Local Density Approximation Method Research Articles

Structural and electronic properties of GaN: Ab initio study within LDA and LDA+U methods

Structural and electronic properties of the GaN were simulated based on Density Functional Theory implementing Local Density Approximation methods. Hubbard U correction gives us an opportunity to find the correct energy gap for GaN in agreement with known experimental results. Choosing more accurate investigation methods leads to calculating accurate electronic band structure and in the future predicting some physical properties of related material. The bottom of the conduction band and the top of the valence band are formed mainly by p-orbitals of host Ga and N atoms. The present study shows the direct band gap character of GaN with a wurtzite structure

Doping effect on electronic and magnetic properties of Ag-doped single-walled (6,0) GaN nanotubes: First-principles study

The electronic and magnetic properties of GaN wurtzite, un-doped, and Ag-doped single-walled (6,0) chiral GaN nanotubes (SWGaNNT) were investigated using Density Functional Theory and Local Spin Density Approximation methods within Hubbard U semiempirical correction (DFT-LSDA + U). In a consecutive DFT-LSDA + U electronic structure simulation, the effects of defects on the nature and origin of ferromagnetism (FM) are studied for an Ag-doped SWGaNNT system. For the Ag-doped GaN nanotube configurations, the energy gap decreases with increasing concentration of impurity. The total energy calculations for doped systems show the stability of the ferromagnetic phase. In the case of the Ag-doping GaN NT system, the wide energy gap is decreasing and the total magnetic moment of this system is ∼2.0 μB. From first-principles calculations, an Ag-doped GaN nanosystem can be made into a magnetic ferromagnetic material, and it is an important candidate for spintronics device applications.

Investigating the molecular crystals of L-Alanine, DL-Alanine, β-Alanine, and Alanine hydrogen chloride: Experimental and DFT analysis of structural and optoelectronic properties

In this study, DFT calculations were employed to assess and contrast the structural, electronic, and optical characteristics of five alanine crystal forms. The Perdew-Burke-Ernzerhof (PBE) generalized gradient approximation (GGA) with the Tkatchenko-Scheffler dispersion correction scheme was employed and compared to the PBE functional and the local density approximation (LDA) without dispersion correction. Experimentally, alanine crystal powders were characterized through X-ray diffraction (XRD) and UV–VIS absorption spectrum. Additionally, UV–VIS absorption measurements were performed on L-α-alanine, DL-alanine, β-alanine, and alanine hydrochloride solvated in water. Significant deviations (>10%) were observed between the experimental crystal lattice parameters and those predicted by the GGA and LDA methods. Incorporating dispersion correction into the GGA functional improved prediction accuracy. Additionally, time-dependent DFT (TD-DFT) computations using the polarizable continuum model were conducted to analyze the UV/VIS absorption spectrum of α-, β-, DL-, and hydrogen chloride alanine in water and interpret experimental data. Different alanine crystals exhibited substantial anisotropic electronic and optical properties, including effective masses, absorption, and dielectric function. The L-α-HCl structure displayed the smallest band gap (4.67 eV), suggesting its suitability for applications such as L-α/L-α-HCl/L-α quantum well structures. The main effective masses of electrons and holes were also determined for each crystal for a better understanding of their electronic transport properties.

Numerical investigation on optical properties of SnO2 by co-doping with Al and N

We investigated the electronic structure and optical properties of the different N-doped position models in Al–N co-doped SnO2 by using the first principles based local density approximation method. By doping Al or N elements, the band gap is broadened linearly and dramatically. Compared with Al:SnO2, the appeared impurity levels induced by N element led to the more transitions from the other levels in the valence band to these impurity levels, leading to an enhanced conductivity and ε2(ω). The absorption edge of the system has red-shifted, the reflectivity is also enhanced.

First-principles calculations of local structure and electronic properties of Er<sup>3+</sup>-doped TiO<sub>2</sub>

Trivalent rare earth erbium ion (Er<sup>3+</sup>) doped titanium oxide (TiO<sub>2</sub>) can possess a very wide range of applications due to its excellent optoelectronic properties, thus standing out among many rare-earth-doped luminescent crystals. However, the issues regarding local structure and electronic properties have not been finalized. To address these problems, the CALYPSO (Crystal structure AnaLYsis by Particle Swarm Optimization) method combined with the first-principles calculations is employed, and many converged structures of Er<sup>3+</sup>-doped TiO<sub>2</sub> are successfully obtained. Further structural optimization is performed by using the VASP (Vienna <i>ab initio</i> simulation package) software package, and we report for the first time that the lowest energy structure of Er<sup>3+</sup>-doped TiO<sub>2</sub> has the <inline-formula><tex-math id="M2">\begin{document}$ P\overline 4 $\end{document}</tex-math><alternatives><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="24-20221847_M2.jpg"/><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="24-20221847_M2.png"/></alternatives></inline-formula><i>m</i>2 symmetry. It can be observed that the doped Er<sup>3+</sup> ions enter into the host crystal and occupy the positions of Ti<sup>4+</sup> ions, resulting in structural distortion, which eventually leads the local Er<sup>3+</sup> coordination site symmetry to reduce from <i>D</i><sub>2<i>d</i></sub> into <i>C</i><sub>2<i>v</i></sub>. We speculate that there are two reasons: 1) the difference in charge between Er<sup>3+</sup> ions and Ti<sup>4+</sup> ions leads to charge compensation; 2) the difference between their electron radii is obvious: the radius is 0.0881 for Er<sup>3+</sup> ion and 0.0881 for Ti<sup>4+</sup> ion. In addition, during the structural search, we also find many metastable structures that may exist at a special temperature or pressure, which play an important role in the studying of structural evolution. When the electronic band structure of the Er<sup>3+</sup>-doped TiO<sub>2</sub> system is calculated, we adopt the method of local density approximation (LDA) combined with the on-site Coulomb repulsion parameter <i>U</i> to accurately describe the strongly correlated system. For the specific value of <i>U</i>, we adopt 3.5 eV and 7.6 eV to describe the strong correlation of 3d electrons of Ti<sup>4+</sup> ions and 4f electrons of Er<sup>3+</sup> ions, respectively. According to the calculation of electronic properties, the band gap value of Er<sup>3+</sup> doped TiO<sub>2</sub> is about 2.27 eV, which is lower than that of the host crystal (<i>E</i><sub>g</sub> = 2.40 eV). The results show that the reduction in the band gap is mainly caused by the f state of Er<sup>3+</sup> ions. The doping of Er ion does reduce the band gap value, but it does not change the conductivity of the system, which have great application prospect in diode-pumped laser. These findings not only provide the data for further exploring the properties and applications of Er<sup>3+</sup>:TiO<sub>2</sub> crystals, but also present an approach to studying other rare-earth-doped crystalline materials.

Ξ hypernuclear states predicted by next-to-leading-order chiral baryon–baryon interactions

Abstract The $\Xi$ single-particle potential obtained in nuclear matter with next-to-leading-order baryon–baryon interactions in chiral effective field theory is applied to finite nuclei by an improved local-density approximation method. As a premise, phase shifts of $\Xi N$ elastic scattering and the results of Faddeev calculations for the $\Xi NN$ bound state problem are presented to show the properties of the $\Xi N$ interactions in the present parametrization. First, the $\Xi$ states in $^{14}$N are revisited because of recent experimental progress, including discussion on the $\Xi N$ spin–orbit interaction that is relevant to the location of the $p$-state. Then the $\Xi$ levels in $^{56}$Fe are calculated. In particular, the level shift which is expected to be measured experimentally in the near future is predicted. The smallness of the imaginary part of the $\Xi$ single-particle potential is explicitly demonstrated.

An ab-initio study of structure and mechanical properties of rocksalt ZrN and its bilayers

In this work, the mechanical properties of transition metal nitrides (TMN) applicable as hard coatings are theoretically studied. Zirconium nitride (ZrN) is chosen as our case study and effects of pressure on its mechanical properties are investigated. Density functional theory (DFT) with general gradient approximation (GGA) and local density approximation (LDA) methods is used to simulate the structural, electronic, phonon and elastic properties of the compound in its bulk (cubic NaCl structure) and thin film states. Density of states (DOS) and band structure calculations show that the compound behaves like a metal. Electronic charge density and phonon calculations reveal the high hardness of the compound. Finally, the lattice constant and volume variations versus the pressure according to Murnaghan equation of state confirm that ZrN is appropriate for hard coating applications.

First principle study of the structural and optoelectronic properties of direct bandgap double perovskite Cs2AgInCl6

Solar cells incorporating double perovskites as absorber layer are currently showing very remarkable progress as a potential environment friendly substitute for lead based hybrid halide perovskites. In the present work we tried to investigate the structural and opto-electronic properties of lead (Pb) free halide double perovskite Cs2AgInCl6. Density Functional Theory (DFT) is employed to systematically analyze the material. For the electron core exchange correlation interaction, the Perdew-Burke- Ernzerhof functional (PBE) within generalized gradient approximation (GGA) was used. The interactions were also analyzed using Local Density Approximation (LDA) method and Perdew Burke Ernzerhof parameterization of the generalized gradient approximation for solid (PBE-sol). The optimized lattice parameters are obtained as 10.565 Å for the material in face-centered cubic structure (Fm-3 m space group). The optimized density is found to be 3.93 gcm−3. The first-principles calculations have given the energy gaps values as 1.1 eV. This energy is slightly lower as an energy underestimation is expected in DFT based calculations. A remarkable factor to mention is the direct bandgap exhibited by the material at Γ(π/a, π/a, π/a), unlike other double perovskites. It is clear from optical studies that the compound is a good absorber in visible region. The optical properties of Cs2AgInCl6 are comparable to hybrid halide perovskites MAPbI3 and halide double perovskites (like Cs2AgBiCl6). Through proper composition grading this material can be envisaged as a possible absorber layer in solar cells.

A density-aware probabilistic interest forwarding method for content-centric vehicular networks

Vehicular networks, compared to other wireless networks, face particular challenges due to the rapidly changing topology and intermittent connections. By eliminating the need to establish and maintain an end-to-end connection, Content-Centric Network (CCN) model has recently become an appropriate solution to meet the challenging demands of vehicular network communications. In this kind of network, the basic method of forwarding interest packets is flooding. This approach will result in excessive redundancy, serious contention, and collision to which it is referred as the broadcast storm problem. In this article, a probabilistic strategy is proposed to alleviate the impact of the broadcast storm on interest forwarding in content-centric vehicular networks. In this density-aware approach, each vehicle dynamically computes the probabilities based on the number of existing neighbors. A local density approximation method is presented, which uses the information provided by the newly modified Pending Interest Table (PIT) entries. Moreover, some time-based techniques are employed to give priority to potential forwarders. The simulation results indicate that the proposed work outperforms the basic CCN. With on average 40% lower network load and 65% fewer interests compared to the basic CCN, it shows only an overall 6% decrease in reachability.

Piezoelectricity and pyroelectricity in hydroxyapatite

The results are based on the first principle modeling and calculations of hydroxyapatite (HAP) nanostructures, especially in ordered hexagonal phase. HAP structures were studied using Local Density Approximation (LDA) method with calculations of Density of States (DOS) and molecular modeling by HyperChem. Computed data show that monoclinic and hexagonal phases can coexist, especially in the ordered states. Calculated piezoelectric coefficient for hexagonal phase d33 ∼6.4 pm/V is consistent with previously obtained value for monoclinic HAP (15.7 pm/V) and experimental data. The estimated pyroelectric coefficient at the order of γ ∼ 10–100 μC/(m2K) is also in line with experimental data.

Insight into Physical and Thermodynamic Properties of X3Ir (X = Ti, V, Cr, Nb and Mo) Compounds Influenced by Refractory Elements: A First-Principles Calculation

The effects of refractory metals on physical and thermodynamic properties of X3Ir (X = Ti, V, Cr, Nb and Mo) compounds were investigated using local density approximation (LDA) and generalized gradient approximation (GGA) methods within the first-principles calculations based on density functional theory. The optimized lattice parameters were both in good compliance with the experimental parameters. The GGA method could achieve an improved structural optimization compared to the LDA method, and thus was utilized to predict the elastic, thermodynamic and electronic properties of X3Ir (X = Ti, V, Cr, Nb and Mo) compounds. The calculated mechanical properties (i.e., elastic constants, elastic moduli and elastic anisotropic behaviors) were rationalized and discussed in these intermetallics. For instance, the derived bulk moduli exhibited the sequence of Ti3Ir < Nb3Ir < V3Ir < Cr3Ir < Mo3Ir. This behavior was discussed in terms of the volume of unit cell and electron density. Furthermore, Debye temperatures were derived and were found to show good consistency with the experimental values, indicating the precision of our calculations. Finally, the electronic structures were analyzed to explain the ductile essences in the iridium compounds.

First-principles study of electronic structure and optical properties of nickel-doped multilayer graphene

Graphene is an ideal two-dimensional crystal with the advantages of high conductivity, unique physical and chemical properties, and high specific surface area. Especially, because of its super excellent electronic properties, graphene may substitute the traditional semiconductor silicon material and carbon nanotube, thus creating a new nanoscale electronic device. In addition, multilayer graphene with ultra-wide spectral absorption characteristics and unique photoelectric properties is an ideal material for photovoltaic devices. However, the zero band gap and semi-metality of graphene both limit its application in space detectors such as the microelectronic industries and satellites. Opening and regulating the graphene band gap by physical methods has become one of the key means to further expand its applications. Research work has shown that the doping of elements can significantly change the electronic structure of graphene, thereby regulating the optical properties of graphene. In order to provide an insight into electronic properties of graphene and tune its electronic band structure and optical properties effectively, electronic and optical properties of Ni-doped multi-layer graphene are studied and a number of interesting results are obtained. The calculation are carried out by the CASTEP tool in Materials Studio software based on the first-principles of ultrasoft pseudopotential of density functional theory. The models of three typical doping positions relative to carbon atoms are constructed. After structural optimization, it is obtained that " above the center of two carbon atoms” is the most stable doping structure. By using the method of local density approximation, the band structure, density of states, dielectric constant, reflectivity and refractive index of the models are calculated. The results show that an enhanced energy band gap can be achieved after nickel-doping, and reach up to 0.604 eV. Besides, peaked phenomenon of density of states at Femi level can be observed, which is accomplished by enhancing the plasma energy. Furthermore, the calculations show that the imaginary part of permittivity and refractive index increase after nickel-doping, suggesting that the optical absorbing performance is improved. All these results provide theoretical guidance for further exploring the optical properties of graphene.

Gapped Nearly Free-Standing Graphene on an SiC(0001) Substrate Induced by Manganese Atoms

The electron band structure of manganese-adsorbed graphene on an SiC(0001) substrate has been studied using angle-resolved photoemission spectroscopy. Upon introducing manganese atoms, the conduction band of graphene completely disappears and the valence band maximum is observed at 0.4 eV below Fermi energy. At the same time, the slope of the valence band decreases, approaching the electron band structure calculated using the local density approximation method. While the former provides experimental evidence of the formation of nearly free-standing graphene on an SiC substrate, concomitant with a metal-to-insulator transition, the latter suggests that its electronic correlations can be modified by foreign atoms. These results pave the way for promising device applications using graphene that is semiconducting and charge neutral.

Entanglement and magnetism in high-spin graphene nanodisks

We investigate the ground-state properties of triangular graphene nanoflakes with zigzag edge configurations. The description of zero-dimensional nanostructures requires accurate many-body techniques since the widely used density-functional theory with local density approximation or Hartree-Fock methods cannot handle the strong quantum fluctuations. Applying the unbiased density-matrix renormalization group algorithm we calculate the magnetization and entanglement patterns with high accuracy for different interaction strengths and compare them to the mean-field results. With the help of quantum information analysis and subsystem density matrices we reveal that the edges are strongly entangled with each other. We also address the effect of electron and hole doping and demonstrate that the magnetic properties of triangular nanoflakes can be controlled by electric field, which reveals features of flat-band ferromagnetism. This may open up new avenues in graphene based spintronics.

First-Principles Investigation of Structural, Electronic and Elastic Properties of HfX (X = Os, Ir and Pt) Compounds

The structural, electronic and elastic properties of B2 structure Hafnium compounds were investigated by means of first-principles calculations based on the density functional theory within generalized gradient approximation (GGA) and local density approximation (LDA) methods. Both GGA and LDA methods can make acceptable optimized lattice parameters in comparison with experimental parameters. Therefore, both GGA and LDA methods are used to predict the electronic and elastic properties of B2 HfX (X = Os, Ir and Pt) compounds. Initially, the calculated formation enthalpies have confirmed the order of thermodynamic stability as HfPt > HfIr > HfOs. Secondly, the electronic structures are analyzed to explain the bonding characters and stabilities in these compounds. Furthermore, the calculated elastic properties and elastic anisotropic behaviors are ordered and analyzed in these compounds. The calculated bulk moduli are in the reduced order of HfOs > HfIr > HfPt, which has exhibited the linear relationship with electron densities. Finally, the anisotropy of acoustic velocities, Debye temperatures and thermal conductivities are obtained and discussed.

Study of the structural and electronic properties of FeO at the LDA and GGA level

Due to its strongly correlated insulating nature, FeO is used in various catalytic applications. Structural and electronic properties of FeO are investigated by using density functional theory (DFT) with the BAND code in Amsterdam Density Functional (ADF) software at ambient conditions. The generalized gradient approximation (GGA) and local density approximation (LDA) without and with the Hubbard potential (U eV) are used to investigate the structural and electronic properties of FeO. Geometric optimization of FeO is carried out by using the LDA and GGA method. CAPZ (Ceperley–Alder–Perdew–Zunger) and PBE (Perdew–Burke–Ernzerhof) are used as exchange correlation potentials for the LDA and GGA, respectively. The Hubbard potential is optimized and U= 0.6eV is used to overcome the under-estimation of the band gap of FeO in both approximations. The band structure shows the metallic nature of FeO with the GGA and LDA while band gap of 2.34 and 2.08eV is observed by using GGA+U and LDA+U, respectively. Our calculated band gap value with the GGA+U is in good agreement with the reported experimental value, i.e. 2.4eV. The total and partial density of states are also plotted to show the band structure. To the best of our knowledge, no study has been reported yet at the U= 0.6eV value of the Hubbard potential.

Assessment on the structural, elastic and electronic properties of Nb3Ir and Nb3Pt: A first-principles study

The pressure dependent behaviors on the structural, elastic and electronic properties of the A15 structure Nb3Ir and Nb3Pt were studied using first-principles calculations based on the density functional theory within generalized gradient approximation and local density approximation methods. Initially, the optimized lattice constants of Nb3Ir and Nb3Pt are consistent with the available experimental and theoretical results. Furthermore, Nb3Ir is found to be more thermodynamically stable than Nb3Pt due to its lower formation enthalpy and higher melting temperature. In addition, the elastic constants of Nb3Ir and Nb3Pt show an increasing tendency, and keep mechanically stable structures under pressures to 40 GPa. Besides, the increasing Cauchy pressures and B/G values have indicated that higher pressures can improve their ductility in both Nb3Ir and Nb3Pt. Finally, the pressure-dependent behaviors on the density of states, Mulliken charges and bond lengths are discussed for both compounds.

Study of the elastic properties and wave velocities of rocksalt Mg1−xFexO: ab initio calculations

The composition dependence of the elastic constants, bulk modulus, shear modulus, Young's modulus, and compression and shear wave velocities along the different directions for the rocksalt Mg1−xFexO material system has been investigated. The calculations are based on the density functional theory using the full-potential augmented-plane-wave method. The exchange and correlation energies have been treated using three approaches, namely, the local density approximation (LDA), the generalized gradient approximation (GGA) of Perdew et al. and the GGA of Wu and Cohen (WC-GGA). It is found that all the approaches used show the same qualitative behavior for the features of interest versus alloy composition x. From the quantitative point of view, the GGA seemed to give better results than the LDA and WC-GGA approaches when compared to experiment. The trend appears to be consistent with those reported in previous calculations of the elastic constants for other semiconductor materials using the LDA and GGA methods.

Study of Structural, Electronic and Optical Properties of Lanthanum Doped Perovskite PZT Using Density Functional Theory

Ferroelectric materials of lanthanum (La) doped PbZrTiO3 (PLZT) were investigated via first principles study. The structural, electronic and optical properties of PLZT in tetragonal structure (P4mm space group) were performed in the framework of density functional theory (DFT) with generalized gradient approximation (GGA) and local density approximation (LDA) methods. The calculated results of structural properties of PLZT were seen to be approximately close to the experimental data. The results of the electronic part were covered with the calculation of energy band gap and density of states (DOS). The highest valence band (VB) which lies at the Fermi level (EF) was dominated by the O 2p at F point. The conduction band (CB) of PLZT occurred at G point, which was primarily dominated by Ti 3d mixed at Pb and La p-state. Whereas the optical part was covered with the refractive index and absorption. The refractive index, n and the extinction coefficient, k were calculated with respect to photon energy. Those results obtained could be such a good prediction in studying parameters and properties of new materials.

Ferromagnetism of Na0.5Bi0.5TiO3 (1 0 0) surface with O2 adsorption

Na0.5Bi0.5TiO3 (NBT) nanocrystalline powders prepared by sol-gel method with annealing at 900°C in air 1h present room-temperature ferromagnetism (FM). The subsequent annealing in vacuum at 900°C for 30min weakens the room-temperature FM, while subsequent treatments in oxygen atmosphere at room-temperature enhances the room-temperature FM, indicating that the room-temperature FM may be induced by the adsorbed oxygen on the NBT nanoparticle surface. The adsorption of O2 molecule on the NBT (1 0 0) surface is studied by using density functional theory within local density approximation plus on-site effect method. The physisorption of configuration R5 is the most stable, whereas the chemisorption of O2 is unfavorable at all adsorption sites. The physisorbed O2 molecule on the NBT (1 0 0) surface with a magnetic moment (MM) closes to that for an isolated O2 molecule. The magnetism of configuration R5 is mainly from the O p orbitals. The stable ferromagnetic coupling mechanism is the direct exchange interaction between the nearest-neighbor O2 molecules adsorbed on the surface. The adsorption of O2 molecule on ferroelectric materials may be a promising approach to achieve multiferroic materials.