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Showing papers on "Dispersion relation published in 2015"


Journal ArticleDOI
TL;DR: Linewidth measurements reveal that the lifetime of the magnons is asymmetric with respect to their counter-propagating directions, and the lifetime asymmetry is dependent on the magnon frequency, being more pronounced, the higher the frequency.
Abstract: The interfacial Dzyaloshinskii-Moriya interaction in an in-plane anisotropic Pt(4 nm)/Co(1.6 nm)/Ni(1.6 nm) film has been directly observed by Brillouin spectroscopy. It is manifested as the asymmetry of the measured magnon dispersion relation, from which the Dzyaloshinskii-Moriya interaction constant has been evaluated. Linewidth measurements reveal that the lifetime of the magnons is asymmetric with respect to their counter-propagating directions. The lifetime asymmetry is dependent on the magnon frequency, being more pronounced, the higher the frequency. Analytical calculations of the magnon dispersion relation and linewidth agree well with experiments.

302 citations


Journal ArticleDOI
TL;DR: In this paper, Ghiba et al. showed that the presence of band-gaps is related to a unique elastic coefficient, the so-called Cosserat couple modulus, which is also responsible for the loss of symmetry of the Cauchy force stress tensor.
Abstract: In this paper, the relaxed micromorphic model proposed in Ghiba et al. (Math Mech Solids, 2013), Neff et al. (Contin Mech Thermodyn, 2013) has been used to study wave propagation in unbounded continua with microstructure. By studying dispersion relations for the considered relaxed medium, we are able to disclose precise frequency ranges (band-gaps) for which propagation of waves cannot occur. These dispersion relations are strongly nonlinear so giving rise to a macroscopic dispersive behavior of the considered medium. We prove that the presence of band-gaps is related to a unique elastic coefficient, the so-called Cosserat couple modulus μ c , which is also responsible for the loss of symmetry of the Cauchy force stress tensor. This parameter can be seen as the trigger of a bifurcation phenomenon since the fact of slightly changing its value around a given threshold drastically changes the observed response of the material with respect to wave propagation. We finally show that band-gaps cannot be accounted for by classical micromorphic models as well as by Cosserat and second gradient ones. The potential fields of application of the proposed relaxed model are manifold, above all for what concerns the conception of new engineering materials to be used for vibration control and stealth technology.

139 citations


Journal ArticleDOI
TL;DR: The lack of Galilean invariance in the spin-orbit-coupled Bose-Einstein condensate loaded into a translating optical lattice is experimentally demonstrated, which leads to anisotropic behavior of the condensates depending on the direction of translation of the lattice.
Abstract: We investigate a spin-orbit-coupled Bose-Einstein condensate loaded into a translating optical lattice. We experimentally demonstrate the lack of Galilean invariance in the spin-orbit-coupled system, which leads to anisotropic behavior of the condensate depending on the direction of translation of the lattice. The anisotropy is theoretically understood by an effective dispersion relation. We experimentally confirm this theoretical picture by probing the dynamical instability of the system.

122 citations


Journal ArticleDOI
TL;DR: In this paper, a quadruple-degenerate state is achieved at the center of the Brillouin zone in a two-dimensional honeycomb lattice phononic crystal, which is a result of accidental degeneracy of two double-deletion states.
Abstract: Artificial honeycomb lattices with Dirac cone dispersion provide a macroscopic platform to study the massless Dirac quasiparticles and their novel geometric phases. In this paper, a quadruple-degenerate state is achieved at the center of the Brillouin zone in a two-dimensional honeycomb lattice phononic crystal, which is a result of accidental degeneracy of two double-degenerate states. In the vicinity of the quadruple-degenerate state, the dispersion relation is linear. Such quadruple degeneracy is analyzed by rigorous representation theory of groups. Using k·p method, a reduced Hamiltonian is obtained to describe the linear Dirac dispersion relations of this quadruple-degenerate state, which is well consistent with the simulation results. Near such accidental degeneracy, we observe some unique properties in wave propagating, such as defect-insensitive propagating character and the Talbot effect.

99 citations


Journal ArticleDOI
TL;DR: In this paper, the authors revisited the hydrodynamic limit of the Langmuir wave dispersion relation based on Wigner-Poisson model in connection with that obtained directly from the original Lindhard dielectric function based on the random-phase approximation.
Abstract: In this paper, we revisit the hydrodynamic limit of the Langmuir wave dispersion relation based on the Wigner-Poisson model in connection with that obtained directly from the original Lindhard dielectric function based on the random-phase-approximation. It is observed that the (fourth-order) expansion of the exact Lindhard dielectric constant correctly reduces to the hydrodynamic dispersion relation with an additional term of fourth-order, beside that caused by the quantum diffraction effect. It is also revealed that the generalized Lindhard dielectric theory accounts for the recently discovered Shukla-Eliasson attractive potential (SEAP). However, the expansion of the exact Lindhard static dielectric function leads to a k4 term of different magnitude than that obtained from the linearized quantum hydrodynamics model. It is shown that a correction factor of 1/9 should be included in the term arising from the quantum Bohm potential of the momentum balance equation in fluid model in order for a correct plas...

92 citations


Journal ArticleDOI
29 Jan 2015-Nature
TL;DR: Assuming that Lorentz symmetry holds for electrons and that the photon dispersion relation governs the Coulomb force, a fivefold-improved limit on anisotropies in the speed of light is obtained.
Abstract: All evidence so far suggests that the absolute spatial orientation of an experiment never affects its outcome. This is reflected in the standard model of particle physics by requiring all particles and fields to be invariant under Lorentz transformations. The best-known tests of this important cornerstone of physics are Michelson-Morley-type experiments verifying the isotropy of the speed of light. For matter, Hughes-Drever-type experiments test whether the kinetic energy of particles is independent of the direction of their velocity, that is, whether their dispersion relations are isotropic. To provide more guidance for physics beyond the standard model, refined experimental verifications of Lorentz symmetry are desirable. Here we search for violation of Lorentz symmetry for electrons by performing an electronic analogue of a Michelson-Morley experiment. We split an electron wave packet bound inside a calcium ion into two parts with different orientations and recombine them after a time evolution of 95 milliseconds. As the Earth rotates, the absolute spatial orientation of the two parts of the wave packet changes, and anisotropies in the electron dispersion will modify the phase of the interference signal. To remove noise, we prepare a pair of calcium ions in a superposition of two decoherence-free states, thereby rejecting magnetic field fluctuations common to both ions. After a 23-hour measurement, we find a limit of h × 11 millihertz (h is Planck's constant) on the energy variations, verifying the isotropy of the electron's dispersion relation at the level of one part in 10(18), a 100-fold improvement on previous work. Alternatively, we can interpret our result as testing the rotational invariance of the Coulomb potential. Assuming that Lorentz symmetry holds for electrons and that the photon dispersion relation governs the Coulomb force, we obtain a fivefold-improved limit on anisotropies in the speed of light. Our result probes Lorentz symmetry violation at levels comparable to the ratio between the electroweak and Planck energy scales. Our experiment demonstrates the potential of quantum information techniques in the search for physics beyond the standard model.

89 citations


Journal ArticleDOI
TL;DR: In this paper, it was shown that periodic traveling waves with sufficiently small amplitudes of the Whitham equation, which incorporates the dispersion relation of surface water waves and the nonlinearity of the shallow water equations, are spectrally unstable to long-wavelengths perturbations if the wave number is greater than a critical value, bearing out the Benjamin-Feir instability of Stokes waves.
Abstract: We show that periodic traveling waves with sufficiently small amplitudes of the Whitham equation, which incorporates the dispersion relation of surface water waves and the nonlinearity of the shallow water equations, are spectrally unstable to long-wavelengths perturbations if the wave number is greater than a critical value, bearing out the Benjamin–Feir instability of Stokes waves; they are spectrally stable to square integrable perturbations otherwise. The proof involves a spectral perturbation of the associated linearized operator with respect to the Floquet exponent and the small-amplitude parameter. We extend the result to related, nonlinear dispersive equations.

87 citations


Journal ArticleDOI
TL;DR: In this article, it was shown that a judicious spatial modulation of gain and loss in near zero metamaterials can induce the propagation of exponentially bound interface modes characterized by zero attenuation.
Abstract: Inspired by the parity-time symmetry concept, we show that a judicious spatial modulation of gain and loss in $\ensuremath{\epsilon}$-near-zero metamaterials can induce the propagation of exponentially bound interface modes characterized by zero attenuation. With specific reference to a bilayer configuration, via analytical studies and parametrization of the dispersion equation, we show that this waveguiding mechanism can be sustained in the presence of moderate gain/loss levels, and it becomes leaky (i.e., radiative) below a gain/loss threshold. Moreover, we explore a possible rod-based metamaterial implementation, based on realistic material constituents, which captures the essential features of the waveguiding mechanism, in good agreement with our theoretical predictions. Our results may open up possibilities for the design of optical devices and reconfigurable nanophotonics platforms.

73 citations


Journal ArticleDOI
TL;DR: In this paper, the surface gravity wave interaction with submerged horizontal flexible porous plate under the assumption of small amplitude water wave theory and structural response was studied by analyzing the complex dispersion relation using contour plots.

71 citations


Journal ArticleDOI
TL;DR: In this article, the authors derived a dispersion equation with appropriate boundary conditions for total pressure (including centrifugal force of tube rotation), which governs the dynamics of incompressible jets and obtained analytical instability criteria of KHI in various cases.
Abstract: Observations show various jets in the solar atmosphere with significant rotational motions, which may undergo instabilities leading to heat ambient plasma. We study the Kelvin–Helmholtz instability (KHI) of twisted and rotating jets caused by the velocity jumps near the jet surface. We derive a dispersion equation with appropriate boundary conditions for total pressure (including centrifugal force of tube rotation), which governs the dynamics of incompressible jets. Then, we obtain analytical instability criteria of KHI in various cases, which were verified by numerical solutions to the dispersion equation. We find that twisted and rotating jets are unstable to KHI when the kinetic energy of rotation is more than the magnetic energy of the twist. Our analysis shows that the azimuthal magnetic field of 1–5 G can stabilize observed rotations in spicule/macrospicules and X-ray/extreme-ultraviolet (EUV) jets. On the other hand, nontwisted jets are always unstable to KHI. In this case, the instability growth time is several seconds for spicule/macrospicules and a few minutes (or less) for EUV/X-ray jets. We also find that standing kink and torsional Alfven waves are always unstable near the antinodes, owing to the jump of azimuthal velocity at the surface, while the propagating waves are generally stable. Kelvin–Helmholtzmore » (KH) vortices may lead to enhanced turbulence development and heating of surrounding plasma; therefore, rotating jets may provide energy for chromospheric and coronal heating.« less

71 citations


Journal ArticleDOI
TL;DR: In this paper, a comparative analysis of the electron whistler-cyclotron and firehose fluctuations based upon anisotropic plasma modeled with Maxwellian and Tsallis-kappa-like particle distributions is presented.
Abstract: Observed electron velocity distributions in the Earth's magnetosphere and the solar wind exhibit a variety of nonthermal features which deviate from thermal equilibrium, for example, in the form of temperature anisotropies, suprathermal tail extensions, and field-aligned beams. The state close to thermal equilibrium and its departure from it provides a source for spontaneous emissions of electromagnetic fluctuations, such as the whistler. Here we present a comparative analysis of the electron whistler-cyclotron and firehose fluctuations based upon anisotropic plasma modeled with Maxwellian and Tsallis-kappa-like particle distributions, to explain the correspondence relationship of the magnetic fluctuations as a function of the electron temperature and thermal anisotropy in the solar wind and magnetosphere plasmas. The analysis presented here considers correlation theory of the fluctuation-dissipation theorem and the dispersion relation of transverse fluctuations, with wave vectors parallel to the uniform background magnetic field, in a finite temperature anisotropic thermal bi-Maxwellian and nonthermal Tsallis-kappa-like magnetized electron-proton plasma. Dispersion analysis and stability thresholds are derived for these thermal and nonthermal distributions using plasma and field parameters relevant to the solar wind and magnetosphere environments. Our results indicate that there is an enhancement of the fluctuations level in the case of nonthermal distributions due to the effective higher temperature and the excess of suprathermal particles. These results suggest that a comparison of the electromagnetic fluctuations due to thermal and nonthermal distributions provides a diagnostic signature by which inferences about the nature of the particle velocity distribution function can be ascertained without in situ particle measurements.

Journal ArticleDOI
TL;DR: In this article, the propagation of shear-horizontal waves near the surface of a piezoelectric semiconductor half-space of crystals of class 6mm with the presence of a biasing electric field in the propagation direction was studied.
Abstract: We study the propagation of shear-horizontal waves near the surface of a piezoelectric semiconductor half-space of crystals of class 6 mm with the presence of a biasing electric field in the propagation direction. The three-dimensional equations of linear piezoelectric semiconductors are used. A transcendental equation that determines the dispersion relation is obtained and solved numerically. Results show that the semiconduction affects the wave speed and causes wave dispersion as well as attenuation, and that the waves can be amplified by the biasing electric field.

Journal ArticleDOI
TL;DR: Raman active, zero-momentum inter-Landau level excitations in graphene are sensitive to electron-electron interactions due to the nonapplicability of the Kohn theorem in this system, with a clearly nonparabolic dispersion relation.
Abstract: We present magneto-Raman scattering studies of electronic inter-Landau level excitations in quasineutral graphene samples with different strengths of Coulomb interaction. The band velocity associated with these excitations is found to depend on the dielectric environment, on the index of Landau level involved, and to vary as a function of the magnetic field. This contradicts the single-particle picture of noninteracting massless Dirac electrons but is accounted for by theory when the effect of electron-electron interaction is taken into account. Raman active, zero-momentum inter-Landau level excitations in graphene are sensitive to electron-electron interactions due to the nonapplicability of the Kohn theorem in this system, with a clearly nonparabolic dispersion relation.

Journal ArticleDOI
TL;DR: In this paper, a general strategy of analysis of the effects of modifications of the dispersion relation in Friedmann-Robertson-Walker spacetimes, applicable both to cases where the relativistic equivalence of frames is spoiled (preferred-frame scenarios'') and to the alternative possibility of ''DSR-relativistic theories,'' theories that are fully relativist but with relativists laws deformed so that the modified dispersion relations is observer independent.
Abstract: In recent years, Planck-scale modifications of the dispersion relation have been attracting increasing interest also from the viewpoint of possible applications in astrophysics and cosmology, where spacetime curvature cannot be neglected. Nonetheless, the interplay between Planck-scale effects and spacetime curvature is still poorly understood, particularly in cases where curvature is not constant. These challenges have been so far postponed by relying on an ansatz, first introduced by Jacob and Piran. We propose here a general strategy of analysis of the effects of modifications of the dispersion relation in Friedmann-Robertson-Walker spacetimes, applicable both to cases where the relativistic equivalence of frames is spoiled (``preferred-frame scenarios'') and to the alternative possibility of ``DSR-relativistic theories,'' theories that are fully relativistic but with relativistic laws deformed so that the modified dispersion relation is observer independent. We show that the Jacob-Piran ansatz implicitly assumes that spacetime translations are not affected by the Planck scale, while under rather general conditions, the same Planck-scale quantum-spacetime structures producing modifications of the dispersion relation also affect translations. Through the explicit analysis of one of the effects produced by modifications of the dispersion relation, an effect amounting to Planck-scale corrections to travel times, we show that our concerns are not merely conceptual but rather can have significant quantitative implications.

Journal ArticleDOI
TL;DR: In this paper, the authors derived a dispersion equation with appropriate boundary condition for total pressure (including centrifugal force of tube rotation), which governs the dynamics of incompressible jets.
Abstract: Observations show various jets in the solar atmosphere with significant rotational motions, which may undergo instabilities leading to heat ambient plasma. We study the Kelvin-Helmholtz (KH) instability of twisted and rotating jets caused by the velocity jumps near the jet surface. We derive a dispersion equation with appropriate boundary condition for total pressure (including centrifugal force of tube rotation), which governs the dynamics of incompressible jets. Then, we obtain analytical instability criteria of Kelvin-Helmholtz instability in various cases, which were verified by numerical solutions to the dispersion equation. We find that twisted and rotating jets are unstable to KH instability when the kinetic energy of rotation is more than the magnetic energy of the twist. Our analysis shows that the azimuthal magnetic field of 1-5 G can stabilize observed rotations in spicule/macrospicules and X-ray/EUV jets. On the other hand, non-twisted jets are always unstable to KH instability. In this case, the instability growth time is several seconds for spicule/macrospicules and few minutes (or less) for EUV/X-ray jets. We also find that standing kink and torsional Alfven waves are always unstable near the antinodes due to the jump of azimuthal velocity at the surface, while the propagating waves are generally stable. KH vortices may lead to enhanced turbulence development and heating of surrounding plasma, therefore rotating jets may provide energy for chromospheric and coronal heating.

Journal ArticleDOI
TL;DR: In this paper, the dynamics of an isolated tip vortex cavity generated at the tip of a wing of an elliptical planform was analyzed using high-speed video shadowgraphy.
Abstract: The dynamic behaviour of vortex cavitation on marine propellers may cause inboard noise and vibration, but is not well understood. The main goal of the present study is to experimentally analyse the dynamics of an isolated tip vortex cavity generated at the tip of a wing of elliptical planform. Detailed high-speed video shadowgraphy was used to determine the cavity deformations in combination with force and sound measurements. The cavity deformations can be divided in different modes, each of which show a distinct dispersion relation between frequency and wavenumber. The dispersion relations show good agreement with an analytical formulation. Finally, experimental support is given to the hypothesis that the resonance frequency of the cavity volume variation is related to a zero group velocity.

Journal ArticleDOI
TL;DR: In this article, a membrane-type resonator consisting of a tensioned elastic membrane and a mass block attached to the center of the membrane was proposed to predict the dispersion relation, band gaps and eigen-modes.
Abstract: This paper deals with flexural wave band gaps in metamaterial beams with membrane-type resonators. The proposed membrane-type resonator consists of a tensioned elastic membrane and a mass block attached to the center of the membrane. Numerical models based on finite element method are presented to predict the dispersion relation, band gaps and eigen-modes. It has shown that the metamaterial beams exhibit unique wave physics. A broad Bragg band gap (BBG) and two low-frequency locally resonant band gaps (LRBGs) can be observed due to the structural periodicity and locally resonant behavior respectively. The first LRBG can be ascribed to the combined resonance of the membranes and the masses, while the second LRBG is caused by the resonance of the membranes. The study of the effective property shows that negative mass density occurs in the LRBGs. The effects of membrane tension and mass magnitude (the weight of mass block) on the LRBGs are further analyzed. It is shown that both the two LRBGs move to high-frequency with the increase of the membrane tension. However, as the mass magnitude increases, the first LRBG moves to low-frequency and the second LRBG almost remains unchanged. It is further demonstrated that, when a larger unit cell with multiple kinds of masses (a larger unit cell incorporating multiple basic unit cells but with different weights of mass blocks within each basic unit cell) are used, the first LRBG can be broadened, which can be employed to achieve broadband vibration attenuation. Moreover, experimental measurements of vibration transmittance are conducted to validate the theoretical predictions. Good agreements between the experimental results and the theoretical predictions are observed.

Journal ArticleDOI
TL;DR: In this paper, a subtracted dispersion relation formalism was applied to improve predictions for the two-photon exchange corrections to elastic electron-proton scattering observables at finite momentum transfers.
Abstract: We apply a subtracted dispersion relation formalism with the aim to improve predictions for the two-photon exchange corrections to elastic electron-proton scattering observables at finite momentum transfers. We study the formalism on the elastic contribution, and make a detailed comparison with existing data for unpolarized cross sections as well as polarization transfer observables.

Journal ArticleDOI
TL;DR: The degree of anisotropy of turbulence as a function of frequency and spatial scale is quantified and the relevance of this theory for rotating turbulence at the moderate Rossby numbers accessible in laboratory experiments is questioned.
Abstract: We present a spatiotemporal analysis of a statistically stationary rotating-turbulence experiment, aiming to extract a signature of inertial waves and to determine the scales and frequencies at which they can be detected. The analysis uses two-point spatial correlations of the temporal Fourier transform of velocity fields obtained from time-resolved stereoscopic particle image velocimetry measurements in the rotating frame. We quantify the degree of anisotropy of turbulence as a function of frequency and spatial scale. We show that this space-time-dependent anisotropy is well described by the dispersion relation of linear inertial waves at large scale, while smaller scales are dominated by the sweeping of the waves by fluid motion at larger scales. This sweeping effect is mostly due to the low-frequency quasi-two-dimensional component of the turbulent flow, a prominent feature of our experiment that is not accounted for by wave-turbulence theory. These results question the relevance of this theory for rotating turbulence at the moderate Rossby numbers accessible in laboratory experiments, which are relevant to most geophysical and astrophysical flows.

Journal ArticleDOI
TL;DR: A new set of linearly stable high-order FR schemes is proposed that minimizes wave propagation errors for the range of resolvable wavenumbers and provides considerably reduced error for advection in comparison to the Discontinuous Galerkin scheme.
Abstract: Modal analysis of the flux reconstruction (FR) formulation is performed to obtain the semi-discrete and fully-discrete dispersion relations, using which, the wave properties of physical as well as spurious modes are characterized. The effect of polynomial order, correction function and solution points on the dispersion, dissipation and relative energies of the modes are investigated. Using this framework, a new set of linearly stable high-order FR schemes is proposed that minimizes wave propagation errors for the range of resolvable wavenumbers. These schemes provide considerably reduced error for advection in comparison to the Discontinuous Galerkin scheme and benefit from having an explicit differential update. The corresponding resolving efficiencies compare favorably to those of standard high-order compact finite difference schemes. These theoretical expectations are verified by a comparison of proposed and existing FR schemes in advecting a scalar quantity on uniform as well as non-uniform grids.

Journal ArticleDOI
TL;DR: In this paper, a theory of gradient index devices in plates that allow the simultaneous control of both S0 and A0 Lamb modes is proposed, which is in contrast to the existing approaches that are mainly limited to the manipulation of only the lowest A0 modes.
Abstract: We propose a theory of gradient index devices in plates that allow the simultaneous control of both S0 and A0 Lamb modes. This is in contrast to the existing approaches that are mainly limited to the manipulation of only the lowest A0 modes. These devices are based on phononic crystal plates, which are studied in the low frequency (homogenization) limit. We demonstrate a direct relationship between the dispersion relation of these two modes in phononic crystal plates that, together with the thickness dependence of the dispersion relation of the A0 mode, allows their simultaneous control. As a matter of illustration, a flat gradient index lens and a circular Luneburg lens are designed by means of the simultaneous variation of the inclusions' radii and the thickness of the plate. Numerical simulations show that the performance of these devices is good for the two modes in a broadband frequency region and that this approach can be used to design more advanced refractive devices for the total control of guide...

Journal ArticleDOI
TL;DR: In this paper, the authors report on the wave transmission characteristics of a hybrid one dimensional medium, which is the result of coupling between a 1D mechanical waveguide in the form of an elastic beam, supporting the propagation of transverse waves and a discrete electrical transmission line, consisting of a series of inductors connected to ground through capacitors.
Abstract: In this paper, we report on the wave transmission characteristics of a hybrid one dimensional (1D) medium. The hybrid characteristic is the result of the coupling between a 1D mechanical waveguide in the form of an elastic beam, supporting the propagation of transverse waves and a discrete electrical transmission line, consisting of a series of inductors connected to ground through capacitors. The capacitors correspond to a periodic array of piezoelectric patches that are bonded to the beam and that couple the two waveguides. The coupling leads to a hybrid medium that is characterized by a coincidence condition for the frequency/wavenumber value corresponding to the intersection of the branches of the two waveguides. In the frequency range centered at coincidence, the hybrid medium features strong attenuation of wave motion as a result of the energy transfer towards the electrical transmission line. This energy transfer, and the ensuing attenuation of wave motion, is alike the one obtained through internal resonating units of the kind commonly used in metamaterials. However, the distinct shape of the dispersion curves suggests how this energy transfer is not the result of a resonance and is therefore fundamentally different. This paper presents the numerical investigation of the wave propagation in the considered media, it illustrates experimental evidence of wave transmission characteristics and compares the performance of the considered configuration with that of internal resonating metamaterials. In addition, the ability to conveniently tune the dispersion properties of the electrical transmission line is exploited to adapt the periodicity of the domain and to investigate diatomic periodic configurations that are characterized by a richer dispersion spectrum and broader bandwidth of wave attenuation at coincidence. The medium consisting of mechanical, piezoelectric, and analog electronic elements can be easily interfaced to digital devices to offer a novel approach to smart materials.

Journal ArticleDOI
TL;DR: In this article, the authors investigated phonon transport in perovskite strontium titanate (SrTiO3), which is stable above its phase transition temperature (~105 K), by using first-principles molecular dynamics and anharmonic lattice dynamics.
Abstract: We investigate phonon transport in perovskite strontium titanate (SrTiO3), which is stable above its phase transition temperature (~105 K), by using first-principles molecular dynamics and anharmonic lattice dynamics. Unlike conventional ground-state-based perturbation methods that give imaginary phonon frequencies, the current calculation reproduces stable phonon dispersion relations observed in experiments. We find that the contribution of optical phonons to the overall lattice thermal conductivity is larger than 60%, which is markedly different from the usual picture with the dominant contribution from acoustic phonons. Mode-dependent and pseudopotential-dependent analyses suggest the strong attenuation of acoustic phonon transport originating from strong anharmonic coupling with the transversely polarized ferroelectric modes.

Journal ArticleDOI
TL;DR: In this paper, the authors present a review of the challenges and opportunities in the field of graded magnonic wave solutions of the Landau-Lifshitz equation (spin waves).
Abstract: The wave solutions of the Landau–Lifshitz equation (spin waves) are characterized by some of the most complex and peculiar dispersion relations among all waves. For example, the spin-wave (“magnonic”) dispersion can range from the parabolic law (typical for a quantum-mechanical electron) at short wavelengths to the nonanalytical linear type (typical for light and acoustic phonons) at long wavelengths. Moreover, the long-wavelength magnonic dispersion has a gap and is inherently anisotropic, being naturally negative for a range of relative orientations between the effective field and the spin-wave wave vector. Nonuniformities in the effective field and magnetization configurations enable the guiding and steering of spin waves in a deliberate manner and therefore represent landscapes of graded refractive index (graded magnonic index). By analogy to the fields of graded-index photonics and transformation optics, the studies of spin waves in graded magnonic landscapes can be united under the umbrella of the graded-index magnonics theme and are reviewed here with focus on the challenges and opportunities ahead of this exciting research direction.

Journal ArticleDOI
TL;DR: In this paper, a theory of helicons propagating through a 3D Weyl semimetal is presented, which relies on the evaluation of the optical conductivity tensor from semiclassical Boltzmann transport theory, with the inclusion of certain Berry curvature corrections.
Abstract: Helicons are transverse electromagnetic waves propagating in three-dimensional (3D) electron systems subject to a static magnetic field. We present a theory of helicons propagating through a 3D Weyl semimetal. Our approach relies on the evaluation of the optical conductivity tensor from semiclassical Boltzmann transport theory, with the inclusion of certain Berry curvature corrections that have been neglected in the earlier literature (such as the one due to the orbital magnetic moment). We demonstrate that the axion term characterizing the electromagnetic response of Weyl semimetals dramatically alters the helicon dispersion with respect to that in nontopological metals. We also discuss axion-related anomalies that appear in the plasmon dispersion relation.

Journal ArticleDOI
TL;DR: In this paper, a frequency comb source based on a mid-infrared quantum cascade laser at λ∼ 9μm with high power output was investigated. But the authors did not consider the effect of interference on the spectrum.
Abstract: We investigate a frequency comb source based on a mid-infrared quantum cascade laser at λ ∼ 9 μm with high power output. A broad flat-top gain with near-zero group velocity dispersion has been engineered using a dual-core active region structure. This favors the locking of the dispersed Fabry-Perot modes into equally spaced frequency lines via four wave mixing. A current range with a narrow intermode beating linewidth of 3 kHz is identified with a fast detector and spectrum analyzer. This range corresponds to a broad spectral coverage of 65 cm−1 and a high power output of 180 mW for ∼176 comb modes.

Posted Content
TL;DR: In this article, the authors prove wave breaking in the nonlinear nonlocal equation which combines the dispersion relation of water waves and a nonlinearity of the shallow water equations, provided that the slope of the initial datum is sufficiently negative, whereby they solve a Whitham's conjecture.
Abstract: We prove wave breaking --- bounded solutions with unbounded derivatives --- in the nonlinear nonlocal equation which combines the dispersion relation of water waves and a nonlinearity of the shallow water equations, provided that the slope of the initial datum is sufficiently negative, whereby we solve a Whitham's conjecture. We extend the result to equations of Korteweg-de Vries type for a range of fractional dispersion.

Journal ArticleDOI
25 Sep 2015-Chaos
TL;DR: For coupled NLS equations, rogue waves will arise even if dispersion and nonlinearity are of opposite signs in each component as new regimes of modulation instability will appear in the coupled system, as demonstrated here for a coupled "AB" system.
Abstract: Rogue waves are unexpectedly large and localized displacements from an equilibrium position or an otherwise calm background. For the nonlinear Schrodinger (NLS) model widely used in fluid mechanics and optics, these waves can occur only when dispersion and nonlinearity are of the same sign, a regime of modulation instability. For coupled NLS equations, rogue waves will arise even if dispersion and nonlinearity are of opposite signs in each component as new regimes of modulation instability will appear in the coupled system. The same phenomenon will be demonstrated here for a coupled “AB” system, a wave-current interaction model describing baroclinic instability processes in geophysical flows. Indeed, the onset of modulation instability correlates precisely with the existence criterion for rogue waves for this system. Transitions from “elevation” rogue waves to “depression” rogue waves are elucidated analytically. The dispersion relation as a polynomial of the fourth order may possess double pairs of complex roots, leading to multiple configurations of rogue waves for a given set of input parameters. For special parameter regimes, the dispersion relation reduces to a cubic polynomial, allowing the existence criterion for rogue waves to be computed explicitly. Numerical tests correlating modulation instability and evolution of rogue waves were conducted.

Journal ArticleDOI
TL;DR: In this paper, the authors investigated the propagation of linear and nonlinear dust acoustic waves in a homogeneous unmagnetized, collisionless and dissipative dusty plasma consisted of extremely massive, micron-sized, negative dust grains.
Abstract: The propagation of linear and nonlinear dust acoustic waves in a homogeneous unmagnetized, collisionless and dissipative dusty plasma consisted of extremely massive, micron-sized, negative dust grains has been investigated. The Boltzmann distribution is suggested for electrons whereas vortex-like distribution for ions. In the linear analysis, the dispersion relation is obtained, and the dependence of damping rate of the waves on the carrier wave number \(k\), the dust kinematic viscosity coefficient \(\eta _{d}\) and the ratio of the ions to the electrons temperatures \(\sigma _{i}\) is discussed. In the nonlinear analysis, the modified Korteweg–de Vries–Burgers (mKdV–Burgers) equation is derived via the reductive perturbation method. Bifurcation analysis is discussed for non-dissipative system in the absence of Burgers term. In the case of dissipative system, the tangent hyperbolic method is used to solve mKdV–Burgers equation, and yield the shock wave solution. The obtained results may be helpful in better understanding of waves propagation in the astrophysical plasmas as well as in inertial confinement fusion laboratory plasmas.

Journal ArticleDOI
TL;DR: Pendry et al. as discussed by the authors demonstrate with the finite-difference time-domain method that radiative heat transfer between two parallel gold plates can be significantly enhanced by engraving periodic grooves with a subwavelength width on the plate surfaces.
Abstract: We demonstrate with the finite-difference time-domain method that radiative heat transfer between two parallel gold plates can be significantly enhanced by engraving periodic grooves with a subwavelength width on the plate surfaces. The enhancement increases with a decrease in the separation distance at near-field regime and it can be further efficiently improved by having a supercell with multiple grooves with different depths. We attribute this near-field enhancement to coupling of thermally excited spoof surface plasmon polaritons, a type of artificial surface wave inherent to structured metal surfaces [J. B. Pendry, L. Martin-Moreno, and F. J. Garcia-Vidal, Science 305, 847 (2004)]. The frequency-dependent contribution to the heat transfer, or transmission-factor spectrum, is confirmed by calculating the dispersion relation of guided modes by the two parallel corrugated plates through a finite-element method. Especially, the photonic density of states derived from the dispersion relation is found to have excellent agreement to the transmission-factor spectrum.