Ab initio calculations of elastic constants and thermodynamic properties of Li2O for high temperatures and pressures

The elastic constants and thermodynamic properties of Li₂O for high temperatures and pressures are calculated by the ab initio unrestricted Hartree-Fock (HF) linear combination of atomic orbital (LCAO) periodic approach. The lattice constant, elastic constants, Debye temperature, and thermal expansion coefficient obtained are in good agreement with the available experimental data and other theoretical results. It is found that at zero pressure the elastic constants C₁₁, C₁₂ and C₄₄, bulk modulus B and Debye temperature Θ_D decrease monotonically over the wide range of temperatures from 0 to 1100 K. When the temperature , C₁₂ approaches zero, consistently with the transition temperature 1200 K. However, with increasing pressure, they all increase monotonically and the anisotropy will weaken.     

Strength of magnesium oxide under high pressure: evidence for the grain-size dependence

X-ray diffraction patterns from magnesium oxide compressed in a diamond anvil cell up to 55 GPa have been recorded and the differential stress (a measure of compressive strength) and grain-size (crystallite size) determined as a function of pressure from the line-width analysis. The strength agrees well with the uniaxial stress component (another measure of compressive strength) derived earlier from the line-shift data. The strength increases while the crystallite size decreases steeply as the pressure is raised from ambient to ∼10 GPa. The increase in strength is much smaller at higher pressures. The strength-pressure data are explained by combining the grain-size dependence of strength and the shear-modulus scaling law. The dependence of strength on grain-size has not been considered in the past in the discussion of high-pressure strength data.     

High pressure X-ray diffraction study of Fe2B

We report the results of a synchrotron based X-ray diffraction study of bct-Fe₂B under quasi-hydrostatic conditions from 0 to 50 GPa. Over this pressure range, no phase change or disproportionation has been observed. A weighted fit of the data to the Birch-Murnaghan equation of state yields a value of the bulk modulus, K, of 164±14 GPa and the first pressure derivative of the bulk modulus, K′, of 4.4±0.5. The compression is found to be anisotropic, with the a-axis being more incompressible than the c-axis.     

Equation of state of cubic hafnium(IV) nitride having Th3P4 -type structure

Pressure dependence of the specific volume, V(P), of the recently discovered high-pressure compound Hf₃N₄ having cubic Th₃P₄-type structure (c- Hf₃N₄) has been measured at room temperature up to 43.9 GPa in a diamond anvil cell using energy-dispersive X-ray powder diffraction combined with synchrotron radiation. A least-square fit of the Birch-Murnaghan equation of state to the experimental V(P)-data yielded for c- Hf₃N₄ the bulk modulus of and its first pressure derivative of . For fixed at 4 the bulk modulus of c- Hf₃N₄ was determined to be . The obtained B₀-value is only insignificantly below that estimated in preliminary measurements. Existing theoretical predictions for B₀ scatter around the present experimental data. The observation of a high bulk modulus of c- Hf₃N₄ supports the suggestion that this compound could have high hardness.     

Theoretical investigations of structural, elastic and thermodynamic properties for PtN2 under high pressure

We have investigated structural and elastic properties of PtN₂ under high pressures using norm-conserving pseudopotentials within the local density approximation (LDA) in the frame of density-functional theory. Calculated results of PtN₂ are in agreement with experimental and available theoretical values. The a/a₀, V/V₀, ductility/brittleness, elastic constants C_ij, shear modulus C′, bulk modulus B, shear modulus G, Young's modulus E, Poisson's ratio σ and anisotropy factor A as a function of applied pressure are presented. Through the quasi-harmonic Debye model, we also study thermodynamic properties of PtN₂. The thermal expansion versus temperature and pressure, thermodynamic parameters X (X=Debye temperature or specific heat) with varying pressure P, and heat capacity of PtN₂ at various pressures and temperatures are estimated.     

Effect of elastic anisotropy on the strain fields and band edges in stacked InAs/GaAs quantum dot nanostructures

The effect of elastic anisotropy on the strain fields and confinement potentials in InAs/GaAs quantum dot (QD) nanostructures was investigated for an isolated dot and a stacked multi-layer dots using finite element analysis and model solid theory. The assumption of isotropy tends to underestimate especially hydrostatic strain that is known to modify confinement potentials in conduction band. Consideration of anisotropy results in a wider band gap and shallower potential well as compared with the isotropic model. Since the band gap and potential well depth would be related to opto-electronic properties of quantum dot systems via quantum mechanical effects, it is suggested that consideration of elastic anisotropy in the calculation of strains and band structures is necessary for the design of QD-based opto-electronic devices.     

Structural and elastic properties under pressure effect of the cubic antiperovskite compounds ANCa3 (A = P, As, Sb, and Bi)

Using first-principles density functional calculations, the effect of high pressures, up to 40 GPa, on the structural and elastic properties of ANCa₃, with A = P, As, Sb, and Bi, were studied by means of the pseudo-potential plane-waves method. Calculations were performed within the local density approximation and the generalized gradient approximation for exchange-correlation effects. The lattice constants are in good agreement with the available results. The elastic constants and their pressure dependence are calculated using the static finite strain technique. We derived the bulk and shear moduli, Young's modulus, Poisson's ratio and Lamé's constants for ideal polycrystalline ANCa₃ aggregates. By analysing the ratio between the bulk and shear moduli, we conclude that ANCa₃ compounds are brittle in nature. We estimated the Debye temperature of ANCa₃ from the average sound velocity. This is the first quantitative theoretical prediction of the elastic properties of PNCa₃, AsNCa₃, SbNCa₃, and BiNCa₃ compounds, and it still awaits experimental confirmation.     

More on shear modulus collapse of lattices at high pressure

In his recent paper, Shear modulus collapse of lattices at high pressure, J. Phys. Cond. Matt. 16 (2004) L125, V.V. Kechin claims that the zero temperature shear modulus of a metallic solid vanishes at a high critical pressure, and the critical pressures for this shear modulus collapse lie in the range 0-250 Mbar for elemental metals. Here we demonstrate that Kechin's arguments contain an erroneous assumption, and therefore, do not prove that all metals become mechanically unstable at high pressures. Ab initio calculations and experimental results on a number of solids are analyzed to confirm our conclusion.     

Electronic parameters and electronic structures in modulation-doped highly strained InxGa1−xAs/InyAl1−yAs coupled double quantum wells

Electronic parameters of a two-dimensional electron gas (2DEG) in modulation-doped highly strained In_xGa_1−xAs/In_yAl_1−yAs coupled double quantum wells were investigated by performing Shubnikov-de Haas (S-dH), Van der Pauw Hall-effect, and cyclotron resonance measurements. The S-dH measurements and the fast Fourier transformation results for the S-dH at 1.5 K indicated the electron occupation of two subbands in the quantum well. The electron effective masses of the 2DEG were determined from the cyclotron resonance measurements, and satisfied qualitatively the nonparabolicity effects in the quantum wells. The electronic subband structures were calculated by using a self-consistent method.     

Model calculations of diffusion limited trapping dynamics in quantum well laser structures

We present a phenomenological theoretical model to treat the trapping of carriers into quantum wells of semiconductor laser structures. We consider explicitely the transport within the barrier layers by solving the continuity equation with the appropriate boundary conditions taking into account surface recombination, radiative and nonradiative recombination in the barrier layers and trapping of carriers into the quantum wells. The experimental findings for the trapping dynamics in GaAs/AlGaAs quantum well structures can be consistently interpreted by the model calculations.     

Time-resolved four-wave-mixing of exciton transitions in GaAs/AlGaAs quantum wells: Novel effects in the lineshape

We investigate coherent exciton transitions in GaAs/Al _x Ga_1–x As quantum wells using subpicosecond four-wave-mixing spectroscopy. In a first part, we show that the four-wave-mixing lineshape deviates considerably from earlier predictions when the detailed structure of the excitons is taken into account, and we study the density dependence and excitation energy dependence in detail. In a second part, we discuss the observation of quantum beats due to the coherent superposition of different excitonic transitions and show how the analysis of the lineshape can yield information about relaxation processes.Dedicated to H.-J. Queisser on the occasion of his 60th birthday     

The influence of H2 plasma treatment on the field emission of amorphous GaN film

The influence of H₂ plasma treatment on the field emission properties of amorphous GaN (a-GaN) films is studied. It is found that the treatment makes little change to the surface morphology. The current density of the treated film decreases from 400 to 30 μA/cm² at the applied field of about 30 V/μm. The treatment can reduce the defects in a-GaN films, and therefore the treatment results in the weakening of the tunneling emission of the a-GaN film at the high field region. The treatment also seems to change the conduction mechanism of the a-GaN film.     

New features in the pressure dependence of photoluminescence spectra of C60 at room temperature

We present new results on the pressure dependence of the electronic band gap of molecular C₆₀ measured by photoluminescence spectroscopy up to 10 GPa at room temperature. In agreement with previous results, the energy gap decreases with increasing pressure up to about 6 GPa. For higher pressures, however, we observe an energy gap that is wider than that at 6 GPa.     

Effects of electric field and hydrostatic pressure on donor binding energies in a spherical quantum dot

The binding energies of a hydrogenic donor in a GaAs spherical quantum dot in the Ga_1−xAl_xAs matrix are presented assuming parabolic confinement. Effects of hydrostatic pressure and electric field are discussed on the results obtained using a variational method. Effects of the spatial variation of the dielectric screening and the effective mass mismatch are also investigated. Our results show that (i) the ionization energy decreases with dot size, with the screening function giving uniformly larger values for dots which are less than about 25 nm, (ii) the hydrostatic pressure increases the donor ionization energy such that the variation is larger for a smaller dot, and (iii) the ionization energy decreases in an electric field. All the calculations have been carried out with finite barriers and good agreement is obtained with the results available in the literature in limiting cases.     

Growth of textured nonstoichiometric MoTe2 films from Mo/Te layers and their use as precursor in the synthesis of MoTe2−xSx films

Molybdenum ditelluride, MoTe₂, thin films have been prepared on a glass substrate by depositing first a layer of Mo of 1 μm thickness by rf sputtering and then layers of tellurium of 4 μm thickness followed by annealing, in a vacuum sealed Pyrex tube, at 823 K for one hour. The films have been studied by X-ray diffraction, electron microprobe analysis, scanning electron microscopy and for optical and electrical properties. This indicated that the films so prepared are well crystallized and textured with a hexagonal structure similar to 2H-MoS₂. However, the film is non stoichiometric due to the presence of small amounts of oxygen. The optical properties are in good agreement with those of single crystals and film deposited by other means. The room temperature conductivity is of the same order of magnitude as that of single crystals. Finally, MoTe₂ has been used as a precursor for the growth of MoTe_2−xS_x films.     

Pressure effects on the donor binding energy in zinc-blende InGaN/GaN quantum dot

Based on the effective-mass approximation, the hydrostatic pressure effects on the donor binding energy of the hydrogenic impurity in zinc-blende (ZB) InGaN/GaN quantum dot (QD) are investigated by means of a variational procedure. Numerical results show that the donor binding energy increases when the hydrostatic pressure increases for any impurity position and QD structure parameter. Moreover, it is found that the hydrostatic pressure has a remarkable influence on the donor binding energy of the hydrogenic impurity located at the vicinity of dot center in ZB InGaN/GaN QD.     

Effects of Shock Pressure on Transition Pressure in Zr

Measurements of free surface velocity profiles of high-purity Zr samples under shock-wave loading are performed to study the dynamic strength and phase transition parameters. The peak pressure of the compression waves is within the range from 9 to 14 GPa, and the Hugoniot elastic limit is 0.5 GPa. An anomalous structure of shock waves is observed due to the α - ω phase transition in Zr. Shock pressure has effects on transition pressure which increases with increasing compression strength, and the stronger shocks have a lower transit time.     

Theory of the photoacoustic effect of semiconductor quantum wells

We develop a theory of the photoacoustic effect of semiconductor quantum wells. Assuming a multilayer system of optically uniaxial media with dissipation we describe the generation of heat and the conduction of the heat through the system by a generalized transfer-matrix method. It is shown by applying this theory to a semiconductor quantum well that the photoacoustic signal is very sensitive on the quantum-size effect and the longitudinal and transverse relaxation times. Our theory predicts that photoacoustics should be a profitable method for the investigation of semiconductor micro- and nanostructures.     

Influence of high pressure on the ferromagnetic transition temperature of CrO2

The shift of the Curie temperature of CrO₂ with pressure was determined by ac-susceptibility measurements under hydrostatic pressure up to 5 GPa. These experiments show that ferromagnetism of CrO₂ is suppressed at a rate , which is close to recent theoretical estimates.