Longitudinal Acoustic Mode Research Articles

Resonance magnetoelectric effects in a layered composite under magnetic and electrical excitations

A novel three-layer composite consisting of a lead zirconate titanate (PZT) substrate, a PZT film on one side, and a film of ferromagnetic alloy on the other side is fabricated and used for studies on the nature of magneto-electric (ME) interactions. The strain mediated ME voltage under an ac magnetic excitation is compared with the voltage produced by pure piezoelectric response of the sample to an ac electric field. The ME voltage response measured across PZT substrate or PZT film under an ac magnetic field shows peaks at bending modes and longitudinal acoustic modes, with a higher voltage at bending mode than for the longitudinal acoustic mode. A variation in the bending mode frequency with bias magnetic field or electric field is observed and is attributed to variation in the Young’s modulus of the magnetic or piezoelectric layers. Estimates of resonance frequencies and ME coefficients are in agreement with the data. Under an ac electric excitation, piezoelectric effects result in a resonance in the voltage measured across the PZT substrate and film. A quantitative agreement is inferred between the measured ME voltage and the voltage produced under an ac electric field.

Micro-Brillouin spectroscopy mapping of the residual density field induced by Vickers indentation in a soda-lime silicate glass

High-resolution Brillouin scattering is used to achieve 3-dimensional maps of the longitudinal acoustic mode frequency shift in soda-lime silicate glasses subject to Vickers indentations. Assuming that residual stress-induced effects are simply proportional to density changes, residual densification fields are obtained. The density gradient is nearly isotropic, confirming earlier optical observations made on a similar glass. The results show that Brillouin micro-spectroscopy opens the way to a fully quantitative comparison of experimental data with predictions of mechanical models for the identification of a constitutive law.

Analysis of the combustion oscillation in a silo-type gas turbine combustor and its suppression

The characteristics of combustion oscillation of a silo-type 79.5 MW gas turbine combustor in commercial operation and its suppression have been investigated. The oscillation of the lean premixed gas turbine combustor resulting from the combustion instability occurred at near full load operation. An FFT analysis of the combustion dynamics showed that the dominant frequency of the oscillation would be that of the 1st longitudinal acoustic resonance mode of the combustor. To suppress the combustion oscillation, a passive control technique for reducing the combustion instability was employed; that is, the fuel to the combustor was redistributed by adjusting the operational schedule of one of six fuel control valves, which would lead the increase of the local operational equivalence ratio near the central recirculation zone of the combustor. By doing so, the oscillation was successfully reduced to the permissible level while the amount of NOx emission met proper regulatory level set by the local government.

Direct and inverse magnetoelectric effect in layered composites in electromechanical resonance range: A review

A model is presented for the increase in magnetoelectric (ME) coupling in magnetostrictive-piezoelectric bilayers in the electromechanical resonance region. The ME voltage coefficients αE have been estimated for transverse field orientations corresponding to minimum demagnetizing fields and maximum αE. We solved the equation of medium motion taking into account the magnetostatic and elastostatic equations, constitutive equations, Hooke's law, and boundary conditions. The resonance enhancement of ME voltage coefficient for the bilayer is obtained at antiresonance frequency. To obtain the inverse ME effect, a pick up coil wound around the sample is used to measure the ME voltage due to the change in the magnetic induction in magnetostrictive phase. The measured static magnetic field dependence of ME voltage has been attributed to the variation in the piezomagnetic coefficient for magnetic layer. The frequency dependence of the ME voltage shows a resonance character due to the longitudinal acoustic modes in piezoelectric layer. The model is applied to specific cases of cobalt ferrite–lead zirconate titanate and nickel–lead zirconate titanate bilayers. Theoretical ME voltage coefficients versus frequency profiles are in agreement with data.

From Bulk to Monolayer MoS2: Evolution of Raman Scattering

AbstractMolybdenum disulfide (MoS2) is systematically studied using Raman spectroscopy with ultraviolet and visible laser lines. It is shown that only the Raman frequencies of $ E_{2{\rm g}}^1 $ and $ A_{{\rm 1g}}^{} $ peaks vary monotonously with the layer number of ultrathin MoS2 flakes, while intensities or widths of the peaks vary arbitrarily. The coupling between electronic transitions and phonons are found to become weaker when the layer number of MoS2 decreases, attributed to the increased electronic transition energies or elongated intralayer atomic bonds in ultrathin MoS2. The asymmetric Raman peak at 454 cm−1, which has been regarded as the overtone of longitudinal optical M phonons in bulk MoS2, is actually a combinational band involving a longitudinal acoustic mode (LA(M)) and an optical mode ($ A_{{\rm 2u}}^{} $). Our findings suggest a clear evolution of the coupling between electronic transition and phonon when MoS2 is scaled down from three‐ to two‐dimensional geometry.

Confinement effects on the vibrational properties of III-V and II-VI nanoclusters

We present a first-principles study of the confinement effects on the vibrational properties of thousand atoms (radii up to 16.2 \AA{}) colloidal III-V and II-VI nanoclusters. We describe how the molecular-type vibrations, such as surface-optical, surface-acoustic, and coherent acoustic modes, coexist and interact with bulk-type vibrations, such as longitudinal and transverse acoustic and optical modes. We link vibrational properties to structural changes induced by the surface and highlight the qualitative difference between III-Vs and II-VIs. We describe the size dependence of the vibrations and find good agreement for Raman shifts and for the frequency of coherent acoustic modes with experiments.

Measurement of the Phonon Density of States of PuO2(+2%Ga)

ABSTRACTInelastic x-ray scattering measurements of the phonon density of states (DOS) of PuO2(+2%Ga) were made and compared to recent predictions from the literature made using three leading theoretical approaches; Density Functional Theory (DFT), DFT plus the Hubbard U (DFT+U), and Dynamical Mean-Field Theory (DMFT). The DFT prediction, which does not account for strong electronic correlations, underestimates the measured energies of most features. The DFT+U and DMFT predictions, which include approximations to strong correlation effects, more accurately reflect the low energy features but exaggerate splitting in the highest energy optic oxygen modes. The exaggeration of the splitting is worse for DFT+U than for DMFT. The transverse acoustic mode shows the least sensitivity to calculation type, and is well reproduced by all three theories. The longitudinal acoustic mode, which is thought to control the thermal conductivity, is more sensitive to calculation type, suggesting an important role for electronic correlations in making application-critical predictions.

Study on characteristics of acoustic modes via stimulated Brillouin scattering in photonic crystal fiber

The effects of optical fiber parameters, pump wavelength and the fiber core refractive index in small-core photonic crystal fibers on characteristics of acoustic modes and the dependency of the acoustic modes coupling are investigated by the effective index method. The results indicate that both the mixed longitudinal acoustic mode and transverse acoustic mode jointly produce the acoustic fields, and their coupling forms the mixed acoustic mode; the propagation constant of acoustic mode can be changed by tailoring the pump wavelength or the PCF core refractive index in the process of SBS; the acoustic mode coupling increase and the phase velocity of the acoustic mode decreases with the increase of the PCF core diameter, and there is also a growing trend of the number of the acoustic modes at the same propagation constant. The velocity decreases with the increase of the frequency of the pump wave.

Intrinsic and extrinsic nonstationary field-driven processes in the spin-ice compound Dy2Ti2O7

Nonequilibrium processes are probed by ultrasound waves in the spin-ice material Dy2Ti2O7 at low temperatures. The sound velocity and the sound attenuation exhibit a number of anomalies versus applied magnetic field for temperatures below the "freezing" temperature of ~500 mK. These robust anomalies can be seen for longitudinal and transverse acoustic modes for different field directions. The anomalies show a broad hysteresis. Most notable are peaks in the sound velocity, which exhibit two distinct regimes: an intrinsic (extrinsic) one in which the data collapse for different sweep rates when plotted as function of field strength (time). We discuss our observations in context of the emergent quasiparticles which govern the low-temperature dynamics of the spin ice.

First principles studies of band structure and electronic properties of ZnSe

The structural and electronic properties of semiconductor ZnSe are investigated by performing first principles calculations using density functional theory (DFT). The exchange correlation potentials were treated within the local density approximation (LDA) and the generalized gradient approximation (GGA) with the quantum espresso package. We calculate the density of state (DOS), projected density of state (PDOS), phonon dispersion frequencies and the electron charge density. Also, the bulk modulus and its pressure derivatives are determined. Where available, the calculated quantities are compared with known results. The electronic band structure revealed an occurrence of a 2.72 eV band gap, while the density of state shows split peak at −2 eV and a minor peak split between 8 and 11 eV. We show that from the gamma points, along the high symmetries Г → X and Г → L directions, there are four dispersion (OL, OT, AL and AT) mode curves which later split into six modes along the X → Г (2OL, OT, 2AT, and AL), L → X (2OT, OL, 2AT, and AL), X → W (2OL, OT, 2AT, and AL) and W → L (2OT, OL, 2AT, and AL) directions. These modes splitting correspond to optical longitudinal mode, optical transverse mode, acoustic longitudinal mode and acoustic transverse mode.

Brillouin Scattering Investigation of Solvation Dynamics in Succinonitrile-Lithium Salt Plastic Crystalline Electrolytes

Temperature dependent Brillouin scattering studies have been performed to ascertain the influence of solvent dynamics on ion-transport in succinonitrile-lithium salt plastic crystalline electrolytes. Though very rarely employed, we observe that Brillouin spectroscopy is an invaluable tool for investigation of solvent dynamics. Analysis of various acoustic (long wavelength) phonon modes observed in the Brillouin scattering spectra reveal the influence of trans-gauche isomerism and as well as ion-association effects on ion transport. Although pristine SN and dilute SN-LiClO(4) samples show only the bulk longitudinal-acoustic (LA) mode, concentrated SN-LiClO(4) (∼0.3-1 M) electrolytes display both the bulk LA mode as well as salt induced brillouin modes at ambient temperature. The appearance of more than one brillouin mode is attributed to the scattering of light from regions with different compressibilities ("compactness"). Correspondingly, these modes show a large decrease in the full width at half-maximum (abbreviated as ν(f)) as the temperature decreases. Anomalous temperature dependent behavior of ν(f) with addition of salt could be attributed to the presence of disorder or strong coupling with a neighbor. The shape of the spectrum was evaluated using a Lorentzian and Fano line shape function depending on the nature and behavior of the Brillouin modes.

Experimental study of the influence of low frequency flow modulation on the whistling behavior of a corrugated pipe

It is well known that airflow in a corrugated pipe can excite whistling at the frequencies of the pipe's longitudinal acoustic modes. This short contribution reports on the results of experiments where a low frequency, oscillating flow with velocity magnitudes of the same order as the airflow has been added. Depending on the oscillation strength, it has been found that this flow may silence the pipe or move the whistling to higher harmonics. It is also shown that the low frequency oscillation itself may excite higher frequency whistling sounds in the pipe.

Acoustic longitudinal mode coupling in w-shaped Al/Ge Co-doped fibre

This paper proposes a novel fibre structure aiming at distributed temperature and strain sensing. Utilizing Al2O3 and GeO2 as dopants to form a w-shaped acoustic waveguide, it realizes modal coupling between longitudinal acoustic modes of its inner and outer core layers, leading to a dual-peak or multi-peak Brillouin gain spectrum. The relationship between the acoustic mode coupling properties and the fibre materials, doping concentrations and structural parameters are investigated, showing that the positions of mode coupling points in acoustic dispersion curves and the coupling intensities can be designed flexibly. A specific fibre design for the discriminative sensing of temperature and strain under a pump wavelength of 1.55 μm is given. The responses of its Brillouin gain properties on temperature and strain are analysed theoretically, demonstrating its potential for distributed fibre Brillouin sensing.

Dispersion relations for circular single and double dusty plasma chains

We derive dispersion relations for a system of identical particles confined in a two-dimensional annular harmonic well and which interact through a Yukawa potential, e.g., a dusty plasma ring. When the particles are in a single chain (i.e., a one-dimensional ring), we find a longitudinal acoustic mode and a transverse optical mode which show approximate agreement with the dispersion relation for a straight configuration for large radii of the ring. When the radius decreases, the dispersion relations modify: there appears an anticrossing of the modes near the crossing point resulting in a frequency gap between the lower and upper branches of the modified dispersion relations. For the double chain (i.e., a two-dimensional zigzag configuration), the dispersion relation has four branches: longitudinal acoustic and optical and transverse acoustic and optical.

Precursor dynamics in the ferroelectric phase transition of barium titanate single crystals studied by Brillouin light scattering

The acoustic anomalies and precursor dynamics of high-quality barium titanate single crystals were investigated by Brillouin light scattering and the birefringence measurements in the paraelectric phase above the cubic-to-tetragonal ferroelectric phase transition temperature (${T}_{c}$). Two elastic stiffness coefficients ${C}_{11}$ and ${C}_{44}$, the related sound velocities, and their absorption coefficients were determined from ${T}_{c}$ to 400${\phantom{\rule{0.16em}{0ex}}}^{\ensuremath{\circ}}$C for the first time. The longitudinal acoustic (LA) mode showed a substantial softening over a wide temperature range above ${T}_{c}$ which was accompanied by a remarkable increase in the acoustic damping as well as growth of central peaks. The broad central peak (CP) exhibited a two-mode and one-mode behavior in the paraelectric and ferroelectric phase, respectively, which was consistent with recent far-infrared reflectivity measurements and first-principle-based calculations [Ponomareva et al., Phys. Rev. B 77, 012102 (2008)]. The acoustic anomalies and CP behavior were correlated with the anomalous birefringence, piezoelectric effect, and the deviation of the Curie-Weiss law observed from the same crystal. This strongly indicates similarity between the dynamics of polar clusters in typical ferroelectrics and the dynamics of polar nanoregions in relaxors, consistent with recent acoustic emission measurements [Dul'kin et al., Appl. Phys. Lett. 97, 032903 (2010)]. The relaxation times estimated from the central peak and the LA mode anomalies exhibited similar temperature dependences with comparable orders of magnitude, indicating that the polarization fluctuations due to the precursor polar clusters couples to the LA mode through density fluctuations. All these anomalies share common microscopic origin, correlated Ti off-centered motions forming polar clusters having local symmetry breaking in the paraelectric phase. The existence of the polar clusters were directly evidenced by the temperature evolution of the precise birefringence map. The narrow central peak within $\ifmmode\pm\else\textpm\fi{}$5 GHz proposed before was not confirmed to exist in the present study.

Brillouin scattering study on crystalline and glassy states of anti-inflammatory racemic S(+)– R(−) ibuprofen

The acoustic properties of racemic S(+)– R(−) ibuprofen single crystals and amorphous ibuprofen were investigated by Brillouin light scattering. The temperature dependence of the longitudinal acoustic (LA) mode in crystalline ibuprofen could be explained by normal anharmonic lattice models. The LA mode frequency of amorphous ibuprofen exhibited a clear change at ∼228 K at which glassy ibuprofen changes into supercooled-liquid state. The acoustic damping of the LA mode in both glassy and crystalline ibuprofen were similar. This indicated the molecular fluctuations remnant in the glassy ibuprofen are comparable to crystalline ibuprofen, which may be attributable to the strong hydrogen-bonded aggregates in both states.

Communication: Are metallic glasses different from other glasses? A closer look at their high frequency dynamics

Using high resolution inelastic x-ray scattering we studied the collective dynamics of the Pd(77)Si(16.5)Cu(6.5) metallic glass, focusing on the energy-momentum region where the boson peak appears. The dispersion relation and the width of the acoustic excitations are determined showing how the longitudinal acoustic modes maintain their dispersive character for frequencies well above the boson peak frequencies. Moreover, we prove that close to these frequencies there is a softening of the apparent sound speed indicating a failure of the Debye continuum approximation at the boson peak frequencies and challenging previous results on other metallic glasses.

Systematic Study of Aggregation Structure and Thermal Behavior of a Series of Unique H-Shape Alkane Molecules

The H-shape alkanes of various arm lengths have been synthesized successfully through the Grignard reaction. The detailed investigation of these novel compounds may allow us to widen the topological chemistry field furthermore. The molecular form and molecular packing structure in the crystal lattice have been revealed successfully on the basis of X-ray structure analysis as well as the analysis of Raman longitudinal acoustic modes (LAM) sensitive to the alkyl zigzag chain segments. The molecular conformation in the crystal lattice is deformed markedly from the originally imagined H-shape. In the cases of C3HOH to C6HOH, for example, the molecules are packed in a complicated manner and the OH···O hydrogen bonds govern the whole intermolecular interactions mainly. Since the alkyl segmental length is not very long, the conformational change is not very drastic, i.e., the small configurational entropy. Synergic effect of the hydrogen bonds and the small configurational entropy gives the higher melting point as known from the thermal data. On the other hand, in the cases of C10HOH and C12HOH, one of the long alkyl chain arms is found to be bent by 90° so that all of the alky chain segments of planar-zigzag conformation can be packed as closely as possible, and the intermolecular OH···O hydrogen bonds are also formed effectively without any mistake. As a result, the contribution of nonbonded intra- and intermolecular van der Waals interactions between the trans-zigzag alkyl chain segments become major, and the coupling of this enthalpy effect with the larger configurational entropy effect of the molecular shape results in the decrement of the melting point which approaches gradually that of longer n-alkane compound. In this way a sensitive balance between the nonbonded van der Waals interactions, the OH···O hydrogen bonds, as well as the configurational entropy effect gives the characteristic thermal behavior of the H-shape compounds. The thus-newly synthesized H-shape alkane compounds should give us new insight into the packing topology of complicated molecules, leading to the development of new functionality unexpected for normal linear alkane compounds.

Raman scattering by folded acoustic phonons in InGaN/GaN superlattices

AbstractWe use Raman scattering to investigate the folded longitudinal acoustic (LA) phonons in a series of InxGa1−xN/GaN superlattices (SLs) grown by molecular beam epitaxy with different compositions (15% < x < 38%) and SL periods (from 8 to 20 nm). A novel, ultralow wavenumber filtering module, which provides access to ultralow wavenumber Raman modes on single‐grating spectrometers, has been used to perform the Raman measurements. Zone‐folding effects are observed, and the wavenumber behavior of the folded LA modes is well reproduced with a linear dispersion for the folded LA modes as predicted by elastic continuum theory. We employ the wavenumber of the doublets to evaluate the period of the SLs. Copyright © 2011 John Wiley & Sons, Ltd.

Application of the Low Frequency Raman Spectroscopy for Studying Ultra‐High Molecular Weight Polyethylenes

AbstractIn the frequency range below ∼150 cm−1, the longitudinal acoustic modes (LAM) localized along straight chain segments (SCS) of macromolecules emerge in the Raman spectra of linear semicrystalline polymers. The LAM frequency is inversely proportional to the SCS length; therefore, the LAM band contour reflects the SCS length distribution in a sample. The opportunities given one by the low‐frequency Raman spectroscopy for studying the ordered structures in polyethylene are demonstrated and discussed. The illustrating material consists of both previously published and original data on nucleation and transformation of the ensemble of SCS in powder‐like, gelled, and drawn ultra‐high molecular weight polyethylene.