Electron mobility for a model Al<Subscript>x</Subscript> Ga<Subscript>1-x</Subscript> As/GaAs heterojunction under pressure

A variational method and a memory function approach are adopted to investigate the electron mobility parallel to the interface for a model Al_xGa_1-xAs/GaAs heterojunction and its pressure effect by considering optical phonon modes (including both of the bulk longitudinal optical (LO) in the channel side and interface optical (IO) phonons). The influence of a realistic interface heterojunction potential with a finite barrier and conduction band bending are taken into account. The properties of electron mobility versus Al concentration, electronic density and pressure are given and discussed, respectively. The results show that the electron mobility increases with Al concentration and electronic density, whereas decreases with pressure from 0 to 40 kbar obviously. The Al concentration dependent and the electron density dependent contributions to the electron mobility from the scattering of IO phonons under pressure becomes more obvious. The variation of electron mobility with the Al concentration and electron density are dominated by the properties of IO and LO phonons, respectively. The effect of IO phonon modes can not be neglected especially for higher pressure and electronic density.     

Correlation effects obtained from optical spectra of Fe-pnictides using an extended Drude-Lorentz model analysis

We introduce an analysis model, an extended Drude–Lorentz model, and apply it to Fe-pnictide systems to extract their electron–boson spectral density functions (or correlation spectra). The extended Drude–Lorentz model consists of an extended Drude mode for describing correlated charge carriers and Lorentz modes for interband transitions. The extended Drude mode can be obtained by a reverse process starting from the electron–boson spectral density function and extending to the optical self-energy and, eventually, to the optical conductivity. Using the extended Drude–Lorentz model, we obtained the electron–boson spectral density functions of K-doped BaFe₂As₂ (Ba-122) at four different doping levels. We discuss the doping-dependent properties of the electron–boson spectral density function of K-doped Ba-122. We also can include pseudogap effects in the model using this approach. Therefore, this approach is very helpful for understanding and analyzing measured optical spectra of strongly correlated electron systems, including high-temperature superconductors (cuprates and Fe-pnictides).     

Specific features of the electronic structure and spectral characteristics of the Gd<Subscript>5</Subscript>Si<Subscript>3</Subscript> compound

The electronic structure and optical properties of the hexagonal intermetallic compound Gd₅Si₃ are investigated. The spin-polarization calculation of the band spectrum is performed in the local spin density approximation, taking account for the strong electron correlations in the 4 f shell of a Gd ion (LSDA + U method). Optical constants of the compound in the wavelength range of 0.22–15 μm are determined by the ellipsometry technique and some spectral characteristics are calculated. The frequency dependence of optical conductivity in the light quantum absorption region is analyzed on the basis of the calculated electron density of states.     

Effect of substrate temperature on transport and optical properties of sprayed Cd1−xZnxS films

Polycrystalline Cd_1?xZn_xS films are prepared by the spray pyrolysis technique on amorphous substrate, at different substrate temperatures. The dark conductivity and thermoelectric power are measured and applied to calculate the electron density and mobility. The electron density decreases with increase in substrate temperature. However, mobility is higher for the films prepared at 400°C substrate temperature. The variation of electron density and mobility with substrate temperature is explained in terms of crystallanity of the film and chlorine concentration in the films. The optical absorption of the films are studied and revealed that there is not much change in band gap with substrate temperature.     

Specific features of the behavior of the optical properties of TbNi<Subscript>5 − <Emphasis Type="Italic">x</Emphasis></Subscript>Cu<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript> intermetallic compounds

The optical properties of the TbNi_{5 ? x} Cu _x intermetallic compounds have been investigated in the spectral range 0.08–5.64 eV by the ellipsometric method. It is shown that substitution of nickel for copper atoms leads to a significant change in the frequency dependence of the optical conductivity; this change is related to modification of the electronic spectrum. The formation of a new interband absorption band has been revealed, whose intensity increases with an increase in the copper content. The concentration dependences of the plasma and relaxation frequencies of conduction electrons in the compounds under study are determined. Self-consistent calculation of the electronic structure of the TbNi₅ binary compound has been performed in the approximation of local electron spin density. The electron density of states for two spin projections and the optical conductivity of this compound have been calculated.     

Influence of aluminum impurity on the electronic structure and optical properties of the TbNi5 intermetallic compound

The electronic structure of the TbNi_{5 ? x} Al _x intermetallic compounds (x = 0, 1, 2) is calculated in the local electron density approximation with the correction to strong electron correlations in 4f shell of terbium ions. Spectral properties of these compounds are measured by ellipsometry in a wavelength range of 0.22–16 μm. Frequency dependences of optical conductivity in the region of interband optical absorption are interpreted based on the results of calculations of electron densities of states. The relaxation and plasma frequencies of conduction electrons are determined.     

First-principles studies of the electronic structure and optical properties of AgBO<Subscript>3</Subscript> (B=Nb,Ta) in the paraelectric phase

The electronic energy-band structure, density of states (DOS), and optical properties of AgBO₃ in the paraelectric cubic phase have been studied by using density functional theory within the local density approximation for exchange-correlation for the first time. The band structure shows a band gap of 1.533 eV (AgNbO₃)and 1.537 eV (AgTaO₃)at (M-⌈)point in the Brillouin zone. The optical spectra of AgBO₃ in the photon energy range up to 30 eV are investigated under the scissor approximation. The real and imaginary parts of the dielectric function and — thus the optical constants such as reflectivity, absorption coefficient, electron energy-loss function, refractive index, and extinction coefficient — are calculated. We have also made some comparisons with related experimental and theoretical data that is available.      

Influence of valence electron concentration on elastic, electronic and optical properties of the alkaline-earth tin oxides A3SnO (A=Ca, Sr and Ba): A comparative study with ASnO3 compounds

By employing first principles method of the plane wave pseudo potential calculations (PP-PW), based on the density functional theory (DFT), within the local density approximation (LDA), the correlation between valence electron concentration and structural, elastic, electronic as well as optical properties of A₃SnO and ASnO₃ compounds where A=Ca, Sr and Ba are investigated. The elastic constants and their pressure dependence are calculated using the static finite strain technique. We derived the bulk, shear and Young's moduli for ideal monocrystalline and for polycrystalline A₃SnO and ASnO₃ aggregates. Band structures reveal that alkaline-earth tin oxides A₃SnO are direct energy band gap (G-G) materials.The hardness of these compounds was explained using chemical bonding properties and Milliken charges transfer. The optical constants, including the dielectric function, optical reflectivity, refractive index and electron energy loss, are calculated for radiation up to 20 eV. We have found that the static dielectric constants of all these compounds are in good agreement with Penn model.     

Optical absorption and electronic structure of intermetallic compound RuIn3

The optical properties of intermetallide RuIn₃ are investigated by ellipsometry in the spectral range of 0.22–10 μm. The experimental data point to the existence of an energy gap of about 0.5 eV in the electronic spectrum of the compound. The density of the electron states and interband optical conductivity are calculated in terms of the density functional theory. The experimental and theoretical spectra of the optical conductivity are compared. It is found that the formation of basic absorption bands is caused by interband transitions of electrons of the d-band of Ru and p-band of In.     

Phonon densities of states of the thorium hydrides

The optical phonon density of states in both Th₄H₁₅ and ThH₂ is presented as measured by inelastic neutron scattering. For Th₄H₁₅ the results indicate a strong electron—optical phonon coupling.     

Optical spectroscopy and electronic structure of the Er5Ge3 compound

The results of the study of the optical properties and electronic structure of the Er₅Ge₃ compound have been presented. In the wavelength range of 0.22–15 μm (0.083–5.64 eV), the optical constants have been measured, and the spectral and electronic characteristics have been determined. The spin-polarization calculations of the band spectrum have been performed in the local electron spin density approximation (LSDA) with a correction for strong correlations in the 4f shell of the rare-earth atom (LSDA + U method). The main features of the experimental dispersion dependence of the optical conductivity in the region of quantum light absorption have been interpreted based on the results of calculations of the electron density of states.     

Ab initio study of the linear and nonlinear optical responses in BiAlO3

The linear and non-linear optical properties of BiAlO₃ are studied by employing the density functional perturbation theory within the local density and generalized gradient approximations. The computations are based on the electronic structure obtained within density functional theory. The optical properties such as the dielectric function, refractive index, spectral reflectivity, absorption coefficient and electron energy-loss spectrum are obtained in the energy region of up to 30 eV. The calculated value of the birefringence for BiAlO₃ shows that it is a uniaxial negative crystal and has a large birefringence. We also report our studies on the second harmonic generation response coefficient over a large frequency range for BiAlO₃ crystal. The predicted second-order optical spectra indicate pronounced structures related to of 1ω and 2ω frequency resonances. Furthermore, the non-linear optic and linear electro-optic coefficients are computed by employing 2n + 1 theorem applied to an electric-field dependent energy functional. The results are compared with the available calculations.     

Nonlinear optics near the metal insulator transition

Large, free-carrier-induced, optical nonlinearities are observed in n-Si:P near the metal-insulator transition. _X⁽³⁾ varies superlinearly with n, suggesting an impurity interaction mechanism. A theory of the effect shows that it measures the pile-up of electron density at the impurities. The experiments imply that the pile-up varies rapidly with electron energy near the transition.     

Si掺杂β-Ga2O3的第一性原理计算与实验研究

采用基于密度泛函理论(DFT)的第一性原理平面波超软赝势(USPP)法, 在广义梯度近似(GGA)下计算了本征β-Ga₂O₃和Si掺杂β-Ga₂O₃的能带结构、电子态密度、差分电荷密度和光学特性. 在蓝宝石衬底(0001)晶面上用脉冲激光沉积(PLD)法制备了本征β-Ga₂O₃和Si掺杂β-Ga₂O₃薄膜, 测量了其吸收光谱和反射光 关键词： 第一性原理 超软赝势 密度泛函理论 2O₃')" href="#">Si掺杂β-Ga₂O₃     

Structural,elastic, electronic and optical properties of the cubic perovskites CaXO3 (X=Hf and Sn)

The structural, elastic, electronic and optical properties of CaXO₃ compounds with the cubic perovskites structure have been investigated, by employing a first principles method, using the plane wave pseudo potential calculations (PP-PW), based on the density functional theory (DFT), within the local density approximation (LDA). The elastic constants and their pressure dependence are calculated using the static finite strain technique. We derived the bulk, shear and Young’s moduli for ideal monocrystalline and for polycrystalline CaXO₃ aggregates which we have classified as ductile in nature. Band structures reveal that these compounds are indirect energy band gap (R-G) semiconductors; the analysis of the site and momentum projected densities, valence charge density bond length, bond population and Milliken charges, shows that bonding is of covalent–ionic nature. We have found that the elastic constants C₁₁, C₁₂, C₄₄ are in good correlation with the bonding properties. The optical constants, including the dielectric function, optical reflectivity, refractive index and electron energy loss, are calculated for radiation up to 20 eV.     

CeO2的电子结构，光学和晶格动力学性质：第一性原理研究

基于考虑了Ce-4f电子间的库仑作用U和交换作用J的LDA+U方案,应用第一性原理计算系统研究了CeO₂的电子结构,光学和晶格动力学性质.电荷密度和电子局域函数的分布特征表明,CeO₂是属于共价键的绝缘体.介电常数、玻恩有效电荷张量和声子色散曲线的计算值和相应的实验结果符合得比较好. 关键词： 电子结构 光学性质 晶格动力学 第一性原理计算     

First-principles study of the optical properties of PbTiO<Subscript>3</Subscript>

The optical properties of PbTiO₃ were studied from first principles using the density functional theory. The dielectric functions and optical constants are calculated using the full potential–linearized augmented plane wave (FP-LAPW) method with the generalized gradient approximation (GGA). The theoretical calculated optical properties and energy loss (EEL) spectrum yield a static refractive index of 2.83 and a plasmon energy of 23.1 eV for cubic phase. The effective electron number at low energy saturates near 20 eV with the value of 18.1 for the effective electron number. In the tetragonal phase the static refractive index decreases to 2.59 and yields a plasmon energy of 22.7 eV.     

Electron energy-loss spectroscopy of LiMn2O4, LiMn1.6Ti0.4O4 and LiMn1.5Ni0.5O4

The dielectric properties of LiMn₂O₄, LiMn_1.6Ti_0.4O₄ and LiMn_1.5Ni_0.5O₄ powders, synthesized by sol-gel method, were determined by analyzing the low-loss region of the electron energy-loss spectroscopy (EELS) spectrum in a transmission electron microscope. From these data, the optical joint density of states (OJDS) was obtained by Kramers-Kronig analysis. Since maxima observed in the OJDS spectra are assigned to interband transitions above the Fermi level, these spectra can be interpreted on the basis of calculated density of states (DOS), carried out with the CASTEP code. Experimental and theoretical results are in good agreement.     

Effect of conduction band nonparabolicity on the optical properties in a single quantum well under hydrostatic pressure and electric field

The effect of conduction band nonparabolicity on the linear and nonlinear optical properties such as absorption coefficients, and changes in the refractive index are calculated in the Al_0.3Ga_0.7As/GaAs heterostructure-based symmetric rectangular quantum well under applied hydrostatic pressure and electric field. The electron envelope functions and energies are calculated in the effective mass equation including the conduction band nonparabolicity. The linear and nonlinear optical properties have been calculated in the density matrix formalism with two-level approximation. The conduction band nonparabolicity shifts the positions of the optical properties and decreases their strength compared to those without this correction. Both the optical properties are enhanced with the applied hydrostatic pressure. While the absorption coefficients are bleached under the combined effect of high pressure and electric field, the bleaching effect is reduced when nonparabolicity is included.     

First-principles study of structural, elastic, electronic and optical properties of SrMO3 (M=Ti and Sn)

We present structural, elastic, electronic and optical properties of the perovskites SrMO₃ (M=Ti, and Sn) for different pressure. The computational method is based on the pseudo-potential plane wave method (PP-PW). The exchange-correlation energy is described in the generalized gradient approximation (GGA). The calculated equilibrium lattice parameters are in reasonable agreement with the available experimental data. This work shows that the perovskites SrTiO₃, and SrSnO₃ are mechanically stable and present an indirect band gaps at the Fermi level. Applied pressure does not change the shape of the total valence electronic charge density and most of the electronic charge density is shifted toward O atom. Furthermore, in order to understand the optical properties of SrMO₃, the dielectric function, absorption coefficient, optical reflectivity, refractive index, extinction coefficient and electron energy-loss are calculated for radiation up to 80 eV. The enhancement of pressure decreases the dielectric function and refractive indices of SrTiO₃ and SrSnO₃.