Phonons and the electronic gap in FeSi

The partial phonon densities of states of iron atoms in the intermetallic compound FeSi have been measured in the temperature range 46–297 K using nuclear resonant inelastic scattering of synchrotron radiation. A significant phonon softening with increasing temperature has been established. The greatest phonon softening for iron atoms is shown to occur in the region of long-wavelength acoustic phonons, for the acoustic branches near the boundary of the Brillouin zone, and for the low-lying weakly dispersive optical branches. The results obtained are analyzed in terms of the views that relate the change in the phonon density of states of iron atoms to the temperature evolution of the electronic density of state for the compound.     

Ab initio lattice dynamics and elastic constants of ZrC

Ab initio calculations and a direct method are applied to derive the phonon dispersion relations and phonon density of states for the ZrC crystal. The results are in good agreement with neutron scattering data. The force constants are determined from the Hellmann-Feynman forces induced by atomic displacements in a 222 supercell. The elastic constants are found using the deformation method and successfully compare with experimental data. Received 2 July 1999 and Received in final form 26 November 1999     

Static and dynamic properties of KCN<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>Cl<Subscript>1−<Emphasis Type="Italic">x</Emphasis></Subscript>

An extended three-body force shell model (ETSM) has been applied to investigate the static and dynamic properties of KCN _x Cl_1−x for the compositionx = 0.56 and 1.0 at 300 K. The phonon dispersion curves computed by us are compared with the single crystal neutron diffraction data. The unusual features of these curves are the upward curvature seen in some of the acoustic branches. This is a result ofK-dependent softening of the phonon due to translation-rotation coupling. The transverse acoustic branch is more soft near the zone centre.     

First-principles study of the phonon spectrum of ɛ-GaSe

The phonon spectrum and phonon density of states of ?-GaSe layered semiconductor have been studied from the first principles in the linear-response approximation. The elastic constants and acoustic velocities along and across layers have been determined. The study of the equilibrium structure and phonon spectrum of the (0001) surface of ?-GaSe has demonstrated that the volume and surface structural dynamic properties of these crystals differ insignificantly. The calculated frequencies and symmetries of the phonon modes in the center of the Brillouin zone are in good agreement with the experimental data obtained from the Raman scattering and infrared spectra.     

Theoretical examination of electron–phonon interaction and superconductivity in the hexagonal pnictide SrPtAs

We have calculated the structural and electronic properties of SrPtAs in a hexagonal KZnAs-type of crystal structure using a generalized gradient approximation of the density functional theory and the ab initio planewave pseudopotential method. These results are used to further calculate the phonon dispersions curves and the phonon density of states using a linear response approach based on the density functional theory. Using the electronic and phonon results, the electron–phonon coupling is computed to be of the intermediate strength of 0.78. In large part, this is contributed by the phonon modes dominated by the vibrations of Pt and As atoms. The superconducting critical temperature is estimated to be 1.9 K, in good accord with its experimental value of 2.4 K.     

Phonon spectra and the one-phonon and two-phonon densities of states of UO<Subscript>2</Subscript> and PuO<Subscript>2</Subscript>

The vibrational spectra of uranium dioxide UO₂ and plutonium dioxide PuO₂, as well as the one-phonon densities of states and thermal occupation number weighted two-phonon densities of states, have been calculated within the framework of the phenomenological rigid ion model. It has been shown that the acoustic and optical branches of the spectra are predominantly determined by vibrations of the metal and oxygen atoms, respectively, because the atomic masses of the metal and oxygen differ from each other by an order of magnitude. On this basis, the vibrational spectra can be represented in two Brillouin zones, i.e., in the Brillouin zone of the crystal and the Brillouin zone of the oxygen sublattice. In this case, the number of optical branches decreases by a factor of two. The two-phonon densities of states consist of two broad structured peaks. The temperature dependences of the upper peak exhibit a thermal broadening of the phonon lines L01 and L02 in the upper part of the optical branches. The lower peak is responsible for the thermal broadening of the lowest two optical (T02, T01) and acoustic (LA, TA) branches.     

Mg2Sn电子结构及热力学性质的第一性原理计算

采用基于第一性原理的赝势平面波方法系统地计算了Mg₂Sn基态的电子结构、弹性常数和热力学性质.计算结果表明Mg₂Sn的禁带宽度为0.1198eV.运用线性响应方法确定了声子色散关系和态密度,得出Mg₂Sn的热力学性质如等容比热和德拜温度.计算Mg₂Sn的热导率并与实验数据相比较. 关键词： 第一性原理 电子结构 弹性常数 热力学性质     

First principles vibrational dynamics of magnesium telluride

The lattice dynamics of large-gap semiconductor MgTe compound at various crystallographic phases; rocksalt (B1), zincblende (B3), NiAs (B8₁) and wurtzite (B4), has been investigated from first principles calculations based on density functional theory (DFT) within plane-wave pseudopotential method and generalized gradient approximation (GGA) of the exchange-correlation functional. The static equation of states of the compound has been studied with Vinet equation of states. The ground state of the compound is a fourfold coordinated wurtzite structure, which is consistent with experiments and recent theoretical calculations. Full phonon dispersion spectra of all related phases of the MgTe have been calculated using density functional perturbation theory within the linear-response approach. In view of the total energy calculations and the obtained vibrational spectra, it can be emphasized that the MgTe polymorphs with tetrahedral coordination (zincblende and wurtzite structures) are of covalent character rather than ionic. The large TO-LO splitting of phonon branches of rocksalt and NiAs phases reflect the high ionicity of these phases.     

Lattice dynamics investigations of SiGe core-shell nanowires

We present results from lattice dynamics calculations on the phonon modes and specific heat of SiGe core-shell nanowires. The results show that phonon dispersion relation of SiGe nanowires consists of four acoustic branches. The frequency of the first optical mode at Γ point shifts to low frequency as the Ge concentration is increasing. There are three strong peaks in the spectra of density of states. The peaks at 9.0 THz and 15.0 THz can be attributed to the high frequency Ge-Ge and Si-Si bond vibration. The broad peak around 3.0 THz of pure silicon nanowire corresponds to the transverse acoustic branch of bulk silicon. Moreover, specific heat of SiGe nanowires increases (decreases) with the increase of the concentration x at low (high) temperature. The specific heat at 300 K can be fitted by C _V = x ² C _Ge + (1 − x)C _Si, where C _Ge and C _Si are specific heat of pure germanium and silicon nanowires respectively.     

Ground state and lattice dynamical study of ionic conductors CaF2, SrF2 and BaF2 using density functional theory

The present paper reports a comprehensive and complementary study on structural, electronic and phonon properties of face centered cubic fluorites, namely CaF₂, BaF₂ and SrF₂, using first principles density functional calculations within the generalized gradient approximation. The calculated lattice constants and bulk modulus are in good agreement with available experimental data. The analysis of band structure and density of states confirms the ionic character for all the three fluorides. The phonon dispersion curves and corresponding phonon density of states obtained in the present work are consistent with the available experimental and other theoretical data. The LO-TO splitting is maximum for CaF₂, which confirms that the ionicity is maximum in the case of CaF₂. The phonon properties for SrF₂ have been calculated for the first time.     

Dispersion and density of states of phonons and electrons in an α-B<Subscript>12</Subscript> crystal determined from first principles

Using the DFT method, we study the phonon properties of an α-B₁₂ rhombohedral crystal in the basis set of plane waves and its electronic structure in the localized basis set of Gaussians. It follows from the phonon dispersion that the crystal possesses a dynamical stability. The effective Born charges, the oscillator strengths, the transverse–longitudinal splitting, and the dielectric functions of dipole modes are calculated. We show that charge transfer from polar to equatorial atoms takes place in a В₁₂ icosahedron, while В–В bonds have predominantly a covalent character. In the density of states of acoustic modes, we reveal a structure that can manifest itself in the spectra of disordered boron compounds. From the dispersion of electronic bands, the occurrence of an indirect energy gap follows. The overlap of partial densities implies the hybridization of s and p electronic states in boron atoms.     

On the lattice dynamics of grey tin

The lattice dynamics of grey tin is studied by fitting a Born-von Karman model to the experimental neutron scattering data of Price and Rowe. In agreement with earlier conclusions drawn for Ge, it is found that fifth neighbor interactions are essential for achieving a good fit. New information is obtained concerning the values of the elastic constants and the Σ₂, Σ₄ branches of the dispersion curves. The density of phonon states is calculated employing the analytical integration method of Gilat and Raubenheimer. Calorimetric data for α-Sn agree reasonably with the results of the computation.     

Structural,electronic and vibrational properties of ordered intermetallic alloys CoZ (Z = Al,Be, Sc and Zr) from first-principles total-energy calculations

Self-consistent band calculations on four intermetallic compounds of the CsCl structure are presented. The calculations were performed employing the self-consistent ultrasoft pseudopotential method based on the density functional theory, within the local density approximation and the generalized gradient approximation. The calculations predicted that the equilibrium lattice constants are in excellent agreement with the experiment for CoAl and are 1% smaller than experimental values for CoBe, CoSc and CoZr, respectively. In the present study, ordered CoAl do not show any magnetic moment, whereas the other three compounds have moderate magnetic moments of about 0.2 and 0.7 Bohr magnetons (μ_B ) per atom. The elastic constants are calculated using two approaches, the energy-strain method and the use of phonon dispersion curves. The values obtained from the two methods are in reasonable agreement for the studied intermetallic compounds CoZ (Z?=?Al, Be, Sc and Zr). The brittleness and ductility properties of CoZ (Z?=?Al, Be, Sc and Zr) are determined by Poisson’s ratio σ criterion and Pugh’s criterion. The calculated elastic constants satisfy the mechanical stability criterion and the ductility of CoZr and CoSc is predicted by Pugh’s criterion. The band structure and density of states, and phonon dispersion curves have been obtained and compared with the available experimental results as well as with existing theoretical calculations. We studied and discussed the position of Fermi level for the selected four intermetallic compounds.     

Analysis of the crystal lattice instability for cage–cluster systems using the superatom model

We have investigated the lattice dynamics for a number of rare-earth hexaborides based on the superatom model within which the boron octahedron is substituted by one superatom with a mass equal to the mass of six boron atoms. Phenomenological models have been constructed for the acoustic and lowenergy optical phonon modes in RB₆ (R = La, Gd, Tb, Dy) compounds. Using DyB₆ as an example, we have studied the anomalous softening of longitudinal acoustic phonons in several crystallographic directions, an effect that is also typical of GdB₆ and TbB₆. The softening of the acoustic branches is shown to be achieved through the introduction of negative interatomic force constants between rare-earth ions. We discuss the structural instability of hexaborides based on 4f elements, the role of valence instability in the lattice dynamics, and the influence of the number of f electrons on the degree of softening of phonon modes.     

A search for high frequency vibrational modes at a stepped surface

In this paper we present a theory of the vibrations of atoms in the vicinity of a stepped surface on a Bravais crystal. The static relaxations in the positions of the atoms in the crystal are determined, and the atomic force constants are then calculated in the relaxed atomic configuration. The general theory is applied to a simple stepped surface, and the local phonon density of states is carried out for atoms at several points on the stepped surface by the real space continued fraction recursion method. No evidence is found for high frequency surface phonons.     

Effects of non-equilibrium polar optic phonons and the band non-parabolicity on small signal high-frequency hot electrons ac mobility in narrow gap semiconductors in high magnetic fields

The small signal high-frequency ac mobility of hot electrons in n-HgCdTe in the extreme quantum limit at low and high temperatures have been calculated considering the non-equilibrium phonon distribution as well as the thermal phonon distribution .The energy loss rate has been calculated considering only optical phonon scattering while the momentum loss rate has been calculated considering acoustic phonon scattering and piezoelectric scattering together with polar optical phonon scattering and separately considering only the polar optical scattering. The results have been discussed and compared. It has been observed that at 20 K, the normalized mobility considering all the three scattering mechanisms differs appreciably from that considering only the polar optical phonon scattering. However, at 77 K, there is no difference in the normalized mobility. This establishes the fact that at higher temperature, the effect of acoustic phonon scattering and piezoelectric coupling is negligible, compared to the polar optical phonon scattering. So the ac mobility considering only polar optical phonon scattering has been studied at 77 and 20 K. The ac mobility is found to remain constant up to 100 GHz and thereafter it started decreasing at higher frequencies at 77 K whereas the ac mobility reduces at much lower frequencies at lower temperature at lower field. The non-parabolicity of the band structure enhances the normalized mobility.     

Lattice dynamics and the incommensurability of the structures of hexagonal polytypes of diamond

In the density functional theory approximation, we have calculated the phonon dispersions and the densities of vibrational states of the 2H, 3C, 4H, 6H, and 8H hexagonal polytypes of diamond. We have found that a one-dimensional incommensurate modulation of the structure arises along the hexagonal axis, the parameter of which not only exceeds the parameters of basic translations of the crystal lattices of polytypes, but also is not a multiple of them. Based on the estimation of interactions between bilayers of carbon atoms in the structure of polytypes, we have assumed that competing interactions between bilayers are the main mechanism by which incommensurability arises. We have shown that optical measurements of vibrational frequencies in the center of the Brillouin zone of polytypes make it possible to retrieve the dispersion of acoustic branches of cubic diamond.     

Lattice dynamics and inelastic neutron scattering experiments on andalusite,Al2SiO5

We report coherent inelastic neutron scattering measurements of the phonon dispersion relations and lattice dynamics shell model calculations of several microscopic and macroscopic properties of andalusite, Al₂SiO₅. Andalusite has an orthorhombic structure with 32 atoms/unit cell. The inelastic neutron scattering measurements (up to energy transfers of 45 meV) were carried out using the triple axis spectrometer at Dhruva reactor, India using a single crystal of andalusite and the phonon dispersion relations along the [100] direction have been measured. The shell model calculations have been used to compute the crystal structure, elastic constants, phonon frequencies, dispersion relations, density of states and the specific heat. The calculated results are in good agreement with available experimental data. The computed one-phonon neutron scattering structure factors based on the shell model have been very useful in the planning and analysis of the inelastic neutron scattering experiments.     

First principles lattice dynamical study of the cubic antiperovskite compounds AsNBa3 and SbNBa3

An ab initio pseudopotential plane wave method using linear response approach has been employed to study the lattice dynamics of two cubic antiperovskites AsNBa₃ and SbNBa₃. The bulk properties, elastic constants, phonon dispersion curves, phonon density of states and temperature dependent thermodynamic quantities of both antiperovskites are obtained. The calculated lattice constants, elastic and bulk properties are compared with the available theoretical data. This is the first systematic and quantitative prediction of phonon and thermodynamical properties of these antiperovskite compounds.     

Structural, electronic and phonon properties’ investigation of YP and YAs compounds

We have studied the structural, electronic and phonon properties of the YP and YAs compounds in NaCl(B1) and CsCl(B2) structures using the density functional theory within the generalized gradient approximation (GGA). The calculated lattice constants, static bulk modulus, first-order pressure derivative of the bulk modulus and transition pressure are reported and compared with previous calculations. We have carried out the calculations of band structure and density of states (DOS) for YP and YAs. Then, a linear-response approach to the density-functional theory is used to derive the phonon frequencies and DOS in both B1 and B2 structures.