Exchange-correlation Effects Research Articles

First principles study on structural, electronic and optical properties of Ga1−xBxP ternary alloys (x = 0, 0.25, 0.5, 0.75 and 1)

The structural, electronic and optical properties of GaP, BP binary compounds and their ternary alloys Ga1−xBxP (x=0.25, 0.5 and 0.75) have been studied by full-potential linearized augmented plane wave (FP-LAPW) method within the framework of density functional theory (DFT) as implemented in WIEN2k package. Local density approximation (LDA) and generalized gradient approximation (GGA) as proposed by Perdew–Burke–Ernzerhof (PBE), Wu–Cohen (WC) and PBE for solid (PBESol) were used for treatment of exchange-correlation effect in calculations. Additionally, the Tran–Blaha modified Becke–Johnson (mBJ) potential was also employed for electronic and optical calculations due to that it gives very accurate band gap of solids. As B concentration increases, the lattice constant reduces and the energy band gap firstly decreases for small composition x and then it shows increasing trend until pure BP. Our results show that the indirect–direct band gap transition can be reached from x=0.33. The linear optical properties, such as reflectivity, absorption coefficient, refractive index and optical conductivity of binary compounds and ternary alloys were derived from their calculated complex dielectric function in wide energy range up to 30 eV, and the alloying effect on these properties was also analyzed in detail.

Density functional studies of magneto-optic properties of Sr2GdReO6

The electronic structure and magneto-optic properties of the Sr2GdReO6 double perovskite were investigated using the full-potential linearized augmented plane wave (FPLAPW) method. Exchange correlation effects are treated using the generalized gradient approximations GGA, GGA + U and GGA + U + SOC approachs. At ambient conditions, these calculation predict a half-metallic character for Sr2GdReO6 material. The electronic band structures and density of states demonstrate that Sr2GdReO6 is HM with a magnetic moment of 9 µB/fu and HM flip gap of 1.82 eV. The half metallicity is attributed by the double-exchange interaction mechanism via the Gd(4f)–O(2p)–Re(t2g) π-bounding. These new double perovskite may become ideal candidate material for future spintronic applications. The analysis charge densities show that bonding character as a mixture of covalent and ionic nature. The optical properties are analyzed and the origin of some peaks in the spectra is described. Besides, the dielectric function ε(ω), refractive index n (ω) and extinction coefficient K (ω) for radiation up to 14 eV have also been reported.

Electron pairing: from metastable electron pair to bipolaron

Starting from the shell structure in atoms and the significant correlation within electron pairs, we distinguish the exchange-correlation effects between two electrons of opposite spins occupying the same orbital from the average correlation among many electrons in a crystal. In the periodic potential of the crystal with lattice constant larger than the effective Bohr radius of the valence electrons, these correlated electron pairs can form a metastable energy band above the corresponding single-electron band separated by an energy gap. In order to determine if these metastable electron pairs can be stabilized, we calculate the many-electron exchange-correlation renormalization and the polaron correction to the two-band system with single electrons and electron pairs. We find that the electron-phonon interaction is essential to counterbalance the Coulomb repulsion and to stabilize the electron pairs. The interplay of the electron-electron and electron-phonon interactions, manifested in the exchange-correlation energies, polaron effects, and screening, is responsible for the formation of electron pairs (bipolarons) that are located on the Fermi surface of the single-electron band.

Inorganic antiferromagnetic semiconductors A2MnWO6 (A = Ba2+, Pb2+) as promising materials for optospintronics applications

Based on density functional theory (DFT), the structural, electronic, magnetic and optical properties of Mn2+ (3d5)–W6+ (5d0) based double perovskite oxides A2MnWO6 (A = Ba2+, Pb2+) were studied using the generalized gradient approximation (GGA) and by adding the exchange correlation effect (GGA + U). The crystal structures of two compounds were found to have cubic symmetry, space group Fm-3m, no 225, at room temperature. Both GGA and GGA + U calculations predicted a semiconducting-antiferromagnetic (AF-type1) behavior in A2MnWO6 compounds with energy gaps (Eg) in spin-up and spin-down except for (A = Pb2+), it exhibits ferrimagnetic (FI) metallic behavior within GGA. The value of Eg decreases with structure changing; Eg in (A = Pb2+; Eg = 0.54–2.11 eV) is smaller than in (A = Ba2+; Eg = 0.85–2.80 eV). Optical calculations show that the AF A2MnWO6 compounds have a good ability to absorb the incident photons due to their narrow Eg. The real and imaginary parts of the dielectric function show that these compounds also possess a great ability to retain the photon energy that it absorbed. It is believed that these materials could be synthesized experimentally, and they would prove useful for optospintronics applications.

Nonlinear quantum ion acoustic shock wave dynamics with exchange-correlation effects

The dynamics of linear and nonlinear electrostatic shock excitations is studied in homogeneous, unmagnetized, unbounded and dissipative quantum plasma consisting of electrons and ions. The dissipation in the system is taken into account by incorporating the ion kinematic viscosity. The system is modelled using the quantum hydrodynamic equations in which the electrons are significantly affected by the quantum forces, viz., the quantum statistical pressure, the quantum Bohm potential and electron exchange-correlations due to electron spin. In the weakly nonlinear limit, using reductive perturbation method deformed Korteweg-de Vries Burgers’s (KdVB) equation, which elegantly combines the effects of nonlinearity, dispersion and dissipation is derived. It is found that the present model predicts the existence of both nonlinear oscillatory and monotonic shock structures. The temporal evolution, stability and phase-space dynamics of nonlinear ion acoustic shocks are investigated numerically to elucidate the effects of quantum diffraction, electron exchange correlation and ion kinematic viscosity.

Magnetosonic solitons in semiconductor plasmas in the presence of quantum tunneling and exchange correlation effects

Low frequency magnetosonic wave excitations are investigated in semiconductor hole-electron plasmas. The quantum mechanical effects such as Fermi pressure, quantum tunneling, and exchange-correlation of holes and electrons in the presence of the magnetic field are considered. The two fluid quantum magnetohydrodynamic model is used to study magnetosonic wave dynamics, while electric and magnetic fields are coupled via Maxwell equations. The dispersion relation of the magnetosonic wave in electron-hole semiconductor plasma propagating in the perpendicular direction of the magnetic field is obtained, and its dispersion effects are discussed. The Korteweg-de Vries equation (KdV) for magnetosonic solitons is derived by employing the reductive perturbation method. For numerical analysis, the plasma parameters are taken from the semiconductors such as GaAs, GaSb, GaN, and InP already existing in the literature. It is found that the phase velocity of the magnetosonic wave is increased with the inclusion of exchan...

Coherent exchange correlation in quantum systems

A number of exchange correlation effects in quantum systems are discussed from a unified point of view. These effects can be observed at both microscopic and macroscopic distances. The analysis of macroscopic correlation effects requires an understanding of the physical mechanism of exchange correlation. This, in turn, requires an analysis of the phase relationships between the wave functions of quantum states that determine the observed physical quantities. The neglect of these phase relationships in the interpretation of correlation experiments gave rise to assumptions such as instantaneous long-range interaction and other similar ideas that contradict the traditional physical principles of short-range interaction, causality, and locality. On the contrary, understanding the physical nature of the correlation mechanism makes it possible to explain these experiments without these kinds of ideas.

Когерентная обменная корреляция в квантовых системах

AbstractA number of exchange correlation effects in quantum systems are discussed from a unified point of view. These effects can be observed at both microscopic and macroscopic distances. The analysis of macroscopic correlation effects requires an understanding of the physical mechanism of exchange correlation. This, in turn, requires an analysis of the phase relationships between the wave functions of quantum states that determine the observed physical quantities. The neglect of these phase relationships in the interpretation of correlation experiments gave rise to assumptions such as instantaneous long-range interaction and other similar ideas that contradict the traditional physical principles of short-range interaction, causality, and locality. On the contrary, understanding the physical nature of the correlation mechanism makes it possible to explain these experiments without these kinds of ideas.

Magnetosonic solitons in dense astrophysical plasmas with exchange-correlation potential effects

Low frequency magnetosonic waves propagation in dense astrophysical electron-ion plasmas are studied in the presence of exchange-correlation potential effects of degenerate electrons. Two fluid quantum magnetohydrodynamic model (QMHD) is employed and quantum effects like temperature degeneracy, Bohm potential and exchange-correlation effects are taken into account the momentum equation of degenerate electrons fluid. The Korteweg-de Vries (KdV) equation for nonlinear magnetosonic waves is derived by employing reductive perturbation method. It is found that the phase velocity of the magnetosonic wave is enhanced in dense plasmas in the presence of exchange-correlation force in the model. The exchange-correlation force due to degenerate electrons also effects on the amplitude and width of the magnetosonic soliton in quantum plasmas, where both positive and negative polarity structures of magnetosonic waves are formed depending on the numerical value of quantum diffraction parameter (i.e., less than or greater than 6). These single pulse structures move with speed greater (for positive polarity structure) or less (for negative polarity structure) than the magnetosonic wave phase speed, respectively. The numerical plots are also illustrated by choosing the values of dense plasma parameters exist in compact stars already published in the literature.

Oscillatory wake potential with exchange-correlation in plasmas

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

Exact functionals for correlated electron–photon systems

For certain correlated electron–photon systems we construct the exact density-to-potential maps, which are the basic ingredients of a density-functional reformulation of coupled matter-photon problems. We do so for numerically exactly solvable models consisting of up to four fermionic sites coupled to a single photon mode. We show that the recently introduced concept of the intra-system steepening (Dimitrov et al 2016 New J. Phys. 18 083004) can be generalized to coupled fermion-boson systems and that the intra-system steepening indicates strong exchange-correlation effects due to the coupling between electrons and photons. The reliability of the mean-field approximation to the electron–photon interaction is investigated and its failure in the strong coupling regime analyzed. We highlight how the intra-system steepening of the exact density-to-potential maps becomes apparent also in observables such as the photon number or the polarizability of the electronic subsystem. We finally show that a change in functional variables can make these observables behave more smoothly and exemplify that the density-to-potential maps can give us physical insights into the behavior of coupled electron–photon systems by identifying a very large polarizability due to ultra-strong electron–photon coupling.

Study of Structural and Electronic Properties of Fluoride Perovskite KCaF3 using FP-LAPW Method

To study the structural and electronic properties of cubical perovskite KCaF3, the first principles calculation within the full potential linearized augmented plane wave (FP-LAPW) method is applied. The exchange correlation effects are included through the GGA exchange potential. The calculated structural properties such as equilibrium lattice constant, the bulk modulus and its pressure derivative are in agreement with the published results of other authors. From our study we have found that the band gap of KCaF3 is 6.4 eV which is the indication of insulating behavior.The Himalayan Physics Vol. 6 & 7, April 2017 (100-103)

Structural, elastic, optoelectronic and magnetic properties of $$\mathbf{CdHo }_\mathbf{2}{} \mathbf{S}_\mathbf{4}$$ CdHo 2 S 4 spinel: a first-principle study

We report the results of the full-potential linearized augmented plane wave (FP-LAPW) calculations on the structural, elastic, optoelectronic and magnetic properties of \(\hbox {CdHo}_{2}\hbox {S}_{4}\) spinel. Both the generalized gradient approximation (GGA) and Trans-Blaha modified Becke-Johnson potential (TB-mBJ) are used to model the exchange-correlation effects. The computed lattice parameter, internal coordinate and bulk modulus are in good agreement with the existing experimental data. According to the calculated elastic moduli, \(\hbox {CdHo}_{2}\hbox {S}_{4}\) is mechanically stable with a ductile nature and a noticeable elastic anisotropy. The ferromagnetic phase of \(\hbox {CdHo}_{2}\hbox {S}_{4}\) is energetically favourable compared to non-magnetic one, with a high magnetic moment of about 8.15 \(\upmu _{\mathrm{B}}\). The calculated band structure demonstrates that the title compound is a direct band gap semiconductor. The TB-mBJ yields a band gap of \(\sim \)1.86 and \(\sim \)2.17 eV for the minority and majority spins, respectively. The calculated optical spectra reveal a strong response in the energy range between the visible light and the extreme UV regions.

Electron capture and excitation processes in collisions of H+ with H in finite-temperature quantum plasmas

Electron capture and excitation processes in H+-H collisions imbedded in warm dense quantum plasmas are studied by the two-center atomic orbital close-coupling (TC-AOCC) method in the energy range 1–200 keV. The screened potential which is used to describe the interaction between charged particles incorporates the quantum electron degeneracy, exchange-correlation and finite-temperature gradient correlation effects. The total and state-selective electron capture and excitation cross-sections for this collision system with this potential are calculated in a wide range of plasma densities and temperatures. It is demonstrated that the screening of Coulomb interactions affects the collision dynamics and the magnitude and energy behavior of the excitation and electron capture cross-sections. The results at very low and high temperatures are also compared with those obtained for zero-temperature quantum plasmas and Debye plasmas, respectively.

Influence of antisite defects and stacking faults on the magnetocrystalline anisotropy of FePt

We present density functional theory (DFT) calculations of the magnetic anisotropy energy (MAE) of FePt, which is of great interest for magnetic recording applications. Our data, and the majority of previously calculated results for perfectly ordered crystals, predict a MAE of $\ensuremath{\sim}3.0\phantom{\rule{0.16em}{0ex}}\mathrm{meV}$ per formula unit, which is significantly larger than experimentally measured values. Analyzing the effects of disorder by introducing stacking faults (SFs) and antisite defects (ASDs) in varying concentrations we are able to reconcile calculations with experimental data and show that even a low concentration of ASDs are able to reduce the MAE of FePt considerably. Investigating the effect of exact exchange and electron correlation within the adiabatic-connection dissipation fluctuation theorem in the random phase approximation (ACDFT-RPA) reveals a significantly smaller influence on the MAE. Thus the effect of disorder, and more specifically ASDs, is the crucial factor in explaining the deviation of common DFT calculations of FePt to experimental measurements.

Shear Alfvén Wave with Quantum Exchange-Correlation Effects in Plasmas

AbstractThe dust shear Alfvén wave is studied in three species dusty quantum plasmas. The quantum effects are incorporated through the Fermi degenerate pressure, tunneling potential, and in particular the exchange-correlation potential. The significance of exchange-correlation potential is pointed out by a graphical description of the dispersion relation, which shows that the exchange potential magnifies the phase speed. The low-frequency shear Alfvén wave is studied while considering many variables. The shear Alfvén wave gains higher phase speed at the range of small angles for the upper end of the wave vector spectrum. The increasing dust charge and the external magnetic field reflect the increasing tendency of phase speed. This study may explain many natural mechanisms associated with long wavelength radiations given in the summary.

Spin polarization and exchange-correlation effects in transport properties of two-dimensional electron systems in silicon

We show that the parallel magnetic field-induced increase in the critical electron density for the Anderson transition in a strongly interacting two-dimensional electron system is caused by the effects of exchange and correlations. If the transition occurs when electron spins are only partially polarized, additional increase in the magnetic field is necessary to achieve the full spin polarization in the insulating state due to the exchange effects.

Propagation of electrostatic surface waves in a thin degenerate plasma film with electron exchange–correlation effects

Propagation of an electrostatic surface wave in a thin degenerate Fermi plasma film in the presence of constant external magnetic field is studied here. Dispersion relations for the symmetric and anti-symmetric modes have been derived and studied quantitatively with the exchange–correlation effects. It has been studied that with the increase in the strength of magnetic field, phase velocity of the waves decreases. Also electron exchange–correlation effects significantly modify the behavior of the surface waves such as frequency of surface wave is found to be downshifted by these effects. Moreover it has been studied that the group velocity of the anti-symmetric mode is greater than the symmetric mode for the whole wave numbers; however, these modes merge into a single mode with the increase of the wave number.

Magnetohydrodynamic shocks in a dissipative quantum plasma with exchange-correlation effects

We investigate the nonlinear propagation of multidimensional magnetosonic shock waves (MSWs) in a dissipative quantum magnetoplasma. A macroscopic quantum magnetohydrodynamic (QMHD) model is used to include the quantum force associated with the Bohm potential, the pressure-like spin force, the exchange and correlation force of electrons, as well as the dissipative force due to the kinematic viscosity of ions and the magnetic diffusivity. The effects of these forces on the properties of arbitrary amplitude MSWs are examined numerically. It is found that the contribution from the exchange-correlation force appears to be dominant over those from the pressure gradient and the other similar quantum forces, and it results into a transition from monotonic to oscillatory shocks in presence of either the ion kinematic viscosity or the magnetic diffusivity.

Theoretical Investigation of Half-Metallic Ferromagnetism in Sodium-Based Fluoro-perovskite NaXF3 (X = V, Co)

We report on the structural, mechanical, magneto-electronic and thermodynamic properties of cubic sodium based fluoro-perovskite NaXF3 (X $=$ V, Co) through density functional theory (DFT). The generalized gradient approximation of Perdew, Burke, and Ernzerh (GGA-PBE) and the Tran-Blaha modified Becke-Johnson potential (TB-mBJ) are used for modeling exchange-correlation effects. The calculated lattice parameters agree well with the experimental measurements. Elastic properties show that these compounds are mechanically stable. The results of spin-polarized band structure show a half metallic behavior of NaVF $_{3}$ and NaCoF $_{3}$ . The magnetic study shows that the nature of these compounds is ferromagnetic and the integer value of the total magnetic moment confirms the half metallicity for both materials. The thermodynamic characters are also investigated through quasi-harmonic approximation.