Lattice dynamics of sapphire (Al2O3)

Using inelastic neutron scattering we have determined all the dispersion branches of the ₁, ₂ and ₃ representations along the three-fold axis as well as the 2 times 15 branches of ₁ and ₂ symmetry along the -A-direction plus some branches along the -D-direction. The experimental data are analyzed using various rigid ion, polarizable ion and shell models. The shell models give a very satisfactory account of the dispersion curves as well as the scattering intensities. Special attention is given to the investigation of dielectric constants and equilibrium conditions.     

Lattice mechanical properties of alkaline earth metals in bcc and fcc phases

A model pseudopotential depending on an effective core radius treated as a parameter is used for alkaline earth metals in bcc and fcc phases to study the Binding energy, Interatomic interactions, phonon dispersion curves, Phonon density of states, Debye-Waller factor, mean square displacement, Debye-Waller temperature parameters, dynamical elastic constants (C₁₁, C₁₂ and C₄₄), bulk modulus (B), shear modulus (C′), deviation from Cauchy relation (C₁₂−C₄₄), Poisson's ratio (σ), Young's modulus (Y), behavior of phonon frequencies in the elastic limit independent of the direction (Y₁), limiting value in the [1 1 0] direction (Y₂), degree of elastic anisotropy (A) and propagation velocities of the elastic waves. The contribution of s-like electrons is incorporated through the second-order perturbation theory due to model potential. The theoretical results are compared with the existing experimental data. A good agreement between theoretical investigations and experimental findings has confirmed the ability of our potential to yield large numbers of lattice mechanical properties of certain alkaline earth metals.     

Theoretical study of phonon spectra in ferromagnetic nanoparticles

Based on the spin-phonon model we analyze the influence of surface and size effects on the phonon properties of ferromagnetic nanoparticles. A Green's function technique in real space enables us to calculate the renormalized phonon energy and its damping depending on the temperature and the anharmonic spin-phonon interaction constants. With decreasing particle size the phonon energy can decrease or increase for different surface spin-phonon interaction constants, whereas the damping increases always. The influence of an external magnetic field is discussed, too. The theoretical results are in reasonable accordance to experimental data.     

Transmission and scattering properties of acoustic waves in phononic band gap materials

We have developed a formula for studying the transmission and scattering properties of finite-sized phononic band gap (PBG) material. We will show that based on the far field approach the transmission coefficients can be obtained by treating PBG samples as scattering objects. We find that the results agree well with the band structure.     

Microscopic foundation of the phenomenological few-level approach to coherent semiconductor optics

We present the results of inelastic neutron scattering experiments for Cr₂O₃ carried out using the single-crystal time-of-flight spectrometer PRISMA at ISIS (U.K.) as well as the three-axis spectrometer TAS-1 at JAERI(J). The collected data are analysed on the basis of phenomenological shell models showing convincing agreement between calculation and experiment both for the frequencies and intensities. All together the dispersion relations of 12 out of 20 phonon branches along the three-fold axis are determined. By a comparison with the Cr₂O₃ iso-type sapphire (Al₂O₃) it is found that the dynamical response of the oxygen ion is only little affected by the ionic substitution. The chromiumd electrons find their expression mainly in a strengthening of the metal-oxygen bonding. The problem of ionic polarizabilities is addressed in detail both for the oxygen and chromium ions.     

Lattice mechanical properties of noble and transition metals

A model pseudopotential depending on an effective core radius but otherwise parameter free is used to study the interatomic interactions, phonon dispersion curves (inq and r-space analysis), phonon density of states, mode Grüneisen parameters, dynamical elastic constants (C ₁₁,C ₁₂ andC ₄₄), bulk modulus (B), shear modulus (C′), deviation from Cauchy relation (C ₁₂–C ₄₄), Poisson’s ratio (σ), Young’s modulus (Y), behavior of phonon frequencies in the elastic limit independent of the direction (Y ₁), limiting value in the [110] direction (Y ₂), degree of elastic anisotropy (A), maximum frequencyω _max, mean frequency 〈ω〉, 〈ω ²〉^1/2=(〈ω〉/〈ω ⁻¹〉)^1/2, fundamental frequency 〈ω ²〉, and propagation velocities of the elastic constants in Cu, Ag, Au, Ni, Pd, and Pt. The contribution of s-like electrons is calculated in the second-order perturbation theory for the model potential while that of d-like electrons is taken into account by introducing repulsive short-range Born-Mayer like term. Very recently proposed screening function due to Sarkar et al. has been used to obtain the screened form factor. The theoretical results are compared with experimental findings wherever possible. A good agreement between theoretical investigations and experimental findings has proved the ability of our model potential for predicting a large number of physical properties of transition metals.     

Neutron scattering study of the intermolecular vibrations in solid C60

We present a detailed account of an inelastic neutron scattering study of the intermolecular vibrations in solid C₆₀ above and below the structural phase transformation. Assignment of the observed phonon peaks to modes of translational and librational character is discussed on the basis of lattice dynamical calculations using phenomenological intermolecular potentials. A new bond charge model is presented which reproduces the observed phonon branches very well. However, the apparently small influence of the orientational disorder on the librational excitations remains to be understood.     

On the properties of real cadmium telluride surfaces

Optical and electrical properties of lapped, mechanically polished, and etched cadmium telluride surfaces have been studied by ellipsometry, infrared transmission, reflectivity, and sheet resistance measurements. They have been related to the characteristics of Schottky barriers realized by deposition of gold on these surfaces. These experiments indicate that the electrical behaviour of devices made of cadmium telluride critically depends on the surface damages produced during the sample preparation: while good diodes can be made when bromine in methanol etching is used, lapping and polishing produce a disturbed layer of poor electrical and optical properties whose thickness is related to the size of the abrasive powder used during the preparation.     

Potentials induced by the electron-optical-phonon interaction in a quantum well

The potential induced by the electron-optical-phonon interaction in a quantum well (QW) is investigated by means of the perturbation theory. We consider the interactions of an electron with both bulklike confined longitudinal optical (LO) phonons and four branches of interface optical (IO) phonons. The spatial distributionV _i(z) of the induced potential for QW structures with different heterolayer compositions and different well widths is calculated in detail. The numerical results show that the heterolayer composition of the QW plays an important role in determining the shape ofV _i(z) and that the existence of IO-phonons is important to the electronic states in QWs.     

Lattice vibrational properties of uranium pnictides

Lattice vibrational properties of uranium pnictides have been studied using breathing shell model (BSM) which includes breathing motion of electrons of the U-atoms due tof−d hybridization. The phonon dispersion curves of U-pnicitides calculated from the present model agree reasonably well with the measured data. A comparison has been made between BSM and our results reported earlier obtained from three-body force rigid ion model to reveal the importance of the short-range electron-phonon interactions in these compounds. We also report, for the first time, the two phonon density of states and specific heat for these compounds.     

Anharmonic properties of TmTe

Data on elastic constants and associated properties at high temperature for TmTe crystal are presented and discussed starting from primary physical parameters viz. nearest neighbour distance and hardness parameter assuming long- and short-range potentials. When the values of the higher order elastic constants are known for a crystal, many of the anharmonic properties of the crystal can be treated within the limit of the continuum approximation in a quantitative manner. In this study, higher order elastic constants and related properties are computed upto 1000 K for TmTe. The first-order pressure derivatives of second- and third-order elastic constants, the second-order pressure derivatives of second-order elastic constants and partial contractions are also evaluated at different temperatures. The results thus obtained are compared with other available data and found in well agreement with present values.     

Phonon Dispersion in Equiatomic Li-Based Binary Alloys

The computations of the phonon dispersion curves （PDC） of four equiatomic Li-based binary alloys, namely Li0.5Na0.5, Li0.5K0.5, Li0.5Rb0.5 and Li0.5Cs0.5, to second order in the local model potential is discussed in terms of the real-space sum of Born von Karman central force constants. Instead of the concentration average of the force constants of metallic Li, Na, K, Rb and Cs, the pseudo-alloy atom （PAA） is adopted to compute directly the force constants of four equiatomic Li-based binary alloys. The exchange and correlation functions due to Hartree （H） and Ichimaru-Utsumi （IU） are used to investigate the influence of screening effects. The phonon frequencies of four equiatomic Li-based binary alloys in the longitudinal branch are more sensitive to the exchange and correlation effects in comparison with the transverse branches. However, the frequencies in the longitudinal branch are suppressed due to IU-screening function than the frequencies due to static H-screening function.     

Debye temperature of cubic crystals

An algorithm for determination of the Debye temperature is presented, which enables the easy computation of the Debye temperature for arbitrary stable cubic crystal.     

Dispersive phonon imaging of InAs

The phonon-focusing patterns of ballistic phonons in InAs are measured in the frequency range 0.1 to 1 THz, in an effort to test the global validity of lattice dynamics models for this semiconductor. Phonon caustic patterns depend sensitively on the shapes of constant frequency surfaces. Several tunnel-junction detectors with sensitivity onsets in this frequency range are used to measure dispersive shifts in the phonon caustics. The measured caustic positions are compared to those predicted by rigid-ion and bond-charge models. Similar to the case of InSb studied by Hebboul and Wolfe, a 6-parameter bond-charge model (BCM)-with force constants determined by neutron, X-ray, and Raman scattering-reproduces the phonon-imaging data both qualitatively and quantitatively. Comparisons of the focusing patterns with an 11-parameter rigid-ion model (RIM) do not show good agreement. New structures are predicted in the phononfocusing patterns at frequencies above about 1.2 THz — presently outside our experimental range-which are highly sensitive to the theoretical modeling.     

Three-body interaction and phonon dispersion relations in aluminium

Based on an ab initio cohesive energy calculation and a model of three-body interaction, the pair potential can be calculated using the Möbius inversion theorem in the theory of numbers. Then the atomic force constants and the phonon dispersion for A1 are evaluated both with and without three-body interaction. Compared with experiments, the results show that taking the three-body interaction into account considerably improves the dispersions. Contrary to previous work, the method for calculating the atomic force constants and phonon dispersions presented here is simple, with only two adjustable parameters.     

Phonons in Excited State for Polydiacetylene

A simple model is used to evaluate the phonon frequencies in both ground and excited state for polydiacetylene. The spring constants K^σ_i in the σ-bond skeleton are distinct from the experimental effective spring constants K_i. The results are compared with experimental date.     

A model for vibrational properties of the atomic interface in A/B fcc metals

The motivation of this theoretical work is to introduce a model calculation for the elastic waves scattering and coherent phonon transport at an atomic nanojunction between face-centered cubic (fcc) leads. The model system A/B consists of two perfect semi-infinite fcc leads A and B, oriented in the same direction and joined by an atomic interface. It is applied to the system Cu/Ni and its inverse Ni/Cu. A theoretical approach based on the matching method is used to study the dynamics of the system A/B.     

Phonons in YBa2Cu3O7−x crystals with site-selective oxygen isotope substitution: Frequencies,normal vectors,non-linear properties

Lattice vibrations of YBa₂Cu₃O_7–x crystals with a site-selective isotope substitution¹⁸O¹⁶O are investigated theoretically. It is shown that shifts of the frequencies of A_1g oxygen vibrations due to this substitution are smaller than under complete one. However, phonon bands and normal vectors are changed much stronger, especially in the case when apical oxygens are substituted. On the basis of the presented analysis anharmonic properties and transition temperatures of the selectively substituted crystals are also briefly discussed.