Complete Dispersion Relation Research Articles

FREE SURFACE TWO-DIMENSIONAL PERIODIC DISTURBANCES IN VARIOUS MODELS OF THE FLUID

The complete dispersion relations of a flat free surface periodic perturbations with a positive definite frequency and a complex wavenumber taking into account spatial attenuation in a viscous stratified charged liquid were obtained in a linear approximation for the first time by methods of the theory of singular perturbations. Regular components of the complete solution describe plane gravitational-capillary waves. Singular components characterize ligaments – thin flows that are absent in the model of an ideal medium. The obtained dispersion relations in extreme cases uniformly transform into known expressions for inviscid stratified, viscous homogeneous and ideal liquids. The calculated dependencies of the wavelength and thickness of the ligament, the group and phase velocity of the components on the frequency at different values of the media parameters are given.

Dispersion relation analysis of the radiative corrections to gA in the neutron \u03b2-decay

We present the first and complete dispersion relation analysis of the inner radiative corrections to the axial coupling constant gA in the neutron β-decay. Using experimental inputs from the elastic form factors and the spin-dependent structure function g1, we determine the contribution from the γW-box diagram to a precision better than 10−4. Our calculation indicates that the inner radiative corrections to the Fermi and the Gamow-Teller matrix element in the neutron β-decay are almost identical, i.e. the ratio λ = gA/gV is almost unrenormalized. With this result, we predict the bare axial coupling constant to be {overset{circ }{g}}_A=-1.2754{(13)}_{mathrm{exp}}{(2)}_{mathrm{RC}} based on the PDG average λ = −1.2756(13).

Phonon dispersion inMoS2

Transition metal dichalcogenides like MoS$_2$, MoSe$_2$, WS$_2$, and WSe$_2$ have attracted enormous interest during recent years. They are van-der-Waals crystals with highly anisotropic properties, which allows exfoliation of individual layers. Their remarkable physical properties make them promising for applications in optoelectronic, spintronic, and valleytronic devices. Phonons are fundamental to many of the underlying physical processes, like carrier and spin relaxation or exciton dynamics. However, experimental data of the complete phonon dispersion relations in these materials is missing. Here we present the phonon dispersion of bulk MoS$_2$ in the high-symmetry directions of the Brillouin zone, determined by inelastic X-ray scattering. Our results underline the two-dimensional nature of MoS$_2$. Supported by first-principles calculations, we determine the phonon displacement patterns, symmetry properties, and scattering intensities. The results will be the basis for future experimental and theoretical work regarding electron-phonon interactions, intervalley scattering, as well as phonons in related 2D materials.

A first-principles study of the vibrational properties of crystalline tetracene under pressure

We present a comprehensive study of the hydrostatic pressure dependence of the vibrational properties of tetracene using periodic density-functional theory (DFT) within the local density approximation (LDA). Despite the lack of van der Waals dispersion forces in LDA we find good agreement with experiment and are able to assess the suitability of this approach for simulating conjugated organic molecular crystals. Starting from the reported x-ray structure at ambient pressure and low temperature, optimized structures at ambient pressure and under 280 MPa hydrostatic pressure were obtained and the vibrational properties calculated by the linear response method. We report the complete phonon dispersion relation for tetracene crystal and the Raman and infrared spectra at the centre of the Brillouin zone. The intermolecular modes with low frequencies exhibit high sensitivity to pressure and we report mode-specific Grüneisen parameters as well as an overall Grüneisen parameter . Our results suggest that the experimentally reported improvement of the photocurrent under pressure may be ascribed to an increase in intermolecular interactions as also the dielectric tensor.

Complete dispersion relations of surface plasmon polaritons at a metal and birefringent dielectric interface

The complete dispersion relations of transverse magnetic surface plasmon polaritons (TM SPPs) at a metal/birefringent dielectric interface are studied using a rigorous electromagnetic treatment. The exact expression of the wave vector normal to the boundary is derived. The propagation properties and components of the Poynting vectors of four types of TM SPPs are demonstrated with real materials in a wide range of photon energies. On the basis of this analysis, new types of TM SPPs are predicted. Furthermore, a perfect surface wave is realized with real materials in this simple structure. Our results may be useful for plasmonic-based devices associated with anisotropic media.

First-principles study of structural, elastic, electronic and vibrational properties of BiCoO3

We used density functional theory (DFT) to study the structural, elastic, electronic, and lattice dynamical properties of tetragonal BiCoO3 applying the “norm-conserving” pseudopotentials within the local spin density approximation (LSDA). The calculated equilibrium lattice parameters and atomic displacements are in agreement with the available experimental and theoretical results. Moreover, the structural stability of tetragonal BiCoO3 were confirmed by the calculated elastic constants. In addition, the elastic properties of polycrystalline aggregates including bulk, shear and Young's moduli, and Poisson's ratio are also determined. The electronic band structure, total and partial density of states (DOS and PDOS) with ferromagnetic spin configuration are obtained. The results show that tetragonal BiCoO3 has an indirect band gap with both up- and down-spin configurations and its bonding behavior is of covalent nature. We compute Born effective charge (BEC) which is found to be quite anisotropic of Bi, Co and O atoms. The infrared and Raman active phonon mode frequencies at the Г point are found. The phonon dispersion curves exhibit imaginary frequencies which lead from the high-symmetry tetragonal phase to low-symmetry rhombohedral phase in BiCoO3. The six independent elastic constants, including bulk, shear and Young's moduli, and Poisson's ratio, complete BEC tensor and phonon dispersion relations in tetragonal BiCoO3 are predicted for the first time. Results of the calculations are compared with the existing experimental and theoretical data.

Optical phonon modes of wurtzite InP

Optical vibration modes of InP nanowires in the wurtzite phase were investigated by Raman scattering spectroscopy. The wires were grown along the [0001] axis by the vapor-liquid-solid method. The A1(TO), E2h, and E1(TO) phonon modes of the wurtzite symmetry were identified by using light linearly polarized along different directions in backscattering configuration. Additionally, forbidden longitudinal optical modes have also been observed. Furthermore, by applying an extended 11-parameter rigid-ion model, the complete dispersion relations of InP in the wurtzite phase have been calculated, showing a good agreement with the Raman experimental data.

The Phonon Thermal Conductivity of Single-Layer Graphene From Complete Phonon Dispersion Relations

In this paper, the phonon scattering mechanisms of single-layer graphene are investigated based on the complete phonon dispersion relations. According to the selection rules that a phonon scattering process should obey the energy and momentum conservation conditions, the relaxation rates of combining and splitting umklapp processes can be calculated by integrating the intersection lines between different phonon mode surfaces in the phonon dispersion relation space. The dependence of the relaxation rates on the wave vector directions is presented with a three-dimensional surface over the first Brillouin zone. It is found that the reason for the optical phonons contributing little to heat transfer is attributed to the strong umklapp processes but not to their low phonon group velocities. The combining umklapp scattering processes involving the optical phonons mainly decrease the acoustic phonon thermal conductivity, while the splitting umklapp scattering processes of the optical phonons mainly restrict heat conduction by the optical phonons themselves. Neglecting the splitting processes, the optical phonons can contribute more energy than that carried by the acoustic phonons. Based on the calculated phonon relaxation time, the thermal conductivities contributed from different mode phonons can be evaluated. At low temperatures, both longitudinal and in-plane transverse acoustic phonon thermal conductivities have T2 temperature dependence, and the out-of-plane transverse acoustic phonon thermal conductivity is proportion to T3/2. The calculated thermal conductivity is on the order of a few thousands W/(m K) at room temperature, depending on the sample size and the edge roughness, and is in agreement well with the recently measured data in the temperature range from about 350 K to 500 K.

Coupled theoretical interpretation and experimental investigation of the anisotropy of the lattice thermal conductivity of Bi2Te3 single crystal

The Debye model is modified for the calculation of the lattice thermal conductivity and used to gain insight into the anisotropy of Bi2Te3. In this work, the Debye temperature is not used to estimate the cutoff frequencies of the phonons that carry heat. The cutoff frequencies are defined by setting an upper limit to the energy of acoustic phonons using the complete dispersion relations. The anisotropy of the thermal conductivity is found to be unrelated to the anisotropy of the sound velocities. It is found that the sound velocity is almost isotropic when the longitudinal and two transversal waves are added together. In addition the relaxation time must be a function of the cutoff frequencies and counterbalances the anisotropy arising from the variation of the number of acoustic phonons traveling in various directions. It is concluded that the anisotropy of the thermal conductivity is mostly related to the Grüneisen’s constant.

Hyperspectral imaging of diffracted surface plasmons

We present the results of far field measurements of the complete 3D dispersion relation of a surface plasmon resonance (SPR) effect induced by an integrated quantum well nanodevice. The light modulations in the far field, where the surface plasmons are extracted by a grating, has been calculated for a continuum of energies and wavevectors injected by the luminescent substrate. We introduce a novel experimental method for direct mapping of the EM wave dispersion that enables the monitoring of massive amounts of light-scattering related information. The quasi-real time method is applied for tracking, in the E(k) space, the SPR peak surfaces generated by the investigated nanodevice. Those additional dimensions, measured with scalable tracking precision, reveal anisotropic surficial interactions and provide spectroscopic response for SPR.

The prediction of the effective charge number in single-walled carbon nanotubes using Monte Carlo simulation

The ensemble Monte Carlo simulation is used to calculate the electron-wind forces per unit length of single-walled carbon nanotubes under an electric field applied through the nanotube axis. The electronic system and the ionic system are decoupled from each other. The rate of momentum transferred from the electronic system to the ionic system in the form of the emission or absorption of longitudinal acoustic and longitudinal optical phonons is calculated stochastically to determine the electron-wind forces. Complete unabridged energy and phonon dispersion relations are included in order to obtain more accurate results. The effect of the temperature and the electric field magnitude on the induced forces is also taken into account. Results are compared with a prediction based on quantum mechanical integral form that calculates the electron occupation probability based on a modified Fermi–Dirac distribution. Results show a quantitative agreement between the two methods, however, the method proposed in here we believe is more accurate, because it does not make simplifications for the electron occupation probability as in the modified Fermi–Dirac distribution.

줄 가열 변화에 따른 박막 트랜지스터 내 포논 열 흐름에 대한 수치적 연구

The anisotropic phonon conductions with varying Joule heating rate of the silicon film in Silicon-on-Insulator devices are examined using the electron-phonon interaction model. It is found that the phonon heat transfer rate at each boundary of Si-layer has a strong dependence on the heating power rate. And the phonon flow decreases when the temperature gradient has a sharp change within extremely short length scales such as phonon mean free path. Thus the heat generated in the hot spot region is removed primarily by heat conduction through Si-layer at the higher Joule heating level and the phonon nonlocality is mainly attributed to lower group velocity phonons as remarkably dissimilar to the case of electrons in laser heated plasmas. To validate these observations the modified phonon nonlocal model considering complete phonon dispersion relations is introduced as a correct form of the conventional theory. We also reveal that the relation between the phonon heat deposition time from the hot spot region and the relaxation time in Si-layer can be used to estimate the intrinsic thermal resistance in the parallel heat flow direction as Joule heating level varies. 기호설명 B : 비국소성 정도를 나타내는 변수 B

Thermal conductivities of single-walled carbon nanotubes calculated from the complete phonon dispersion relations

The thermal conductivity of an individual single-walled carbon nanotube is calculated with the analysis of all possible combining and splitting umklapp scattering processes based on complete phonon dispersion relations. The relaxation rate for the transverse acoustic phonons that undergo the combination umpklapp process is found to increase with many singularities as temperature rises. The calculated mean free path suggests that the combining (splitting) umklapp process is predominant in the low (high) frequency regime. The phonons with a very high frequency contribute little to thermal conduction due to the splitting umklapp process. The calculated thermal conductivity for (10,10) carbon nanotube is $474\phantom{\rule{0.3em}{0ex}}\mathrm{W}∕\mathrm{m}\phantom{\rule{0.2em}{0ex}}\mathrm{K}$ at $300\phantom{\rule{0.3em}{0ex}}\mathrm{K}$.

Inelastic neutron scattering study of frustrated Heisenberg triangular magnetCuFeO2

We have studied the magnetic excitation spectrum of thegeometrically frustrated triangular lattice antiferromagnets ofCuFeO2 with collinearground state and CuFe1−xAlxO2 (0.014≤x≤0.030) with noncollinear ground state, using inelastic neutron scattering measurements on thesingle crystals. We have reported almost complete spin-wave dispersion relations and thetemperature dependence of the magnetic excitation spectrum in both pure and slightlydiluted samples. While the higher-energy spin-wave branches remain almost unchangedbetween the pure and diluted samples, the energy gap of the lowest-energy branch inCuFeO2 becomes zero as a result of the dilution, suggesting the retrievalof the original Heisenberg spin character of the orbital singletFe3+. Furthermore, by comparing with recent elastic neutron diffraction and x-raymeasurements, we discuss the interrelation among the magnetic ordering, crystal latticedistortion and magnetic excitation qualitatively.

Determination of elastic properties and interatomic force constants using inelastic neutron scattering

Neutron scattering is the most powerful technique for measuring vibrational excitation spectra. With single‐crystal samples, the complete phonon dispersion relation can be measured. At small phonon wavelengths, the slope of the dispersion curve is related directly to the elastic constants of the material. Information from shorter wavelength phonons can be used to determine parameters for interatomic force models or compared to first‐principles calculations. Model parameters can then be used to calculate phonon densities‐of‐states and consequently connect to the thermodynamics. Polycrystalline samples yield less information, but the data is directly related to the phonon density‐of‐states for Bravais lattices. Measurements on polycrystalline samples have the advantage of being fast and are more useful for parametric studies of the lattice dynamics (with temperature, for example). These ideas will be used to discuss several interesting systems, most notably Pu metal and Fe‐Ga magnetostrictive alloys. [This work is supported by the U.S. Department of Energy.]

Low-Frequency Waves and Instabilities in Collisionless Plasmas

We present a theoretical overview of low-frequency waves and instabilities in collisionless, multi-component plasmas with gyrotropic (\(P_\perp \not= P_\parallel \)) thermal pressure. We show that the complete dispersion relation can be obtained in the framework of a mixed magnetohydrodynamic (MHD)-kinetic formalism, which uses the MHD mass, momentum, and induction equations, together with the kinetically corrected version of the double-adiabatic equations of state. The complete dispersion relation contains not only the three standard modes (fast, slow, and Alfven) from double-adiabatic MHD, but also the mirror mode from kinetic theory. We examine the stability properties of these four modes, firstly in the case of a uniform medium, and secondly in the case of a stratified and rotating medium. We also discuss the connections with the quasi-interchange modes (interchange and translation) often referred to in the context of magnetospheric physics.

Slow Z-mode radiation from sounder-accelerated electrons

Quasi-electrostatic Z-mode waves created by the transmitting part of the OEDIPUS-C payload were measured on the receiving part at 1200 m distance. Solutions of the complete electromagnetic dispersion relation for a hot magnetoplasma reveal, however, that there is no solution that provides direct ray paths along the transmitter-receiver separation direction with the observed signal group delays. An interpretive model is therefore proposed in which sounder-accelerated electrons (SAE) radiate incoherently as they spiral along the magnetic field direction in the general direction of the receiving subpayload. Test-particle theory combined with the hot-plasma dispersion solution is used to predict the total electric field for previously reported SAE flux levels. It is found that voltage levels measured on the receiving dipoles have about the same order of magnitude as the predicted ones.

Vibrational properties of the high-pressureCmcmphase ofZnTe

A lattice dynamical model based on ab initio methods has been developed for the common Cmcm-type orthorhombic high-pressure phase of binary semiconductors. It is applied to ZnTe. The model supplies the zone-center phonon frequencies which are compared to experimental Raman scattering results. The complete phonon dispersion relations and the one-phonon density of states of Cmcm-ZnTe are calculated. The phonon eigenvectors at the G point of the Brillouin zone are given.

Collective dynamics in phospholipid bilayers investigated by inelastic neutron scattering: exploring the dynamics of biological membranes with neutrons

We present the first inelastic neutron scattering study of the short wavelength dynamics in a phospholipid bilayer. We show that inelastic neutron scattering using a triple-axis spectrometer at the high-flux reactor of the ILL yields the necessary resolution and signal to determine the dynamics of model membranes. The results can quantitatively be compared to recent Molecular Dynamics simulations. Reflectivity, in-plane correlations and the corresponding dynamics can be measured simultaneously to gain a maximum amount of information. With this technique, complete dispersion relations can be measured with a high-energy resolution. Structure and dynamics in phospholipid bilayers, and the relation between them, can be studied on a molecular length scale.

Collective dynamics in phospholipid bilayers investigated by inelastic neutron scattering: exploring the dynamics of biological membranes with neutrons

We present the first inelastic neutron scattering study of the short wavelength dynamics in a phospholipid bilayer. We show that inelastic neutron scattering using a triple-axis spectrometer at the high-flux reactor of the ILL yields the necessary resolution and signal to determine the dynamics of model membranes. The results can quantitatively be compared to recent Molecular Dynamics simulations. Reflectivity, in-plane correlations and the corresponding dynamics can be measured simultaneously to gain a maximum amount of information. With this technique, complete dispersion relations can be measured with a high-energy resolution. Structure and dynamics in phospholipid bilayers, and the relation between them, can be studied on a molecular length scale.