Effect of vibrational states on nuclear level density

Simple methods to calculate a vibrational enhancement factor of a nuclear level density with allowance for damping of collective state are considered. The results of the phenomenological approach and the microscopic quasiparticle—phonon model are compared. The practical method of calculation of a vibrational enhancement factor and level density parameters is recommended. The text was submitted by the authors in English     

Effect of densification on the density of vibrational states of glasses

We studied the effect of densification on the vibrational dynamics of a Na(2)FeSi(3)O(8) glass. The density of vibrational states (DOS) has been measured using nuclear inelastic scattering. The corresponding changes in the microscopic, intermediate-range, and macroscopic properties have also been investigated. The results reveal that, in the absence of local structure transformations, the Debye level and the glass-specific excess of vibrational states above it have the same dependence on density, and the evolution of the DOS is fully described by the transformation of the elastic medium.     

Calculation of the vibrational spectra of copper crystals with vacancies

The local vibrational spectra of copper crystals containing vacancies are calculated using the pair atomic potential derived in the framework of the resonance pseudopotential theory. The calculations are performed by a recursive method with due regard for the symmetry of the defect region. The frequencies of the vacancy-induced resonance vibrations of different symmetries are determined.     

Effect of vacancies and interstitials on the dynamic properties of the La2−x SrxCuO4 superconductor

The effect of vacancies and interstitials in the CuO₂ layer on the vibrational spectrum in the La_2−x Sr_xCuO₄ system has been calculated by molecular dynamics. It is shown that the excitation probability for local ∼0.4-eV high-frequency vibrations of nonphonon origin in the vicinity of Sr impurity atoms decreases if copper vacancies are introduced at a concentration x=0.17, which corresponds to the maximum superconducting transition temperature, this decrease being still more effective (by about ten times) if interstitial atoms are present. The appearance of interstitials makes a considerable region around them (five to six nearest neighbors) quasi-amorphous. A comparison with available experimental data is made. It is concluded that the behavior of the system under irradiation is accounted for primarily by interstitials, which bring about strong perturbation in the lattice (∼1 nm) up to making it completely amorphous. Fiz. Tverd. Tela (St. Petersburg) 40, 984–988 (June 1998)     

Resonant modes due to anion interstitials and anion vacancies in fluorites

The resonant modes due to anion interstitials and anion vacancies in fluorites are computed on the basis of a Green's function formalism. The Green's functions are calculated on the basis of shell model fitted to the phonon dispersion curves of the respective crystals. The force constant changes are calculated by finding the relaxations of the different host atoms and then using the short range potential due to Catlow and Norgett. The calculated frequencies of the resonant mode due to anion interstitials in BaF₂; shows good agreement with the experimental results. The results discussed in a comparative fashion for the three crystals CaF₂, SrF₂; and BaF₂; along with the possibility of their experimental detection.     

Diffusion coefficients of vacancies and interstitials along tilt grain boundaries in molybdenum

The diffusion coefficients of vacancies and interstitials along symmetrical tilt grain boundaries in molybdenum have been calculated using the molecular dynamics method. The migration energies of defects have been obtained. The activation energy and coefficients of grain boundary self-diffusion have been deter-mined. A comparison of the obtained results with the studies of other authors indicates that boundaries formed between particles in the powder in sintering experiments have a higher diffusion activity as compared to stable grain boundaries in polycrystals.     

Simple models of the density of vibrational states in polyatomic molecules

An analytic approach to calculations of the density of vibrational states in a system consisting of independent anharmonic oscillators was developed. The method worked at all energies from zero point energies to the complete dissociation energies of the selected oscillators.     

Electric quadrupole interaction due to vacancies and interstitials in cubic host metals

A procedure has been developed to evaluate from first principle the electric field gradient (efg) due to vacancies and interstitials in cubic host metals. For this purpose the screening charge distribution in the valence-effect has been constructed in scattering theory using appropriate muffin-tin potential. In the size-effect, the efg has been evaluated in a more realistic model of the lattice than the commonly used elastic continuum. Unlike other calculations, no adjustable parameter has been used in the present procedure.     

The density of unoccupied electronic states in copper

The density of unoccupied electronic states in Cu has been measured employing soft X-ray appearance-potential spectroscopy (SXAPS). The L_III le     

Elastic metamaterials with simultaneously negative effective shear modulus and mass density

We propose a type of elastic metamaterial comprising fluid-solid composite inclusions which can possess a negative shear modulus and negative mass density over a large frequency region. Such a material has the unique property that only transverse waves can propagate with a negative dispersion while longitudinal waves are forbidden. This leads to many interesting phenomena such as negative refraction, which is demonstrated by using a wedge sample and a significant amount of mode conversion from transverse waves to longitudinal waves that cannot occur on the interface of two natural solids.     

Split interstitials in computer models of single-crystal and amorphous copper

The effect of interstitial atoms in a dumbbell configuration on elastic moduli of single crystal copper has been investigated using molecular dynamics simulation. It has been shown that shear components of the dipole tensor and λ-tensor increase when the concentration of split interstitials exceeds 0.6–0.7%. This is associated with their interaction responsible for a significant change in the distribution of orientations of split interstitials and, hence, for a change in the type of the induced local symmetry breakings of the face-centered cubic structure. It has been found that, in the model of amorphous copper, there are “defects” (elastic dipoles) with properties similar to those of split interstitials in the single crystal. The deviatoric component of their λ-tensor is more than an order of magnitude greater than the dilatation component and is responsible for the decreased value of the shear modulus and thermal effects in noncrystalline metallic materials.     

Study of temperature dependence of density, bulk modulus, shear modulus and thermal pressure for mantle minerals

The temperature dependence of different parameters i.e. density ρ(T), bulk modulus K_T(T), shear modulus G_T(T) and thermal pressure ΔP_th for mantle minerals i.e. X₂SiO₄ (X=Mg, Fe, Co, Mn) have been studied in high temperature range on the basis of semi-phenomenological isobaric equation of state. The calculated values of these parameters are showing good agreements with experimental value in case of each mantle minerals.     

Effect of edge vacancies on localized states in a semi-infinite zigzag graphene sheet

The effect of vacancies on the robustness of zero-energy edge electronic states in zigzag-type graphene layer is studied at different concentrations and distributions of defects. All calculations are performed by using the Green’s function method and the tight-binding approximation. It is found that the arrangement of defects plays a crucial role in the destruction of the edge states. We have specified a critical distance between edge vacancies when their mutual influence becomes significant and affects markedly the density of electronic states at graphene edge.     

Effect of vacancies on the spin density waves onset in Fe/Cr superlattices

The spin density wave’s onset in Cr based superlattices is considered within proximity of Fe interlayer boundaries and the effect of randomly located vacancies in Cr monolayers is examined. The study is performed for Fe/Cr, Fe/Cr/V superlattices with odd and even number of Cr monolayers. It is shown that the number of Cr monolayer determines the spin density wave’s nodes onset in the perfect Fe/Cr super lattices. Pinning of Cr magnetic moments on vacancies destroys this determination and leads to appearance or disappearance of nodes.     

Anharmonic analysis of the vibrational states of pyrimidine by the density functional method

The quartic force field of pyrimidine is calculated in the approximation of the hybrid density functional B3LYP/6-31G(d). On the basis of this force field, the IR spectrum of pyrimidine in the range 250–3800 cm^?1 is interpreted. The Darling-Dennison and Fermi resonances are taken into account and their spectral manifestations are analyzed. A combined method for the anharmonic analysis of the vibrational states of polyatomic molecules that employs the theoretical anharmonicity constants and experimental frequencies is proposed. The method ensures a higher prediction accuracy.     

Effect of working temperature on young's modulus of electrolytic copper

Electrolytic copper wires of different grain diameters were given variable amounts of plastic strain, and then subjected to increased tensile loadings at different working temperatures. The associated changes in Y. M. were measured. Plastic strain decreased Y. M. at first to a minimum value, then it increased Y. M. This effect was interpreted in terms of texture variations and mutual dislocation interactions. Increasing working temperature decreased Y. M. because of accompanying increase in slip distances.     

Dependence of vibrational density of states and Raman spectra on size in non-polar and polar nano-crystalline semiconductors

The vibrational density of states (VDOS) of Si, diamond, SiC, and InSb clusters has been calculated using the cluster model for various cluster sizes. The results show that the peak frequency of optical-like bands of VDOSs of non-polar nano-crystalline (NC) semiconductors varies with size, but that of polar NC semiconductors does not vary with size. We attribute the origin of this different behavior of non-polar and polar NC semiconductors to different interactions in the optical-like modes of them. That is, the deformation potentials for the non-polar NC semiconductors and electrostatic potentials for the polar NC semiconductors. According to the amorphous crystal (aC) model, Raman spectra are directly related to VDOS. In this Letter, it is verified that the aC model can be applied to NC semiconductors. Calculated VDOS are compared with observed Raman spectra of corresponding samples, which show agreement.     

High-temperature behaviour of average structure and vibrational density of states in the ternary superionic compound AgCuSe

Results of simultaneous thermal analysis (STA), synchrotron powder diffraction (in the range 300–973 K) and inelastic neutron scattering (at 285 and 505 K) on non-superionic β- and superionic α-AgCuSe are reported. The sample is stable in argon on heating. The volume change at the superionic phase transition is about 5%. A model for the average structure of α-AgCuSe is proposed. No anomalies in the temperature dependence of the parameters of the average structure were revealed. Ionic conductivity in α-AgCuSe can originate from cation jumps in “skewed” 〈100 〉 directions between nearest-neighbour tetrahedral sites via the peripheries of the octahedral cavities. A correlation between the temperature dependence of the cation redistribution in α-AgCuSe and the temperature dependence of the ionic conductivity is supposed. Various contributions (anharmonic effects, time-average static disorder and phonon-phonon scattering) to the widths of individual phonons upon temperature increase lead to pronounced changes in the neutron-weighted densities of states of β- and α-AgCuSe and accompany the superionic phase transition as well.