Virtual Crystal Approximation Research Articles

Электронная структура и магнитный фазовый переход в геликоидальных ферромагнетиках Fe-=SUB=-1-x-=/SUB=-Co-=SUB=-x-=/SUB=-Si

AbstractLSDA + U + SO calculations of the electronic structure of helicoidal Fe_1 - x Co_ x Si ferromagnets within the virtual crystal approximation have been supplemented with the consideration of the Dzyaloshinski-Moriya interaction and ferromagnetic fluctuations of the spin density of collective d electrons with the Hubbard interactions at Fe and Co atoms randomly distributed over sites. The magnetic-state equation in the developed model describes helicoidal ferromagnetism and its disappearance accompanied by the occurrence of a maximum of uniform magnetic susceptibility at temperature T _ C and chiral fluctuations of the local magnetization at T > T _ C . The reasons why the magnetic contribution to the specific heat at the magnetic phase transition changes monotonically and the volume coefficient of thermal expansion (VCTE) at low temperatures is negative and has a wide minimum near T _ C have been investigated. It is shown that the VCTE changes sign when passing to the paramagnetic state (at temperature T _ S ).

A Critical Study of the Elastic Properties and Stability of Heusler Compounds: Phase Change and Tetragonal <i>X<sub>2</sub>YZ</i> Compounds

In the present work, the elastic constants and derived properties of tetragonal and cubic Heusler compounds were calculated using the high accuracy of the full-potential linearized augmented plane wave (FPLAPW). To find the criteria required for an accurate calculation, the consequences of increasing the numbers of $k$-points and plane waves on the convergence of the calculated elastic constants were explored. Once accurate elastic constants were calculated, elastic anisotropies, sound velocities, Debye temperatures, malleability, and other measurable physical properties were determined for the studied systems. The elastic properties suggested metallic bonding with intermediate malleability, between brittle and ductile, for the studied Heusler compounds. To address the effect of off-stoichiometry on the mechanical properties, the virtual crystal approximation (VCA) was used to calculate the elastic constants. The results indicated that an extreme correlation exists between the anisotropy ratio and the stoichiometry of the Heusler compounds, especially in the case of Ni$_{2}$MnGa.

Optical phonon modes and polaron related parameters in GaxIn1−xP

Based on a pseudopotential approach under the virtual crystal approximation that includes the effect of compositional disorder, the optical lattice vibration frequencies and polaron related parameters in zinc-blende GaxIn1−xP have been studied. Our findings showed generally reasonably good accord with data in the literature. Other case, our results are predictions. The composition dependence of longitudinal optical (LO) and transverse optical (TO) phonon modes, LO-TO splittings, Frӧhlich coupling parameter, Debye temperature of LO phonon frequency, and polaron effective mass has been analyzed and discussed. While a non-monotonic behavior has been noticed for the LO and TO phonon frequencies versus Ga concentration x, a monotonic behavior has been observed for the rest of the features of interest. The information derived from this investigation may be useful for optoelectronic technological applications.

Mechanical properties of AlxIn1-xSb ternary alloys under the effect of pressure and temperature

The pseudo-potential method (EPM) within the virtual crystal approximation (VCA) including disorder effect was used to determine the mechanical properties of AlxIn1-xSb alloys in zinc-blende structure. The studied properties as elastic constants and their related quantities as bulk, shear and Young moduli are calculated and compared with the published data and showed generally good agreement. Other related quantities are also successfully studied. The variation of all studied quantities with composition, pressure and temperature has been examined. Most of our results can give an important reference especially in the region of high pressure and temperature.

Rethinking phonons: The issue of disorder

Current understanding of phonons treats them as plane waves/quasi-particles of atomic vibration that propagate and scatter. The problem is that conceptually, when any level of disorder is introduced, whether compositional or structural, the character of vibrational modes in solids changes, yet nearly all theoretical treatments continue to assume phonons are still waves. For example, the phonon contributions to alloy thermal conductivity (TC) rely on this assumption and are most often computed from the virtual crystal approximation (VCA). Good agreement is obtained in some cases, but there are many instances where it fails—both quantitatively and qualitatively. Here, we show that the conventional theory and understanding of phonons requires revision, because the critical assumption that all phonons/normal modes resemble plane waves with well-defined velocities is no longer valid when disorder is introduced. Here we show, surprisingly, that the character of phonons changes dramatically within the first few percent of impurity concentration, beyond which phonons more closely resemble the modes found in amorphous materials. We then utilize a different theory that can treat modes with any character and experimentally confirm its new insights.

Effects of Ni d-levels on the electronic band structure of NixCd1-xO semiconducting alloys

NixCd1-xO has a ∼3 eV band edge offset and bandgap varying from 2.2 to 3.6 eV, which is potentially important for transparent electronic and photovoltaic applications. We present a systematic study of the electronic band structure of NixCd1-xO alloys across the composition range. Ion irradiation of alloy samples leads to a saturation of the electron concentration associated with pinning of the Fermi level (EF) at the Fermi stabilization energy, the common energy reference located at 4.9 eV below the vacuum level. The composition dependence of the pinned EF allows determination of the conduction band minimum (CBM) energy relative to the vacuum level. The unusually strong deviation of the CBM energy observed from the virtual crystal approximation is explained by a band anticrossing interaction between localized 3d states of Ni and the extended states of the NixCd1-xO alloy host. The resulting band structure explains the dependence between the composition and the electrical and optical properties of the alloys—the rapid reduction of the electron mobility as well as previously observed positive band gap bowing parameter. X-ray photoelectron spectroscopy studies confirm that the L-point valence band maximum in the Cd-rich alloys are unaffected by the interaction with Ni d-states.

Unifying first-principles theoretical predictions and experimental measurements of size effects in thermal transport in SiGe alloys

In this work, we demonstrate the correspondence between first principle calculations and experimental measurements of size effects on thermal transport in SiGe alloys. Transient thermal grating (TTG) is used to measure the effective thermal conductivity. The virtual crystal approximation under the density functional theory (DFT) framework combined with impurity scattering is used to determine the phonon properties for the exact alloy composition of the measured samples. With these properties, classical size effects are calculated for the experimental geometry of reflection mode TTG using the recently-developed variational solution to the phonon Boltzmann transport equation (BTE), which is verified against established Monte Carlo simulations. We find agreement between theoretical predictions and experimental measurements in the reduction of thermal conductivity (as much as $\sim$ 25\% of the bulk value) across grating periods spanning one order of magnitude. This work provides a framework for the tabletop study of size effects on thermal transport.

The magnetic phase transition in Mn2-xFexB alloys: First-principles calculations

Based on the density-functional theory (DFT) and the generalized gradient approximation (GGA), Mn2-xFexB (0 ≤ x ≤ 2) alloys are investigated for their magnetic, electronic and structural properties by the first principles calculations with the virtual crystal approximation (VCA). Two extremes at x = 0 and 2, respectively, show that anti-ferromagnetic Mn2B and ferromagnetic Fe2B have the same electronic structure as I4/mcm but with different lattice constants. There is a critical point for phase transition from antiferromagnetic (AFM) to ferromagnetic (FM) at x = 0.5 according to calculations under the condition of the lowest energy for the four states of non-magnetic (NM), FM, AFM1 and AFM2. It is in agreement with the Stoner model in the range of the FM state.

Magnetocrystalline anisotropy of Laves phase Fe2Ta1−xWx from first principles: Effect of 3d−5d hybridization

The magnetic properties of Fe$_2$Ta and Fe$_2$W in the hexagonal Laves phase are computed using density functional theory in the generalised gradient approximation, with the full potential linearised augmented plane wave method. The alloy Fe$_2$Ta$_{1-x}$W$_x$ is studied using the virtual crystal approximation to treat disorder. Fe$_2$Ta is found to be ferromagnetic with a saturation magnetization of $\mu_0 M_\text{s} = 0.66~\mathrm{T}$ while, in contrast to earlier computational work, Fe$_2$W is found to be ferrimagnetic with $\mu_0 M_\text{s} = 0.35~\mathrm{T}$. The transition from the ferri- to the ferromagnetic state occurs for $x \leq 0.1$. The magnetocrystalline anisotropy energy (MAE) is calculated to $1.25~\mathrm{MJ/m^3}$ for Fe$_2$Ta and $0.87~\mathrm{MJ/m^3}$ for Fe$_2$W. The MAE is found to be smaller for all values $x$ in Fe$_2$Ta$_{1-x}$W$_x$ than for the end compounds and it is negative (in-plane anisotropy) for $0.1 \leq x \leq 0.9$. The MAE is carefully analysed in terms of the electronic structure. Even though there are weak 5d contributions to the density of states at the Fermi energy in both end compounds, a reciprocal space analysis, using the magnetic force theorem, reveals that the MAE originates mainly from regions of the Brillouin zone with strong 3d-5d hybridisation near the Fermi energy. Perturbation theory and its applicability in relation to the MAE is discussed.

Martensitic transformation from β to α′ and α″ phases in Ti–V alloys: A first-principles study

Abstract

And semiconductor alloys as competent materials for opto-electronic and solar cell applications: a comparative analysis

ZnO1−xTex ternary alloys have great potential to work as a photovoltaic (PV) absorber in solar cells. ZnO1−xSx is also a ZnO based alloy that have uses in solar cells. In this paper we report the comparative study of various parameters of ZnO1−xTex and ZnO1−xSx for selecting it to be a competent material for solar cell applications. The parameters are mainly being calculated using the well-known VCA (virtual crystal approximation) and VBAC (Valence Band Anti-Crossing) model. It was certainly being analysed that the incorporation of Te atoms produces a high band gap lower than S atoms in the host ZnO material. The spin-orbit splitting energy value of ZnO1−xTex was found to be higher than that of ZnO1−xSx. Beside this, the strain effects are also higher in ZnO1−xTex than ZnO1−xSx. The remarkable notifying result which the paper is reporting is that at a higher percentage of Te atoms in ZnO1−xTex, the spin-orbit splitting energy value rises above the band gap value, which signifies a very less internal carrier recombination that decreases the leakage current and increases the efficiency of the solar cell. Moreover, it also covers a wide wavelength range compared to ZnO1−xSx.

Effect of doping on lattice dynamics and electron–phonon coupling of the actinides Ac–Th alloy

We have studied the electronic, lattice dynamical, and electron–phonon properties of the actinides Thx alloy within the framework of density functional perturbation theory. The self-consistent virtual crystal approximation is used for the alloy modeling, and spin–orbit coupling is included in the calculation of all relevant quantities. An overall decrease of the electron–phonon coupling (λ) by from Ac to Th was observed. However, its dependence on x shows a non-linear behavior. λ reduces just 6% from Ac to a Th content of , then drops drastically (∼) from there until . The large decrease of λ for is due to the reduction of the density of states at the Fermi level (), combined with a general phonon hardening. On contrast, the behavior for is the result of a subtle balance between an enhancement of phase space and the above mentioned effects on and the phonons. The phase-space enhancement is related to the appearance of Kohn anomalies, which fade away as the Th concentration increases.

Thermal expansion coefficient of WRe alloys from first principles

We calculate the coefficient of thermal expansion (CTE) in tungsten-rhenium random alloys for Re concentrations between 0% and 50% and for temperatures up to 2400 K by employing the quasiharmonic approximation within the ab initio framework of density functional theory. We treat chemical disorder by the virtual crystal approximation and compute the phonon density of states at two levels of sophistication. While the traditional Debye-Gr\uneisen (DG) model fails to account for the experimentally observed increase in CTE upon Re addition for concentrations above 10% Re, explicit phonon calculations within density functional perturbation theory lead to an overall good agreement with experiment. Thereby we identify the pronounced phonon softening and anisotropy between transversal and longitudinal modes in W-Re to be responsible for the breakdown of the DG model.

Σ3(111)grain boundary of body-centered cubic Ti-Mo and Ti-V alloys: First-principles and model calculations

The energetics and atomic structures of $\mathrm{\ensuremath{\Sigma}}3[1\overline{1}0](111)$ grain boundary (GB) of body-centered cubic (bcc) Ti-Mo and Ti-V alloys are investigated using density-functional-theory calculations and virtual crystal approximation. The electron density in bcc structure and the atomic displacements and excess energy of the GB are correlated to bcc-$\ensuremath{\omega}$ phase stability. Model calculations based on pairwise interplanar interactions successfully reproduce the chemical part of GB energy. The chemical GB energy can be expressed as a sum of excess pairwise interactions between bcc (111) layers, which are obtained from Gaussian elimination of the total energies of a number of periodic structures. The energy associated with the relaxation near the GB is solved by numerical minimization using the derivatives of the excess interactions. Anharmonic interlayer interactions are necessary for obtaining accurate relaxation energy and excess GB volume from model calculations. The effect of GB on vibrational spectrum is also investigated. Segregation energies of B and Y to a substitutional site on the GB plane are calculated. Preliminary results suggest that Y tends to segregate, while B tends to antisegregate.

Atomistic tight-binding calculations of near infrared emitting CdxHg1−xTe nanocrystals

I present the structural and optical calculations for CdxHg1−xTe zinc-blende nanocrystals with the experimentally synthesized Cd compositions (x) in the framework of atomistic tight-binding model (TB) and configuration interaction description (CI). This atomistic tight-binding model incorporates the sp3s∗ orbital and the first neighbouring interaction, in addition to the non-linear dependence of compositions on the band gaps the bowing factor is involved into this model through the extended virtual crystal approximation (VCA). The information of DOS highlights that the contribution of the conduction and valence bands is subjected by cation and anion atoms, respectively. The improvement of the optical band gaps in CdxHg1−xTe nanocrystals is presented with the increasing Cd compositions. The optical band gaps can be used to tune their optical properties across a technologically useful range from 688nm to 2755nm which can be implemented for the near infrared emitting devices. In addition, the enhancement of the optical property is reported with the increasing Cd contents. With the increasing Cd compositions, the atomistic electron-hole Coulomb interaction is mainly increased, whereas the atomistic electron-hole exchange interaction is reduced. The Stokes shift and fine structure splitting are progressively reduced with the increasing compositions and diameters. The application of fine structure splitting is utilized to implement as a source of entangled photon pairs in the quantum information. Finally, the comprehensive computations of alloy CdxHg1−xTe nanocrystals effectively determine the composition- and size-dependent structural and optical properties which render these nanocrystals promising candidates as the near infrared emitting devices and optical amplifiers.

Phonon scattering mechanisms dictating the thermal conductivity of lead zirconate titanate (PbZr1−xTixO3) thin films across the compositional phase diagram

This work represents a thorough investigation of the thermal conductivity (κ) in both thin film and bulk PbZr1–xTixO3 (PZT) across the compositional phase diagram. Given the technological importance of PZT as a superb piezoelectric and ferroelectric material in devices and systems impacting a wide array of industries, this research serves to fill the gap in knowledge regarding the thermal properties. The thermal conductivities of both thin film and bulk PZT are found to vary by a considerable margin as a function of composition x. Additionally, we observe a discontinuity in κ in the vicinity of the morphotropic phase boundary (MPB, x = 0.48) where there is a 20%–25% decrease in κ in our thin film data, similar to that found in literature data for bulk PZT. The comparison between bulk and thin film materials highlights the sensitivity of κ to size effects such as film thickness and grain size even in disordered alloy/solid-solution materials. A model for the thermal conductivity of PZT as a function of composition (κ(x)) is presented, which enables the application of the virtual crystal approximation for alloy-type material systems with very different crystals structures, resulting in differing temperature trends for κ. We show that in the case of crystalline solid-solutions where the thermal conductivity of one of the parent materials exhibits glass-like temperature trends the compositional dependence of thermal conductivity is relatively constant for most values of x. This is in stark contrast with the typical trends of thermal conductivity with x in alloys, where the thermal conductivity increases dramatically as the composition of the alloy or solid-solution approaches that of a pure parent materials (i.e., as x = 0 or 1).

Competing magnetic instabilities in the weak itinerant antiferromagnetic TiAu

The binary intermetallic compound TiAu is distinguished by an extremely sharp and narrow density of states peak at the Fermi level that has been proposed, via scattering between mirrored van Hove singularities, to be the mechanism for antiferromagnetic ordering versus the more fundamental ferromagnetic Stoner instability. Here we study, using density functional theory methods, magnetic tendencies and effects of doping, the latter within the virtual crystal approximation (VCA). Ferromagnetic tendencies are quantified using the fixed spin moment approach, illustrating the strong Stoner instability that does not, however, provide the ground state. Use of VCA results allows the identification of the value of the Stoner exchange constant $\mathcal{I}=0.74\phantom{\rule{0.16em}{0ex}}\mathrm{eV}$ for Ti. Magnetic fluctuations not included with semilocal density functionals are quantified with the procedure provided by Ortenzi and coauthors, with alloy concentrations corresponding to the quantum critical points reduced by a factor of three to five. Our results provide useful guidelines for experimental doping studies of TiAu.

Effects of composition on the properties of mixed CdSi1-xGexAs2 chalcopyrites as explored by the first-principles calculations

The structural, electronic, optical and elastic properties of the CdGeAs2 and CdSiAs2 chalcopyrites and their CdSi1-xGexAs2 mixtures were calculated using the first-principles methods. Good agreement with the available experimental data was achieved for both neat compounds. Based on this, the mixed CdSi1-xGexAs2 compounds were explored in details. Two different models - the virtual crystal approximation (VCA) and the supercell (SC) model - were used to analyze the impact of the second cation substitution on the above-mentioned properties of the mixed materials in the whole range of the Ge/Si concentration from x=0 to x=1. It has been shown that the lattice constants, band gaps, refractive indexes, elastic constants and Debye temperature all exhibit linear variation with the second cation composition. The linear dependences of all these parameters allow for their numerical estimation for any composition in the studied range.

Polariton excitations in a non-ideal array of microcavities with quantum dots

The polariton spectrum of a one-dimensional non-ideal array of coupled microcavities containing quantum dots has been studied. The specific features of the dispersion of electromagnetic excitations, which are induced in this system both by a variation in the distance between the adjacent microcavities and by a variation in the composition of the quantum dots, have been investigated using the numerical simulation within the framework of the virtual crystal approximation. The density of states of the quasiparticles under consideration has been determined.

Structural and electronic properties of Pb-doped Bi2Sr2Ca2Cu3O10: Comparison of LDA and GGA calculations

We use Density Functional Theory to study the effects on the crystal structure and the electronic band structure of substituting Pb for Bi in Bi2Sr2Ca2Cu3O10. We further use the Local Density Approximation (LDA) and the Generalized Gradient Approximation (GGA). The Virtual Crystal Approximation (VCA) was used to account for the substitution. We found that GGA reproduces better the lattice parameters although in both cases the internal coordinates were reproduced with some uncertainties. We further looked at the behavior of the so called Bi–O pockets, some electronic states that originate on the Bi–O planes and that appear on the Fermi surface (FS) in contradiction to the experimental evidence. We found that LDA and GGA differ on that subject. With 26% Pb and using LDA, the Bi–O pockets run away from the FS. But when GGA is used, it is needed up to 35% Pb to make the Bi–O pockets disappear from the FS. In the last case, once the Bi–O pockets are removed from the FS, we get a very good agreement with angular resolved photo-emission (ARPES) and nuclear magnetic resonance (NMR) experiments.