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Using the microscopic s-f model and Green's function theory, we study the temperature dependence of the band gap energy E g and the phonon energy ω and damping γ of ferro- and antiferromagnetic semiconductors, i.e., with different signs of the s-f interaction constant I. The band gap is a fundamental quantity which affects various optical, electronic and energy applications of the materials. In the temperature dependence of E g and the phonon spectrum, there is a kink at the phase transition temperature T C or T N due to the anharmonic spin–phonon interaction (SPI) R. Moreover, the effect of the SPI R and electron–phonon interaction (EPI) A on these properties is discussed. For I > 0 , R > 0 , E g decreases with increasing SPI and EPI, whereas for I < 0 , R > 0 , there is a competition; E g increases with raising the EPI and decreases for enhanced SPI. For R < 0 , in both cases, the SPI and EPI reduce E g . The magnetic field dependence of E g for the two signs of I and R is discussed. The SPI and EPI lead to reducing the energy of the phonon mode ω = 445 cm−1 in EuO ( I > 0 , R < 0 ), whereas ω = 151 cm−1 in EuSe ( I > 0 , R > 0 ) is enhanced with increasing EPI and reduced with SPI. Both the SPI and EPI lead to an increasing of the phonon damping in EuO and EuSe. The results are compared with the existing experimental data. [ABSTRACT FROM AUTHOR]

Recently, there has been renewed interest in thermoelectric materials based on germanium telluride, which demonstrate high efficiency in mid-temperature range. This paper discusses the theoretical description of the phonon spectrum and lattice thermal conductivity in GeTe using ab initio methods. Using these methods, the temperature dependence of the lattice thermal conductivity in the rhombohedral phase was calculated and effects of scattering by point defects and nanostructuring were estimated. The modification of the phonon spectrum upon the transition to the high-temperature cubic phase is investigated. The calculated temperature dependences of the lattice thermal conductivity are compared with the available experimental data on GeTe and its solid solutions. [ABSTRACT FROM AUTHOR]

This study reveals unexpected strain-dependent friction, when applied strain to one of two layers of relatively sliding black phosphorus, friction is likely to exhibit characteristic rise and subsequent fall, forming a distinct friction peak. Existence and location of such peak are closely related to magnitude of normal loads. Interfacial pinning is weak at lower normal load, matched dissipation channels are easy to be destroyed, resulting in a monotonic decrease of friction. At higher normal load, tip is also pulled apart once applying a small strain to substrate, stored deformation energy leads to greater energy dissipation than at strain of zero, causing friction to decrease non-monotonically. Moreover, ability of sliding layers to withstand strain decreases with increasing normal loads or temperatures. [Display omitted] • When strain is applied to one of the two sliding layers, the friction increases and then decreases to form a distinct peak. • The ability of sliding layers to withstand strain is influenced by normal load and temperature. • A method for controlling two-dimensional materials interlayer phononic friction is investigated using phonon spectrum. [ABSTRACT FROM AUTHOR]

The paper is devoted to model phonon spectra calculations of argyrodite family crystals, namely: Ag7GeSe5I and Cu7GeSe5I (Ag7SiS5I and Cu7SiS5I). The specificity of the crystalline structure of these crystals is realization of variable partial populating different crystallographic orbits with Cu, I and Ag atoms (in 24 (g) and 48 (h) Positions Multiplicity, Wyckoff letter, Site symmetry of the space group F 43m ), which are associated with high superionic conductivity in crystals of this family. Compared in this paper are model phonon spectra calculated with account of three different configurations of orbital populations with Cu and Ag atoms (6 + 1, 5 + 2 and 4 + 3), by choosing the same lattice model for the unit cell and the set of force constants of each crystal. The calculations were performed by finding the eigenvalues of the generalized dynamic matrix perturbed by the modulation function of the mass defect obtained in the concept over spatial symmetry. It is shown that the phonon spectra do not undergo significant qualitative changes, which can serve as indirect evidence of the possibility to realize the jumping mechanism of super-ionic conductivity in these crystals in the high-temperature phase. [ABSTRACT FROM AUTHOR]

In the CeO2, ThO2, and NpO2 crystals, the acoustic branches of the phonon spectra correspond predominantly to the vibrations of metal ions, and the optical branches correspond mainly to the vibrations of oxygen ones, due to a significant difference in the masses of these components of the compounds. An important crystallographic phenomenon is that metal and oxygen are located in sublattices corresponding to different Bravais types, which makes it possible to analyze vibrations in the corresponding Brillouin zones (BZ). This analysis was carried out by unfolding the optical branches of the phonon spectrum from the BZ of the crystal into the BZ of the oxygen sublattice. The number of optical branches in this BZ is half that in the BZ of the crystal. The effect of anharmonicity on the thermal broadening of the spectral lines of the phonon frequencies is studied in the approximation of two-phonon densities of states. For all three compounds, the calculated two-phonon densities of states represent two broad structured peaks. The temperature dependences of the upper peak show thermal broadening of the upper part of the optical branches and the lower peak shows thermal broadening of the low-energy optical and acoustic branches. [ABSTRACT FROM AUTHOR]

The different translational symmetry of the cuprite crystal sublattices is taken into account by unfolding the phonon spectrum from the Brillouin zone of the crystal into the Brillouin zones of the anion and metal. Such a representation of the spectrum is reasonable because 6 optical branches are essentially caused by the vibrations of anions and 12 acoustic branches are due to the vibrations of metal owing to the great value difference between their masses. The volumes of the Brillouin zones of the anion and metal sublattices are two and four times, respectively, as large as those of the crystal resulting in the qualitative differences in the corresponding unfolded spectra because of the metal tetrahedron vibrations of the distortion type in the acoustic part of the spectrum. These vibrations located on the Х′(1,0,0)–W′(1,0,1/2) and Х′(0,1,0)– W′(0,1,1/2) lines of the metal Brillouin zone are responsible for the negative thermal expansion. The unfolded spectra have only three branches in the corresponding Brillouin zones and this simplifies the selection rules for the physical processes with the participation of the sublattice particle vibrations. [ABSTRACT FROM AUTHOR]

Cu6PS5Br compound belongs to the large family of complex chalcohalides crystallizing in the argyrodite structures. The main peculiarity of the copper-containing argyrodites is high ionic conductivity of Cu+ ions, which makes it possible to use as the functional electronic materials. In the present study, the crystal structure of Cu6PS5Br argyrodite have been analyzed and described using the superspace symmetry concept. The program operating under the Maple environment and suitable for theoretically calculating the phonon spectra of the CuPS5 Br argyrodite crystal has been developed. Zone structures have been calculated and presented for a series of model Cu6PS5 Br phases. The eigenvalues of the generalized dynamic matrix have been found as well as the dispersion dependences have been built for the directions G-X-M-R-G-M of the Brillouin zone. [ABSTRACT FROM AUTHOR]

In this paper, first-principles calculations and density functional perturbation theory (DFPT) based on the ab-initio approach, have been utilized to investigate mechanical, elastic anisotropy, dynamical, and thermal properties of nuclear fuel UO. The results reveal that the obtained equilibrium lattice parameter at zero temperature and pressure has a good consistency with the experimental lattice constant. The elastic constants are evaluated using the strain-stress method. These coefficients are estimated to be positive and satisfied the Born-Huang relations, which is indicative of the mechanical stability of this compound. The mechanical properties such as shear modulus, bulk modulus, and Young's modulus of UO are calculated, and the results show the compound's stiffness. Pugh's criterion (B/G) is obtained to be approximately 3.5, which confirms that UO is a ductile substance. Elasticity analysis including universal anisotropy and Zener's anisotropy factor reveals that UO falls into the category of anisotropic materials. The phonon spectrum curve is calculated using DFPT along the high symmetric paths and no negative frequencies are observed in the dispersion relations, implying the dynamical stability of the crystalline structure. The quasi-harmonic Debye (QHD) model is employed to study the thermophysical properties such as Debye temperature, isochoric heat capacity, coefficient of thermal expansion, entropy, and Grüneisen parameter at high pressures and temperatures. We found that molar specific heat was more affected by the temperature than pressure. [Display omitted] • The quasi-harmonic Debye model (QHD) has been used in this study. • Phonon dispersion diagram reveals that UO is dynamically stable in cubic phase. • The obtained elastic constants have shown that this ceramic is mechanically stable. • Thermodynamic properties under high pressures and temperatures are studied. • The elastic anisotropy properties of UO alloy are evaluated for the first time. [ABSTRACT FROM AUTHOR]

The structural, vibrational, phonon dispersion spectrum, and thermodynamic properties of LaCoO 3 were investigated using first-principles calculations using density functional theory. Calculations were performed using the projector augmented wave (PAW) pseudopotentials within the generalized-gradient approximation (GGA) and local density approximation (LDA) models for the exchange and correlation functional. In addition, GGA + U and LDA + U were used to describe the 3 d electrons. The phonon dispersion spectrum and phonon density of states of LaCoO 3 were calculated with the finite displacement method. The related thermodynamic properties such as heat capacity, entropy, free energy, and the coefficient of thermal expansion were calculated at various temperatures from the lattice dynamical data obtained by quasi-harmonic approximation. The phonon dispersion spectrum results indicate that the LaCoO 3 structure is dynamically stable. The phonon dispersion and the peaks of the infrared (IR) and Raman spectrum in the frequency range of 0–700 cm −1 were analyzed and assigned. The calculated thermodynamic properties agree well with the experimental results. [ABSTRACT FROM AUTHOR]

• Excitation of coherent optical phonon modes with near-infrared laser pulses. • Identification of optical phonons spectrum in garnets within the range of 3–11 THz. • Detection of phonon modes with a different decay time up to 13 ps. The excitation of optical phonon modes at resonance has been recently identified as offering a promising energy-efficient pathway for the ultrafast switching of magnetization. Here, using time-resolved pump–probe measurements, we explore the characteristics of different optical phonon modes in ferrimagnetic cobalt-doped yttrium iron garnet. By rotating the linear polarization of both the pump and probe, we identify a large spectrum of optical phonons within the frequency band of 3–11 THz. We determine the lifetimes of the different phonons using band filtration, revealing that the phonon mode with the eigenfrequency 3.4 THz survives for up to 13 ps. Its long lifetime, comparable with the relaxation time of photo-magnetic anisotropy in garnets, renders this phonon mode the most promising candidate for ultrafast phononic switching of magnetization. [ABSTRACT FROM AUTHOR]

For a further understanding of the phase transitions mechanism in type-I silicon clathrates K8Si46, ab initio self-consistent electronic calculations combined with linear-response method have been performed to investigate the vibrational properties of alkali metal K atoms encapsulated type-I silicon-clathrate under pressure within the framework of density functional perturbation theory. Our lattice dynamics simulation results showed that the pressure induced phase transition of K8Si46 was believed to be driven by the phonon instability of the calthrate lattice. Analysis of the evolution of the partial phonon density of state with pressure, a legible dynamic picture for both guest K atoms and host lattice, was given. In addition, based on phonon calculations and combined with quasi-harmonic approximation, the specific heat of K8Si46 was derived, which agreed very well with experimental results. Also, other important thermal properties including the thermal expansion coefficients and Grüneisen parameters of K8Si46 under different temperature and pressure were also predicted. [ABSTRACT FROM AUTHOR]

The phonon spectrum of hypothetical superhard cp-BC2N is calculated based on ab initio method of density functional in the center of the Brillouin zone and interpolated over the entire Brillouin zone using the Keating phenomenological model. The interaction parameters are determined by optimization of the IR- and Ramanactive frequencies for a phenomenological model by their comparison with the results of ab initio calculations. Numerical values of short-range interaction constants and charges are in agreement with the characteristics of the chemical bond calculated ab initio. These parameters have transparent physical meaning and chemical nature and can further be used for both qualitative estimations of any physical and physico-chemical quantities and quantitative calculations of the phonon spectra of a number of isostructural compounds. The Keating phenomenological model is used to study the genesis of the phonon spectrum from the spectra of sublattices. [ABSTRACT FROM AUTHOR]

To measure a heat capacity of a single carbon nanotube is impossible nowadays and experiments are performed with the nanotubes bundles. However, the existing theories of carbon nanotubes heat capacity do not take into account an interaction between an individual nanotube and its environment. Here we show that due to this fact the known theories overestimate the heat capacity in the ultra-low temperature limit. In this region the main contribution to heat capacity of a single free nanotube is caused by the bending phonon mode with a quadratic dispersion. At T =1 K different known models of the phonon dynamics lead to the theoretical specific heat which is at least 10 times greater than the experimentally measured value. To overcome this contradiction we take into account the interaction between an individual nanotube and its environment. As a result, the nanotube loses its Goldstone degrees of freedom and essential reconstruction of the lowest-energy part of its phonon spectrum occurs. The theory proposed explains experimental data on thermal capacity in the ultra-low temperature range and simultaneously demonstrates better agreement with experiments in the region 2.5 K< T <40 K than the preceding models. [ABSTRACT FROM AUTHOR]

Abstract: We study the dispersion characteristics of strained graphene using many-body interatomic potentials and find: (i) borders of the structural stability of a flat graphene in the three-dimensional space of the strain components (ε xx , ε yy , ε xy ); (ii) sound velocities of strained graphene; and (iii) phonon density of states (DOS) of strained graphene. The border of structural stability of flat graphene is also presented in the space of components of normal and shear membrane forces (T x , T y , T xy ). We find that flat graphene is structurally stable under elastic strain up to 0.3–0.4, but it becomes unstable to a shear strain in the absence of tensile components of strain. Also graphene cannot remain flat under compressive membrane forces because its bending stiffness vanishes. We employ the molecular dynamics simulations to study the post-critical behavior of graphene. We demonstrate that ripples with controllable amplitude and orientation can be generated under simultaneous action of shear and tensile membrane forces. Gaps in the phonon DOS are observed when graphene is strained close to the appearance of ripples. Sound velocities of unstrained graphene do not depend on the propagation direction but application of strain makes graphene anisotropic. One of the sound velocities vanishes at the border of the structural stability of graphene meaning that vanishing of sound velocity (or corresponding elastic constant) predicts impending instability. [Copyright &y& Elsevier]

In this paper, we report ab initio calculations of the vibrational and structural properties of ZnAl2O4 and ZnGa2O4 spinel structures. The calculated vibrational modes at zero pressure, at the Γ point and the complete phonon spectrum of both compounds are presented. Also, we report our findings for the high-pressure structure dependences of spinel and post-spinel structures. [ABSTRACT FROM AUTHOR]

We study the Schrödinger equation for an off-diagonal tight-binding hamiltonian, as well as the equations of motion for out-of-plane vibrations on the separable square Fibonacci quasicrystal. We discuss the nature of the spectra and wave functions of the solutions. [ABSTRACT FROM AUTHOR]

The polarized Raman spectra of Zn or Ca doped NdBa2Cu3O7−δ single crystals have been investigated. It was found that the linewidth of Ba mode in Zn doped Nd 1:2:3 is broader than those in pure Nd 1:2:3. This result suggests that the Nd substitution on Ba sites occurred in Zn doped Nd 1:2:3. In x(zz)x¯ geometry the relative intensity of the O(4) mode in Zn doped Nd 1:2:3 is smaller than those in pure or Ca doped Nd 1:2:3. The difference between Zn doped and Ca doped Nd 1:2:3 in the relative intensity of the O(4) mode may be produced by the disordering in the CuO2 plane. [Copyright &y& Elsevier]

• The positive energies of all phonon branches and large eigenvalues of the optical branch indicate that WTe 2 is thermodynamically stable. • The acoustic and optical phonon branches overlap for phonon energies from 0 to 10 meV, and thus there is no gap between them. • The grouping of acoustic phonons and optical branches has implications for the thermal transport properties of WTe 2. We investigate the electronic structures of intrinsic and doped WT e 2 by first-principles calculations based on the density functional theory. We first calculate the phonon dispersion and phonon density of states of WT e 2. We found that all the phonon modes have positive energies, and that the optical phonon branches have high eigenvalues, which indicates that the structure of WT e 2 is thermodynamically stable. In the range of phonon energy from 0 to 10 meV, the acoustic and optical branches overlap, and thus there is no gap between them. The grouping of acoustic phonons and optical branches has implications for the thermal transport properties of WT e 2. We next analyzed the electronic band structures of intrinsic and doped WT e 2. We show that doping with other chalcogens in the same group of elements as Te reduces the energy band gap but leaves the overall band structure relatively unchanged. However, doping with elements from other groups, such as C and H, greatly modifies the electronic band structure, especially near the Fermi level. In fact, doping with such elements can elicit a transition of WT e 2 from a gapped to a gapless phase. [ABSTRACT FROM AUTHOR]

In this work, lattice dynamics is utilized to calculate the phonon spectrum of L10-FePt crystal and molecular dynamic (MD) method is constructed to calculate the phonon density of state (PDOS), which can be compared to the result of lattice dynamics. By the phonon spectrum and PDOS, the optimum mode and frequency of the laser to activate phonons is found in L10-FePt, and the heating rate using different frequency and mode of the laser has been schematically simulated by MD method. [ABSTRACT FROM PUBLISHER]

In this paper, we study the phonon thermal properties of a ladder nanostructure in harmonic approximation. We present a model consisting of two infinite chains with different masses. Then, we investigate the effect of different masses on the phonon spectrum. Moreover, as a specific case, in the absence of the second neighbor interaction, we calculate the phonon density of states/modes. Finally, we consider the thermal conductivity of the system. The results show that the phonon spectrum shifts down to the lower frequencies by increasing the masses. Furthermore, a frequency gap appears in the phonon spectrum. By increasing the springs constants, the thermal conductance decreases. [ABSTRACT FROM AUTHOR]