Melting of the cooperative Jahn-Teller distortion in LaMnO 3 single crystal studied by Raman spectroscopy

We have studied the behavior of the Raman phonons of a stoichiometric LaMnO₃ single crystal as a function of temperature in the range between 77 K and 900 K. We focus on the three main phonon peaks of the Pbnm structure, related to the tilt, antisymmetric stretching (Jahn-Teller mode) and stretching modes of Mn-O octahedra. The phonon frequencies show a strong softening that can be fit taking into account their renormalization because of three phonon anharmonic effects in the pseudoharmonic approximation. Thermal expansion effects, in particular the variation of Mn-O bond lengths with temperature, are not relevant above 300 K. On the contrary, phonon width behavior deviates from the three phonon scattering processes well bellow T _c . The correlation between the magnitude of the cooperative Jahn-Teller distortion, that disappears at 800 K, and the amplitude of the Raman phonons in the orthorhombic phase is shown. Nevertheless, Pbnm phonons are still observable above this temperature. Phonon width and intensity behavior around T _c can be explained by local melting of the orbital order that begins quite below T _c and by fluctuations of the regular Mn-O octahedra that correspond to dynamic Jahn-Teller distortions. Received 25 January 2001 and Received in final form 14 March 2001     

Soft phonons and the central mode at structural phase transitions

Applying Landau's quasi-particle picture to an interacting phonon system it is shown that the softening of phonon modes is connected with an enhancement of the quasi-particle interaction. This in turn leads to a critical slowing-down of fluctuations around local equilibrium which may give rise to an additional central peak in the scattering function.     

Lattice dynamics of crystalline In4Se3

Calculations of the phonon spectrum of crystalline In₄Se₃ in a model of central-pair interactions with neglect of the long-range forces are presented. The model developed contains five unknown parameters, which are determined from experimental values of the elastic moduli without consideration of the internal displacement of the sublattices. The phonon spectrum obtained contains a large number of low-frequency modes, which deform the acoustic branches. Some common features are discovered in the dispersion curves of the electron and phonon spectra. Fiz. Tverd. Tela (St. Petersburg) 40, 2103–2108 (November 1998)     

Effect of the fluctuations on the TLS parameters

The influence of the phonon fluctuations on the equilibrium parameters of the TLS potential curve is investigated. The distribution range of the transparency parameter of the TLS potential barriers is restricted. The effect of the phonon fluctuations results in a strong reduction of the second maximum temperature of the ultrasonic attenuation, which markedly improves the agreement between the theory and experiment.     

Strong pairing fluctuations and lattice anomalies in the pseudogap phase of the cuprates

Abstract

The pseudogap state in underdoped cuprates shows many very anomalous features. Among them are an extended temperature region of pairing fluctuations above the superconducting transition temperature and an unusual giant phonon anomaly in the same temperature and hole density range. A recent theoretical proposal that ascribes these anomalies to the presence of strong phase fluctuations related to a Leggett mode, is summarised.     

Dynamical parameters and phonon spectrum of crystal In4Te3

Calculations of the phonon spectrum of an In₄Te₃ crystal in the central pair-interaction model neglecting long-range forces are reported. The model developed contains 19 unknown parameters, which were determined from experimental values of the displacements of individual sublattices in the unit cell and from the change in the basis vectors as a result of hydrostatic pressure. The phonon spectrum obtained contains a large number of low-frequency optical branches, deforming the acoustic branches. Fiz. Tverd. Tela (St. Petersburg) 41, 1843–1847 (October 1999)     

Second sound in SrTiO3

We study collective phonon excitations in SrTiO3 by low-frequency light scattering. We employ extended thermodynamics for phonon gas to construct a theoretical spectral function that is applicable regardless of local thermal equilibrium. Our analysis reveals the temperature dependence of tauN, the relaxation time for the momentum-conserving phonon collisions (normal processes), in SrTiO3. These results indicate that the previously reported anomalous soundlike spectrum originates from second sound, which is a wavelike propagation of heat.     

Quantum 1/f noise associated with ionized impurity scattering and electron-phonon scattering in condensed matter

Scattering of charged particles is accompanied by the emission of soft photons. Handel's theory of 1/f noise, based on the infrared quasi-divergent coupling of the system to the electromagnetic field, indicates that the current associated with a beam of scattered particles will exhibit 1/f noise. His derivation is valid in a vacuum. Here we extend his results and obtain the fluctuation spectrum for the fluctuations in cross-section and for the scattering rates w _kk′ in k-space, using the Born approximation. Next we consider mobility fluctuations due to these scattering rates, employing the relaxation time solutions of the Boltzmann transport equation, valid in non-degenerate semiconductors. Explicit results are obtained for the mobility-fluctuation noise caused by ionized impurity scattering, acoustic phonon scattering, optical phonon scattering, polar optical phonon scattering, and intervalley scattering. We derive Hooge's law, and the Hooge parameters for the above-mentioned processes are obtained in detail. This is then applied to n-type silicon and n-type gallium arsenide; the overall Hooge parameter, which is a weighted average of the partial α-parameters, is computed as a function of temperature and compared with experiment. For silicon, good agreement is obtained with available data. As a byproduct we also find the mobilities as function of temperature for these materials. Excellent agreement with the well-known experimental data is observed.

We still note that this is the first theoretical derivation of Hooge's law and that the magnitude of the noise is obtained in detail without adjustable parameters. We believe that quantum 1/f noise gives the limiting value of 1/f noise that can be observed.     

Electrical fluctuations in normal metal point-contacts

The low-frequency electrical fluctuations are measured for point-contacts of Cu, Ag, and Au, the contact diameter being less than the electron mean free path. The $\text{1}\text{?}$ spectral density and a nonmonotonic (oscillating) dependence of r.m.s. noise voltage, V_N, on the d.c. bias, eV, are found. The positions of minima in the V_N(eV) curves are in certain connection with the characteristic phonon energies of metals studied. The phenomenon is attributed to the coherent phonon emission by hot electrons moving along the contact axis.     

Light scattering spectra of polarization fluctuations and models of the phase transition in KDP type ferroelectrics

Abstract

Recent light scattering studies in KH₂PO₄ type hydrogen-bonded crystals are reviewed. Dynamical properties of these crystals have been discussed in terms of the coupled tunneling proton-optical phonon model up to date. Light scattering spectra in several GHz and cm^?1 regions with various scattering geometries observed in KH₂PO₄, KD₂PO₄ and their mixed crystals, however, revealed that all these spectra are well analysed in terms of a coupled polarization relaxation mode-acoustic phonon system. Since most of the spontaneous polarization is due to shifts of K, P and O ions, relaxation motions of H₂PO₄ dipoles arc concluded to be the origin of these low-frequency spectra. Models of the phase transition mechanism are discussed on the basis of these results and other recent experimental studies including the explanation of large isotope effects in these crystals.     

First-principles calculation of the phonon frequencies in γ Fe

The total energy, the equilibrium lattice constant, and the bulk modulus of the fcc phase of iron have been calculated by the full-potential LMTO method. The use of a generalized gradient approximation in the calculation of the electronic structure and lattice properties of γ-Fe is discussed. Next, the transverse phonon frequency at the W point of the Brillouin zone of a fcc lattice is calculated by the “frozen-phonon” method in the harmonic approximation. A local minimum has been found in the curve of the variation of the total energy of the system as a function of the amplitude of the atomic displacements corresponding to the chosen normal mode. To take account of anharmonic effects, a pseudoharmonic approximation is used and an effective potential that approximates the curve of the variation of the total energy of the system and depends on the temperature via the correlation function of the mean-square displacement of atoms from their equilibrium positions is constructed. The theoretical temperature dependence of the effective frequency of the phonon mode responsible for the structural phase transition corresponds qualitatively to the experimentally observed dependence. A new interpretation is given for the structural phase transition as a transition of the corresponding phonon mode from the excited to the ground state. Fiz. Tverd. Tela (St. Petersburg) 39, 171–175 (January 1997)     

On combustion noise related to chemical reactions

An inhomogeneous pressure wave equation has been derived for chemically reacting multicomponent gas mixtures to predict the acoustic field associated with combustion. Out of three source terms for the generation of combustion noise—namely, fluctuations in the heat release rate, in the momentum flow rate and in the viscous and diffusive working—the first is examined to relate this acoustic source strength to the concentration and temperature fluctuations through the chemical kinetics of reaction processes. The relationships between the spectral features of combustion noise and the statistical aspects of these fluctuations are obtained. The essential feature that combustion noise has substantial low-frequency components is explained by the dependence of the contribution of reaction fluctuations on the integral of the time correlation with respect to the correlation time.     

Dependence of the phonon spectrum of a metal on electron temperature in a nonequilibrium electron-phonon system

The dependence of the phonon spectrum of a crystal and the associated thermodynamic functions on electron temperature in the absence of equilibrium between the electrons and the lattice is investigated. The treatment is performed within the Thomas-Fermi approximation for a body-centered cubic crystal at high pressures. Zh. éksp. Teor. Fiz. 115, 231–242 (January 1999)     

Size effect on phonon transport in two-dimensional silicon film

Phonon transport in two-dimensional silicon film is investigated and frequency dependent Boltzmann transport equation is solved numerically using discrete ordinate method. The transient effects of phonon transport in the film are incorporated in the analysis. The influence of film size on phonon transport is examined through equivalent equilibrium temperature in the film. It is found that increasing film thickness enhances phonon scattering and dispersion in the film while increasing equivalent equilibrium temperature. The rate of equivalent equilibrium temperature increase is high in the early heating period ( \(\hbox {t} \le 50\,\hbox {ps}\) ) and the rate of temperature increase becomes gradual in the film as the heating period progresses.     

On the low-frequency limit of the Schönfeld pulse 1/f law

On the basis of propositions of the common fluctuation theory, peculiarities of small fluctuations in real physical systems with limited sizes are analyzed. It is established that small fluctuations should necessarily be divided into two types of fluctuations: “small” and “very small”. It is shown that the damping process of “small” fluctuations has relaxation character, while the damping process of “very small” fluctuations is of random character, i.e., it represents a random rectangular signal. The probability density of “very small” fluctuations is shown to be Gaussian. The agreement of the obtained results with experimental data acquired from semiconductor-based devices is analyzed. A relation between the generation–recombination noise and phonon number fluctuations in semiconductors is studied. On the basis of this consideration it is shown that the Schönfeld pulse spectrum preserves its well-known 1/f form only in the range of intermediate frequencies; at lower frequencies the spectrum gets saturated. An expression for the low-frequency limit of Schönfeld pulse 1/f law is obtained.     

声子库的量子态对介观电路量子特性影响的研究

考虑电子与声子间相互作用,研究了两种声子库纯初始态(正则系综与粒子数态)下耗散介观电路的动力学特性.长时间极限下(t→∞)：当环境处于热平衡态时,电路系统中的电流和电荷的平均值只与电路所处初始量子态中的平均值有关,与环境无关;环境初态为粒子数态时,电荷与电流平均值随时间的演化特性与环境初始处于热平衡态下时完全一样,表明介观电路中的电荷与电流的平均值与环境量子态的某组占有数无关.电路中电流和电荷的量子涨落不仅与系统的初态有关,还与系统所处环境的量子态及温度有关.一般地说,电路系统与环境的纠缠会 关键词： 介观耗散电路 声子库 量子初态 量子态纯度     

Raman phonon spectra and lattice dynamics of 9,10-dinitroanthracene under pressure

Abstract

Raman phonon spectra of 9, 10-dinitroanthracene have been recorded in the pressure range 0-6GPa. No phase transition is detected up to the maximum pressure studied. Quasi Harmonic Lattice Dynamics calculations, based on an atom-atom potential previously modeled on homologous 9,10-disubstituted anthracenes, have been performed. The optimized potential was used to calculate the equilibrium geometry and the lattice phonon frequencies as a function of pressure. The calculated structure at ambient conditions closely resembles the experimental one. The calculated phonon frequencies show a good agreement with the experimental values at all pressures measured.     

Cutoff of long-wave phonons in a nanocrystal due to a nonuniform strain field

This paper considers the effect of extended monopole and dipole strain fields on the low-frequency boundary of the phonon spectrum in a crystal of finite dimensions. The boundary shift depends on the dynamical volume of the nonuniform strain region, which is determined by the parameters of the crystal and the sources of stress. An increase in the volume of the deformed region leads to a decrease in the undistorted part of the crystal, where a phonon with the largest wavelength can be produced. A monopole strain field is more efficient in cutting off long-wave phonons than a dipole strain field, and can “soften” the phonon spectrum. If a source generates stresses of the order of those on an interatomic scale, these effects can be the strongest and most diverse in crystals or phase precipitates with dimensions of less than 10⁻²⁶ cm. Zh. éksp. Teor. Fiz. 111, 1845–1857 (May 1997)     

Hydrogen-/propane-hydrate decomposition: thermodynamic and kinetic analysis

ABSTRACT

The dissociation of planar mixed hydrogen-/propane-hydrate in contact with water has been studied by molecular dynamics between 260 and 320?K. Two modes of termination of the hydrate interface were chosen: direct cleavage to produce an 001-surface, as well as one with completed cages. Substantial differences in melting points and initial dissociation kinetics were evident between both kinds of surface. These break-up rates were significantly dependent on temperature. A kinetic model was applied to fit dissociation profiles, clarifying crisply two hydrate dissociation regions; after a temperature-sensitive decomposition stage, a well-characterised lattice-collapse process is, by and large, insensitive to temperature. For hydrate-liquid systems at thermal equilibrium, consideration of relaxation times of auto-correlation functions of clathrate-molecule-number fluctuations shows significant differences between the two modes of interface: a unifying relationship was established between the non-equilibrium break-up rate from outright melting and that inferred from fluctuation-dissipation at equilibrium.