Academic literature on the topic 'Lattice Vibrational Modes'

Abstract Vibration dynamics of crystalline borophene is considered in the framework of the Born–von Karman model. The vibrations perpendicular to the plane of 2D borophen lattice (flexural modes) are studied.

AbstractThe structural, mechanical and vibrational properties of Ru2TiZ (Z = Si, Ge, Sn) Full Heusler Alloys (FHAs) are computed using PBE-GGA as an exchange-correlation functional in Kohn–Sham equations. The calculated lattice constants of these alloys in L21 phase deviate from experimental values upto 0.85 % which shows a good agreement between the model and the experiments. These lattice constants are then used to compute the second order elastic constants C11, C12 and C44 with Wien2k-code. Elastic moduli and mechanical parameters are also calculated by these three independent elastic constants. Mechanical parameters Pugh’s and Poisson’s ratio indicate non-brittle nature of these alloys. Furthermore, the Debye temperature where the collective vibrations shift to an independent thermal vibration, longitudinal and transverse sound velocities, melting temperatures, and thermal conductivities are also obtained to investigate the phonon modes of oscillation. These phonon modes confirm the stability of these alloys as there exists no imaginary phonon frequency in the phonon-dispersion curves.

A discrete breather (DB) is a spatially localized vibrational mode of large amplitude in a defect-free anharmonic lattice. Generally, zero-dimensional DB is considered to be localized in all [Formula: see text] directions of the [Formula: see text]-dimensional lattice. However, the question of existence of DBs localized in [Formula: see text]–[Formula: see text] directions and delocalized in other [Formula: see text] directions remains open. In the present paper, for the first time, the case of [Formula: see text] and [Formula: see text] is considered by constructing a two-dimensional (2D) DB in the fcc nickel lattice using molecular dynamics methods. In order to excite such DB, one of the delocalized vibrational modes of the triangular lattice was used (the (111) plane in fcc crystal is a triangular lattice). All simulations were carried out at zero temperature. The investigated 2D DB demonstrates hard-type nonlinearity, when its oscillation frequency increases with increasing amplitude. The oscillation frequencies of the DB are above the upper edge of the phonon spectrum for nickel, which is 10.3[Formula: see text]THz. The maximum DB lifetime is found to be 9.5[Formula: see text]ps. The obtained results expand our understanding of diversity of nonlinear spatially localized vibrational modes in nonlinear lattices.

A phenomenological model of anisotropic lattice vibrations is proposed, using a temperature-dependent spectral cutoff and varying Debye temperatures for the vibrational normal components. The internal lattice energy, entropy and Debye–Waller B factors of non-cubic elemental crystals are derived. The formalism developed is non-perturbative, based on temperature-dependent linear dispersion relations for the normal modes. The Debye temperatures of the vibrational normal components differ in anisotropic crystals; their temperature dependence and the varying spectral cutoff can be inferred from the experimental lattice heat capacity and B factors by least-squares regression. The zero-point internal energy of the phonons is related to the low-temperature limits of the mean-squared vibrational amplitudes of the lattice measured by X-ray and γ-ray diffraction. A specific example is discussed, the thermodynamic variables of the hexagonal close-packed zinc structure, including the temperature evolution of the B factors of zinc. In this case, the lattice vibrations are partitioned into axial and basal normal components, which admit largely differing B factors and Debye temperatures. The second-order B factors defining the non-Gaussian contribution to the Debye–Waller damping factors of zinc are obtained as well. Anharmonicity of the oscillator potential and deviations from the uniform phonon frequency distribution of the Debye theory are modeled effectively by the temperature dependence of the spectral cutoff and Debye temperatures.

The far-infrared and Raman spectra of a series of alkali, ammonium and alkylammonium salts of Mo2X93- (X = Cl , Br, I) have been measured. All observed infrared-active vibrations, including lattice modes, together with certain Raman-active modes, have been assigned for the alkali salts of Mo2Cl93- on the basis of the D6h unit cell symmetry which applies. The vibrational assignments for the corresponding bromide and iodide complexes follow in the same relative order as those of Mo2Cl93-. The splitting of degenerate modes for certain alkylammonium salts of Mo2Cl93- has been interpreted on the basis of C2v symmetry applicable to these particular salts. By correlating the spectral changes observed with variation of the cation, it has been possible to distinguish the bridging-halogen deformation modes from neighbouring lattice modes.

Symmetry-determined nonlinear normal modes, which describe large-amplitude vibrations of diamond lattice, are studied by specific group-theoretical methods. For two of these modes, corresponding to vibrational state with and without multiplication of the unit cell, we present their atomic patterns and dynamical properties obtained with the aid of the ab initio simulations based on the density functional theory (DFT).

LiNiPO4, multifunctional material with potential ionic current and magnetoelectric applications, was studied by IR optical spectroscopy and lattice dynamic shell-model calculations. We present IR phonon parameters of lithium-nickel-phosphate, both experimental (at T = 30 K) and calculated ones. Multiferroic properties of lithium-nickel-phosphate were confirmed by temperature evolution of phonon band parameters due to spin-phonon interaction revealed for a number of phonon modes. The most pronounced effect was registered for B2 u IR-vibrational mode near 202 cm−1, which demonstrates anomalous temperature behavior in the vicinity of magnetic ordering ( TN = 21.8 K) indicating the coupling to the magnetic subsystem. This mode is assigned to the complex normal vibration including the librations of the PO4 tetrahedra, out-of-plane vibrations of the Li atoms, and the in-plane vibrations of the magnetic Ni atoms along the b axis. The detected phonon shift at TN can be explained by the striction phenomena.

AbstractThe dynamics of a three-component nonlinear delocalized vibrational mode in graphene is studied with molecular dynamics. This mode, being a superposition of a root and two one-component modes, is an exact and symmetrically determined solution of nonlinear equations of motion of carbon atoms. The dependences of a frequency, energy per atom, and average stresses over a period that appeared in graphene are calculated as a function of amplitude of a root mode. We showed that the vibrations become periodic with certain amplitudes of three component modes, and the vibrations of one-component modes are close to periodic one and have a frequency twice the frequency of a root mode, which is noticeably higher than the upper boundary of a spectrum of low-amplitude vibrations of a graphene lattice. The data obtained expand our understanding of nonlinear vibrations of graphene lattice.

Temperature dependence of phonons spectra and allied properties of rhombohedral La 0.7 Sr 0.3 MnO 3 are investigated by using the lattice dynamical method. A tendency of phonon mode to instability causing JT lattice distortion is reflected in a softening of the stretching mode in the phonon dispersion curve of La 0.7 Sr 0.3 MnO 3 at both 1.6 and 300 K. While the A1g mode softens because of gradual decrease in MnO 6 rotations, the stretching mode hardens upon reduction in temperature. The distinct features of phonon modes at different temperatures are also reflected in the calculated phonon density of states. Other thermal properties such as specific heat, the Debye temperature, and Grüneisen parameter are also presented. The decrease in the Debye temperature at 300 K indicates the effect of temperature in lattice softening. Anomalously high value of the Grüneisen parameter points out the presence of anharmonic lattice modes.

La principale motivation de cette these a ete l'etude de la dynamique collective des verres et des systemes aptes a former un verre a une echelle spatiale de l'ordre de la distance entre particules. Cette region n'etait pas facilement accessible avant le developpement de la technique de diffusion inelastique de rayons x (ixs), qui permet de mesurer directement le facteur de structure dynamique des particules dans les systemes. Afin d'etudier par ixs les excitations analogues a des modes phonons dans la region mesoscopique, une tres haute resolution en energie est necessaire. Dans ce contexte une importante realisation de cette these a ete la mise au point d'une technique pour la construction d'analyseurs spheriques adaptes a un spectrometre pour la diffusion inelastique de rayons x. Les analyseurs ainsi construits ont ouvert la possibilite d'effectuer des experiences de diffusion inelastiques de rayons x analysant les fluctuations de densite dans la matiere condensee. L'etude des modes collectifs dans les systemes aptes a former un verre a montre l'existence d'excitations inelastiques dont l'energie augmente avec l'impulsion transferee. L'observation d'une vitesse de groupe non nulle a prouve que les caracteristiques inelastiques observees sont encore dues a la propagation de modes analogues a des modes acoustiques. La comparaison avec des simulations de dynamique moleculaire a permis d'avoir une plus grande comprehension de la nature des vecteurs propres d'un systeme desordonne tel qu'un verre. Plus exactement, ces vecteurs propres sont bien representes par une onde plane et une composante aleatoire, et ils pourraient expliquer l'observation de l'exces en densite des etats vibrationnels dans les verres. Une etude de la dependance en temperature des modes a haute frequence dans le glycerol a permis de deduire le comportement en temperature de la vitesse du son a haute frequence, dans la phase liquide. Cette etude a montre que la ixs peut etre utilisee pour determiner experimentalement la dependance en temperature de la vitesse du son non-relaxee, ce qui constitue un parametre tres important pour toute approche hydrodynamique a la relaxation structurale dans les systemes aptes a former un verre.

Low-temperature heat capacity data contain information on the physical properties of materials, and new models continue to be developed to aid in the analysis and interpretation of heat capacity data into physically meaningful properties. This work presents the development of two such models and their application to real material systems. Equations describing low-energy vibrational modes with a gap in the density of states (DOS) have been derived and tested on several material systems with known gaps in the DOS, and the origins of such gaps in the DOS are presented. Lattice vacancies have been shown to produce a two-level system that can be modeled with a sum of low-energy Schottky anomalies that produce an overall linear dependence on temperature in the low-temperature heat capacity data. These two models for gaps in the vibrational DOS and the relationship between a linear heat capacity and lattice vacancies and many well-known models have been applied to several systems of materials to test their validity and applicability as well as provide greater information on the systems themselves. A series of bulk and nanoscale Mn-Fe and Co-Fe spinel solid solutions were analyzed using the entropies derived from heat capacity data, and excess entropies of mixing were determined. These entropies show that changes in valence, cation distribution, bonding, and the microstructure between the mixing ions is non-ideal, especially in the nanoparticles. The heat capacity data of ten Al doped TiO2 anatase nanoparticle samples have also been analyzed to show that the Al3+ dopant ions form small regions of short-range order, similar to a glass, within the TiO2 particles, while the overall structure of TiO2 remains unchanged. This has been supported by X-ray diffraction (XRD) and electron energy-loss spectroscopy and provides new insights to the synthesis and characterization of doped materials. The final investigation examines nanocrystalline CuO using heat capacities, magnetization, XRD, and electron microscopy and compares the findings to the known properties of bulk CuO. All of these measurements show transitions between antiferromagnetic and paramagnetic states in the temperature range of about 150-350 K that are greater in number and higher in temperature than the transitions in bulk CuO. These changes are shown to cause an increase in the temperature range of multiferroicity in CuO nanoparticles.

The natural dynamics are not ideal or linear. To understand their complex behavior, we needs to study the nonlinear dynamics in more simple models. This thesis is consist of two main setups. Both setups are simplified models for the behavior occurs in the complex systems. We studied in both systems the same nonlinear dynamics such as higher-harmonics, sub-harmonics, solitary waves,...etc. In Chapter (2), the propagation of nonlinear waves in a lattice of repelling particles is studied theoretically and experimentally. A simple experimental setup is proposed, consisting in an array of coupled magnetic dipoles. By driving harmonically the lattice at one boundary, we excite propagating waves and demonstrate different regimes of mode conversion into higher harmonics, strongly in influenced by dispersion. The phenomenon of acoustic dilatation of the chain is also predicted and discussed. The results are compared with the theoretical predictions of FPU equation, describing a chain of masses connected by nonlinear quadratic springs. The results can be extrapolated to other systems described by this equation. We studied theoretically and experimentally the generation and propagation of kinks in the system. We excite pulses at one boundary of the system and demonstrate the existence of kinks, whose properties are in very good agreement with the theoretical predictions, that is the equation that approaches, under the conditions of our experiments, the one corresponding to full model describing a chain of masses connected by magnetic forces. The results can be extrapolated to other systems described by this equation. Also, In the case of a lattice of finite length, where standing waves are formed, we report the observation of subharmonics of the driving wave. In chapter (3), we studied the propagation of intense acoustic waves in a multilayer crystal. The medium consists in a structured fluid, formed by a periodic array of fluid layerswith alternating linear acoustic properties and quadratic nonlinearity coefficient. We presents the results for different mathematicalmodels (NonlinearWave Equation,Westervelt Equation and Constitutive equations). We show that the interplay between strong dispersion and nonlinearity leads to new scenarios of wave propagation. The classical waveform distortion process typical of intense acoustic waves in homogeneous media can be strongly altered when nonlinearly generated harmonics lie inside or close to band gaps. This allows the possibility of engineer a medium in order to get a particular waveform. Examples of this include the design of media with effective (e.g. cubic) nonlinearities, or extremely linear media. In chapter (4), the oscillatory behavior of a microbubble is investigated through an acousto-mechanical analogy based on a ring-shaped chain of coupled pendula. Observation of parametric vibration modes of the pendula ring excited at frequencies between 1 and 5 Hz is considered. Simulations have been carried out and show spatial mode, mixing and localization phenomena. The relevance of the analogy between a microbubble and the macroscopic acousto-mechanical setup is discussed and suggested as an alternative way to investigate the complexity of microbubble dynamics.
La dinámica natural no es ideal ni lineal. Para entender su comportamiento complejo, necesitamos estudiar la dinámica no lineal en modelos más simples. Esta tesis consta de dos configuraciones principales. Ambas configuraciones son modelos simplificados de el comportamiento que se produce en los sistemas complejos. Estudiamos en ambos sistemas la misma dinámica no lineal como son la generación de armónicos superiores, los sub-armónicos, las ondas solitarias, etc. En elCapítulo (2), se estudia, tanto teórica comoexperimentalmente, la propagación de ondas no lineales en sistemas periodicos de partículas acopladas mediante fuerzas repulsivas. Se propone una configuración experimental simple, que consiste en una matriz de dipolos magnéticos acoplados. Inyectando armónicamente la señal en un extremo, excitamos ondas de propagación y demostramos diferentes regímenes de conversión de modos en armónicos, fuertemente influenciados por la dispersión. También se predice y se discute el fenómeno de dilatación acústica de la cadena. Los resultados se comparan con las predicciones teóricas de la ecuación FPU, describiendo una cadena de masas conectadas por muelles cuadráticos no lineales. Los resultados pueden ser extrapolados a otros sistemas descritos por esta ecuación. Estudiamos también teórica y experimentalmente la generación y propagación de kinks. Excitamos pulsos en la frontera del sistema y demostramos la existencia de kinks cuyas propiedades están en muy buen acuerdo con las predicciones teóricas, es decir, con la ecuación que aproxima bajo las condiciones de nuestros experimentos la correspondiente al modelo completo que describe un cadena de masas conectadas por fuerzas magnéticas. Los resultados pueden ser extrapolados a otros sistemas descritos por esta ecuación. Además, en el caso de una red finita, donde se forman ondas estacionarias, describimos la observación de subarmónicos del armónico principal. En el capítulo (3), estudiamos la propagación de ondas acústicas intensas en un cristal multicapa. El medio consiste en un fluido estructurado, formado por un conjunto periódico de capas fluidas con propiedades acústicas lineales alternas y coeficiente de no linealidad cuadrática. Presentamos los resultados de diferentes modelos matemáticos (ecuación de ondas no lineal, ecuación de Westervelt y ecuaciones constitutivas). Mostramos que la interacción entre la fuerte dispersión y la no linealidad conduce a nuevos escenarios de propagaciónde ondas. El proceso de distorsión de la onda clásica, típico de las ondas acústicas intensas en medios homogéneos, puede ser alterado de forma importante cuando los armónicos generados no linealmente se encuentran dentro o cerca del gap. Esto permite la posibilidad de diseñar un medio con el fin de obtener una forma de onda en particular. Ejemplos de esto incluyen el diseño demedios con no linealidad efectiva (por ejemplo, cúbica) o medios extremadamente lineales. En el capítulo (4), el comportamiento oscilatorio de una microburbuja se investiga a través de una analogía acusto-mecánica basada en una cadena en forma de anillo de péndulos acoplados. Se estudian los modos de vibración paramétrica del anillo pendular excitado a frecuencias entre 1 y 5 Hz. Se han llevado a cabo simulaciones que muestran la presencia de modos espaciales, mixtos y fenómenos de localización. Se discute la relevancia de la analogía entre una microburbuja y la configuración macroscópica acústico-mecánica y se sugiere como una vía alternativa para investigar la complejidad de la dinàmica de microburbujas.
La dinàmica natural no és ideal ni tampoc lineal. Per entendre el seu comportament complex, es necessita estudiar la dinàmica no lineal dels models més simples. Aquesta tesi consisteix en l'estudi de dues configuracions principals, que són models simplificats del comportament que es produeix en els sistemes complexos. Estudiem en ambdós sistemes la mateixa dinàmica no lineal, com és la generació d'harmònics superiors, sub-harmònics, ones solitàries, etc. En el capítol (2), estudiem, tant teòrica com experimentalment, la propagació de les ones no lineals en sistemes periòdics de partícules acoblades mitjançant forces repulsives. Es proposa una configuració experimental simple, que consisteixen en una matriu de dipols magnètics acoblats. En conduint harmònicament la xarxa en un límit, excitemla propagació de les ones i demostrem diferents règims de conversió de modes en harmònics més alts, força influenciada per la dispersió. El fenomen de la dilatació acústica de la cadena també es prediu i es discuteix. Els resultats es comparen amb les prediccions teòriques que descriu una cadena de masses conectades per molls quadràtics no lineals. Els resultats es poden extrapolar a altres sistemes descrits per aquesta equació. Hem estudiat teòrica i experimentalment la generació i propagació de Kinks. Excitem polsos a la frontera del sistema i demostrem l'existència d'Kinks, les propietats desl quals estan en molt bon acord amb les prediccions teòriques, és a dir, de l'equació que aproxima sota les condicions dels nostres experiments la corresponent al model complet que descriu un cadena demasses connectades per forcesmagnètiques. Els resultats es poden extrapolar a altres sistemes descrits per aquesta equació. A més, en el cas d'una xarxa finita, on es formen ones estacionàries, descrivim l'observació de subarmónicos de l'harmònic principal. En el capítol (3), s'estudia la propagació d'ones acústiques intenses en un medi multicapa. El medi consisteix en un fluid estructurat, format per una matriu periòdica de capes de fluid amb l'alternança de propietats acústiques lineals i coeficient de no linealitat de segon grau. Es presenten els resultats per a diferents models matemàtics no lineals (equació d'ones no lineal, equació de Westervelt i les equacions constitutives). Es demostra que la interacciò entre la forta dispersió i no linealitat condueix a nous escenaris de propagació de l'ona. El procés de distorsió en formad'ona clàssica, típica de les ones acústiques intenses en medis homogenis, es pot alterar de manera significativa quan els harmònics generats de forma no lineal es troben dins o a prop del gap. Això obri la possibilitat de dissenyar unmedi per tal d'obtenir una forma d'ona particular. Exemples d'això inclouen el disseny delsmedis amb una no linealitat efectiva (per exemple cúbica), o medis extremadament lineals. En el capítol (4), el comportament oscilatori d'una micro-bombolla és investigat a través d' una analogia acústica-mecànica basada en una cadena en forma d'anell de pèndols acoblats. Es considera l'observació dels modes de vibració paramètriques de l'anell pendular excitat amb freqüències entre 1 i 5 Hz. S'han dut a terme simulacions que mostren la presència de moes espacilas, mixtes i fenòmens de localització. Es discuteix la relevància de l'analogia entre les microbambolles i la configuració macroscòpica acústica-mecànica i es suggereix una formaalternativa per a investigar la complexitat de la dinàmica demicrobombolles.
Mehrem Issa Mohamed Mehrem, A. (2017). Nonlinear acoustics in periodic media: from fundamental effects to applications [Tesis doctoral no publicada]. Universitat Politècnica de València. https://doi.org/10.4995/Thesis/10251/80289
TESIS

Thesis (Ph. D.)--Worcester Polytechnic Institute.
Keywords: fluid-structure interaction; low dimensional models; coupled map lattices; vortex shedding; cylinder wake patterns; flow control; multi-variable least squares algorithm; neural networks; adaptive estimation. Includes bibliographical references (p. 147-149).

Etude entre 0 et dollar GPA, permettant de mettre en évidence une transition de basse pression à 1,6 GPA; mesures de l'absorption optique et de l'indice de réfraction, de la diffusion Raman et de la diffusion Brillouin; analyse des variations a la transition. Description satisfaisante des variations des modes de vibration au moyen d'un modèle de dynamique réticulaire a forces centrales.

The discovery and control of new phases of matter are a central endeavor in materials research. Phase transition in two-dimensional (2D) materials has been achieved through laser irradiation, strain engineering, electrostatic doping, and controlled chemical vapor deposition growth, and laser irradiation is considered as a fast and clean technique for triggering phase transition. By using first-principles calculations, we predict that the monolayer MoTe2 exhibits a photo-induced phase transition (PIPT) from the semiconducting 2H phase to the topological 1T′ phase. The purely electronic excitations by photon soften multiple lattice vibrational modes and lead to structural symmetry breaking within sub-picosecond timescales, which is shorter than the timescale of a thermally driven phase transition, enabling a controllable phase transition by means of photons. This finding provides deep insight into the underlying physics of the phase transition in 2D transition-metal ditellurides and show an ultrafast phase-transition mechanism for manipulation of the topological properties of 2D systems. More importantly, our finding opens a new avenue to discover the new families of PIPT materials that are very limited at present but are essential to design the next generation of devices operated at ultrafast speed.

Many of the thermal properties of materials are intimately related to the fundamentally relevant characteristic, the heat capacity. In electrically insulating materials, the heat capacity arises primarily from the lattice vibrations; the conduction electrons also contribute to the heat capacity in metals. This chapter introduces the essential concepts of lattice vibrations, followed by the basic models of heat capacity. Finally, typical attributes like thermal expansion and thermal conductivity are discussed. Thermoelectric effects are noted briefly along with the discussion of semiconductors in the following chapter.

Pristine graphite crystallizes according to the D46h space group. There are twelve modes of vibration associated with the three degrees of freedom of the four atoms in the primitive cell. The hexagonal Brillouin zone and the phonon dispersion curves of pristine graphite, calculated by Maeda et al. (1979), are shown in Figure 4.1. The zone-center (Γ point) modes are labeled as three acoustic modes (A2u + Elu), three infrared active modes (A2u + Elu), four Raman active modes (2E2g), and two silent modes (2Blg). The first calculation of phonon dispersion for the stage-1 compounds KC8 and RbC8 was presented by Horie et al. (1980) on the basis of the model of Maeda et al. (1979) for the lattice dynamics of pristine graphite. Although the calculated phonon energies do not agree well with the experimental data, the model has most of the ingredients for describing the lattice dynamics of stage-1 GICs. A simple review of their work is presented as follows. The primitive cell of KC8, having a p(2 × 2)R0° superlattice, contains 16 carbon atoms and two K atoms. Note that only an αβ stacking sequence is assumed here (see Section 3.6.1). The primitive translation vectors are given by t1 (0, a, 0), t2 = (−√3a/2, a/2, 0), and t3 = (−√3a/4, −a/4, c), where a = 2aG = 4.91 Å and c = 5.35 × 2 = 10.70 Å. The corresponding Brillouin zone is shown in Figure 4.2b. The phonon dispersion for KC8 has been calculated by Horie et al. (1980) on the basis of the Born-von Karman force constant model. This dispersion curve is compared with that of pristine graphite by folding the dispersion curves of graphite into the first Brillouin zone of KC8. Since the side of the Brillouin zone in KC8 is not flat in two directions, as shown in Figure 4.2b, it is a little difficult to transfer the information on the dispersion curves in the first Brillouin zone of graphite into the Brillouin zone of KC8. For simplicity, nevertheless, we assume that the side of the Brillouin zone in KC8 is flat like that of graphite.

Phonons are introduced as an example of quasi-particles that can only exist in matter. Debye’s quantum model for heat capacity of solids and comparison with experimentin different temperature ranges is presented. The dispersion relations of lattice vibration (phonons) and quantization for chains of atoms presented, revealing the optical and acoustic modes; anharmonic effects are discussed. Crystal lattice structures and Brillouin zones are introduced. Phonon scattering and the Umklapp process described. The variation of the thermal conductivity of dielectrics with temperature is interpreted. X-ray scattering studies of phonon dispersion relations are described. Coupling between phonons with photons in polaritons is explained: Raman scattering studies of GaN used to exhibit the cross-over of their dispersion relations. The Mössbauer effect, a recoilless process, and its dependence on temperature are explained.

Reliability of structures is the most important problem in modern meсhanical engineering. Its solution poses the task of creating materials with high stable physical, mechanical, chemical and other properties. The materials that make up the basis of structures are, as a rule, operated in variable fields of various nature (mechanical, temperature, etc.), therefore they continuously change their properties. In this regard, progress in the field of mechanical engineering cannot be achieved without full knowledge of the real structure of materials, without a deep understanding of the microprocesses that occur in the materials during their processing and operation.Therefore, the purpose of the work is to describe methods and techniques for determining damping, structural and mechanical characteristics of materials and to evaluate the nature of their changes under the influence of temperature, time and force factors.The most promising non-destructive physical research methodsinclude the method of mechanical spectroscopy (internal friction) and the microhardness method. With the help of the method of mechanical spectroscopy, it is possible to reliably determine such seemingly different properties of materials as the ability to dampen vibrations arising during the operation of machine parts, the diffusion mobility of atoms in the crystal lattice and along dislocations. Internal friction (IF), which characterizes the ability to dissipate the energy of mechanical vibrations with a large and small amplitude in the material, can often be unambiguously associated with the number and mobility of dislocations and point defects of the crystal lattice. At the same time, it allows you to assess the material's susceptibility to another consequence, creep and grain boundary relaxation. Processes such as separation and dissolution of residual phases, relaxation, rest, hardening and tempering, dispersion hardening, plastic deformation and deformation aging and return have been successfully studied for many years by the method of internal friction. IF not only makes it possible to determine damping, structural and mechanical characteristics, but also to follow the nature of their changes in the fields of various nature [1–4]. Microhardness, which characterizes the energy of the interatomic bond of the crystal, its mechanical strength or the resistance of the crystal lattice to large plastic deformations and destruction, was chosen as a controlling characteristic in the study of the results.The research results showed that thermal cycling leads not only to the for-mation of high density dislocations in alloys, but also to their redistribution into polygonal walls. This is due to the fact that the available impurities, their complexes or separate dispersed allocations significantly block the newly formed dislocations and the already formed substructure, and thus increase its thermal stability and increase the heat resistance of the material. At the same time, as the strength characteristics increase, the damping properties are not significantly lost.Value/originality. Application of thermocycling under load is possible not only for materials made of dispersion-hardening alloys, which serve as raw materials for technology and industry, but also for finished parts and technical structures made of them. This will simplify the processing technology, reduce energy consumption and allow to obtain a significant economic effect.

The interaction of laser radiation with chemical species adsorbed on solid surfaces is a process of relevance to vibrationally activated desorption and other reactive processes. For desorption, photons can either excite an internal mode of the adsorbate, A, forming A* which can convert to translational motion and removal, or the photons can directly excite the adspecies-surface bond A-S above the dissociation limit. In both cases the important competing process is vibrational deactivation due to the coupling with the modes of the solid lattice. The central question is the amount of energy that can be localized in the surface bond at a given laser intensity.

Ultrafast infrared spectroscopy was used to unveil the rates at which different vibrational modes of atomic-scale defects in diamond interact with their environment. We adopted the N3VH0 defect as a model system, and studied perturbed free induction decay (PFID) and population decay features. The faster dephasing times of the fundamental stretch mode in comparison to the observed bend mode (first overtone) was a result of its more direct perturbation of the crystal lattice. We further demonstrate the versatility of ultrafast spectroscopy by investigating the energy level structure and dynamics of an unknown defect that is infrared active at 3237 cm−1.

We report on strong coupling between molecular vibrational resonances of polymethyl methacrylate (PMMA) molecules and gap plasmon resonance of an ultrathin plasmonic cavity in the midinfrared range. The strong coupling is achieved when the molecular vibrational mode and plasmonic cavity exchange energy faster than their relaxation rates and it is maximum when two relaxation rates are equal [1]. In this work, we designed, fabricated and characterized a composite medium consisting of a thin PMMA layer sandwiched between the nanoantenna array and a continuous metallic thin film to achieve vibration strong coupling. The spectral position and the relaxation rate of gap plasmonic resonance are tuned through the molecular resonance of the PMMA molecules (at 1730 cm−1) to go from weak to strong coupling regime. Strong coupling between vibrational modes and gap plasmon mode leads to the formation of new hybrid light-matter states called polaritonic states (@ 1690 cm−1 & 1810 cm−1), separated by the vacuum Rabi splitting (120 cm−1). Thin film coupled nanoantennas with sub-wavelength gaps have shown great potential in nanophotonic applications because they offer the ultimate electric field confinement in the gap. Our work is complementary to earlier work using microcavities and provides a new approach to achieve strong coupling with a nanoscale plasmonic cavity (λ/25) and the possibility to modulate the strong coupling regime by changing the gap thickness of the cavity and the lattice period of the nanoantenna array.

Soft Modes, ISRS, and Perovskite Ferroelectrics The dynamics of cooperative ordering in crystals near structural phase transitions have long been elusive. Most picosecond and femtosecond time-resolved measurements have involved rapid laser heating of a sample whose phase transition (e.g. melting) dynamics are then monitored1. In this case many lattice degrees of freedom are excited, and little information is provided about the roles or dynamics of particular lattice motions. In structural phase transitions, the order parameter is described by one or a very few specific ("soft") lattice modes whose motions bring crystalline constituents from their initial positions into their positions in a new crystalline phase. Experimental characterization of the dynamics is possible in some cases through Raman spectroscopy, but often the highly damped character of the soft mode leads to a central peak in the Raman spectrum which cannot be analyzed accurately. In many cases it is not even possible to tell whether the ordering motion is a heavily damped vibration, Debye relaxation, or some combination of the two. This information is essential for understanding the microscopic mechanism of a transition since vibrational or relaxational character is an indication of ionic motion within a single potential energy minimum (i.e. a displacive transition) or hopping between different sites (an order-disorder transition) respectively.

IR excitation of selected vibrational modes of a variety of molecules trapped in solid rare gases causes a photoisomerization. The reactions are generally single photon processes which involve a rotation about a single bond. The quantum yields for photoisomerization have now been measured in both the forward and re verse directions for several molecules. The range of excitation includes both the fundamental and first overtone vibrations. These quantum yields vary from 0 to 1. The vibrational photochemistry is not consistent with a statistical redistribution of energy in the guest molecule before reaction. The guest molecules are not isolated in a gas phase sense; vibrational energy flow to the lattice competes with reaction and intramolecular (guest) energy redistribution. It is likely that relaxation of the guests can occur via specific pathways. Evidence is presented to show that the products and the quantum yields for their formation depend on the relaxation rates and pathways. It is thus possible to create product distributions by even broadband (nonlaser) IR irradiation that are impossible thermally.

The elastic continuum model is resorted to study the lattice specific heat of individual crystalline silicon nanowires with 22 and 37 nm in diameter. Incorporating both longitudinal and transverse modes, a model based on the power-law phonon dispersion is developed to illustrate the characteristics of phonon density of states (DOS) in bulk and low-dimensional systems. The results show that there exist multiple and explicit van Hove singularities for thin wire and step-like enhancement for thin film, attributing to the quantized phonon modes in the particular dimensionality. Furthermore, we examine the phonon dispersion and DOS of silicon nanowires and find infinite phonon branches. A cutoff frequency based on the reduced phonon group velocity is defined to guarantee that the phonon wavelength can not be shorter than the lattice constant. The softening of vibrational modes in low-frequency region and the deficit in high-frequency region occur, resulting in the excess specific heat of silicon nanowire compared with that of bulk silicon. This phenomena becomes more significant at low temperatures. In addition, the dependence of specific heat of nanowires on temperature departs from that of bulk material in view of the quasi one-dimensional feature. Qualitative support is presented to our present work.