency window by variation of symmetry parameter, s. The maximum group velocity is obtained 0.1 ps in communication band corresponded to 30 μm traveling of light in the vacuum which is comparable to previous articles. The proposed structure may have potential applications in optical memory and delay blocks in designing integrated optical circuits. [ABSTRACT FROM AUTHOR]

We study the state transfer through quantum walks placed on a bounded one-dimensional path. We first consider continuous-time quantum walks from a Gaussian state. We find such a state when superposing centered on the starting and antipodal positions preserves a high fidelity for a long time and when sent on large circular graphs. Furthermore, it spreads with a null group velocity. We also explore discrete-time quantum walks to evaluate the qubit fidelity throughout the walk. In this case, the initial state is a product of states between a qubit and a Gaussian superposition of position states. Then, we add two σ x gates to confine this delocalized qubit. We also find that this bounded system dynamically enables periodic recovery of the initial separable state. We outline some applications of our results in dynamic graphs and propose quantum circuits to implement them based on the available literature. [ABSTRACT FROM AUTHOR]

Ultrasonic-guided waves (GWs) have shown a high potential to be applied to the structural integrity of timber utility poles to detect and assess distinct types of defects. Yet, there are many challenges associated with this method, which may hinder its application, including the orthotropic nature of the timber, the presence of natural cracking, and the effect of environmental factors such as temperature and moisture content (MC) on the propagation of GWs. This study aims to scrutinize the effect of MCs ranging from 0 to 24%, and temperatures between − 20 and 100 °C on the propagation characteristics of GWs (longitudinal and circumferential) in cylinder timber structures excited over a range of frequencies between 10 and 20 kHz, experimentally and numerically. The numerical analysis was carried out using COMSOL Multiphysics, and Macro Fiber Composites were used to excite and sense the GWs. The anisotropic nature of timber poles was modeled using a transversely isotropic behavior. The results showed that the effect of timber's temperature and MC on the GWs should be assessed simultaneously. Three-dimensional maps were generated to present the relationship between various wave modes, temperature, and MC. The work observed a larger critical role of MC than the temperature on the propagating GWs and timber's material properties. The effect of temperature is more critical when timber's MC increases above the fiber saturation point (FSP) due to high stiffness variations. This is seen as the group velocities of the longitudinal and flexural waves (as well as the bulk wave) shifted more at MCs above FSP than below FSP. When MC varies above FSP with no temperature variations the velocity difference is negligible with 1.5% due to complete timber saturation at all temperature values. The results showed that the change in the mode velocities in dry wood is not significant with varying temperatures, as the stiffness changes by 0.07%. Experimental validation, for the numerical results, showed low differences for the bulk wave, and flexural modes were within [7.1 15]% and [1.3 4.7]% for the 2nd flexural branch at 12.5 kHz, respectively. Based on the above results, the environmental conditions can highly impact the GWs characteristics in timber structures, hence this should be carefully considered in the application phase. [ABSTRACT FROM AUTHOR]

Longitudinal wave propagation in axially graded nanotubes was explored. The effect of shear deformation and lateral inertia on nanorods was considered using the nonlocal Raylegh–Bishop rod theory. As a novel approach, a nonlocal parameter was assumed in the graded formulation. The higher order Haar wavelet method was utilized for solving the governing equation of motion. The effects of material grading power-law index and nonlocal parameters on the longitudinal wave response of axially graded nanorods were investigated. Phase and group velocity variations of the axially graded nanorod were obtained. The present study may be useful in the modeling of advanced functional composite nanowires. [ABSTRACT FROM AUTHOR]

The current work focuses on investigating how various external and structural factors can affect the electronic structure and optical properties of a GaAs/Ga1−xAlxAs quantum ring. Specifically, we have studied the effects of spin–orbit interaction (SOI), pressure, temperature, and dimensions on the energy levels, dipole transition matrix elements (DTMEs), susceptibilities, optical absorption coefficients (OACs), refractive index changes (RICs), and group velocity of the quantum ring. These results show that the ground state energy of the system increases with the concentration of aluminum, while increasing temperature and ring size leads to a decrease in DTMEs due to a decrease in wave function overlap. We have also found that the presence of Rashba SOI, pressure, temperature, as well as the ring’s size dominate the electronic structure and subsequently affect the OACs and RICs of the system. A significant enhancement of the OACs can be witnessed when the Rashba coefficient and ring size are enlarged. Furthermore, the results represent a red shift with enhanced Rashba coefficient and relaxation time, whereas the growing ring size results in a blue shift in Optical sensitivities. Finally, we have analyzed the dependence of the group velocity of probe light pulses on the ring's size, photon frequency, and incident optical intensity. Our findings suggest that by controlling these parameters appropriately, it may be possible to achieve effective control of light speed, which could have interesting applications in electro-optical quantum communication networks. Overall, this work sheds light on the potential applications of these findings in the design and construction of spin-based devices and may offer new avenues for further research in this field. [ABSTRACT FROM AUTHOR]

The current work focuses on investigating how various external and structural factors can affect the electronic structure and optical properties of a GaAs/Ga1−xAlxAs quantum ring. Specifically, we have studied the effects of spin–orbit interaction (SOI), pressure, temperature, and dimensions on the energy levels, dipole transition matrix elements (DTMEs), susceptibilities, optical absorption coefficients (OACs), refractive index changes (RICs), and group velocity of the quantum ring. These results show that the ground state energy of the system increases with the concentration of aluminum, while increasing temperature and ring size leads to a decrease in DTMEs due to a decrease in wave function overlap. We have also found that the presence of Rashba SOI, pressure, temperature, as well as the ring's size dominate the electronic structure and subsequently affect the OACs and RICs of the system. A significant enhancement of the OACs can be witnessed when the Rashba coefficient and ring size are enlarged. Furthermore, the results represent a red shift with enhanced Rashba coefficient and relaxation time, whereas the growing ring size results in a blue shift in Optical sensitivities. Finally, we have analyzed the dependence of the group velocity of probe light pulses on the ring's size, photon frequency, and incident optical intensity. Our findings suggest that by controlling these parameters appropriately, it may be possible to achieve effective control of light speed, which could have interesting applications in electro-optical quantum communication networks. Overall, this work sheds light on the potential applications of these findings in the design and construction of spin-based devices and may offer new avenues for further research in this field. [ABSTRACT FROM AUTHOR]

A numerical model of wave packets on the surface of the reservoir was built with slowly changing depth, concentrated in the vicinity of moving points along space-time rays with group velocity (such wave packets are called quasiphotons). At the bottom, profile of the reservoir was taken as the bottom profile of the Gulf of Finland in the coastal zone of the Sestroretsk approach fairway. [ABSTRACT FROM AUTHOR]

In this work, internal gravity waves are considered that propagate in a stratified medium of finite depth with variable background shear flows. In the linear formulation, using the Fourier method, the author constructed the Green function of the corresponding equation of internal gravity waves. The asymptotic of the Green function in the far zone expressed through the square of the Airy function is studied. It is shown that, for real distributions of buoyancy frequency and shear flow vector, the wave zone is bounded by two closed curves: head and rear fronts. In the case when the components of shear velocity vector less vary over the vertical, the head wave front is a circle expanding with a maximum group velocity of the separate wave mode without flows and shifted by the flow with a velocity equal to the average value of the shear velocity vector taken with some weight. [ABSTRACT FROM AUTHOR]

Quantum technology has the potential to revolutionize sensors and the Internet of Things (IoT). Efficient and secure data transfer among sensors and IoT devices is a major challenge. To address this, the manuscript presents coherent manipulation of the scattering cross section and its sensitivity based on subluminal propagation and rotary photon drag, using an atomic medium with two levels coupled with a cavity. The group index of the proposed atomic medium is measured in the range of - 0.1 ≤ n g ≤ 150 and group velocity in the range of 2 × 10 6 m / s ≤ v g ≤ ± 2 × 10 9 m/s. The maximum delay time is reported to t g = 0.3 μ s and rotary photon drag to θ d = ± 5 micro radian. This effect of photon drag is used in sensing, energy harvesting, and optical switching in IoT. The scattering cross-section of 0.1 × 10 21 m 2 is reported. The scattering cross-section impacts the efficiency and reliability of quantum-based communication for IoT. The maximum cross-sectional sensitivity is measured to S σ = 0.4 × 10 - 19 m 2 / n r . The cross-sectional sensitivity potentially impacts secure communication and highly precise detection in sensors and IoT. [ABSTRACT FROM AUTHOR]

This work proposes a noise-reduction structure that integrates phase-modulating metasurface (PMM) with acoustic liners (ALs) to enhance the narrow band absorption performance of a duct with relatively small length-diameter ratio. The PMM manipulates the wavefront by introducing different transmission phase shifts based on an array of Helmholtz resonators, so that the spinning wave within the duct can be generated. Compared with the plane wave, the generated spinning wave has a lower group velocity, which results in a greater traveling distance over the ALs in the duct. The optimization design is performed to determine the final structural parameters of the PMM, which is based on the predictions of the amplitude and phase shift of the acoustic wave at the outlet of the PMM using the theory of passive phased array. With the manipulation of the PMM, the incident plane wave is modulated into a spinning wave, and then enters into the acoustic liner duct (ALD), whose structural parameters are optimized by maximizing the transmission loss using the mode-matching technique. Finally, the noise-reduction performance of this combined structure is evaluated by numerical simulations in the presence of grazing flow. The results demonstrate that, compared with the traditional ALD, the proposed structure exhibits a significant increase in transmission loss within the considered frequency band, especially near the peak frequency of the narrow band noise. [ABSTRACT FROM AUTHOR]

A methodology is proposed for high-precision local measurement of the group velocity of longitudinal waves in solid samples with millimeter thickness. Achievement of the required accuracy involves laser thermo-optical excitation of submicrosecond ultrasonic video pulses and ultrawideband piezoelectric recording of acoustic signals reflected from the test sample. Plane-parallel samples made of duralumin, quartz, and steel with a thickness of 2–6 mm are studied. To achieve the required accuracy in measuring the group velocity of ultrasound, the signal shape is mathematically processed with compensation for diffraction of the ultrasonic beam as it propagates through the sample. The possibility of ensuring the uncertainty in measuring the group velocity of ultrasound in the 1–15 MHz frequency range at a level of 0.1% in samples with millimeter thickness is demonstrated. [ABSTRACT FROM AUTHOR]

The article presents the results of studies of internal waves in the shelf zone of the Black Sea in the water area near the mouth of the Kodor River, conducted in June 2021. Spatial surveys were carried out with the Rio Grande 600 kHz ADCP, which revealed the main characteristics of currents and the presence of internal waves generated by river inflow into the sea. Internal waves were recorded on two transects oriented along the normal to the coastline and extending to depths of 100 m. The observed waves had the features of nonlinear waves, were located on the near-surface thermocline, and had heights of up to 3–6 m. Also, short-period internal waves in the form of a wave train with an apparent period of about 3.5 min were recorded by a thermistor chain in the coastal zone of the sea. The dispersion curves and eigenfunctions of internal waves were calculated by numerically solving the internal wave equation, taking into account the velocity shift in the sea. Theoretical data are compared with experimental data. The significant influence of river inflow on the parameters of the generated internal waves moving with the flow was revealed. The experimentally observed decrease in wave periods and increase in their group and phase velocities were confirmed. [ABSTRACT FROM AUTHOR]

Surface wave azimuthal anisotropy in the uppermost crust could be essential to obtain the patterns of the local stress field and shallow Earth's interior structures. We used the micro-earthquakes (M ≤ 4) waveforms recorded by the Tehran Disaster Mitigation and Management Organization (TDMMO) and Iranian Seismological Center (IrSC) networks between 2006 and 2018. We applied the multiple filter analysis, FTAN, to measure Rayleigh wave group velocities and then inverted them to obtain isotropic and fast anisotropic direction maps at periods of 0.5 to 3.0 s. After obtaining the local dispersion curve for each geographic grid point, we applied a 1D VS inversion procedure. We inserted the resulting model into the original grid point to obtain a quasi-3D VS model in the Tehran region. According to these results, we divided the study area into five local fast anisotropic direction sectors. The resulting velocity maps indicate that the Tehran basin, with relatively low velocity, has been filled out by alluvial deposits with thicknesses between 0.4 km (in the north) and 1.2 km (in the south). In contrast, the fast-direction pattern in this basin changes from W-E (sector #2-west) to N-S (sector #4). A low-to-high-velocity anomaly change inside the basin and near-surface depths (up to 3 km) can illustrate the secondary faults, such as the Pardisan fault. The northern Tehran mountains appear with a high velocity in these maps with three different fast anisotropy directions (sectors #1, part of #2-west, and #5). This feature has also been observed in other stress field studies. In general, our tomographic results in the uppermost crust indicate that the azimuthal anisotropy can provide the velocity structure and illustrate the local stress field. [ABSTRACT FROM AUTHOR]

Propagation characteristics of Love waves in a layered structure composed of a piezomagnetic layer attached to an elastic substrate exposed to a magnetic field and compressive stress are investigated theoretically in this article. The effective elastic, piezomagnetic and magnetic permeability constants of a piezomagnetic material can affect by a magnetic field and compressive stress. The effects of magnetic field and compressive stress on the phase velocity, group velocity, mode shape, and magnetic potential of the Love wave are discussed in detail. It is found that the number of modes increases as the intensity of magnetic field increases while this tendency is reverse when applying compressive stress. As the intensity of magnetic field increases, the group velocity decreases but the magnitude of surface displacement of a piezomagnetic layer increases. The findings presented in this article are useful for improving the performance of surface acoustic wave (SAW) devices. [ABSTRACT FROM AUTHOR]

It is shown that the average meridional velocities of sunspot groups linearly depend on their average longitudinal velocities (solar rotation) with a high correlation coefficient of -0.95. The relationship differs for small, short-lived and large, long-lived groups. The meridional motions of sunspots do not have any pronounced global distribution law with latitude, but depend on their individual longitudinal velocities in a rotating coordinate system close to the Carrington one. The found relations indicate that the Coriolis force may play a role in driving the meridional motions of sunspot groups. [ABSTRACT FROM AUTHOR]

The Internet of Things (IoT) is transitioning towards Quantum Computing soon, relying on electromagnetic waves for data transfer within circuits and between IoT devices. This paper introduces a four-level atomic medium controlled by electromagnetic fields to manipulate the speed of light, resulting in subluminal and superluminal information transfer for IoT. Subluminal propagation transmits data at an average rate, while superluminal propagation transfers data faster than conventional methods. The differentiation between these two modes of propagation is determined by the sign of the group index, which is positive for subluminal and negative for superluminal. Our proposed model reports a maximum positive and negative group index of ± 5000. This modified group velocity to become v g = ± c / 5000 enhances IoT information transferring in quantum-based communication. The group delay time is measured to t d = p m 10 μ s in the medium, which increases the capacity for IoT information storage. The maximum value of normal and anomalous phase shift is reported to Φ = ± 20000 radians at 60 c m lengths of the medium; this effect is used to divide information in sub-set that protect IoT information from different types of attacks and losses. [ABSTRACT FROM AUTHOR]

The manuscript describes coherent control of the anomalous optical drag in a three-levels atomic medium driven by probe and control fields. Generally, anomalous optical drag occurs in a medium with a negative group index and faster group velocity. The condition for anomalous optical drag is that the group index is less than the inverse of the refractive index. The group index is negative and its maximum negative value is measured to − 10,000. The study investigates the effect of probe field detuning, decay rates, and Rabi frequency of the control field on the anomalous optical drag. The results show that a maximum of − 100 μm linear anomalous photon drag is achieved with a control field in a medium moving with a velocity of 10 m/s. The maximum linear anomalous photon drag is found to be sensitive to probe detuning and decay rate, with a minimum value of − 0.8 μm. The modified results of this study could be useful for spatial modes imaging coding and sensing technology. The ability to control and manipulate the anomalous optical drag can enable the design of novel optical devices and systems with enhanced performance. [ABSTRACT FROM AUTHOR]

Time-multiplexed Coherent Ising Machines (CIMs) have demonstrated promising results in rapidly solving large-scale combinatorial problems. However, CIMs remain relatively large and power-demanding. Here, we demonstrate a spinwave-based Ising machine (SWIM) that due to the low spinwave group velocity allows for sufficient miniaturization and reduced power consumption. The SWIM is implemented using a 10-mm-long 5-μm-thick Yttrium Iron Garnet film with off-the-shelf microwave components and can support an 8-spin MAX-CUT problem and solve it in less than 4 μs consuming only 7 μJ. As the SWIM minimizes its energy, we observe that the spin states can demonstrate both uniform and domain-propagation-like switching. The developed SWIM has the potential for substantial further miniaturization with reduction of power consumption, scalability in the number of supported spins, increase of operational speed, and may become a versatile platform for commercially feasible high-performance solvers of combinatorial optimization problems. The authors propose and experimentally demonstrate a magnonic version of a coherent Ising machine that implements a thin film Yttrium Iron Garnet spin-wave delay-line combined with microwave components. The work emphasizes the relative advantages that a slower more compact spin-wave system has over optical machines using similar principles. [ABSTRACT FROM AUTHOR]

The spectral characteristics of the photocurrent in the near-infrared range in vertical Ge/Si p–i–n photodiodes with Ge quantum dots embedded in a two-dimensional photonic crystal are investigated. The interaction of the quantum dots with photonic Bloch modes leads to the resonant enhancement of the sensitivity of photodiodes. The dependences of the photocurrent on the angle of incidence of light are used to determine the dispersion relations of the Bloch modes. Regions in the dispersion characteristics where the group velocity of photons is close to zero are revealed. It is established that the maximum enhancement of the photocurrent relative to a photodiode without photonic crystal, which can be up to a factor of ~60, results from the interaction of quantum dots with "slow" Bloch modes. [ABSTRACT FROM AUTHOR]

The properties of acoustic-gravity waves (AGWs) in the atmosphere can be determined to a greater extent by the features of the propagation medium than by the sources of these disturbances. In the presence of spatial inhomogeneities of atmospheric parameters, significant deviations of AGW characteristics from theory are observed. This complicates the experimental diagnosis of waves and the search for a connection with their potential sources. AGW observations from the Dynamics Explorer 2 satellite indicates the predominance of waves with certain spectral characteristics in the polar thermosphere. It has been found that AGWs with large amplitudes are spatially consistent with areas of strong winds, while AGWs move mainly toward the wind. In order to explain the observed AGW properties, we investigate the filtering of the spectrum of these waves in the presence of a spatially inhomogeneous wind. It is shown that the direction and magnitude of the wave vector change in a special way in the oncoming inhomogeneous wind. In this case, with an increase in the speed of the headwind, the wave vector gradually tilts toward the horizontal plane. The vertical component of the wave vector decreases rapidly, and its horizontal component tends to some threshold value, which is predominant in observations. In addition, in the oncoming inhomogeneous flow, the frequencies and amplitudes of the waves increase. As a result, high-frequency wave harmonics with a small angle of inclination of the wave vector to the horizontal plane and a characteristic horizontal wavelength will prevail in a strong headwind from the continuous spectrum of atmospheric AGWs that can be generated by a hypothetical source. Since the wave vector and the group velocity vector in AGWs are almost perpendicular to each other, such waves provide efficient energy transfer in the vertical direction. In this regard, AGWs play an important role in the energy balance of the polar atmosphere by redistributing the energy of horizontal wind currents in the vertical direction. [ABSTRACT FROM AUTHOR]

The concept of Alfvén waves, introduced into science 80 years ago, played an important role in the formation and development of cosmic electrodynamics. Alfvén waves differ in that at each point in space the group velocity vector and the vector of the external magnetic field are collinear to each other, due to which the waves can carry momentum, energy, and information over long distances. In memory of the outstanding event, we briefly describe two Alfvén resonators, one of which is located high above the Earth, in the radiation belt, and the second, in the ionospheric layers. Both resonators have a discrete spectrum in the upper part of the range of ultralow frequency oscillations of natural origin (approximately from 0.2 Hz to 7 Hz). The close connection between the concept of Alfvén waves and the current problems of the electrodynamics of geophysical media is especially emphasized. [ABSTRACT FROM AUTHOR]

We investigated forced Lamb waves using the method of multiple scales and Green's function method. With the former method, we derived a solvability condition containing terms describing forced effects. In Lamb waves problems, the solvability condition is a first-order partial differential equation in contrast to the governing equations describing Lamb waves, which are second-order partial differential equations and boundary conditions. With the latter method, we obtained the Green's function for the solvability condition. This way of deriving the Green's function is easier than solving the governing equations for Lamb waves. Finally, we obtained the amplitudes of the forced Lamb waves using the Green's function. The proposed method may be used to obtain the amplitudes from arbitrary forced distributions in both the time and space domains. Furthermore, the method may be used to obtain the frequency and wave number responses that take into consideration the detuning effects and may be used to consider dispersion effects. As a result, we found out that some kind of resonance occurs when the relation between input position and time delay corresponds to the group velocity of the input propagation mode. Solutions obtained by our method were verified using the frequency-domain-based integral transform method. [ABSTRACT FROM AUTHOR]

Based on the barometric data recorded by the seismic monitoring network on the Chinese mainland, Lamb wave periods and relative arrival times of 462 stations are obtained with the aid of Meyer's wavelet decomposition, Welch's periodogram spectrum estimation and waveform cross-correlation. By extracting the seismic Rayleigh waves of two seismic stations in the South Pacific and comparing them with the synthetic seismograms, the occurrence time of the first two large volcanic eruptions and the largest volcanic eruption are credibly deduced, and then the travel times and propagation speeds of the Lamb waves are obtained. In order to further explain the attributes of the pressure disturbance associated with the complex acoustic gravity wave (AGW) phases on pressure, a series of numerical simulations with the QSSP program are applied to yield highly similar waves corresponding to the barometric records, which indicates that the eruption source may contain more than 10 sub-events within an hour. According to our detailed analyses, major conclusions are obtained as follows: (1) Although the primary pressure disturbance of the Tonga volcanic eruption appears to be a simple bulge, it is in fact a complex wave composed of multiple eruptions. The largest eruption occurred about 13 min after the first large eruption. (2) In addition to the propagation of the traditional Lamb mode, the Pekeris mode phase that propagates with a lower speed is also observed in some stations, and its amplitude is about a fifth of the Lamb wave. The Pekeris waves may have been more significantly delayed as they traveled against the westerly wind due to their much shorter periods and lower velocities. (3) The group velocity of the Lamb wave is about 308 m/s. Its average period is 70 min, and the wavelength is about 1300 km. The arrival time deviation at each station is negatively correlated with the difference in the near-surface air temperature between North and South China. However, accurately estimating the parameters of the Pekeris waves or the antipodal Lamb waves is challenging due to their low signal-to-noise ratio (SNR), even though the horizontal propagation speed of the Pekeris wave toward China is estimated to be approximately 225 m/s. (4) Much shorter periods and later arrivals at the stations within 200 km of Beijing may be related to the significant cooling (i.e. 12 °C temperature drop), which occurred from Ulantoba, Outer Mongolia, to Beijing beginning at noon on January 15 (Beijing time). [ABSTRACT FROM AUTHOR]

We present a theoretical and numerical study of the dynamics of diffractionless three-dimensional (3D) extremely short optical pulses with a Bessel cross-section in an optically anisotropic medium with a spatially modulated refractive index (known as photonic crystal) based on carbon nanotubes of the zig-zag type. The nonlinear absorption is taken into account based on experimental data, as well as the effects associated with the external pump field. Through numerical simulations, it was found that in such an optically anisotropic photonic crystal, Bessel pulses propagate stably over times of several dispersion lengths, while dissipative solitons arise during the evolution of pulses. The influence of the parameters of the anisotropic photonic crystal—e.g., the depth and period of modulation of the refractive index and anisotropy parameters—on the evolution of pulses, in particular, their shape and group velocity, is established. [ABSTRACT FROM AUTHOR]

We have investigated the optical responses of a microwave-mediated pump–probe laser-driven five-level Kobrak–Rice (KR5)-like closed-loop atomic system involving three Rydberg states. The atomic transitions between the Rydberg states are driven by electric field components of two microwave fields. It is observed that multiple electromagnetically induced transparency (EIT) windows appear in the probe absorption profile for the Doppler-free medium. The position of the EIT windows is controllable by altering the ratio of the two microwave field strengths and the relative phase between them when the pump and probe strengths are fixed. For Doppler-broadened EIT-induced medium, atoms with different groups of velocities further affect the atomic coherence substantially. Corresponding dispersion profiles owing variation in the group refractive index of the medium show possible switching between superluminal and subluminal probe light propagation. Finally, we have demonstrated the generation of four-wave mixing (FWM) signal in the said five-level system. The amplitude of the generated FWM signal is found to be influenced by the strength ratios of the two microwave fields and their relative phase. [ABSTRACT FROM AUTHOR]

In this paper, we present a novel method to design an ultra-small photonic integrated biosensor to detect cancerous cells. The proposed biosensor is based on the self-phase modulation in PhC-SOA, inducing a frequency shift on a pulse traveling through the device. The amount of the frequency chirp depends on the group velocity of the active medium waveguide being determined by the refractive index of the microfluidic infiltrating the holes around the waveguide. The refractive index of the microfluidic is also determined by the cell type that can be normal or cancerous. Since the refractive index of a cancerous cell is higher than that of a normal one, the group index of the waveguide and the output chirp will decrease. By measuring the amount of the output chirp, we can detect the cell type. The Simulation results showed that for a 0.02 change in the refractive index of the cell, a 3.71 nm central wavelength shift occurred for a 10-ps 7-mW gaussian pulse input with a central wavelength of 1533.53 nm. In terms of the wavelength shift, the sensitivity and figure of merit are 185.5 and 530, respectively. To detect the cell type, we integrated a PhC channel drop filter to drop the chirped signal due to the cancerous cell infiltration. Designing an appropriate PhC-CDF leads to achieving an ultra-small cancerous detection cell biosensor with more than 97% precision. [ABSTRACT FROM AUTHOR]

That the speed of light in vacuum is constant is a cornerstone of modern physics. However, recent experiments have shown that when the light field is confined in the transverse plane, the observed propagation speed of the light is reduced. This effect is a consequence of the transverse structure which reduces the component of wavevector of the light in the direction of propagation, thereby modifying both the phase and group velocity. Here, we consider the case of optical speckle, which has a random transverse distribution and is ubiquitous with scales ranging from the microscopic to the astronomical. We numerically investigate the plane-to-plane propagation speed of the optical speckle by using the method of angular spectrum analysis. For a general diffuser with Gaussian scattering over an angular range of 5°, we calculate the slowing of the propagation speed of the optical speckle to be on the order of 1% of the free-space speed, resulting in a significantly higher temporal delay compared to the Bessel and Laguerre–Gaussian beams considered previously. Our results have implications for studying optical speckle in both laboratory and astronomical settings. [ABSTRACT FROM AUTHOR]

Abstract—The paper addresses the study of changes in the seismic process associated with a volcanic eruption and evaluation of the energy parameters and structure of the wave field from seismic data. Three types of disturbances in the structure of the wave field are identified. Firstly, this is the Rayleigh surface waves with an average oscillation period of 23 s, which are clearly traceable at distances up to 100 degrees. The group velocity of the Rayleigh waves is 3.6–3.8 km/s. The magnitude calculated from these waves for the stations with mainly oceanic propagation paths is Ms = 5.52 ± 0.18, the corresponding seismic energy is on the order of Ec = (1–7) × 1013 J, and the scalar seismic moment is M0 = 2.4 × 1017 J. Two other types of oscillations are detected on seismic channels with a passband between 0.0003 and 0.1 Hz. In the first oscillation types, the apparent velocity of the propagation of the disturbance lies between 0.28 and 0.37 km/s and a characteristic period is 268 s. This type of disturbance is associated with the gravitational response of the seismometer to the acoustic perturbation in the atmosphere. For the second type of seismic disturbances, the propagation velocity is 0.21–0.28 km/s with the characteristic period of 450 s on the horizontal components of seismic receivers. This type of seismic disturbance is likely to be caused by the interaction of a gravity wave in the hydrosphere with the coast of the islands where seismic stations are located. [ABSTRACT FROM AUTHOR]

The propagation of bending waves in a plate resting on a two-parameter elastic foundation is considered. In contrast to the classical Kirchhoff model, the mathematical model used here takes into account not only the kinetic and potential energies of bending vibrations, but also the kinetic energy due to the inertia of rotation of the plate elements during bending. The dispersion equation, phase velocity, energy transfer velocity, and energy characteristics of waves propagating in the plate are analyzed depending on the ratio of the coefficients determining the shear and compression stiffness of the elastic foundation. Conditions are found under which waves with phase and group velocities having opposite directions (frequently called "backward" waves), can exist in the plate. It is demonstrated that such waves significantly change the behavior of the energy flux. In addition, relations are found that relate the kinematic and average energy characteristics of the waves. [ABSTRACT FROM AUTHOR]

The Eastern Indian Ocean is a tectonically and geodynamically active region that has experienced deformations due to rifting, uplifting, and plume activity. The earlier Rayleigh wave studies in the East Indian Ocean were mainly focused on the structure of the Bay of Bengal, Ninety East Ridge, and Broken Ridge. The structure of other region of the East Indian Ocean is not much explored. In the present study, Rayleigh wave dispersion analysis is performed to observe the signatures of upper mantle deformation in terms of shear wave velocity of the East Indian Ocean using global search method. The fundamental mode Rayleigh wave group velocities are estimated between 15 and 100 s using the multiple filter technique. The group velocities of the raypaths that traverse the same region are clustered (E1–E8) to get an average dispersion curve. Using a genetic algorithm, each cluster's group velocities are inverted for shear velocity structure. The observed dispersion curve of E6, E7, and E8 indicates the lower group velocities between 35 and 100 s relative to E1, E2, E3, E4, and E5, with an average variation of about 0.07–0.18 km/s. The crustal thickness obtained in the study region is ~ 26 km and is due to the increased thickness of the lower crust (9.1–12.4 km) having Vs 3.95–4.04 km/s. The theoretical Vs have been calculated for serpentinite rock at uppermost lithospheric conditions and found to be similar to the Vs of the lower crust in the present study. Hence, it is assumed that unusual crustal thickness is due to the progressive development of the upper lithosphere formation (Ultramafic rock) into material (serpentinite rock) with crustal-like shear velocity or moderately lower than sub-Moho shears velocity. The undeformed lithosphere is evidenced by the high-velocity (Vs 4.62–4.77 km/s) layer beneath the Moho, whose thickness ranges from 41.3 to 51.6 km. The high-velocity lithosphere is followed by a low-velocity zone that extends up to 160 km; however, the variation in Vs (4.57–4.31 km/s) indicates that the low-velocity zone is deformed. [ABSTRACT FROM AUTHOR]

The autowaves of localized plastic flow are considered for the stages of easy glide and linear work hardening in a number of metals. The propagation rates were determined experimentally for the autowaves in question with the help of speckle photography. The quadratic dispersion relations and the wave number dependences of phase and group velocities derived for the localized plastic flow autowaves are discussed. [ABSTRACT FROM AUTHOR]

Anisotropic elastic materials containing multiple elliptical cracks of different orientation distributions are considered using the (self-consistent) effective field method. The method allows, in particular, accounting for mutual positions of cracks. The derived effective elastic stiffness tensor applies to a broad range of the crack density. Group velocity surfaces (wave surfaces) of cracked materials are constructed. It is shown that, for the isotropic materials with parallel penny shaped cracks, the wave surfaces are close to ellipsoidal. A method of solution of the inverse problem and determination of fracture parameters from acoustical data (group velocities of quasi-longitudinal and quasi-transverse waves of various directions in damaged rock materials) is proposed. Examples of application of the method are presented, and its accuracy is assessed. The results can be used for determination of crack density in rock materials by acoustical methods. [ABSTRACT FROM AUTHOR]

Internal structure imaging of the Earth, along with determining basin structure, can aid in evaluating potential seismic hazards. However, the high operating cost limits the current geophysical exploration methods; moreover, it is difficult to apply these techniques over a large area, which limits our understanding of the Quaternary structure and the development of earthquake prevention science. A combination of dense array observation technology and ambient noise surface wave tomography is being rapidly developed as a high-resolution urban detection method. Here, we report the ambient noise imaging results of a high-density array experiment. In the ambient noise surface wave tomography method (e.g., surface wave tomography; Eikonal tomography), the signal is assumed to be a single mode. However, several multimode signals were detected in this dataset. With the use of traditional methods to measure the dispersion, mode confusion occurs and the extracted dispersion curve jumps. To solve this problem, by combining the advantages of phase-matched filtering and dispersion compensation, we realized the automatic pickup of fundamental group velocity using reference phase velocity. From this, a Rayleigh wave group velocity map was obtained. The regional average phase velocity information was included in the inversion steps to reduce the uncertainty in the inversion of shear wave velocity. Finally, an S-wave velocity structure model was obtained within a depth of 500 m. The velocity structure was roughly layered and grew with depth. In the depth range of 240–320 m, obvious decreases in the S-wave velocity were observed. Compared with geothermal drilling data, this was speculated to be the reflection of a water-rich (confined water) sand layer. This study provides a technical approach for and a processing example of a high-density array, and its velocity model can be used as a reference for urban subsurface structure, underground space utilization, and earthquake disaster prevention and control. [ABSTRACT FROM AUTHOR]

It is found that the problem of boundary layer stability with a diffusion flame under the conditions of a constant molecular weight over the boundary layer in the inviscid approximation and in the Dunn–Lin approximation with identical Schmidt numbers can be reduced to a similar problem for a single-component gas. Approximately, this conclusion is also valid for different Schmidt numbers. Calculations of the steady-state parameters of the boundary layer show that there are two generalized inflection points testifying to a necessary condition of the "inviscid" Rayleigh instability. Simulations show that the boundary layer with a diffusion flame is most unstable to two-dimensional disturbances. As the frequency is increased, the phase velocity of the growing wave tends to the value at the generalized inflection point. Despite sufficiently large growth rates, the degree of spatial enhancement of the wave is equal with high accuracy to the ratio of the time degree of enhancement to the group velocity. [ABSTRACT FROM AUTHOR]

By analytically solving the density matrix equations of a three-level atomic system interacting with two coherent laser fields and an incoherent pumping field under steady-state condition, we have derived the expressions for the absorption and dispersion (and thus group index and group velocity), and population difference as a function of the intensity, frequency, polarization and phase parameters of the laser fields. We have demonstrated that the appearance of spontaneously generated coherence—SGC (characterized by the polarization and non-orthogonality of the electric dipole moments) fundamentally changes the atomic optical responses. By adjusting the strength of SGC and/or incoherent pumping rate, the medium can switch between absorption and amplification regimes, between normal dispersion and anomalous dispersion, and thus light propagation in the medium can switch from slow light to fast light and vice versa. In particular, we have also found that in amplification regime the light is slowed down considerably. Amplifying and slowing light also become more efficient as incoherent pumping rate increases. In addition, the presence of the SGC and incoherent pumping field leads to a sensitive dependence of the atomic responses on the relative phase of the laser fields. Namely, the absorption, dispersion, and group index vary periodically with relative phase. [ABSTRACT FROM AUTHOR]

The quasistatic approximation (QSA) is an efficient method of simulating laser- and beam-driven plasma wakefield acceleration, but it becomes imprecise if some plasma particles make long longitudinal excursions in a strongly nonlinear wave, or if waves with non-zero group velocity are present in the plasma, or the plasma density gradients are sharp, or the beam shape changes rapidly. We present an extension to QSA that is free from many of its limitations and retains its main advantages of speed and reduced dimensionality. The new approach takes into account the exchange of information between adjacent plasma layers. We introduce the physical model, describe its numerical implementation, and compare the simulation results with available analytical solutions and other codes. [ABSTRACT FROM AUTHOR]

We use numeric-analytical methods to study theoretically the reflective properties of the layered media of the seasonally thawed active layer (AL) in the process of their freezing and thawing in the ultrawide frequency band from 10 MHz to 10 GHz, as well as the process of interaction of 2.5-ns ultrawide band pulses in the megahertz frequency range with such media, for the light loam soil with the clay-fraction weight content equal to 20% as an example. The spectral amplitudes of the reflection coefficient, the time waveforms and delays, and the propagation velocity of the pulse are calculated in the case of a monostatic radar configuration with probing the AL to the nadir by an electromagnetic wave with a plane front. It is shown that the AL soil is a low-pass filter for the reflected waves, and the cutoff frequency of this filter decreases nonlinearly with increasing AL thickness. In this case, the average frequency in the spectrum of the probing pulse decreases from 527 MHz (before the pulse enters the medium) by 38% and 23% per meter during propagation through the thawed and frozen active layers, respectively. The velocity of the ultrawideband pulse and the group velocity of the pulse propagation, which is calculated at the average frequency of the probing-pulse spectrum before the pulse enters the medium, coincide in both the thawed and frozen ALs of the light loam soil. It is shown that the energy potential of the radar system should exceed 40 dB per 0.5 m of the AL thickness in the frequency range below 1 GHz to ensure probing of the lower boundary of the AL in the freezing or thawing light loam soil. The performed studies demonstrate the basic possibility and good prospects for developing pulsed ultrawideband radar systems in the megahertz frequency range for remote probing of the geophysical parameters of the layered structure of ALs in light loam soils. [ABSTRACT FROM AUTHOR]

Based on first-principles and Boltzmann transport equation, the electronic structure and thermoelectric properties of derivative TlAgX (X = S, Se) monolayers of KAgSe monolayer are predicted. The results show that the TlAgX (X = S, Se) single layers are indirect bandgap semiconductors. TlAgS and TlAgSe monolayers have the low lattice thermal conductivity of 0.20 and 0.17 W m−1 K−1 at 700 K, respectively. Such ultra-low lattice thermal conductivity is ascribed to the small phonon group velocity, large Grüneisen parameters, and short phonon relaxation time, with greatly suppressed phonon transport. The n-doping TlAgS and TlAgSe monolayers have high ZT values in wide temperature range (300–700 K) and the maximal ZT value reaches 9.1 and 10.3 at 700 K, respectively. The excellent thermal and electrical transport properties and resulting good thermoelectric performance indicate that the TlAgX (X = S, Se) monolayers are good thermoelectric candidate materials. [ABSTRACT FROM AUTHOR]

Perturbations of attenuation coefficients and group velocities of acoustic modes is considered in a shallow-water waveguide with an inhomogeneous bottom relief. Explicit formulas are constructed for calculating the first and second derivatives of the aforementioned quantities with respect to the sea depth on the basis of the perturbation theory developed by the authors in previous publications. Application of the proposed formulas is illustrated by a model example. It is shown that they allow highly accurate approximation of depth dependences for group velocities and modal attenuation coefficients. The proposed formulas can be used to obtain simple analytic estimates of acoustic propagation loss in irregular waveguides, as well as to allow for inhomogeneities of the bottom relief in geoacoustic inversion according to the data on waveguide dispersion of signals. [ABSTRACT FROM AUTHOR]

Due to the unique physical properties, graphene and its derivatives are often used as the reinforcements to improve and modulate mechanical performances and dynamic behaviors of composite materials. In this paper, a semi-analytical finite element method is utilized to analyze the wave properties of functionally graded graphene-reinforced piezoelectric nanocomposite cylindrical shells with graphene agglomeration effects. The effective micromechanics model, the first-order shear deformation shell theory, and the Hamiltonian principle are organically intergrated to derive the wave equation to analyze the modulation of wave propagation of functionally graded graphene-reinforced piezoelectric nanocomposite cylindrical shells by regulating agglomeration effects. The results indicate that the decrease in volume fraction of agglomeration phase and the raising of the amount of graphene sheets in agglomeration phase lead to the reduction of phase and group velocities and hinder the transmission of waves and wave energy. The higher the graphene total volume fraction, the more obvious the influence of the agglomeration effects on the structural wave properties. Meanwhile, it is found that the agglomeration effects can inhibit the modulation of the concentration and functional gradient distribution patterns of graphene on the wave properties. It is noted that the center conversion frequency of out-of-plane modes and the group velocity of the conversion intersection point of out-of-plane modes are sensitive to agglomeration effects, the values of the center conversion frequency and the group velocity of the conversion intersection point increase with the weakening of the agglomeration effects. This work can provide guidance and new thought for the design of smart nanocomposites and the application of elastic wave characteristics in non-destructive testing. [ABSTRACT FROM AUTHOR]

Linear localized waves evolution in a discrete mass-in-mass lattice is studied. The presence of the attached mass in the model contributes to dispersion giving rise to appearance of both the acoustic and optic wave modes. Important features of the waves are described using the dispersion relation, which is obtained in a continuum limit of the original discrete equations using a harmonic wave solutions. We study numerically the localized initial perturbations evolution and compare the features of the numerical solutions with those obtained for the analytical harmonic wave ons. We have found differences in the wave dynamics depending on the parameters of the initial conditions. One scenario describes almost permanent shape and velocity counterpart localized waves propagation with oscillating standing wave around the position of the initial pulse. Another wave evolution accounts for a decrease in the moving wave amplitude with the developing oscillating tail behind the localized wave. Contrary to the periodic analytical solution, no evidence of a band gap is found in the simulations of localized waves. [ABSTRACT FROM AUTHOR]

This document is a correction notice for an article titled "Effects of skin moisturization on various aspects of touch showing differences with age and skin site" published in Scientific Reports. The correction addresses an error in Figure 5, panel d, where the values were swapped round. The original figure and accompanying legend are provided in the correction notice. The authors of the article are Mariama Dione, Roger Holmes Watkins, Jean-Marc Aimonetti, Roland Jourdain, and Rochelle Ackerley. [Extracted from the article]

In this paper we study the propagation of two-dimensional periodic waves in an inviscid continuously stratified fluid with a free surface in the frequency range from to Hz. Dispersion relations, as well as expressions for phase and group velocities for surface and internal waves in physically observable variables, are given. The fluid is shown to behave as homogeneous when the wavelength reaches values on the order of the stratification scale. As the wave frequency approaches the buoyancy frequency, the energy transfer rate decreases: the group velocity of surface waves tends to zero, while the phase velocity tends to infinity. In the case of infinitesimal perturbations of a stratified fluid with a free surface, surface and internal waves exist in separated frequency intervals. [ABSTRACT FROM AUTHOR]