First-principles calculations on the origins of the gap bowing in InAs1-xPx alloys

We perform self-consistent ab-initio calculations to study the structural, electronic and thermodynamic properties of InAs_1-xP_x alloy. The full potential-linearized augmented plane wave (FP-LAPW) method was employed within density functional theory (DFT). The ground-state properties are determined for the bulk materials (InAs and InP) as well as for the different concentration of their alloys. Deviations of the lattice constants from Vegard's law and the bulk modulus from linear concentration dependence (LCD) were observed. The microscopic origins of the gap bowing were explained by using the approach of Zunger and co-workers. The gap bowing for the alloy of interest was found to be mainly caused by the charge-exchange contributions. In addition, the thermodynamic stability of InAs_1-xP_x alloy was investigated by calculating the excess enthalpy of mixing ΔH_m and the calculated phase diagram showed a broad miscibility gap with a high critical temperature.     

Theoretical investigations on KClxBr1−x, KClxI1−x and KBrxI1−x: A first-principles study

Using first-principles total energy calculations within the full-potential linearized augmented plane wave (FP-LAPW) method, we have investigated the structural, electronic and thermodynamic properties of potassium halides (KCl_xBr_1−x, KCl_xI_1−x and KBr_xI_1−x), with x concentrations varying from 0% up to 100%. The effect of composition on lattice constants, bulk modulus, band gap and dielectric function was investigated. Deviations of the lattice constants from Vegard's law and the bulk modulus from linear concentration dependence (LCD) were observed for the three alloys. The microscopic origins of the gap bowing were explained by using the approach of Zunger and coworkers. On the other hand, the thermodynamic stability of these alloys was investigated by calculating the excess enthalpy of mixing ΔH_m as well as the phase diagram.     

Theoretical study of CuxAg1−xI alloys

We have performed first-principles calculations using full potential linearized augmented plane wave (FP-LAPW) method within density functional theory (DFT) to investigate the fundamental properties of Cu_xAg_1−xI alloys. We used both GGA96 [J.P. Perdew, K. Burke, M. Ernzerhof, Phys. Rev. Lett. 77 (1996) 3865.] and EVGGA [E. Engel, S.H. Vosko, Phys. Rev. B. 47 (1993) 13164.] generalized gradient approximations of the exchange-correlation energy that are based on the optimization of total energy and corresponding potential. Quantities such as lattice constants, bulk modulus, band gap, density of occupied states and effective mass were calculated as a function of copper molar fraction x. These parameters were found to depend non-linearly on alloy composition x, except the lattice parameter, which follows Vegard's law. The microscopic origins of the gap bowing were explained using the approach of Zunger and co-workers; we have concluded that the band-gap energy bowing was mainly caused by the chemical charge-transfer effect and the volume deformation , while the structural relaxation contribute to the gap bowing parameter at smaller magnitude. The calculated phase diagram shows a broad miscibility gap for this alloy with a high critical temperature.     

Noise Characteristics of Optocouplers on Neutron Radiation

Neutron dose radiation experiment is designed to study the optocoupler's displacement effects and the noise characteristics. The burst noise is introduced in optocouplers on neutron radiation, which is indicated from experiments. With the increasing neutron radiation the displacement defects in space-charge region increase, the scattering enhances and the noise signal mutations increase. All these represent the noise time series mutations, the random pulses and the increasing noise complexity. The burst noise becomes evident, and the power spectrum density, the characteristic frequency and the fractal dimension of time series of noise greatly increase.     

Strongly Correlated Effect in TiS2

The thermoelectric compound TiS2 is studied by using the full-potential linearized augmented plane-wave method on the density functional theory with the generalized gradient approximation （GGA） as well as the on-site Coulomb interaction correction （＋U）. The Seebeck coefficient of TiS2 is calculated based on the electronic structure obtained within the GGA under the consideration of the on-site Coulomb interaction. The calculated Seebeck coefficient at 300K shows that Coulomb interaction U in the range of 4.97-5.42eV is important to reproduce the experimental data. The obtained energy gap Eg around 0.05 eV indicates that TiS2 is an indirect narrow-gap semiconductor.     

Metal-free spin channels in graphitic boron-nitrogen nanostructures

Evidence is given for the existence of metal-free spin channels in an insulating medium. First-principles calculations indicate the presence of an unpaired spin, in a ground state boron-nitrogen nanostructure with a carbon zig-zag chain generated by the inclusion of a disclination with either negative or positive Gaussian curvature. The spin-polarized states are delocalized on the carbon chain suggesting possible spintronics applications.     

Ab Initio Study on Hypothetical Silver Nitride

We perform the ab initio calculations based on norm-conserving pseudopotentials and density functional theory to investigate the structural, elastic, and thermodynamical properties for silver nitride （AgN） compound that is a member of the 4d transition metal group and has not been synthesized yet. The obtained results are compared with the other available theoretical data, and the agreement is, generally, quite good. We also present the pressure-dependent behaviour of some mechanical and thermodynamical properties for the same compounds.     

Experimental and theoretical studies on the magnetic property of carbon-doped ZnO

Intrinsic room-temperature ferromagnetism was detected over n-type carbon-doped ZnO prepared through solid-state reaction. Our results of first-principle calculations based on density functional theory revealed that the CZn₄O₁₂ unit is the origin of magnetic moment in the carbon-doped ZnO system. The carbon component has a significant contribution to the net magnetic moment, and any oxygen vacancy present in CZn₄O₁₂ has a negative effect on the magnetic properties of the system. Moreover, both antiferromagnetic and ferromagnetic interactions are predicted among carbon atoms located at different CC distances. The result suggests that the defect density influenced by the distribution of carbon has a significant effect on the magnetic properties of the carbon-doped ZnO system.     

Finite element method for solving Kohn-Sham equations based on self-adaptive tetrahedral mesh

A finite element (FE) method with self-adaptive mesh-refinement technique is developed for solving the density functional Kohn-Sham equations. The FE method adopts local piecewise polynomials basis functions, which produces sparsely structured matrices of Hamiltonian. The method is well suitable for parallel implementation without using Fourier transform. In addition, the self-adaptive mesh-refinement technique can control the computational accuracy and efficiency with optimal mesh density in different regions.     

X-ray characterization of precipitates in cerium-doped PbTe and SnTe crystals

The homogeneity, solid solubility, and chemical bonds in the new materials PbTe, SnTe doped with Ce were investigated. Scanning electron microscope observation and electron probe microanalysis carried out on PbTe crystals doped with Ce, revealed three types of Ce-rich precipitates with following compositions: CeTe₂, Ce₃Te₇, Ce₂Te₅ and small admixture of PbTe in precipitates. The solubility of Ce in PbTe matrix was estimated as 0.5±0.1 at. %. The solubility of PbTe in CeTe₂ and Ce₃Te₇ was found to be 3±0.5 at. %, but 7±0.5 at. % in the case of Ce₂Te₅. In SnTe crystal doped with Ce only one kind of precipitate with composition Ce₂SnTe₅ was found. Cerium solubility in SnTe matrix was estimated to be 1±0.25 at. %. According to our knowledge this is the first report of the identification of Ce₂SnTe₅ compound. The similar compounds Ce₂SnS₅ and Ce₂SnSe₅ are known. Received: 9 December 1998 / Accepted: 9 February 1999 / Published online: 28 April 1999     

Optical properties of the alkaline-earth fluorohalides matlockite-type structure SrFX (X=Cl, Br, I) compounds

The optical properties of the SrFX (X=Cl, Br, I) compound have been reported using the full potential linearized augmented plane wave (FP-LAPW) method as implemented in the WIEN2K code. We employed the generalized gradient approximation (GGA), which is based on exchange-correlation energy optimization to calculate the total energy. Also we have used the Engel-Vosko GGA formalism, which optimizes the corresponding potential for band structure calculations. Our calculations show that the valence band maximum (VBM) and conduction band minimum (CBM) are located at Γ resulting in a direct energy gap. We present calculations of the frequency-dependent complex dielectric function ε(ω) and its zero-frequency limit ε₁(0). We find that the value of ε₁(0) increases on decreasing the energy gap. The reflectivity spectra and absorption coefficient have been calculated and compared with the available experimental data.     

Correlations in the ground state of the one-dimensional Hubbard model

With eigenfunctional theory and a rigorous expression of exchange-correlation energy of a general interacting electron system, we study the ground state properties of the one-dimensional Hubbard model, and calculate the ground-state energy as well as the charge gap at half-filling for arbitrary coupling strength u=U/(4t) and electron density nc. We find that the simple linear approximation of the phase field works well in weak coupling case, but it becomes inappropriate as the on-site Coulomb interaction becomes strong where the fluctuations of the bosonic auxiliary field are strong. Then we propose a new scheme by adding Gutzwiller projection which suppresses the density fluctuations and the new results are quite close to the exact ones up to considerably strong coupling strength u=3.0 and for arbitrary electron density nc. Our calculation scheme is proved to be effective for strongly correlated electron systems in one dimension, and its extension to higher dimensions is straightforward.     

Electron Orbital Magnetic Moments in the Armchair Carbon Nanotubes

Based on the density functional theory, we calculate the band structure of an armchair carbon nanotube in an axial magnetic field. The result shows that there are two kinds of magnetic moments with different symmetries. One is the Aharonov--Bohm-type magnetic moment which can be easily understood with classical picture, the other belonging to the valence, and conduction sub-bands should be explained by quantum mechanics. We use an effective mass model to analyse the magnetic moments and by comparing with the result of first-principle calculation, we conclude that the effective mass model is reasonable to estimate the change of the band gap in magnetic fields.     

First Principles Calculation of Universal Conductance Fluctuation in Monatomic Metal Chains

Combining the non-equilibrium Green＇s function method and density functional theory, we provide a first-principle scheme to calculate the universal conductance fluctuation （UCF） in quasi one-dimensional monatomic chains subject to a magnetic field. Our results show that for these monatomic chains, the amplitude of the UCF is much smaller than the previous theoretical prediction for mesoscopic conductors by Lee et al. [Phys. Rev. Lett. 55 （1985） 1622; Phys. Rev. B 36 （1987） 1039] The reason is that the ergodic hypothesis fails in these nanowires due to the confinement of geometry. We ascribe the phenomenon to the flux-dependent density of states fluctuation.     

First-Principles Calculation of Electronic Structure and Optical Properties of Sb-Doped ZnO

The geometric structure, electronic structure, optical properties and the formation energy of Sb-doped ZnO with the wurtzite structure are investigated using the first-principles ultra-soft pseudo-potential approach of plane wave based upon the density functional theory. The calculated results indicate that the volume of ZnO doped with Sb becomes larger, and the doping system yields the lowest formation energy of Sb on the interstitial site and the oxygen site. Furthermore, Sb dopant first occupies the octahedral oxygen sites of the wurtzite structure. It is found that Sb substituting on oxygen site behaves as a deep acceptor and shows the p-type degenerate semiconductor character. After doping, the electron density difference demonstrates the considerable electron charge density redistribution, which induces the effect of Sb-doped ZnO to increase the charge overlap between atoms. The density of states move towards lower energy and the optical band gap is broadened. Our culated results are in agreement with other experimental results and could make more precise monitoring and controlling possible during the growth of ZnO p-type materials.     

Optimization of (002)-Oriented ZnO Film Synthesis in Sol-Gel Process and Film Photoluminescence Property

By orthogonal design theory, technological parameters of the (002)-oriented ZnO film prepared in sol-gel process are optimized. A set of technological parameters for growing highly (002)-oriented ZnO film is obtained. As a result, it is proven that the Zn²⁺ concentration is the most important factor to grow a highly (002)-oriented ZnO film. We take an appropriate Zn²⁺ concentration 0.35mol/L for the aimed film, of which photoluminescence property is better than those of the films derived from other Zn²⁺ concentrations precursor solution. The Zn²⁺ concentration either larger or smaller than 0.35mol/L leads to the (002)-oriented degree decrease of films. By employing an atom force microscope, a hexagonal atom arrangement, which indicates that the film site detected is a ZnO single crystal, is observed in the surface of the highly (002)-oriented film.     

Optical Properties of Zinc-Blende InGaN/GaN Quantum Well Structures and Comparison with Experiment

Optical properties of zinc-blende InGaN/GaN Q W structures are investigated using the multiband effective-mass theory. The transition wavelength values at 300 K ranged from 440 to 570nm in the investigated range of the In composition and the well width. The theoretical wavelengths show reasonable agreement with the experimental results. The optical gain decreases with the increasing well width. This is mainly due to the reduction in the quasi-Fermi-level separation because the optical matrix element increases with the well width.     

Ferromagnetism of Ni cluster in Ni-doped ZnO by solid state reaction

Ni-doped ZnO samples with ferromagnetism at room temperature have been prepared by solid state reaction. It is found that the ferromagnetism originates from the nanosized Ni clusters formed from the decomposition of NiO during calcination. The magnetic properties can be explained by the microstructure of sparsely distributed, randomly oriented and magnetically saturated Ni clusters. Experiments show that the addition of Cu ions will block the decomposition of NiO because Cu²⁺ is more likely reduced to Cu⁺. Hall effect confirms the absence of exchange coupling between local spins and charge carriers.     

Exact confined eigenstates in one-dimensional finite periodic systems with arbitrary symmetric basis

A set of eigenenergy equations for complete confinement of N periodic one-dimensional system with arbitrary symmetric potential in each cell is presented. Exact eigenenergies and their eigenfunctions are solved analytically. We find different solutions on the band edge states comparing early results solved by conventional methods. Our results show that higher band edge state of each bandgap is dependent on the confined length, but lower band edge state is independent of the length. Moreover, wavefunctions for lower band edge states are Bloch waves but those for other eigenstates, including higher band edge states, do not obey Bloch waves.