Transverse Sound Speed Research Articles

Mobility of twinning dislocations in copper up to supersonic speeds

Understanding the mobility of twinning dislocations is important for multiscale modeling of crystal plasticity, especially at high strain rates, where such dislocations may reach transonic or supersonic speeds. We used molecular dynamics simulations to investigate the relationship between dislocation velocity and the applied resolved shear stress of an edge twinning dislocation in copper up to supersonic speeds. The twinning dislocation mobility relation is composed of two branches separated by a band of forbidden velocities. The lower velocity branch is limited by the first transverse sound speed ∼2000 m/s while the upper branch stretches from ∼3500 m/s in the transonic regime to supersonic velocities. Twinning dislocations cannot undergo uniform steady-state motion at velocities within the forbidden band. Our simulation results also reveal that edge twinning dislocation motion in copper is kink-mediated. We discuss the implications of our findings for the motion of twins, twinning dislocations, and twinning dislocation kinks in copper.

Экспериментальные исследования акустического поля поперечных волн в модели биологической ткани

This article develops the ultrasound diagnostics principles using elastography technology and presents the experimental setup diagram. Biological tissue is modeled using silicone material. Research has been carried out and the longitudinal and transverse sound speeds have been determined. Further research will create a database of pathological and healthy tissues various samples.

Universal scaling of transverse sound speed and its isomorphic property in Yukawa fluids.

Molecular dynamical simulations are performed to investigate the scaling of the transverse sound speed in two-dimensional (2D) and 3D Yukawa fluids. From the calculated diagnostics of the radial distribution function, the mean-squared displacement, and the Pearson correlation coefficient, the approximate isomorphic curves for 2D and 3D liquidlike Yukawa systems are obtained. It is found that the structure and dynamics of 2D and 3D liquidlike Yukawa systems exhibit the isomorphic property under the conditions of the same relative coupling parameter Γ/Γ_{m}=const. It is demonstrated that the reduced transverse sound speed, i.e., the ratio of the transverse sound speed to the thermal speed, is an isomorph invariant, which is a quasiuniversal function of Γ/Γ_{m}. The obtained isomorph invariant of the reduced transverse sound speed can be useful to estimate the transverse sound speed, or determine the coupling strength, with applications to dusty (complex) plasma or colloidal systems.

Observation of fast sound in two-dimensional dusty plasma liquids.

Equilibrium molecular dynamics simulations are performed to study two-dimensional (2D) dusty plasma liquids. Based on the stochastic thermal motion of simulated particles, the longitudinal and transverse phonon spectra are calculated, and used to determine the corresponding dispersion relations. From there, the longitudinal and transverse sound speeds of 2D dusty plasma liquids are obtained. It is discovered that, for wavenumbers beyond the hydrodynamic regime, the longitudinal sound speed of a 2D dusty plasma liquid exceeds its adiabatic value, i.e., the so-called fast sound. This phenomenon appears at roughly the same length scale of the cutoff wavenumber for transverse waves, confirming its relation to the emergent solidity of liquids in the nonhydrodynamic regime. Using the thermodynamic and transport coefficients extracted from the previous studies, and relying on the Frenkel theory, the ratio of the longitudinal to the adiabatic sound speeds is derived analytically, providing the optimal conditions for fast sound, which are in quantitative agreement with the current simulation results.

Revealing the supercritical dynamics of dusty plasmas and their liquidlike to gaslike dynamical crossover

Dusty plasmas represent a powerful playground to study the collective dynamics of strongly coupled systems with important interdisciplinary connections to condensed matter physics. Due to the pure Yukawa repulsive interaction between dust particles, dusty plasmas do not display a traditional liquid-vapor phase transition, perfectly matching the definition of a supercritical fluid. Using molecular dynamics simulations, we verify the supercritical nature of dusty plasmas and reveal the existence of a dynamical liquid-like to gas-like crossover which perfectly matches the salient features of the Frenkel line in classical supercritical fluids. We present several diagnostics to locate this dynamical crossover spanning from local atomic connectivity, shear relaxation dynamics, velocity autocorrelation function, heat capacity, and various transport properties. All these different criteria well agree with each other and are able to successfully locate the Frenkel line in both 2D and 3D dusty plasmas. In addition, we propose the unity ratio of the instantaneous transverse sound speed $C_T$ to the average particle speed $\bar{v}_{p}$, i.e., $C_T / \bar{v}_{p} = 1$, as a new diagnostic to identify this dynamical crossover. Finally, we observe an emergent degree of universality in the collective dynamics and transport properties of dusty plasmas as a function of the screening parameter and dimensionality of the system. Intriguingly, the temperature of the dynamical transition is independent of the dimensionality, and it is found to be always $20$ times of the corresponding melting point. Our results open a new path for the study of single particle and collective dynamics in plasmas and their interrelation with supercritical fluids in general.

Origin of viscosity at individual particle level in Yukawa liquids

The transition of the viscosity $\ensuremath{\eta}$ from a collisional gas through a minimum value to a correlated liquid is investigated using computer simulations with the Green-Kubo relation. It is discovered that, as the temperature varies, the transition of $\ensuremath{\eta}$ is well described by the unity ratio of the instantaneous transverse sound speed ${C}_{T}$ to the average particle speed ${\overline{v}}_{p}$. While ${C}_{T}/{\overline{v}}_{p}<1, \ensuremath{\eta}$ increases with the temperature, since in this regime the viscosity is dominated by the gaslike individual dynamics. However, when ${C}_{T}/{\overline{v}}_{p}>1$ where the cooperative dynamics dominates, the fundamental origin of viscosity of liquids is found to be just losing or gaining neighbors for individual particles, so that the viscosity of a typical liquid reasonably decreases with the temperature. Our results reveal that the viscosity transition point of ${C}_{T}/{\overline{v}}_{p}=1$ is just $\ensuremath{\approx}20$ times the corresponding melting point for both two-dimensional and three-dimensional Yukawa liquids with various screening parameters, which probably can be used as a new criterion to distinguish the strong and weak couplings in plasma physics.

Abnormal interactions between high-speed edge dislocation and microvoid in BCC metals

The interaction between a high speed edge dislocation and a microvoid has been investigated by molecular dynamics (MD) simulations in BCC crystals of Ta, Fe, V and W in the present work. The focus is placed on the dynamic effect on the dislocation-microvoid interaction. Three interaction scenarios different from static cases, i.e., repeated dislocation oscillations around microvoids, dislocation depinning from microvoid like releasing an elastic slingshot, and formation of abnormal “pull forward” dislocation configuration, have been found for the first time. The “repeated oscillations” can be attributed to the dislocation line tension and dynamic effect, which is analogous to under-damped mechanical oscillator system. For the “releasing slingshot” case, the dislocation first bows out after breaking away from the microvoid, and then a stable “drag backward” dislocation configuration is formed. When further increasing the applied shear stress, the dislocation can accelerate to subsonic speed of roughly0.7CT, with CT being the transverse sound speed, leading to a stable “pull forward” configuration. These different dislocation configurations are closely related to the formation of superjog driven by the dislocation-microvoid interaction. By careful analysis, it is found that the variation of relative mobility between the original edge dislocation on the (110) plane and the superjog segments on the (112) plane with shear stresses perfectly coincides with the above three dislocation configurations. In fact, when the applied shear stress is high enough and the dislocation speed is subsonic, the friction stress on the superjog segments turns to be smaller than that of the original edge dislocation, resulting in the formation of abnormal “pull forward” configuration.

Realistic anisotropic Karmarkar stars in Rastall gravitational framework

In this study, we discuss the anisotropic matter distribution of compact stars in the context of Rastall modified theory of gravity by applying Karmarkar condition of embedding technique. For this purpose, we consider the spherical symmetric geometry and solve the resulting Rastall field equations by taking different values of Rastall parameter, i.e., ξ=0.05,ξ=−0.05, and ξ=−0.15 into account. Since the Karmarkar condition links both metric potentials through a differential equation and allows one to pick one of the metric potential, so we pick a well-known interesting form of grr component of metric as eλ(r)=1+cr21+ar2n(1+br2)2. We elaborate the properties of compact stars Vela X-1, 4U 1820-30 and SAX J 1808.4-3658 for different positive values of n, i.e., 1.8≤n<7, where n≠2,4,6. It is interesting to mention here that results are not justified for n≥7. Also, we find no solution for even integral values of n, but for odd integers, we obtain physically admissible solutions representing the compact stars. For greater values of n, we obtain stiff fluid equations of state in which adiabatic index shows the increasing behavior and speed of sound approximates to speed of light. It is seen that the causality condition is violated for all values of n≥7 and the values of n between 0 and 1.8 generate complex values of transverse sound speed vt.

Melting, freezing, and dynamics of two-dimensional dipole systems in screening bulk media.

This paper reports on the molecular dynamics simulations of classical two-dimensional (2D) electric dipole systems. The properties of 2D systems with bare (nonscreened) and screened dipole-dipole interactions have been investigated. Based on the polygon construction method, we present simulation results on the phase transition, and we locate the melting and freezing points of 2D dipole systems in terms of a polygon disorder parameter, with the polygon disorder parameter being the sum of nontriangular polygon order parameters. It was found that the phase transition of the system occurs when the polygon disorder parameter has a value 0.165. This result was cross-checked by using both local and overall orientational order parameters. We also identified that the value of the average local orientational order parameter at the phase transition point is 0.67. These results are valid for the ordinary (bare) dipole-dipole interaction as well as the screened dipole-dipole interaction, and they are expected to be general for other 2D systems with repulsive pair interaction. We observed that both melting and freezing points shift to lower values of temperature due to screening. In the liquid state, the radial distribution function and polygon construction method show the loss of order in a structure as screening becomes more severe. Furthermore, the impact of screening on the system's collective excitation spectra and diffusive characteristics at liquid and solid states has been studied. Results show the decrease in the values of both longitudinal and transverse sound speeds and the emergence of anomalous superdiffusive motion in the liquid state due to screening.

The emergence of inertial waves from coherent vortex source in strongly coupled dusty plasma

The evolution of isotropic, nondispersive, inertial waves emerging from an unsteady initial coherent vortex source is studied for strongly correlated dusty plasma using two-dimensional molecular dynamics simulation. In this study, the effects of azimuthal speed of a vortex source, strong correlation, large screening, and the compressibility of the medium on the propagation of generated inertial waves have been presented. It has been observed that these inertial waves only exist when the angular speed or azimuthal speed of the vortex source (U0) is larger than the transverse sound speed (Ct) of the system. The estimated speed of the nonlinear wave (CNLW) is found to be always larger and close to longitudinal sound speed (Cl) of the system for the range of coupling and screening parameters studied. We find that spontaneously generated inertial wave speed in dusty plasma is suppressed by the compressibility and dust-neutral drag of the system and is less sensitive to coupling strength. We also report a transition from “incompressible to compressible” flow. This transition is found to depend on the screening parameter and azimuthal speed of the vortex source. The existence of a critical Mach number Mc≈0.35 is found (where Mc=U0/Cl), above which inertial waves are found to exist, indicating the compressible nature of the wave.

Dislocation drag from phonon wind in an isotropic crystal at large velocities

ABSTRACTThe anharmonic interaction and scattering of phonons by a moving dislocation, the photon wind, imparts a drag force on the dislocation. In early studies, the drag coefficient B was computed and experimentally determined only for dislocation velocities v much less than transverse sound speed, . In this paper, we derive analytic expressions for the velocity dependence of B up to in terms of the third-order continuum elastic constants of an isotropic crystal, in the continuum Debye approximation. In so doing we point out that the most general form of the third order elastic potential for such a crystal and the dislocation-phonon interaction requires two additional elastic constants involving asymmetric local rotational strains, which have been neglected previously. We compute the velocity dependence of the transverse phonon wind contribution to B in the range 1–90% for Al, Cu, Fe, and Nb in the isotropic Debye approximation. The drag coefficient for transverse phonons scattering from screw dislocations is finite as , whereas B is divergent for transverse phonons scattering from edge dislocations in the same limit. This divergence indicates the breakdown of the Debye approximation and sensitivity of the drag coefficient at very high velocities to the microscopic crystalline lattice cutoff. We compare our results to experimental results wherever possible and identify ways to validate and further improve the theory of dislocation drag at high velocities with realistic phonon dispersion relations, inclusion of lattice cutoff effects, MD simulation data, and more accurate experimental measurements.

Shear modulus of two-dimensional Yukawa or dusty-plasma solids obtained from the viscoelasticity in the liquid state.

Langevin dynamical simulations of two-dimensional (2D) Yukawa liquids are performed to investigate the shear modulus of 2D solid dusty plasmas. Using the known transverse sound speeds, we obtain a theoretical expression of the shear modulus of 2D Yukawa crystals as a function of the screening parameter κ, which can be used as the candidate of their shear modulus. The shear relaxation modulus G(t) of 2D Yukawa liquids is calculated from the shear stress autocorrelation function, consisting of the kinetic, potential, and cross portions. Due to their viscoelasticity, 2D Yukawa liquids exhibit the typical elastic property when the time duration is much less than the Maxwell relaxation time. As a result, the infinite frequency shear modulus G_{∞}, i.e., the shear relaxation modulus G(t) when t=0, of a 2D Yukawa liquid should be related to the shear modulus of the corresponding quenched 2D Yukawa solid (with the same κ value), with all particles suddenly frozen at their locations of the liquid state. It is found that the potential portion of the infinite frequency shear modulus for 2D Yukawa liquids at any temperature well agrees with the shear modulus of 2D Yukawa crystals with the same κ obtained from the transverse sound speeds. Thus, we find that the shear modulus of 2D Yukawa solids can be obtained from the motion of individual particles of the corresponding Yukawa liquids using their viscoelastic property.

Properties of Dislocation Drag from Phonon Wind at Ambient Conditions.

It is well known that, under plastic deformation, dislocations are not only created but also move through the crystal, and their mobility is impeded by their interaction with the crystal structure. At high stress and temperature, this “drag” is dominated by phonon wind, i.e., phonons scattering off dislocations. Employing the semi-isotropic approach discussed in detail in a previous paper (J. Phys. Chem. Solids 2019, 124, 24–35), we discuss here the approximate functional dependence of dislocation drag B on dislocation velocity in various regimes between a few percent of transverse sound speed and (where is the effective average transverse sound speed of the polycrystal). In doing so, we find an effective functional form for dislocation drag for different slip systems and dislocation characters at fixed (room) temperature and low pressure.

Embedded class solutions compatible for physical compact stars in general relativity

We have explored a family of new solutions satisfying Einstein’s field equations and Karmarkar condition. We have assumed an anisotropic stress-tensor with no net electric charge. Interestingly, the new solutions yield zero values of all the physical quantities for all even integer $n > 0$ . However, for all $n >0$ ( $n \neq $ even numbers) they yield physically possible solutions. We have tuned the solution for neutron star Vela X-1 so that the solutions matches the observed mass and radius. For the same star we have extensively discussed the behavior of the solutions. The solutions yield a stiffer equation of state for larger values of n since the adiabatic index increases and speed of sound approaches the speed of light. It is also found that the solution is physically possible for Vela X-1 if $1.8 \leq n < 7$ (with $n\neq 2,4,6$ ). All the solutions for $n \geq 7$ violates the causality condition and all the solutions with $0 < n < 1.8$ lead to complex values of transverse sound speed $ v_{t}$ . The range of well-behaved n depends on the mass and radius of compact stars.

Conformal solids and holography

We argue that a SO(d) magnetic monopole in an asymptotically AdS space-time is dual to a d-dimensional strongly coupled system in a solid state. In light of this, it would be remiss of us not to dub such a field configuration solidon. In the presence of mixed boundary conditions, a solidon spontaneously breaks translations (among many other symmetries) and gives rise to Goldstone excitations on the boundary — the phonons of the solid. We derive the quadratic action for the boundary phonons in the probe limit and show that, when the mixed boundary conditions preserve conformal symmetry, the longitudinal and transverse sound speeds are related to each other as expected from effective field theory arguments. We then include backreaction and calculate the free energy of the solidon for a particular choice of mixed boundary conditions, corresponding to a relevant multi-trace deformation of the boundary theory. We find such free energy to be lower than that of thermal AdS. This suggests that our solidon undergoes a solid-to-liquid first order phase transition by melting into a Schwarzschild-AdS black hole as the temperature is raised.

Anomalous elastic properties across the \u03b3 to \u03b1 volume collapse in cerium

The behavior of the f-electrons in the lanthanides and actinides governs important macroscopic properties but their pressure and temperature dependence is not fully explored. Cerium with nominally just one 4f electron offers a case study with its iso-structural volume collapse from the γ-phase to the α-phase ending in a critical point (pC, VC, TC), unique among the elements, whose mechanism remains controversial. Here, we present longitudinal (cL) and transverse sound speeds (cT) versus pressure from higher than room temperature to TC for the first time. While cL experiences a non-linear dip at the volume collapse, cT shows a step-like change. This produces very peculiar macroscopic properties: the minimum in the bulk modulus becomes more pronounced, the step-like increase of the shear modulus diminishes and the Poisson’s ratio becomes negative—meaning that cerium becomes auxetic. At the critical point itself cerium lacks any compressive strength but offers resistance to shear.

Soft porous silicone rubbers with ultra-low sound speeds in acoustic metamaterials

Soft porous silicone rubbers are demonstrated to exhibit extremely low sound speeds of tens of m/s for these dense materials, even for low porosities of the order of a few percent. Our ultrasonic experiments show a sudden drop of the longitudinal sound speed with the porosity, while the transverse sound speed remains constant. For such porous elastomeric materials, we propose simple analytical expressions for these two sound speeds, derived in the framework of Kuster and Toksöz, revealing an excellent agreement between the theoretical predictions and the experimental results for both longitudinal and shear waves. Acoustic attenuation measurements also complete the characterization of these soft porous materials.

A well-behaved charged anisotropic Tolman VII space–time

We present a new exact solution of charged anisotropic Tolman VII type solution representing compact stars. Here the transverse pressure and sound speed are decreasing in nature and well behaved. These solutions can be used to model both neutron star and quark star within the range of observed masses and radii. For particular values of constant parameters, we present a neutron star of mass 0.924 [Formula: see text] with radius 11.55 km and a quark star of mass 0.7 [Formula: see text] with radius 8.76 km. The model predicts a surface gravitational redshift of 0.145 for a particular choice of constant parameters. Also, the compactness parameter (i.e., M/R = 0.119) is less than 4/9 and satisfies the Buchdahl–Andréasson limit. Indeed, our charged anisotropic solution also satisfies the Cooperstock and De La Cruz condition as M/Q = 13.74 &gt; 1, which is needed for a configuration to be at equilibrium.

Quantitative analysis of temperature dependent acoustic trapping characteristics by using concentric annular type dual element ultrasonic transducer

This paper presents the temperature dependence of lateral acoustic trapping capability by probing the speed of sound in individual lipid droplets at a given temperature of water and measuring its corresponding displacement, a value for quantitatively evaluating a spring-like behavior of the acoustic trap with certain strength. A 20/40MHz dual element LiNbO3 ultrasonic transducer is fabricated to simultaneously perform both transverse trapping and sound speed measurement for each droplet over a discrete temperature range from 20°C to 30°C. Time of flight method is employed for pulse tracking that determines the arrival time of an echo reflected back from either a trapped droplet or a mylar film.The estimated speeds of sound in water and droplets are 1484.8m/s and 1431.6m/s at 20°C, while 1506.0m/s and 1400.6m/s at 30°C, respectively. As the temperature rises, the sound speed in droplets decreases at an average rate of 3.1m/s/°C, and the speed in water increases at 2.1m/s/°C. The average displacement varies from 150.0μm to 179.0μm with an increasing rate of 2.9μm/°C, and its standard deviation is obtained between 1.0μm and 2.0μm over the same temperature range. Reduced sound speed as a function of rising temperature results in increased displacement, indicating that the trapping strength is adjustable by regulating ambient temperature in water as well as by changing transducer excitation parameters. Therefore, the results suggest that the temperature dependence of this trapping technique can be exploited for developing a remote manipulation tool of micron-sized particles in a thermally fluctuating environment. It is also shown that any deviated trapping strength caused by thermal disturbance near the trap can be restored to its desired level by compensating either temperature difference or trapping system condition.

Shear shocks in fragile networks.

A minimal model for studying the mechanical properties of amorphous solids is a disordered network of point masses connected by unbreakable springs. At a critical value of its mean connectivity, such a network becomes fragile: it undergoes a rigidity transition signaled by a vanishing shear modulus and transverse sound speed. We investigate analytically and numerically the linear and nonlinear visco-elastic response of these fragile solids by probing how shear fronts propagate through them. Our approach, which we tentatively label shear front rheology, provides an alternative route to standard oscillatory rheology. In the linear regime, we observe at late times a diffusive broadening of the fronts controlled by an effective shear viscosity that diverges at the critical point. No matter how small the microscopic coefficient of dissipation, strongly disordered networks behave as if they were overdamped because energy is irreversibly leaked into diverging nonaffine fluctuations. Close to the transition, the regime of linear response becomes vanishingly small: the tiniest shear strains generate strongly nonlinear shear shock waves qualitatively different from their compressional counterparts in granular media. The inherent nonlinearities trigger an energy cascade from low to high frequency components that keep the network away from attaining the quasi-static limit. This mechanism, reminiscent of acoustic turbulence, causes a superdiffusive broadening of the shock width.