Effective Number Of Valence Electrons Research Articles

Thermoelectric properties and low-temperature transport anomalies in the p -type full-Heusler compounds Fe2−xCrxVAl

Elemental substitutions are successfully used for the optimization of thermoelectric properties of a specific material; it requires, however, a deep understanding of its impact. For this purpose, based on the full-Heusler material ${\mathrm{Fe}}_{2}\mathrm{VAl}$, various compounds (${\mathrm{Fe}}_{2\ensuremath{-}x}{\mathrm{Cr}}_{x}\mathrm{VAl}$) were synthesized by substituting Cr for Fe in a wide range from $x$ = 0.005 up to $x$ = 0.4. X-ray diffraction analysis revealed full solubility of Cr for all concentrations. Bulk thermoelectric properties, such as electrical resistivity, Seebeck coefficient, thermal conductivity, and Hall resistivity were measured from 2 K up to 780 K, and all results were discussed in the context of the outcome of density functional calculations. Transport anomalies, resembling Kondo scattering, were observed for all samples below 30 K. Finally, an increased effective number of valence electrons of 7 for Cr was phenomenologically determined, which revealed good agreement with other $p$-type doping studies of ${\mathrm{Fe}}_{2}\mathrm{VAl}$.

Remark on the correlation between the refractive index and the optical band gap in some crystalline solids

The correlation between static refractive index (ns) and optical band gap (Eg) based on the Wemple-DiDomenico (WD) model and the empirical correlation between the energy of an effective dispersive oscillator and optical band gap is examined for 52 crystalline solids. A comparison with two recently published empirical relationships between ns and Eg indicates that the use of the WD model provides better results and, more importantly, connects the ns versus Eg dependence with some chemical characteristics of solids such as the coordination number of cation, the valence of anion, the effective number of valence electrons and ionicity.

Structural, electronic, dynamic, optic and elastic properties of MgScGa via density functional theory

The structural, electronic and phonon properties of the MgScGa compound were investigated by density functional theory using the generalized gradient approximation. Some basic structural properties of this compound, such as the lattice constants, bulk modulus and pressure derivative of the bulk module have been studied. Electronic properties were investigated by calculating and analyzing the electronic band structure and total density of states graphs for the MgScGa compound. Electronic band structure calculations showed that MgScGa compound has a semiconductor structure. Phonon spectra were calculated using a linear response method within the framework of the total predicted state density, density functional perturbation theory for the MgScGa compound. A factor group analysis has been performed in order to get the decomposition of whole representation of Γ of MgScGa compound into irreducible representation. For investigation of optic properties; real and imaginary components of complex dielectric function, reflectivity (R), refractive index (n), extinction coefficients (k), energy-loss functions for volume (LV) and surface (LS), the effective number of valence electrons per unit cell (Neff) were calculated. Elastic properties are revealed by calculating the elastic stiffness constants, Bulk, Shear and Young modulus, Poisson Ratio, Flexibility Coefficient and Zener anisotropy constant.

Probing the spin-glass freezing transition in Cu1−xMnx alloy by spin current

In this study, we used the thermally driven spin current to investigate the spin frustrations and spin fluctuations in spin-glass (SG) $\mathrm{C}{\mathrm{u}}_{1\ensuremath{-}x}\mathrm{M}{\mathrm{n}}_{x}$ alloys. Tuning the $\mathrm{C}{\mathrm{u}}_{1\ensuremath{-}x}\mathrm{M}{\mathrm{n}}_{x}$ composition results in a transition of the alloys from the SG state to the antiferromagnetic state; these states have different spin-freezing temperatures $({T}_{f})$. Most spins randomly freeze at temperatures lower than the ${T}_{f}$ of the alloy. For each alloy composition, we obtained a temperature-dependent inverse spin Hall voltage with a peak at ${T}_{p}$. Crucially, ${T}_{p}$ had nearly identical composition dependence as that of ${T}_{f},$, with ${T}_{p}$ being nine times larger than ${T}_{f}$. Similar behavior was captured using the SG insulator, amorphous ${\mathrm{Y}}_{3}\mathrm{F}{\mathrm{e}}_{5}{\mathrm{O}}_{12}$. These results indicated that the maximum spin fluctuation in both conducting and insulating SGs occurred at temperatures considerably higher than the ${T}_{f}$ of each. In addition, we demonstrated the importance of the effective number of valence electrons in tailoring the spin Hall angle in binary alloys.

The first principle calculation of elemental-vacancy-assisted modifications on structural properties and optical responses of ZnO system

The structural properties and optical responses of ZnO0.94 and Zn0.94O systems as the possible models of ZnO system containing elemental defect states were studied using the plane-wave method within the generalized gradient approximation. The pristine ZnO system was used as the reference. The simulated XRD patterns showed the three highest intensity for (100), (002), and (101) orientations for all the systems. Compared to O vacancy, Zn vacancy mostly provided higher structure factor shifts of ZnO system. Moreover, the strongest local-symmetry distortion at ZnO4 tetrahedra was implied in Zn0.94O system. The optical responses showed that all the systems exhibited the optical dichroism based on the extinction coefficient spectra. Interestingly, Zn0.94O system showed the lowest energy levels of absorption and high-reflectance edges. Additionally, the highest saturation threshold energy of the effective number of valence electrons was obtained for this system. The result emphasizes the importance of Zn vacancy in providing the significant effect on the optical responses of ZnO system.

Investigation of Structural, Electronic, Optic and Elastic Properties of Perovskite RbGeCl3 Crystal: A First Principles Study

Some physical properties of RbGeCl3 crystal are investigated with ABINIT computer program within the generalized gradient approximation (GGA) and the local density approximation (LDA), using density functional theory (DFT). We studied the geometry optimization, electronic band structure, electron density of states, optical properties such as the dielectric functions, reflectivity, refractive index, extinction coefficients, energy-loss functions for volume, the effective number of valence electrons per unit cell and elastic properties of RbGeCl3. The calculated electronic band structure shows that the RbGeCl3 has a direct band gap and this compound is a semiconducting material with a wide bandgap.

Parameters of a Complex Structure of Optical Functions of Dimagnesium Stannide

The spectra of sixteen optical functions of Mg2Sn crystal are determined at 77 K in the range 1–11 eV. The calculations were performed using known experimental reflection spectra R(E), software packages created on the basis of the Kramers–Kronig relations, and analytical formulas for the communication between optical functions. Their main peculiarities and general regularities have been established. The integral spectra of the imaginary part of the dielectric constant e2(E) are decomposed into elementary components in the range from 2 to 5.5 eV using an improved nonparametric method of combined Argand diagrams taking into account the effective number of valence electrons participating in the formation of individual bands. The energies of maxima and the strengths of oscillators of elementary components of the transition bands are determined and their assumed nature and localization are proposed on the basis of the data of well-known theoretical calculations.

Electronic and optical properties of the supercell of 8-Pmmn borophene modified on doping by H, Li, Be, and C: a DFT approach

Borophene nanosheet has been doped with lithium, beryllium, carbon, and hydrogen atom(s). Modification of electronic properties through the splitting and shifting of the Dirac point has been explored within the framework of ab initio density functional theory (DFT). Doping the Pmmn borophene structure with lithium, beryllium, carbon, and hydrogen atoms leads to semiconductor–metallic transitions as well as absorption of light within the different parts of the electromagnetic spectrum. Indirect band-gap opening is noticed when the system is doped with one hydrogen atom. But when the doping is made double, the system becomes metallic. For lithium and beryllium doping, the split Dirac point has been observed to be situated at the conduction band (CB). However, for single-site carbon atom doping, it is seen to be situated at the valence band (VB), while, for double doping, the system becomes semiconducting with an indirect bandgap of 528 meV. Optical properties calculations show that, for parallel and perpendicular polarization, the excitations have mainly occurred within the low- and high-energy regions, respectively. The effective number of valence electrons does not show any saturation effect for all the doped systems in parallel as well as for perpendicular polarizations.

Structural, Electronic, and Mechanical Properties of A3Mn2O7 (A = Sr, Ca): Ab Initio Calculation

In the present study, the structural, electronic, optical, and mechanical properties of the Ruddlesden-Popper type oxide compounds are investigated by means of density functional theory. In our calculation, spin polarized electron band structures and density of the state were identified by adding to the spin contribution of the Mn-atom. For A3Mn2O7 compounds, the real and imaginary parts of the dielectric function and other optical properties, such as energy loss function, effective number of valence electrons, and effective optical dielectric constant, were calculated accordingly. In addition, the bulk modules, shear modules, Young's modulus and Poisson ratios, anisotropy factors, sound velocities, and Debye temperatures for these compounds were calculated too.

Effect of pressure on physical properties of LuAsO4 crystal

The physical properties of LuAsO4, such as structural, electronic, linear optical, dynamic, and thermodynamic properties, have been studied under pressure using the density functional theory within the local density approximation. The calculations of electronic band structure of the crystal show that it is an insulator with an indirect band gap value of 4.1977 eV at zero pressure and that the value of band gap increase by increasing pressure. The linear optical properties of the crystal, dependent on the frequency of incoming light, have been calculated. The components of dielectric tensor, static dielectric constant, and refractive index have been calculated under pressure while some optical constants such as energy loss function, extinction coefficient, absorption coefficient, reflectivity, effective dielectric constant, and effective number of valence electrons have been calculated at zero pressure. Moreover, the phonon dispersion, some thermodynamic potentials such as free energy, internal energy, and some thermodynamic properties such as entropy and heat capacity have been calculated under pressure.

Properties of the Fundamental Optical Functions of Magnesium Silicide

The spectra of 16 optical Mg2Si functions in the range from 0 to 11 eV at 77 K are determined. They contain five maxima and shoulders due to interband transitions and metastable excitons, as well as the maxima of bulk and surface plasmons at ~9.56 and ~7.30 eV. Their peculiarities and general patterns of behavior are established. Calculations were performed based on a well-known experimental reflection spectrum in the range from 0 to 11 eV at 77 K. The integral spectra of the imaginary parts of the permittivity e2(E), characteristic bulk (–Im e–1), and surface (–Im (1 + e)–1) energy losses of electrons are expanded into elementary components in the region from 2 to 5 eV. The main parameters of the transition components were determined, including the energies of their maxima and oscillator strengths. Calculations were carried out using computer programs based on the Kramers–Kronig relations and known analytical formulas for the correlation between optical functions, as well as an improved nonparametric method of combined Argand diagrams taking into account the effective number of valence electrons participating in the formation of individual bands. Instead of five maxima and shoulders of the integral spectra, 17 elementary bands with oscillator strengths in the range from 0.0003 to 1.92 were established. They are due to transverse and longitudinal type excitonic and interband transitions. Based on known theoretical calculations, we propose herein the localization and assumed nature of the optical transition bands.

Local-symmetry distortion, optical properties, and plasmonic states of monoclinic Hf0.5Zr0.5O2 system: a density-functional study

We study structural, electronic, and optical properties of the monoclinic Hf0.5Zr0.5O2 (space group: P21/c). Monoclinic ZrO2 and HfO2 systems are used as the references. This study employs the plane-wave density-functional method within the generalized gradient approximation. The structural properties emphasize the strongest local-symmetry distortion in Hf0.5Zr0.5O2 system. The bandgap (3.67 eV) and valence bandwidth (∼4.94 eV) of the system are between that of ZrO2 and HfO2 systems. Furthermore, the negative average defect formation energy confirms the stability of Hf0.5Z0.5O2 system. In all the systems, the significant differences between n0x(n0y) and n0z imply the strong optical dichroism between xy plane and z axis. On the other hand, the small differences between n0x and n0y imply the weak optical dichroism along xy plane. In Hf0.5Zr0.5O2 system, the optical properties show the zero-frequency refractive indices of (n0x, n0y, n0z) = (2.039, 2.086, 1.851). The system also shows the highest intensity and weakest dichroism of plasmonic states along y axis and xy plane, respectively. Moreover, we find the lowest energy level of maximum transmittance (T ≈ 1), strong-reflectance width, absorption width, and saturation threshold energy (∼9.6 eV) of the effective number of valence electrons. Our result emphasizes the importance of the doping treatment to the significant tuning of optical properties of Hf1−xZrxO2 system. This study presents the essential properties to guide future experiments to develop novel optical devices.

Temperature-dependent dispersion model of float zone crystalline silicon

In this paper, we present the temperature dependent dispersion model of float zone crystalline silicon. The theoretical background for valence electronic excitations is introduced in the theoretical part of this paper. This model is based on application of sum rules and parametrization of transition strength functions corresponding to the individual elementary phonon and electronic excitations. The parameters of the model are determined by fitting ellipsometric and spectrophotometric experimental data in the spectral range from far infrared (70cm-1) to extreme ultraviolet (40eV). The ellipsometric data were measured in the temperature range 5–700K. The excitations of the valence electrons to the conduction band are divided into the indirect and direct electronic transitions. The indirect transitions are modeled by truncated Lorentzian terms, whereas the direct transitions are modeled using Gaussian broadened piecewise smooth functions representing 3D and 2D van Hove singularities modified by excitonic effects. Since the experimental data up to high energies (40eV) are available, we are able to determine the value of the effective number of valence electrons. The Tauc–Lorentz dispersion model is used for modeling high energy electron excitations. Two slightly different values of the effective number of valence electrons are obtained for the Jellison–Modine (4.51) and Campi–Coriasso (4.37) parametrization. Our goal is to obtain the model of dielectric response of crystalline silicon which depends only on photon energy, temperature and small number of material parameters, e.g. the concentration of substituted carbon and interstitial oxygen. The model presented in this paper is accurate enough to replace tabulated values of c-Si optical constants used in the optical characterization of thin films diposited on silicon substrates. The spectral dependencies of the optical constants obtained in our work are compared to results obtained by other authors.

Energies of Maxima and Oscillator Strengths of CaO Elementary Transition Bands Over a Wide Energy Range

Integral spectra of the imaginary parts of the dielectric permittivity e2(E) and characteristic volume (–Im e–1) and surface [–Im (1 + e)–1] energy losses of calcium oxide were deconvoluted into elementary components in the range 6–40 eV. The main component parameters including the energies of maxima and oscillator strengths were determined using an improved non-parametric method of united Argand diagrams and the method of the effective number of valence electrons participating in the transitions. A total of 41 components with oscillator strengths in the range 0.001–0.22 were identified instead of the 14 maxima and shoulders of the integral spectra. They were caused by transverse and longitudinal exciton and interband transitions.

Experimental investigation of off-stoichiometry and 3d transition metal (Mn, Ni, Cu)-substitution in single-crystalline FePt thin films

In L10 (fct)-FePt thin films, both tuning Fe and Pt concentrations and substitution with third-metal were studied for magnetic characteristic optimization. We investigated single-crystalline FePt-X (X = Mn, Ni, Cu) thin films grown epitaxially on MgO(001) substrates at a substrate temperature of 350 °C by changing Fe, Pt, and X contents, and explored the effects of off-stoichiometry and 3d-metal-substitution. The magnetic moment per atom (m) of FePt-X films as a function of the effective number of valence electrons (neff) in 3d metal sites follows the Slater-Pauling-type trend, by which m decreases by the neff deviation from neff = 8, independently of the X metal and the Pt concentration. The magnetic anisotropy (Ku) exhibits neff dependence similar to m. This trend was almost independent of the Pt concentration after compensation using the theoretical prediction on the relation between Ku and Fe/Pt concentrations. Such a trend has been proved for stoichiometric FePt-X films, but it was clarified as robust against off-stoichiometry. The compensated Ku (Kucomp) of FePt-Mn and FePt-Cu followed a similar trend to that predicted by the rigid-band model, although the Kucomp of the FePt-Mn thin films dropped more rapidly than the rigid band calculation. However, it followed the recent first-principles calculation.

Electronic and Optical Properties of Rare Earth Oxides: <i>Ab Initio</i> Calculation

In this work, we have investigated the electronic and optical properties of the technologically important rare earth oxide compounds—X2O3 (X: Gd, Tb) using the density functional theory within the GGA. The band structure of X2O3 have been calculated along high symmetry directions in the first brillouin zone. The real and imaginary parts of dilectric functions and the other optical responses such as energy-loss function, the effective number of valence electrons and the effective optical dielectric constants of the rare earth sesquioxides (Gd2O3 and Tb2O3) were calculated.

First principles prediction of the elastic, electronic, and optical properties of Sb2S3 and Sb2Se3 compounds

We have performed a first principles study of structural, mechanical, electronic, and optical properties of orthorhombic Sb2S3 and Sb2Se3 compounds using the density functional theory within the local density approximation. The lattice parameters, bulk modulus, and its pressure derivatives of these compounds have been obtained. The second-order elastic constants have been calculated, and the other related quantities such as the Young's modulus, shear modulus, Poisson's ratio, anisotropy factor, sound velocities, Debye temperature, and hardness have also been estimated in the present work. The linear photon-energy dependent dielectric functions and some optical properties such as the energy-loss function, the effective number of valence electrons and the effective optical dielectric constant are calculated. Our structural estimation and some other results are in agreement with the available experimental and theoretical data.

Derivation of the linear relationship between SWCNTs functionalization energies and sidewall curvature

A simple linear relationship between the functionalization reaction energies for the exohedral monovalent addition on the surface of an ideal, infinitely long, single-walled carbon nanotube (SWCNT) and the reciprocal SWCNT radius has been derived employing the hard–soft acid basis principle and the tight binding model. The slope of the derived linear relationship is a function of the effective number of valence electrons involved in the functionalization reaction. The intercept of the derived linear relationship, equal to the reaction energies on a planar graphite surface, is a function of the electrophilicity of the monovalent addend and of the condensed Fukui function of its reacting atom. The theoretical predictions of this simple formula are coherent with the computational density functional theory data reported in the literature.

超伝導臨界温度と擬ポテンシャル半径の関係

The relation between the superconducting critical temperature, Tc, and the effective pseudopotential radius, r(eff), was examined for elements, AB-type compounds, and Fe- and Cu-based compounds. Here, assuming arbitrarily their bonding states hybridized with valence electrons, we estimated r(eff) for the materials from the pseudopotential radii given for s, p, and d electrons. It is suggested that Tc can be estimated from the linear relation between Tc and N(v)r(eff)3, where N(v) is the effective number of valence electrons. For the elements with relatively high Tc such as Nb, Tc, and V, for those in AB compounds, and for Cu and Fe in their cuprates, pnictides, and chalcogenides, the value of Tc was found to vary almost linearly to N(v)r(eff)3 by assuming the p3dn or sp2dn hybridization. These findings suggest that the p3dn or sp2dn hybridization is a key bonding character to realize high Tc. For some of the compounds containing transition elements, however, the data deviate largely from the linear relation, indicating that the validity of the present argument is limited in such a category.

Approximation of Range in Materials as a Function of Incident Electron Energy

A simple composite analytic expression has been developed to approximate the electron range in materials. The expression is applicable over more than six orders of magnitude in energy ( ; MeV) and range ( 10-9-10-2 m), with an uncertainty of ≤ 20% for most conducting, semiconducting, and insulating materials. This is accomplished by fitting data from two standard NIST databases [ESTAR for the higher energy range and the electron inelastic mean free path (IMFP) for the lower energies]. In turn, these data have been fit with well-established semiempirical models for range and IMFP that are related to standard material properties (e.g., density, atomic number, atomic weight, stoichiometry, and bandgap energy). Simple relations between the IMFP and the range, based on the continuous-slow-down approximation, are used to merge results from the two databases into a composite range expression. A single free parameter, termed the effective number of valence electrons per atom Nv, is used to predict the range over the entire energy span.