Exchange-correlation Effects Research Articles

Ab initio study of electronic structure and magnetic properties of CoMnTaZ (Z = Si, Ge) quaternary Heusler compounds

The electronic structures and magnetic properties of CoMnTaSi and CoMnTaGe Heusler alloys with LiMgPbSb-type structure investigated firstly by using the first-principles calculations. We have applied the full-potential linearized augmented plane waves plus local orbitals (FP-L/APW+LO) method. Exchange-correlation effects are treated using the generalized gradient approximations GGA and GGA+U. The GGA calculation shows the CoMnTaSi and CoMnTaGe compounds at its equilibrium lattice constant are HM ferromagnet with an indirect band gap Γ → X of 0.34 and 0.39 eV and a HM gap of 0.34 and 0.13 eV in the spin-down channel. The CoMnTaZ (Z = Si, Ge) compounds have an integer total magnetic moment of 1.00 μB, satisfying the Slater–Pauling rule mtot = (Nv − 24). The similar results are also obtained by GGA+U calculation. Therefore, these new materials are good candidates for potential applications in spintronic.

On low-lying excited states of extended nanographenes.

Low-lying excited states of planarly extended nanographenes are investigated using the long-range corrected (LC) density functional theory (DFT) and the spin-flip (SF) time-dependent density functional theory (TDDFT) by exploring the long-range exchange and double-excitation correlation effects on the excitation energies, band gaps, and exciton binding energies. Optimizing the geometries of the nanographenes indicates that the long-range exchange interaction significantly improves the CC bond lengths and amplify their bond length alternations with overall shortening the bond lengths. The calculated TDDFT excitation energies show that long-range exchange interaction is crucial to provide accurate excitation energies of small nanographenes and dominate the exciton binding energies in the excited states of nanographenes. It is, however, also found that the present long-range correction may cause the overestimation of the excitation energy for the infinitely wide graphene due to the discrepancy between the calculated band gaps and vertical ionization potential (IP) minus electron affinity (EA) values. Contrasting to the long-range exchange effects, the SF-TDDFT calculations show that the double-excitation correlation effects are negligible in the low-lying excitations of nanographenes, although this effect is large in the lowest excitation of benzene molecule. It is, therefore, concluded that long-range exchange interactions should be incorporated in TDDFT calculations to quantitatively investigate the excited states of graphenes, although TDDFT using a present LC functional may provide a considerable excitation energy for the infinitely wide graphene mainly due to the discrepancy between the calculated band gaps and IP-EA values. © 2017 Wiley Periodicals, Inc.

Propagation and interaction of two soliton in a quantum semiconductor plasma with exchange correlation effects

Collisions of solitary pulses in a four species quantum semiconductor plasma consisting of degenerate electrons, degenerate holes, and non-degenerate ions are investigated. The electron and hole exchange-correlation forces between the identical particles when their wave functions overlap due to the high number densities are considered. Using the extended Poincarê–Lighthill-Kue method in opposite directions, two Korteweg-de Vries equations are derived. Hirota's method is used to derive the analytical phase shifts after the collision of one soliton and two soliton. Typical values for GaAs, GaSb, GaN, and InP semiconductors are considered to analyze the effects after collisions.

Comment on “Effects of electron exchange-correlation potential on electrostatic oscillations in single-walled carbon nanotubes” [J. Appl. Phys. 115, 204304 (2014)

In a recent article, Khan and Hassan [J. Appl. Phys. 115, 204304 (2014)] investigated the dispersion properties of electrostatic electron plasma oscillations on the surface of single-walled carbon nanotubes by considering the exchange-correlation potential. They showed that the surface plasmon energies are blue-shifted by the exchange-correlation effects. Here, we derive the correct form of the dispersion relation and show that the energies of plasmonic modes are red-shifted by the exchange-correlation effects.

Spectral properties of hydrogen-like ions in finite-temperature quantum plasmas

The oscillator strengths and radiative transition probabilities for the Lyman and Balmer series for transitions up to n = 5 of a hydrogen-like ion embedded in a finite-temperature quantum plasma are calculated in a wide range of plasma densities and temperatures for the first time. The plasma screened Coulomb interaction includes the quantum electron degeneracy, exchange-correlation, and finite-temperature gradient correction effects. The results for the oscillator strengths at temperatures 0.35 eV and 3.5 keV are compared with those for zero-temperature quantum plasmas and for Debye plasmas, respectively.

The effects of exchange–correlation on high-frequency electrostatic surface wave in magnetized quantum plasma through a porous medium

In this paper the propagation of an electrostatic surface wave at the interface between a vacuum and quantum plasma through a Brinkman porous medium is studied by considering exchange–correlation effects. A general analytical expression for dispersion relation is derived using the linearized quantum hydrodynamic model in conjunction with Poisson’s equation in the presence of a static and constant magnetic field. The growth and instability rates of electrostatic surface waves are obtained and separated. Numerical values are used to summarize and analyze the normalized dispersion relations for overcritical dense plasma condition in different cases. The results show that the behavior of surface plasmon waves can be significantly modified by the exchange–correlation effects which have different influences on the system stability. It is shown that the exchange–correlation effects caused the frequency of such waves to down-shift. It is found that the down-shift of the real part of frequency Re(Ω) by the exchange–correlation effect may increase by either increasing the plasmonic coupling H or increasing the porosity effects. In addition, it is shown that by increasing the magnetic field strength the group velocity is increased. Although the instability of the surface wave is decreased by increasing the plasmonic coupling H, it is increased by increasing the porosity effects (ν). The obtained results can help us in the physical understanding of the surface magnetized quantum wave on a semi-bounded quantum plasma through a porous media.

The optoelectronic properties of a solar energy material: Ag2HgSnS4

We used an ab initio full potential-linearized augmented plane wave technique within the density functional theory to study the structural and optoelectronic properties of Ag2HgSnS4 in a wurtzite-stannite phase. The exchange correlation effects are included through the generalized gradient approximation and modified Becke-Johnson exchange potential. Various physical quantities, such as lattice parameter, bulk modulus, band structure and density of states, are given. Also, we have presented the results of the effective mass for the electrons in the CB and the holes in the BV. We show that the modified Becke-Johnson exchange potential can predict the energy band gap in better agreement with the experiment. In addition the dielectric function and energy-loss function are presented for the energy range of 0-26 eV. The electronic and optical properties indicate that this compound can be successfully used in optoelectronic devices

Ab-initio calculations on half-Heusler NiXSn (X = Zr, Hf) compounds: electronic and optical properties under pressure

In this study, we have investigated the electronic and optical properties of half-Heusler NiXSn (X = Zr, Hf) compounds under pressure by means of first principles calculations. The generalized gradient approximation is used to model exchange–correlation effects. We have estimated a transition from indirect band gap to direct band gap at 50 and 127 GPa for NiZrSn and NiHfSn, respectively. We have also plotted the static dielectric constant versus pressure for both compounds. The obtained results are in agreement with the available experimental and theoretical data.

Mode Coupling and Two-Stream Instabilities in Semiconductors

The quantum hydrodynamic model is used to investigate the arbitrary degenerate electron/hole stream instabilities in ballistic semiconductors or semiconductor plasmas (semiplasmas) for a wide range of impurity doping, number density, and temperatures. It is observed that, while the instability in classical plasma regime is always of single $k$ -range type, the double-range instability can occur in degenerate plasma regime. Different types of carrier counter-streaming instabilities are classified and parametrically studied in arbitrarily doped ballistic semiconductors. The parametric study of counter-stream instability in silicon at room temperature shows that the doping scheme has fundamental effect on electron–electron and hole–hole instabilities but no effect on electron–hole instability. Other parameters, such as the electron temperature and stream speed, have significant effects on the unstable wavenumber range and the growth rate. It is remarked that the electron stream in n-type semiconductors becomes unstable at somewhat lower limit of the stream speed than that of holes in a p-type silicon semiconductors. A description based on the energy exchange between unstable modes is given for different instability types. It is found that the instability in asymmetric doped semiconductors (electron–electron and hole–hole) is of symmetric type without energy exchange between the modes, while, for symmetric doped semiconductors (electron–hole), it is of asymmetric type with the energy exchange. Finally, it is found that the electron and hole spin exchange–correlation effects enhance the two-stream instability.

Optical absorption spectrum and electronic structure of multiferroic hexagonal YMnO3 compound

Optical absorption (OA) spectrum and electronic structure of the hexagonal YMnO3 compound have been investigated by employment of the first-principles calculations based on density functional theory. The calculations were performed upon the ferroelectric structure of the YMnO3, by testing various approximations of the exchange-correlation effects between the Mn d-electrons and considering two types of magnetic ordering of the Mn sub-lattice: (1) collinear anti-ferromagnetic order of the G-type and (2) non-collinear antiferromagnetic order that correspond to magnetic space group P63. The results demonstrate that satisfactory agreement between the theoretical and the experimental OA spectrum can be achieved only if both non-collinear anti-ferromagnetic order of the Mn spins and strong correlations between the Mn d-electrons are taken into account. The latter is found to be best described by effective Hubbard parameter Ueff = 2.55 eV. The principal features of the OA spectrum are interpreted in terms of calculated electronic structure. It is found that the most important, threshold 1.6 eV OA peak is generated by electron transitions from strongly hybridized occupied Mn d- and its neighboring in-plane O p-states to unoccupied Mn d-states. It is also concluded that the electronic gap (calculated as ∼1.1 eV) should be smaller than the optical one (∼1.6 eV).

Effect of exchange-correlation and GW approximations on electrical property of cubic, tetragonal and orthorhombic CH3NH3PbI3

ABSTRACTA first-principle calculations based on van der Waals-corrected Density Functional Theory (vdW-DFT) is performed to investigate atomic structure of methyl ammonium lead iodine (CH3NH3PbI3) which is a key material for high efficiency solid-state solar cell. A temperature dependent symmetry was previously reported which also included in this study. DFT calculation of electronic and optical properties are systematically studied with semi-local, non-local exchange-correlation and post-DFT approximation including PBE, HSE06 hybrid functional and GW. Relativistic effect in lead ion was taken into account by incorporating spin-orbit coupling (SOC) effect to obtain more accurate band gap of this material. With GW-SOC functional, our results of band gap calculations showed good agreement between DFT calculations and experimental studies which confirmed that this computational scheme is suitable for high accuracy material design, e.g. for solar cell applications.

Understanding electronic and optical properties of strontium titanate at both PBE and HSE06 levels

The structural parameters, electronic structure and optical properties of strontium titanate have been investigated by the first-principles. Exchange–correlation effects are treated by the generalized gradient approximation(GGA) employing Perdew–Burke–Ernzerhof (PBE) functional and hybrid density functional Heyd-Scuseria-Ernzerhof(HSE06). The direct-band gaps are equal to 2.06 and 3.73 eV under PBE and HSE06 level, respectively. The indirect-band gaps are equal to 1.67 and 3.33 eV under PBE and HSE06 level, respectively. The optical properties including complex dielectric function, absorption coefficient, refractivity and reflectivity index have been calculated. Meanwhile, the origin of the spectral peaks on the basis of the electronic band structures has been interpreted.

Principle Investigation of Structural, Electronics and Chemical Properties of Sn Doped PbX (X=S, Se, Te)

The Structural, electronic and chemical bonding of the Sn doped PbX (X=S, Se, Te) are calculated by full potential linearized augmented plane wave method (FP-LAPW) within density functional theory (DFT). Generalized gradient approximation of Wu and Cohen (WC-GGA) is used for calculating the exchange correlation effects. The structural parameters changes with respect to doping. The calculations show that the band gap decreases with the increase of Sn concentration. It has observed that the s, p and d states of PbX (X=S, Se, Te) control their electronic properties. The alloys Pb1–xSnxS, Pb1–xSnxSe and Pb1–xSnxTe show covalent bonding nature which enhances by increasing the Sn concentration.

RELATIVISTIC THEORY OF EXCITATION AND IONIZATION OF HEAVY ALKALI RYDBERG ATOMS IN A BLACK-BODY RADIATION FIELD: NEW DATA

The combined relativistic energy approach and relativistic many-body perturbation theory with the zeroth Dirac-Fock potential approximation are used for computing the thermal Blackbody radiation ionization characteristics of the alkali Rydberg atoms, in particular, the rubidium and caesium in Rydberg states with principal quantum number n = 20-100. Preliminary application of theory to computing ionization rate for the Rydberg sodium atom in the have demonstrated physically reasonable agreement between the theoretical and experimental data. The accuracy of the theoretical data is provided by a correctness of the corresponding relativistic wave functions and accounting for the exchange-correlation effects.

SNS potential with exchange field in quantum dusty plasmas

The shielding potential of a static test charge is studied in quantum dusty plasmas. The plasma system consisting upon electrons, ions and negatively static charged dust species, is embedded in an ambient magnetic field. The modified equation of dispersion is derived using quantum hydrodynamic model (QHD) for magnetized plasmas. The quantum effects are inculcated through Fermi degenerate pressure, tunneling effect and exchange-correlation effects. The study of shielding is important to know the existence of the silence zones in space and astrophysical objects as well as crystal formation. The graphical description of the normalized potential depict the significance of the exchange and correlation effects arising through spin and other variables on the shielding potential.

First principles study of the structural and optoelectronic properties of the A2InSbO6 (A = Ca, Sr, Ba) compounds

Different physical properties of the double perovskite A2InSbO6 (A=Ca, Sr, Ba) compounds are investigated using the full potential linear augmented plane wave method within the density functional theory (DFT) where different generalized gradient approximations are used for approximating the exchange-correlation effects. Importantly, the DFT calculated structural indexes are in close agreement with the experimental data. The compounds show a semiconductor nature with a direct bandgap where the upper valence band is mainly consist of the O-p state, and the lower part of the conduction band is composed of the Sb-s state. Furthermore, the electron charge density plots show that the chemical bonding is partially covalent and ionic. From the DFT calculated optical properties, it is clear that the A2InSbO6 (A=Ca, Sr, Ba) compounds strongly absorb and reflect the incident radiation in the visible and ultraviolet range and can therefore be effectively used in optoelectronic devices .

Two-dimensional modeling and analysis of a nanometer transistor as a THz emitter

In this paper, we report on the influences of quantum effects, electron exchange-correlation, Fermi velocity, gate to channel distance and viscosity on the plasma frequency and instability of the plasma waves in a nanometer transistor. By extending the analysis to two-dimensional case, allowing oblique wave propagation, including viscosity and departing from gradual channel approximation, we obtain a general analytical expression for dispersion relation, plasma frequency, and “increment.” We found that, while the plasma frequency decreases with the electron exchange-correlation effect, it increases with quantum effects and Fermi velocity. It is shown that the spectrums of plasma waves are discrete both in longitudinal and lateral (transverse) direction. We also express the total radiated power in terms of transistor parameters especially the lateral dimension. Viscosity which is inherently presented in the structure and cannot be neglected, dramatically decrease the emitted power and set a lower limit on the length of transistor. We show that a nanometer transistor with a long width (a long lateral dimension) has advantages for the realization of practical terahertz emitters.

Nonlinear ion acoustic dissipative shock structure with exchange-correlation effects in quantum semiconductor plasmas

Ion acoustic shocks in the electron-hole-ion semiconductor plasmas have been studied. The quantum recoil effects, exchange-correlation effects and degenerate pressure of electrons and holes are included. The ion species are considered classical and their dissipation is taken into account via the dynamic viscosity. The Korteweg de Vries Burgers equation is derived by using reductive perturbation approach. The excitation of shock waves in different semiconductor plasmas is pointed out. For numerical analyses, the plasma parameters of different semiconductors are considered. The impact of variation of the plasma parameters on the strength of the shock wave in the semiconductor plasmas is discussed.

First-Principles Study of Structural, Optical, and Thermodynamic Properties of ZnIn2X4 (X = Se, Te) Compounds with DC or DF Structure

Structural and optoelectronic properties of ZnIn2Se4 and ZnIn2Te4 compounds in defect chalcopyrite (DC) and defect famatinite (DF) structures have been calculated by the full-potential linearized augmented plane-wave (FP-LAPW) method within density functional theory (DFT) as implemented in the WIEN2K package. For the exchange correlation effects, we adopted the Perdew–Burke–Ernzerhof (PBE) generalized gradient approximation (GGA) for structural calculations and the Tran–Blaha-modified Becke–Johnson (TB-mBJ) functional for electronic properties. The lattice parameters (a, c) and internal parameters (x, y, z) are in good agreement with available results. The band structures prove that these kinds of material have a direct bandgap (Γ–Γ) in both structures. Optical properties such as the dielectric function e(ω) and refractive index n(ω) were calculated in the energy range from 0 eV to 14 eV. Thermodynamic properties were also analyzed using the quasiharmonic Debye model.

Elliptically polarized electromagnetic waves in a magnetized quantum electron-positron plasma with effects of exchange-correlation

The dispersion properties of elliptically polarized electromagnetic waves in a magnetized electron-positron-pair (EP-pair) plasma are studied with the effects of particle dispersion associated with the Bohm potential, the Fermi degenerate pressure, and the exchange-correlation force. Two possible modes of the extraordinary or X wave, modified by these quantum effects, are identified and their propagation characteristics are investigated numerically. It is shown that the upper-hybrid frequency and the cutoff and resonance frequencies are no longer constants but are dispersive due to these quantum effects. It is found that the particle dispersion and the exchange-correlation force can have different dominating roles on each other depending on whether the X waves are of short or long wavelengths (in comparison with the Fermi Debye length). The present investigation should be useful for understanding the collective behaviors of EP plasma oscillations and the propagation of extraordinary waves in magnetized dense EP-pair plasmas.