Showing papers on "Dispersion relation published in 2008"
TL;DR: The phonon dispersion relation in the phonon glass-electron crystal material Ba(8)Ga(16)Ge(30) using neutron triple-axis spectroscopy is investigated, disclosing unambiguously the theoretically predicted avoided crossing of the rattler modes and the acoustic-phonon branches.
Abstract: Engineering of materials with specific physical properties has recently focused on the effect of nano-sized ‘guest domains’ in a
‘host matrix’ that enable tuning of electrical, mechanical, photo-optical or thermal properties. A low thermal conductivity is a
prerequisite for obtaining effective thermoelectric materials, and the challenge is to limit the conduction of heat by phonons, without
simultaneously reducing the charge transport. This is named the ‘phonon glass–electron crystal’ concept and may be realized in
host–guest systems. The guest entities are believed to have independent oscillations, so-called rattlermodes,which scatter the acoustic
phonons and reduce the thermal conductivity. We have investigated the phonon dispersion relation in the phonon glass–electron
crystal material Ba8Ga16Ge30 using neutron triple-axis spectroscopy. The results disclose unambiguously the theoretically predicted
avoided crossing of the rattler modes and the acoustic-phonon branches. The observed phonon lifetimes are longer than expected,
and a new explanation for the low L is provided.
517 citations
TL;DR: In this article, a homogenization model was developed to characterize textured surfaces formed by a periodic arrangement of thin metallic pins attached to a conducting ground plane: the ldquoFakir's bed of nailsrdquo substrate.
Abstract: We develop a homogenization model to characterize textured surfaces formed by a periodic arrangement of thin metallic pins attached to a conducting ground plane: the ldquoFakir's bed of nailsrdquo substrate. It is demonstrated that the textured surface can be accurately modeled using a dielectric function, provided spatial dispersion effects are considered as well as additional boundary conditions. We derive closed analytical formulas for the reflection coefficient, and for the dispersion characteristic of the surfaces waves. In addition, it is demonstrated that the artificial substrate may mimic almost exactly the behavior of an ideal impedance surface boundary, and that the only physical factor that may limit this remarkable property is the skin depth of the metal. The reported results are supported by full wave simulations as well as by experimental data.
222 citations
TL;DR: In this paper, the authors investigated the dispersion relation and Landau damping of ion acoustic waves in the collisionless magnetic-field-free plasma when the plasma is described by the nonextensive q -distributions of Tsallis statistics.
Abstract: We investigate the dispersion relation and Landau damping of ion acoustic waves in the collisionless magnetic-field-free plasma when the plasma is described by the nonextensive q -distributions of Tsallis statistics. We show that the increased numbers of superthermal particles and low velocity particles can explain the strengthened and weakened modes of Landau damping, respectively, with the q -distribution. When the ion temperature is equal to the electron temperature, the weakly damped waves are found to be the distributions with small values of q .
193 citations
TL;DR: In this article, the authors investigated the plane wave propagation in hexagonal and reentrant lattices through the application of Bloch's theorem and derived two-dimensional dispersion relations to illustrate peculiar properties of re-entrant configurations and investigate the directional behavior of hexagonal lattices for varying construction angles.
188 citations
TL;DR: In this paper, an analytic, kinetic model with one-, two-, and three-dimensional particle-in-cell simulations was used to confirm the existence of the cosmic ray current-driven instability in the kinetic regime and determine its saturation mechanisms.
Abstract: The cosmic ray current-driven (CRCD) instability, predicted by Bell (2004), consists of non-resonant, growing plasma waves driven by the electric current of cosmic rays (CRs) that stream along the magnetic field ahead of both relativistic and non-relativistic shocks. Combining an analytic, kinetic model with one-, two-, and three-dimensional particle-in-cell simulations, we confirm the existence of this instability in the kinetic regime and determine its saturation mechanisms. In the linear regime, we show that, if the background plasma is well magnetized, the CRCD waves grow exponentially at the rates and wavelengths predicted by the analytic dispersion relation. The magnetization condition implies that the growth rate of the instability is much smaller than the ion cyclotron frequency. As the instability becomes non-linear, significant turbulence forms in the plasma. This turbulence reduces the growth rate of the field and damps the shortest wavelength modes, making the dominant wavelength, \lambda_d, grow proportional to the square of the field. At constant CR current, we find that plasma acceleration along the motion of CRs saturates the instability at the magnetic field level such that v_A ~ v_{d,cr}, where v_A is the Alfven velocity in the amplified field, and v_{d,cr} is the drift velocity of CRs. The instability can also saturate earlier if CRs get strongly deflected by the amplified field, which happens when their Larmor radii get close to \lambda_d. We apply these results to the case of CRs in the upstream medium of supernova remnants. Considering only the most energetic CRs that escape from the shock, we obtain that the field amplification factor of ~10 can be reached. This confirms the CRCD instability as a potentially important component of magnetic amplification process in astrophysical shocks.
177 citations
TL;DR: Direct observation of the strong exciton-photon coupling in a ZnO tapered whispering gallery (WG) microcavity at room temperature is reported, providing a comprehensive picture for coupling of different excitons with differently polarized WG modes.
Abstract: We report direct observation of the strong exciton-photon coupling in a ZnO tapered whispering gallery (WG) microcavity at room temperature. By scanning excitations along the tapered arm of the ZnO tetrapod using a micro-photoluminescence spectrometer with different polarizations, we observed a transition from the pure WG optical modes in the weak interaction regime to the excitonic polariton in the strong coupling regime. The experimental observations are well described by using the plane wave model including the excitonic polariton dispersion relation. This provides a direct mapping of the polariton dispersion, and thus a comprehensive picture for coupling of different excitons with differently polarized WG modes.
168 citations
TL;DR: In this article, the properties of single layer graphene have been investigated and the existence of unusual charge car-riers, analogous to massless, chiral Dirac particles having a Berry's phase ofand resulting in a new, half-integer quantum Hall effect.
Abstract: Experiments into the properties of single layer graphene have demonstrated the existence of unusual charge car- riers, analogous to massless, chiral Dirac particles having a Berry's phase ofand resulting in a new, half-integer quantum Hall effect (1,2). Bilayer graphene has shown equally exciting properties, creating a system of massive chiral particles with a Berry's phase of 2� and exhibiting another, distinct quantum Hall effect (3). Both phenomena derive from the unusual energy dispersion of graphene leading to the presence of a distinctive Landau level (LL) at zero energy (4 -7). The peculiar dispersion relations of graphene create unique LL spectra in the presence of a magnetic field, B. In single layer graphene, the linear dispersion leads to distinctive ���
164 citations
TL;DR: In this paper, the authors explain a method for computing the bulk viscosity of strongly coupled thermal plasmas dual to supergravity backgrounds supported by one scalar field, which requires only the leading frequency dependence of an appropriate Green's function in the low-frequency limit.
Abstract: We explain a method for computing the bulk viscosity of strongly coupled thermal plasmas dual to supergravity backgrounds supported by one scalar field. Whereas earlier investigations required the computation of the leading dissipative term in the dispersion relation for sound waves, our method requires only the leading frequency dependence of an appropriate Green's function in the low-frequency limit. With a scalar potential chosen to mimic the equation of state of QCD, we observe a slight violation of the lower bound on the ratio of the bulk and shear viscosities conjectured in [1].
163 citations
TL;DR: In this article, a new derivation of the quantum-gravitational vacuum refraction index was proposed based on a stringy analogue of the interaction of a photon with internal degrees of freedom in a conventional medium.
157 citations
TL;DR: Calculations within the standard formalism demonstrate that the phonon-induced renormalization of the electron energies and the Fermi velocity is almost one order of magnitude smaller than the effect observed in photoemission experiments, which rules out electron–phonon interaction in bulk La2 - xSrxCuO4 as the exclusive origin of the measured kink.
Abstract: This paper reports first-principles calculations of the role of phonons in La2−xSrxCuO4 (LSCO). It is demonstrated that the phonon-induced renormalization of the electron energies and the Fermi velocity is almost one order of magnitude smaller than the effect observed in photoemission experiments. Therefore the present finding rules out electron–phonon interaction in bulk LSCO as the exclusive origin of the measured kink. Despite over two decades of intense research efforts, the origin of high-temperature superconductivity in the copper oxides remains elusive. Angle-resolved photoemission spectroscopy experiments1,2 have revealed a kink in the dispersion relations (energy versus wavevector) of electronic states in the copper oxides at binding energies of 50-80 meV, raising the hope that this anomaly could be a key to understanding high-temperature superconductivity. The kink is often interpreted in terms of interactions between the electrons and a bosonic field. Although there is no consensus on the nature of the bosons (or even whether a boson model is appropriate), both phonons1 and spin fluctuations2 have been proposed as possible candidates. Here we report first-principles calculations of the role of phonons and the electron–phonon interaction in the photoemission spectra of La2 - xSr x CuO4. Our calculations within the standard formalism demonstrate that the phonon-induced renormalization of the electron energies and the Fermi velocity is almost one order of magnitude smaller than the effect observed in photoemission experiments. Therefore, our result rules out electron–phonon interaction in bulk La2 - xSr x CuO4 as the exclusive origin of the measured kink. Our conclusions are consistent with those reached independently in a recent study3 of the related compound YBa2Cu3O7.
149 citations
TL;DR: In this paper, the authors studied the numerical dispersion of spectral element methods of arbitrary order for the isotropic elastic wave equation in two and three dimensions by a simplified modal analysis of the discrete wave operators.
TL;DR: Simulations using 3D finite-element method calculations reveal harmonic sets of longitudinal standing waves in cavities between nanorods, reminiscent of acoustic waves generated by musical instruments.
Abstract: Hexagonal 2D arrays of Au nanorods support discrete plasmon resonance modes at visible and near-infrared wavelengths when coupled with light at normal incidence (kz). Reflectance spectra of nanorod arrays mounted on a thin Au baseplate reveal multiple resonant attenuations whose spectral positions vary with nanorod height and the dielectric medium. Simulations using 3D finite-element method calculations reveal harmonic sets of longitudinal standing waves in cavities between nanorods, reminiscent of acoustic waves generated by musical instruments. The nodes and antinodes of these quarter-wave plasmon modes are bounded, respectively, at the base and tips of the array. The number of harmonic resonances and their frequencies can be adjusted as a function of nanorod height, diameter-spacing ratio, and the refractive index of the host medium. Dispersion relations based on these standing-wave modes show strong retardation effects, attributed to the coupling of nanorods via transverse modes. Removal of the metal ...
TL;DR: It is shown that tailoring the geometry of the microtube bottle resonators enables the realization of a wide variety of mode distributions and dispersion relations.
Abstract: We report on the realization of optical microtube resonators with a bottlelike geometry. The measured eigenenergies and the measured axial field distributions of the modes can be described by a straight and intuitive model using an adiabatic separation of the circulating and the axial propagation. The dispersion of the axial mode energies follows a photonic quasi-Schrodinger equation including a quasipotential which can be determined for the actual geometry of the microtube in a precise and simple way. We show that tailoring the geometry of the microtube bottle resonators enables the realization of a wide variety of mode distributions and dispersion relations.
TL;DR: In this article, the authors explain a method for computing the bulk viscosity of strongly coupled thermal plasmas dual to supergravity backgrounds supported by one scalar field, which requires only the leading frequency dependence of an appropriate Green's function in the low-frequency limit.
Abstract: We explain a method for computing the bulk viscosity of strongly coupled thermal plasmas dual to supergravity backgrounds supported by one scalar field. Whereas earlier investigations required the computation of the leading dissipative term in the dispersion relation for sound waves, our method requires only the leading frequency dependence of an appropriate Green's function in the low-frequency limit. With a scalar potential chosen to mimic the equation of state of QCD, we observe a slight violation of the lower bound on the ratio of the bulk and shear viscosities conjectured in arXiv:0708.3459.
TL;DR: The onset of rectification in the system is directly related to the nonlinearity of the electron gas dispersion relation, which is relevant for understanding radiative heat transfer and vibrational energy flow in electrically insulating molecular junctions.
Abstract: We study heat transfer between conductors, mediated by the excitation of a monomodal harmonic oscillator. Using a simple model, we show that the onset of rectification in the system is directly related to the nonlinearity of the electron gas dispersion relation. When the metals have a strictly linear dispersion relation, a Landauer-type expression for the thermal current holds, symmetric with respect to the temperature difference. Rectification becomes prominent when deviations from linear dispersion exist, and the fermionic model cannot be mapped into a harmonic bosonized representation. The effects described here are relevant for understanding radiative heat transfer and vibrational energy flow in electrically insulating molecular junctions.
TL;DR: In this article, the phase of high-order harmonic generation from transiently aligned molecules was measured using extreme ultraviolet interferometry and the dispersion relation of the returning electron wave packet as a function of harmonic order was analyzed.
Abstract: We use extreme-ultraviolet interferometry to measure the phase of high-order harmonic generation from transiently aligned ${\mathrm{CO}}_{2}$ molecules. We unambiguously observe a reversal in phase of the high-order harmonic emission for higher harmonic orders with a sufficient degree of alignment. This results from molecular-scale quantum interferences between the molecular electronic wave function and the recolliding electron as it recombines with the molecule, and is consistent with a two-center model. Furthermore, using the combined harmonic intensity and phase information, we extract accurate information on the dispersion relation of the returning electron wave packet as a function of harmonic order. This analysis shows evidence of the effect of the molecular potential on the recolliding electron wave.
TL;DR: In this article, the one-loop chiral perturbation theory π π -scattering amplitude and dispersion theory in the form of the inverse amplitude method was used to study the quark-mass dependence of the two lightest resonances of the strong interactions, the f_0(600) (σ) and the ρ meson.
Abstract: We use the one-loop chiral perturbation theory π π -scattering amplitude and dispersion theory in the form of the inverse amplitude method to study the quark-mass dependence of the two lightest resonances of the strong interactions, the f_0(600) (σ) and the ρ meson. As the main results, we find that the rho π π coupling constant is almost quark mass independent and that the ρ mass shows a smooth quark-mass dependence while that of the σ shows a strong nonanalyticity. These findings are important for studies of the meson spectrum on the lattice.
TL;DR: In this article, the full frequency dependence of the refractive index using world-line sigma model techniques together with the Penrose plane wave limit of spacetime in the neighbourhood of a null geodesic is calculated.
TL;DR: In this article, a fracture is modeled as an infinite layer of viscous fluid bounded by two elastic half-spaces with identical parameters, and a simple dispersion equation for wave-propagation velocity is obtained.
Abstract: Stoneley guided waves in a fluid-filled fracture generally have larger amplitudes than other waves, and therefore, their properties need to be incorporated in more realistic models. In this study, a fracture is modeled as an infinite layer of viscous fluid bounded by two elastic half-spaces with identical parameters. For small fracture thickness, I obtain a simple dispersion equation for wave-propagation velocity. This velocity is much smaller than the velocity of a fluid wave in a Biot-type solution, in which fracture walls are assumed to be rigid. At seismic prospecting frequencies and realistic fracture thicknesses, the Stoneley guided wave has wavelengths on the order of several meters and an attenuation Q factor exceeding 10, which indicates the possibility of resonance excitation in fluid-bearing rocks. The velocity and attenuation of Stoneley guided waves are distinctly different at low frequencies for water and oil. The predominant role of fractures in fluid flow at field scales is supported by permeability data showing an increase of several orders of magnitude when compared to values obtained at laboratory scales. These data suggest that Stoneley guided waves should be taken into account in theories describing seismic wave propagation in fluid-saturated rocks.
TL;DR: In this article, a semiclassical polarization potential was used to study the effect of the Coulomb dipole polarizability on the elastic scattering of 6 He on 208 Pb at laboratory energies of 14, 16, 18 and 22 MeV.
TL;DR: The numerical analysis of finite planar metal-insulator-metal waveguide structures using the transfer-matrix formalism reveals both bound and leaky surface plasmon (SP) modes.
Abstract: The numerical analysis of finite planar metal-insulator-metal waveguide structures using the transfer-matrix formalism reveals both bound and leaky surface plasmon (SP) modes. The dispersion relations, propagation lengths and confinement factors of these SP modes are presented. The highest energy SP mode consists of non-radiative (bound) and radiative (leaky) portions separated by a spectral gap. The leaky regime is further divided into antenna and reactive mode regions. The antenna mode may be used for both free-space coupling and beam steering devices.
TL;DR: In this article, a number of existing models for surface wave phase speeds (linear and nonlinear, breaking and non-breaking waves) are reviewed and tested against phase speed data from a large-scale laboratory experiment.
TL;DR: In this paper, the phase and group velocities of piezomagnetic (PM) and piezoelectric (PE) half-space medium are investigated for two cases: a PM layer on a PE halfspace and the reverse configuration.
TL;DR: In this article, the dynamics of receding contact lines is investigated experimentally through controlled perturbations of a meniscus in a dip-coating experiment, and it is shown that the dynamics can be interpreted as a quasi-steady succession of stationary states.
Abstract: The dynamics of receding contact lines is investigated experimentally through controlled perturbations of a meniscus in a dip-coating experiment. We first describe stationary menisci and their breakdown at the coating transition. Above this transition where liquid is deposited, it is found that the dynamics of the interface can be interpreted as a quasi-steady succession of stationary states. This provides the first experimental access to the entire bifurcation diagram of dynamical wetting, confirming the hydrodynamic theory developed in Part 1. In contrast to quasi-static theories based on a dynamic contact angle, we demonstrate that the transition strongly depends on the large-scale flow geometry. We then establish the dispersion relation for large wavenumbers, for which we find a decay rate σ proportional to wavenumber |q|. The speed dependence of σ is described well by hydrodynamic theory, in particular the absence of diverging time scales at the critical point. Finally, we highlight some open problems related to contact angle hysteresis that lead beyond the current description.
TL;DR: In this paper, the authors generalize these constraints to all three Lorentz invariance-breaking parameters that can occur in the dispersion relations for photons, electrons and positrons at first-and second-order suppression with the Planck scale.
Abstract: The propagation of photons, electrons and positrons at ultrahigh energies above $\ensuremath{\sim}{10}^{19}\text{ }\text{ }\mathrm{eV}$ can be changed considerably if the dispersion relations of these particles are modified by terms suppressed by powers of the Planck scale. We recently pointed out that the current nonobservation of photons in the ultrahigh-energy cosmic ray flux at such energies can put strong constraints on such modified dispersion relations. In the present work we generalize these constraints to all three Lorentz invariance-breaking parameters that can occur in the dispersion relations for photons, electrons and positrons at first- and second-order suppression with the Planck scale. We also show how the excluded regions in these three-dimensional parameter ranges would be extended if ultrahigh-energy photons were detected in the future.
TL;DR: In this article, the magneto-inertial wave was identified as the largest contribution to the observed fluctuations, and all of them travel in the retrograde direction in the frame of reference bound to the fluid.
Abstract: The magnetostrophic regime, in which Lorentz and Coriolis forces are in balance, has been investigated in a rapidly rotating spherical Couette flow experiment. The spherical shell is filled with liquid sodium and permeated by a strong imposed dipolar magnetic field. Azimuthally travelling hydromagnetic waves have been put in evidence through a detailed analysis of electric potential differences measured on the outer sphere, and their properties have been determined. Several types of wave have been identified depending on the relative rotation rates of the inner and outer spheres: they differ by their dispersion relation and by their selection of azimuthal wavenumbers. In addition, these waves constitute the largest contribution to the observed fluctuations, and all of them travel in the retrograde direction in the frame of reference bound to the fluid. We identify these waves as magneto-inertial waves by virtue of the close proximity of the magnetic and inertial characteristic time scales of relevance in our experiment.
TL;DR: It is shown that the motion of the plate is indeed a superposition of bending waves following the theoretical dispersion relation of the linear wave equation and the nonlinearities seem to efficiently break the coherence of the waves so that no modal structure is observed.
Abstract: An thin elastic steel plate is excited with a vibrator and its local velocity displays a turbulentlike Fourier spectrum. This system is believed to develop elastic wave turbulence. We analyze here the motion of the plate with a two-point measurement in order to check, in our real system, a few hypotheses required for the Zakharov theory of weak turbulence to apply. We show that the motion of the plate is indeed a superposition of bending waves following the theoretical dispersion relation of the linear wave equation. The nonlinearities seem to efficiently break the coherence of the waves so that no modal structure is observed. Several hypotheses of the weak turbulence theory seem to be verified, but nevertheless the theoretical predictions for the wave spectrum are not verified experimentally.
TL;DR: In this paper, the structure, band structure, total density of states, dielectric function, reflectivity, refractive index and loss function have been calculated for spinel MgAl2O4 oxide using density functional theory.
Abstract: The structure, band structure, total density of states, dielectric function, reflectivity, refractive index and loss function have been calculated for spinel MgAl2O4 oxide using density functional theory. The full-potential linearized augmented plane wave method was used with the generalized gradient approximation. Calculations of the optical spectra have been performed for the energy range 0–40 eV. It is shown that the material is transparent at visible wavelengths and the dispersion curve of the refractive index is fairly flat in the long-wavelength region and rises rapidly towards shorter wavelengths. The refractive index is 1.774 at 800 nm near the visible region. (© 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)
TL;DR: In this paper, the one-electron potential, carrier concentration profile, quantized subband state energies, and parallel dispersion relations are calculated for an accumulation layer at a semiconductor surface by solving Poisson's equation within a modified Thomas-Fermi approximation and numerically solving the Schrodinger equation for the resulting potential well.
Abstract: The one-electron potential, carrier concentration profile, quantized subband state energies, and parallel dispersion relations are calculated for an accumulation layer at a semiconductor surface by solving Poisson's equation within a modified Thomas-Fermi approximation and numerically solving the Schrodinger equation for the resulting potential well. A nonparabolic conduction band, described within the Kane k.p approximation, is incorporated in the model. Example calculations are performed for a typical clean InN surface and for a variety of surface state densities and bulk carrier concentrations. Agreement is found between the model calculations and experimental measurements of the subband energies and dispersions at c-plane InN surfaces from electron tunneling spectroscopy and angle resolved photoemission spectroscopy.
TL;DR: In this article, the vibrational characteristics of single-walled carbon nanotubes (SWNTs) with initial axial loading based on the theory of nonlocal elasticity were investigated.
Abstract: This paper studies the vibrational characteristics of single-walled carbon nanotubes (SWNTs) with initial axial loading based on the theory of nonlocal elasticity. The consistent equations of motion for the nonlocal Euler-Bernoulli and Timoshenko beam models are provided taking into account the initial axial stress. The small scale effect on CNT wave propagation dispersion relation is explicitly revealed for different CNT wave numbers and diameters by theoretical analyses and numerical simulations. In addition, the applicability of the two beam models is explored by numerical simulations. The research work reveals the significance of the effects of small scale, transverse shear deformation and rotary inertia on wave propagation in short SWCNTs with initial axial loading.