Elastic properties of compressed rare-gas crystals in a model of deformable atoms

Physics of the Solid State - The elastic properties of compressed Ne, Ar, Kr, and Xe rare-gas crystals were studied in a model of deformable and polarizable atoms. The second-order Fuchs elasticity...     

Elastic properties of heavy rare-gas crystals under pressure in the model of deformable atoms

The quantum-mechanical model of deformable and polarizable atoms has been developed for the purpose of investigating the elastic properties of crystals of rare gases Kr and Xe over a wide range of pressures. The inclusion of the deformable electron shells in the analysis is particularly important for the shear moduli of heavy rare-gas crystals. It has been shown that the observed deviation from the Cauchy relation δ(p) for Kr and Xe cannot be adequately reproduced when considering only the many-body interaction. The individual dependence δ(p) for each of the rare-gas crystals is the result of two competitive interactions, namely, the many-body and electron-phonon interactions, which manifests itself in a quadrupole deformation of the electron shells of the atoms due to displacements of the nuclei. The contributions of these interactions in Kr and Xe are compensated with good accuracy, which provides a weakly pressure-dependent value for the parameter δ. The ab initio calculated dependences δ(p) for the entire series Ne-Xe are in good agreement with the experiment.     

Atomic properties for Ne,Ar, Kr from an optimised potential model for atoms

A variationally optimised effective atomic central potential has been used to calculate atomic form factors, 〈r²〉, ionization potentials, and dipole polarizabilities for Ne, Ar and Kr. The predictions agree very closely with Hartree-Fock results.     

Elastic scattering of electrons from He, Ne and Ar atoms at 35 keV

The eikonal Born series (EBS) method is applied to the elastic scattering of electrons by He, Ne and Ar atoms at 35 keV. The differential cross-sections are compared with the numerical results obtained by the partial-wave analysis. A simple analytical Dirac-Hartree-Fock-Slater (DHFS) field is used for these atoms. The results are also obtained by Wallace, Das and modified Das method. An oscillatory nature and a strong forward peak in the cross-section are not found at 35 keV. The results are nearer to the experimental data of Coffmann and M. Fink as well as numerical results based on relativistic partial-wave treatment.     

Elastic and ultrasonic properties of single crystalline nickel nanowires

In the present paper, we have theoretically calculated the non linear elastic constants of single crystalline Ni NWs at very broad temperature range 20–300 K validating simple interaction potential model. The temperature dependent ultrasonic attenuation and other related properties are determined using their second and third order elastic constants (SOECs/TOECs). Where possible, the results are compared with experiments from literature. There is a correlation between the thermal conductivity and ultrasonic attenuation in the temperature range 100–300 K. Also, a correlation between the resistivity and ultrasonic attenuation in the temperature range 40–100 K has been established validating the theoretical approach.     

Elastic properties of crystalline and liquid gallium at high pressures

The elastic properties of gallium, such as the bulk modulus B, the shear modulus G, and the Poisson’s ratio σ, are investigated and the relative change in the volume is determined in the stability regions of the Ga I, Ga II, and liquid phases at pressures of up to 1.7 GPa. The observed lines of the Ga I-Ga II phase transition and the melting curves of the Ga I and Ga II phases are in good agreement with the known phase diagram of gallium; in this case, the coordinates of the Ga I-Ga II-melt triple point are determined to be 1.24 ± 0.40 GPa and 277 ± 2 K. It is shown that the Ga I-Ga II phase transition is accompanied by a considerable decrease in the moduli B (by 30%) and G (by 55%) and an increase in the density by 5.7%. The Poisson’s ratio exhibits a jump from typically covalent values of approximately 0.22–0.25 to values of approximately 0.32–0.33, which are characteristic of metals. The observed behavior of the elastic characteristics is described in the framework of the model of the phase transition from a “quasi-molecular” (partially covalent) metal state to a “normal” metal state. An increase in the Poisson’s ratio in the Ga I phase from 0.22 to 0.25 with an increase in the pressure can be interpreted as a decrease in the degree of covalence, i.e., the degree of spatial anisotropy of the electron density along the bonds, whereas the large value of the pressure derivative of the bulk modulus (equal to approximately 8) observed up to the transition to the Ga II phase or the melt is associated not only with the quasicovalent nature of the Ga I phase but also with the structural features. In view of the presence of seven neighbors for each gallium atom in the Ga I phase, the gallium lattice can be treated as a structure intermediate between typical open-packed and close-packed structures. Premelting effects, such as a flattening of the isothermal dependence of the shear modulus G(p) with increasing pressure and an increase in the slope of the isobaric dependences G(T) with increasing temperature, are revealed in the vicinity of the melting curve. The bulk modulus of liquid gallium near the melting curve proves to be rather close to the corresponding values for the normal metal Ga II.     

Nonlocal shear deformable shell model for thermal postbuckling of axially compressed double-walled carbon nanotubes

An investigation is reported of the thermal buckling and postbuckling of axially compressed double-walled carbon nanotubes (CNTs) subjected to a uniform temperature rise. The double-walled carbon nanotube is modeled as a nonlocal shear deformable cylindrical shell, which contains small-scale effects and van der Waals interaction forces. The governing equations are based on higher order shear deformation shell theory with a von Kármán–Donnell-type of kinematic nonlinearity and include thermal effects. Temperature-dependent material properties, which come from molecular dynamics (MD) simulations, and an initial point defect, which is simulated as a dimple on the tube wall, are both taken into account. The small-scale parameter, e ₀ a, is estimated by matching the buckling temperature of CNTs observed from the MD simulation results with the numerical results obtained from the nonlocal shear deformable shell model. The numerical illustrations concern the thermal postbuckling response of perfect and imperfect, single- and double-walled CNTs with different values of compressive load ratio. The results show that buckling temperature and postbuckling behavior of nanotubes are very sensitive to the small-scale parameter. The results reveal that temperature-dependent material properties have a significant effect on the thermal postbuckling behavior of both single- and double-walled CNTs.     

Conduction bands of compressed crystalline neon

The study is made of an excess electron in a perfect crystal of neon. Conduction bands (excess electron energy spectrum) are calculated ab initio using the OPW method. The consistent account is taken of the change in electronic functions of atoms in the course of their amalgamation to form a crystal and the effect of this amalgamation upon the excess electron behaviour is studied. These effects are weak at normal pressure but with the pressure increase their role becomes more important. When a crystal is compressed up to 16–20 kbar the conduction bands do not qualitatively change their forms but become broader in energy, the actual minimum remains in the centre of the Brillouin zone and the effective mass at this point decreases. At further compression strong deformation of the band is observed.     

Elastic scattering of20Ne on24Mg

High spin states in⁸⁵Rb and⁸⁵Sr were populated in the reactions⁷⁶Ge(¹²C,p2n) and⁷⁶Ge(¹²C,3n), respectively. The lifetimes of twelve states were measured via the recoil distance Doppler shift technique. The positive parity bands in both nuclei associated with theg _9/2 orbit exhibit decrasingE2 strengths for increasing spin value marking the onset of multiparticle excitation.     

Elastic scattering ofα-particles from20Ne

A relatively simple procedure using nuclear interaction calculated microscopically from two-nucleon potential employing equivalence of resonating group method and generator coordinate method has been used to calculate the differential cross-sections (DCS) forα +²⁰Ne elastic scattering atE _lab=18.0, 20.2, 21.9, 23.0 and 27.2 MeV. The absorption effects due to the opening of the non-elastic channels are taken into account approximately by the sharp cut-off of lower partial waves. The anomalous large oscillations of the DCS at backward angles atE _lab=18.0 and 27.2 MeV are reproduced. The calculated results are in fair agreement with the experimental data.     

Intermediate Energy Pion-20Ne Elastic Scattering in the α+16O Model of 20Ne

We present an analysis of π-²⁰Ne elastic scattering at intermediate energy basing on the α+¹⁶O model of the ²⁰Ne nucleus and in the framework of Glauber multiple scattering theory. Satisfactory agreement with the general features of the experimental data of pion elastic scattering on the neighboring 4N-type nuclei is obtained without any free parameters. Compared with the experimental angular distributions of pion elastic scattering on ¹²C, ¹⁶O, ²⁴Mg, and ²⁸Si nuclei, the diffractive patterns and the positions of the dips and peaks in the angular distributions of π-²⁰Ne elastic scattering are reasonably predicted by the calculations.     

Effect of Debye plasma on compressed exotic atoms

The effect of plasma environment on the energy levels and structural properties of exotic atomic systems p⁺μ⁻ and μ⁺μ⁻ bound by Coulomb force and confined at the centre of an impenetrable spherical box has been thoroughly studied. The plasma environment is assumed to produce a Debye type of screening in the potential energy. Compression is generated by confining the exotic system in a spherical box. The effect of plasma and compression on system properties like orbital energies, excited states, dipole polarizabilities, oscillator strength and transition probabilities has been studied using time dependent variation perturbation theory. Ionisation pressures at different plasma screening have been estimated. The data obtained may be useful in the astrophysical context where different plasma environments exist.     

Elastic properties of metastable crystalline and amorphous gasb-ge semiconductors synthesized under high pressure

We present the systematic study of the elastic shear G and bulk B moduli in amorphous and crystalline metastable ternary solid solutions (GaSb)_1?x Ge_2x . It is found that the moduli of crystalline phases initially decrease with Ge concentration, falling down to minimum values at 20-30% Ge. The minimal values of elastic moduli for amorphous samples are observed at 50-60% Ge. Elastic softness of crystalline solid solutions is assumed to be related to the increase of chemical disorder and, consequently, of static (non-thermal) geometrical disorder in positions of atoms. An additional topological disorder in amorphous solid solutions leads to additional elastic softening.     

Elastic properties of graphene: The Keating model

The elastic properties of graphene have been described in terms of the Keating model. It has been shown that the two-dimensional structure of graphene is described by two independent elastic constants, like an isotropic solid. The Young’s modulus and the Poisson’s ratio have been determined. The results are compared with the experimental data obtained for graphite.     

The concentration of excited Ne atoms in a hollow-cathode discharge in He-Ne

The absolute populations of Ne levels have been measured spectroscopically for gas pressures of 0.1–8.0 mm Hg.I am indebtedto V. P. Chebotaev for direction in this work and to I. M. Beterov for collaboration in deriving the ratio of the probabilities for the 6328 and 6096 Å lines.     

Elastic properties and model atomic interactions in Fe-Si alloys

The elastic constants and the lattice parameter of the Fe-Si alloy crystals are calculated in the concentration range 0–25 at.% Si. The interaction between atoms is described by two series of three potentials for the atomic pairs Fe-Fe, Si-Si, Fe-Si which are determined by fitting to the experimental data. The ordering reaction above 10 at.% Si causes such a change in the interaction that the second series of three potentials has to be introduced. A good agreement between theoretical and experimental concentration dependences of the elastic constants and the lattice parameter is reached.     

Elastic scattering of two ground-state N atoms

An interaction potential for an N₂(X¹σ_g⁺) molecule is constructed by using the highly accurate valence internally contracted multireference configuration interaction method and the largest basis set, aug-cc-pV6Z, in the valence range. The potential is used to investigate the elastic scattering of two N atoms at energies from 1.0× 10^-11 to 1.0× 10^-4 a.u. The derived total elastic cross sections are very large and almost constant at ultralow temperatures, and the shape of total elastic cross section curve is mainly dominated by the s-partial wave at very low collision energies. Three shape resonances are found in the total elastic cross sections. Concretely, the first one is very sharp and strong. It results from the g-partial-wave contribution and the resonant energy is 3.645× 10^-6 a.u. The second one is contributed by the h-partial wave and the resonant energy is 1.752× 10^-5 a.u. This resonance is broadened by those from the d- and f-partial waves. The third one comes from the l = 6 partial wave contribution and the resonant energy is 3.522× 10^-5 a.u. This resonance is broadened by those from the g- and h-partial waves. The N₂(X¹σ_g⁺) molecular parameters, which are determined at the current theoretical level, achieve very high accuracy due to the employment of the largest correlation-consistent basis set in the valence range.     

Elastic scattering of 800-MeV protons by 20Ne nuclei

The differential cross section and polarization observables are calculated for the elastic scattering of 800-MeV protons by ²⁰Ne nuclei. The assumption that the ²⁰Ne nucleus has an alpha-cluster structure is shown to lead to results that agree with the measured values of observables of the above scattering process.