Fermi resonance of optical one-phonon and acoustic two-phonon vibrations in light metals

It is shown that the anharmonicity of crystal lattice vibrations in light metals such as beryllium, can give rise to a Fermi resonance of optical one-phonon and acoustic two-phonon vibrations. New hybridized vibrational states are formed as a result of such a resonance interaction: biphonon and quasibiphonon vibrations and renormalized optical vibrations. Depending on the wave vector, these vibrational states can be both damped and stationary. The corresponding dispersion equation is obtained, whose solution made it possible to determine the spectrum of these vibrations (dispersion curves and the wave vector dependence of the damping for damped vibrations). It is shown that ultrafast damping of optical vibrations, similar to the well-known superradiance effect for Frenkel’ and Wannier-Mott excitons, is possible. Fiz. Tverd. Tela (St. Petersburg) 39, 542–546 (March 1997)     

Spectroscopy of vibrational modes in metal nanoshells

In this work we study the spectrum of vibrational modes in metal nanoparticles with a dielectric core. Vibrational modes are excited by the rapid heating of the particle lattice that takes place after laser excitation, and can be monitored by means of pump-probe spectroscopy as coherent oscillations of transient optical spectra. In nanoshells, the presence of two metal surfaces results in a substantially different energy spectrum of acoustic vibrations than for solid particles. We calculated the energy spectrum as well as the damping of nanoshell vibrational modes. The oscillator strength of the fundamental breathing mode is larger than that in solid nanoparticles. At the same time, in very thin nanoshells, the fundamental mode is overdamped due to instantaneous energy transfer to the surrounding medium. PACS 78.67.-n; 78.67.Bf; 63.22.+m     

Low-frequency Raman scattering from CeO<Subscript>2</Subscript> nanoparticles

Raman scattering measurements were performed on CeO₂ nanoparticles at room temperature. Low-frequency modes are assigned to confined acoustic vibrations of spherical CeO₂ nanoparticles. Frequencies of these vibrational modes have been calculated in the elastic continuum approximation, which considers a nanoparticle as a homogeneous elastic sphere. We assumed stress-free boundary conditions. The specific dependence of the vibrational frequency on the particle diameter enables the determination of the particle size from the experimental Raman frequency. The particle size value calculated in this way agrees well with the value acquired from the phonon confinement model. PACS 61.46.Df; 73.63.Bd; 63.22.+m     

Anomalous delay of phonons reflected from the surface of a superlattice

We study theoretically the propagation of acoustic phonons in a superlattice (SL) with a free surface. A phonon incident normally on the SL from a substrate is perfectly reflected, but it comes back to the substrate either with a time delay or with a time advance. Specifically the time delay is enhanced considerably if the frequency of the incident phonon coincides with an eigenfrequency of the vibrational modes localized at the surface of the SL. This suggests the observability of the surface vibrational modes by a time-resolved phonon reflection experiment.     

The violin: Chladni patterns,plates, shells and sounds

In this article we consider the vibrations and radiated sound of the bowed violin. The vibrations are discussed in terms of the normal modes of the instrument involving the coupled vibrations of the bowed string, the supporting bridge, the hollow shell comprising the body of the instrument and, ultimately, the acoustic modes of the performance space in which the instrument is played. We show that damping plays an important role in characterizing the normal modes in what can be distinguished as weak and strong coupling limits. The historic and modern application of Chladni pattern measurements to enhance our understanding of the acoustics and as an aid to the making of violins is highlighted, alongside the modern equivalents of experimental modal and computational finite-element analysis. The symmetry-breaking properties of the internal soundpost is shown to have a profound affect on the intensity and quality of sound radiated by the bowed instrument.     

Direct observation of confined acoustic phonons in the photoluminescence spectra of a single CdSe-CdS-ZnS core-shell-shell nanocrystal

We report on the direct observation of confined acoustic phonons in the photoluminescence spectra of single CdSe-CdS-ZnS nanocrystals, whose ligands were exchanged to poly(ethylene oxide) (PEO) before they were embedded in a PEO matrix. Modeling a nanocrystal as an elastic sphere, the confined acoustic modes can be assigned to purely radial vibrations: the breathing mode and its two first radial harmonics. In addition to acoustic modes, we also observe longitudinal optical modes of the core material and, remarkably, also of both shell materials.     

The influence of surfaces and interfaces on coherent phonons in semiconductors

Experiments have shown that ultrafast optical excitation of semiconductors can produce oscillating changes in the optical properties of the material. The frequency of the oscillations in transmission or reflection usually matches one of the phonon modes, typically theq = 0 optical mode. These oscillations are known as coherent phonons. We discuss the role of surfaces and interfaces on the coherent phonon signal. We show that: (1) the coherent phonon signal can be used as a probe of the surface depletion field and (2) multiple interfaces as in a superlattice, can drastically alter the coherent phonon spectrum: screening of the modes in the superlattices is reduced and acoustic modes can now be excited.     

Performance and testing of a four channel high-resolution heterodyne interferometer

When studying complex vibrations, simultaneous measurements at several points are indispensable if one is dealing with objects whose vibrational behavior is not guaranteed to be stable over longer periods of time, such as biological specimens, micro-mechanical elements or objects characterized by nearly resonant normal modes with different vibrational patterns. Obviously, both amplitude and phase need to be measured at each point to obtain a full characterization of the vibration. We introduce birefringent beam displacers as a highly efficient beam multiplying method to create a system of four heterodyne interferometers operating in parallel from a single laser source. The design and the performance characteristics (resolution, cross-talk) of the instrument will be discussed. The system revealed the existence of running vibrational modes on an electrically driven plate clamped along its outer edge.     

Self-organization of irregular nanoelectromechanical vibrations in multimode shuttle structures

We investigate theoretically multimode electromechanical "shuttle" instabilities in dc voltage-biased nanoelectromechanical single-electron tunneling devices. We show that initially irregular (quasiperiodic) oscillations that occur as a result of the simultaneous self-excitation of several mechanical modes with incommensurable frequencies self-organize into periodic oscillations with a frequency corresponding to the eigenfrequency of one of the unstable modes. This effect demonstrates that a local probe can selectively excite global vibrations of extended objects.     

Ultrafast vibrational dynamics and stability of deuterated amorphous silicon

Infrared four-wave mixing experiments performed upon deuterated amorphous silicon layers (a-Si:D) reveal profound differences in the dynamics of Si-D stretch vibrations compared to those of analogous Si-H vibrational modes in hydrogenated amorphous silicon (a-Si:H). Remarkably, transient-grating measurements of the population decay rate of the Si-D vibrations show single-exponential decay directly into collective modes of the a-Si host, bypassing the local bending modes of the defect into which the Si-H vibrations decay. Photon-echo measurements of the vibrational dephasing suggest at low temperature contributions from TO nonequilibrium phonons and at elevated temperatures elastic phonon scattering of TA phonons.     

Probing the acoustic vibrations of single metal nanoparticles by ultrashort laser pulses

The strong interaction of metal nanoparticles with light makes it possible to detect individual particles by far‐field optical methods. In this article, the interaction of a metal nanoparticle with a short laser pulse is discussed, with the emphasis on the coherent excitation of mechanical (acoustic) modes and the optical detection of these vibrations. The literature on acoustic vibrations of single metal nanoparticles of different shapes (spheres, dumbbells, rods, cubes, wires, prisms) is reviewed, and the modes that have been excited and detected in these particles are discussed. Finally, the insights and potential applications enabled by these studies are summarized.     

Nonlinearity of magnetoacoustic excitations in planar structures

This paper reports on the results of experimental investigations into the threshold power of the onset of nonlinearity of magnetoacoustic vibrations in planar structures (such as a ferrite film-dielectric substrate structure) in the range of phase matching of the higher bulk magnetostatic and acoustic modes. Under the experimental conditions, the wavelength of the higher bulk magnetostatic modes is of the order of 1 μm and shorter. On this basis, the energy of these vibrations with respect to the origin of the magnetostatic wave spectrum is determined by the energy of the inhomogeneous exchange interaction. The standing magnetoacoustic waves are examined in conventional yttrium iron garnet films with free surface spins in which, under standard conditions, only dipole magnetostatic vibrations are excited in planar resonators. Consideration is given to the threshold power of the onset of precession instability of the dipole exchange acoustic modes which, as was shown earlier by the authors, are excited in the range of the phase matching of the exchange and acoustic modes. A comparative analysis is performed for the threshold powers of dipole magnetostatic, exchange acoustic, and dipole exchange acoustic modes. It is demonstrated that the threshold power of the instability of magnetostatic modes decreases significantly when the natural frequencies of the dipole modes coincide with those of the exchange acoustic modes. __________ Translated from Fizika Tverdogo Tela, Vol. 44, No. 7, 2002, pp. 1285–1289. Original Russian Text Copyright ? 2002 by Bugaev, Gorsky.     

Thermal conductance associated with six types of vibration modes in quantum wire modulated with quantum dot

We study the ballistic phonon transport and thermal conductance of six low-lying vibration modes in quantum wire modulated with quantum dot at low temperatures. A comparative analysis is made among the six vibrational modes. The results show that the transmission rates of the six vibrational modes relative to reduced frequency display periodic or quasi-periodic oscillatory behavior. Among the four acoustic modes, the thermal conductance contributed by the torsional mode is the smallest, and the thermal conductances of other acoustic modes have adjacent values. It is also found that the thermal conductance of the optical mode increases from zero monotonously. Moreover, the total thermal conductance in concavity-shaped quantum structure is lower than that in convexity-shaped quantum structure. These thermal conductance values can be adjusted by changing the structural parameters of the quantum dot.     

Torsional Splittings in Small-Amplitude Vibrational Fundamental States of Methanol-Type Molecules

Group-theoretical methods are used to show that inverted torsional splittings in fundamental levels of small-amplitude vibrations of methanol-like molecules can be parameterized and understood in terms of the energy level patterns induced when a pair of high-barrier torsionally split components of given v(t) and (t)A+(t)E symmetry species in the molecular symmetry group G(6) is allowed to interact with small-amplitude vibrational modes of symmetry (v)E. Such doubly degenerate (v)E vibrational modes arise rather naturally in G(6) (isomorphic with the point-group C(3v)) for those methyl-group vibrations in point-group-C(s) asymmetric tops such as CH(3)-CHO that are analogs of the degenerate methyl-group stretch, bend, and rocking vibrations in point-group-C(3v) symmetric tops such as CH(3)-C identical withC-H. The present group-theoretical treatment is somewhat different from, but (as a comparison of model parameters shows) still fundamentally similar to, the recent local mode explanation of inverted torsional splittings in the C-H stretching fundamental region in methanol. Copyright 2001 Academic Press.     

Supra- and nanocrystallinities: a new scientific adventure

Nanomaterials exist in the interstellar medium, in biology, in art and also metallurgy. Assemblies of nanomaterials were observed in the early solar system as well as silicate particle opals. The latter exhibits unusual optical properties directly dependent on particle ordering in 3D superlattices.The optical properties of noble metal nanoparticles (Ag, Au and Cu) change with the ordering of atoms in the nanocrystals, called nanocrystallinity. The vibrational properties related to nanocrystallinity markedly differ with the vibrational modes studied. Hence, a drastic effect on nanocrystallinity is observed on the confined acoustic vibrational property of the fundamental quadrupolar modes whereas the breathing acoustic modes remain quasi-unchanged. The mechanical properties characterized by the Young's modulus of multiply twinned particle (MTP) films are markedly lower than those of single nanocrystals.Two fcc supracrystal growth mechanisms, supported by simulation, of Au nanocrystals are proposed: heterogeneous and homogeneous growth processes. The final morphology of nanocrystal assemblies, with either films by layer-by-layer growth characterized by their plastic deformation or well-defined shapes grown in solution, depends on the solvent used to disperse the nanocrystals before the evaporation process.At thermodynamic equilibrium, two simultaneous supracrystal growth processes of Au nanocrystals take place in solution and at the air-liquid interface. These growth processes are rationalized by simulation. They involve, on the one hand, van der Waals interactions and, on the other hand, the attractive interaction between nanocrystals and the interface.Ag nanocrystals (5 nm) self-order in colloidal crystals with various arrangements called supracrystallinities. As in bulk materials, phase diagrams of supracrystals with structural transitions from face-centered-cubic (fcc) to hexagonal-close-packed (hcp) and body-centered-cubic (bcc) structures are observed. They depend on the chain length of the coating agent and on the solvent used to disperse the nanocrystals before evaporation. The transition from fcc to hcp is attributed to specific stacking processes depending on evaporation kinetics whereas the formation of bcc supracrystals is attributed to van der Waals attractions.These results open up a new research area, which currently suffers from an extensive lack of knowledge.     

FORCED AND FREE VIBRATIONS OF RECTANGULAR SANDWICH PLATES WITH PARAMETRIC STIFFNESS MODULATION

An active control of the resonant vibrations of a rectangular sandwich plate performed by the parametric stiffness modulation is analyzed. The controlled vibrations are those of the dominantly flexural type excited by the transverse force acting at the first resonant frequency of dominantly flexural vibrations. The stiffness modulation is performed at a comparatively high frequency identified by the resonance of a mode of the dominantly shear type. The method of direct partition of motions is used that predicts an existence of the modal interaction between these two modes of vibrations due to the parametric stiffness modulation. It is shown that such a parametric control can provide a significant shift of the first eigenfrequency of a controlled plate (the one subjected to the stiffness modulation) from its nominal value for an uncontrolled plate. Heavy fluid loading conditions are accounted for as well as the energy dissipation in the material of a plate. It is demonstrated that although heavy fluid loading reduces resonant frequencies of forced vibrations, the suggested mechanism of control remains valid in these cases. Dynamics of an elementary two-degree-of-freedom model mechanical system is considered to illustrate the mechanism of modal interaction, which is involved in the suggested way of an active control of vibrations of sandwich plates.     

Correlation between local vibrations and metal mass in AlB2‐type transition‐metal diborides

Lattice vibrations have been investigated in TiB₂, ZrB₂ and HfB₂ by temperature‐dependent extended X‐ray absorption fine structure (EXAFS) experiments. Data clearly show that the EXAFS oscillations are characterized by an anomalous behavior of the Debye–Waller factor of the transition‐metal–boron pair, which is suggested to be associated with a superposition of an optical mode corresponding to phonon vibrations induced by the B sublattice and an acoustic mode corresponding to the transition‐metal (TM) sublattice. Data can be interpreted as a decoupling of the metal and boron vibrations observed in these transition‐metal diborides (TMB₂), a mechanism that may be responsible for the significant reduction of the superconducting transition temperature observed in these systems with respect to the parent MgB₂ compound. The vibrational behavior of TM–TM bonds has also been investigated to study the occurrence of anisotropy and anomalies in the lattice vibrational behavior of TM–TM bonds.     

Low-frequency vibrations of bacteriochlorophyll

Frequencies and normal vibrational modes of bacteriochlorophyll are calculated using the semiempirical quantum-mechanical MNDO-PM3 method. To analyze the structure of normal modes, the indices of delocalization of the vibrations and the distribution functions of normal modes over atoms in the molecule are introduced. It is shown that normal vibrational modes of bacteriochlorophyll in the region from 3 to 20 cm^?1 represent “intermolecular” vibrational modes of phytol and tetrapyrrole macrocycle. As the vibrational frequency increases, the normal modes delocalized both on phytol and tetrapyrrole atoms alternate with the modes that are delocalized only on phytol atoms or only on tetrapyrrole atoms. The structural properties of some modes are considered in the aspect of their possible involvement in the formation of absorption spectra of the pigments of reaction centers in photosynthesis and in the formation in them of the coordinate of a primary reaction of the intermolecular electron transfer.     

Intrinsic Localized Spin-Wave Modes in an Anisotropic Ferromagnet with Dzyaloshinsky-Moriya Interaction

The existence and properties of intrinsic localized spin-wave modes in a ferromagnetic XXZ spin chain with Dzyaloshinsky-Moriya interaction are investigated analytically in the semiclassical limit. The model Hamiltonian is quantized by introducing the Dyson-Maleev transformation and the coherent state representation is chosen as the basic representation of the system. By making use of the method of multiple scales combined with a quasidiscreteness approximation, the equation of motion for the coherent-state amplitude is reduced to the nonlinear Schrödinger equation. It is shown that a bright intrinsic localized spin-wave mode whose eigenfrequency lies below the bottom of the magnon frequency band can exist in the ferromagnetic system. We also show that the system can produce a dark intrinsic localized spin-wave mode, i.e., nonpropagating kink, whose eigenfrequency is below the upper of the magnon frequency band. In addition, we find that the introduction of the Dzyaloshinsky-Moriya interaction changes wave numbers in the Brillouin-zone corresponding to the appearance of intrinsic localized spin-wave modes.