Local Density Approximation Research Articles

Electronic K x rays emitted from muonic atoms: An application of relativistic density-functional theory

We develop a method using relativistic density-functional theory with a self-interaction correction, which is simple and fast yet has reasonable accuracy. A comparison with measured $K\ensuremath{\alpha}$ lines and their hypersatellites of several atoms, from low $Z$ to high $Z$, reveals that the relativistic local density approximation is suitable for $K\ensuremath{\alpha}$ lines. In contrast, the relativistic local spin density approximation with a self-interaction correction is better for ${K}^{h}\ensuremath{\alpha}$ hypersatellites. Compared with the nonrelativistic density-functional theory, we found that the relativistic effect is significant (about 100 eV) even for middle-$Z$ atoms, such as Cu. The screening effects, from inner shell to outer shell, and the conduction band, are also discussed. The present paper provides all the transition lines of muonic atoms, which can be used to narrow down the possible transitions by comparing them with the measurements.

Dynamical analysis of attosecond molecular modes

Attosecond charge migration (CM) modes in halogenated hydrocarbon chains are investigated from first principles. The initial hole state on the Br site is created by the constrained density functional theory (CDFT) and the following CM dynamics is tracked by the time-dependent density functional theory (TDDFT). We find the attosecond mode is mainly recorded in the spin channel where the electron is initially ionized. By employing approximate functionals at different theory levels, we find the local density approximation gives satisfactory results to reproduce the attosecond mode and more advanced exchange-correlation functionals do not affect the mode significantly. By alleviating the self-interaction error with an average density correction or allowing nuclei to relax, it is shown that the nuclear motion is a key factor to alter the mode. In addition, by a detailed analysis of single-electron orbitals, we find the attosecond mode is attributed to the electronic collective motion in the outermost energy shell. Our results will be useful in understanding the mechanism of molecular modes and motivating the ultrafast CM detection methods for future experiments.

Proximity of superconducting LaCoSi to a ferromagnetic quantum critical point.

The physical characteristics of the 4 K superconductor LaCoSi are studied using first-principles density functional theory inside the local density approximation (LDA) framework. We discover that LDA predicts a ferromagnetic ground state for LaCoSi, which is in disagreement with the experiments. Even though LDA rarely overestimates the local magnetic moment associated with the magnetic ion in itinerant systems, such occurrences highlight the significance of spin fluctuations in the system. In this view, the Ginzburg-Landau free energy expansion and its variation as a function of pressure are used to calculate the amplitude of the zero-point fluctuations. Based on our calculations, we contend that the superconductivity associated with LaCoSi is closely related to a ferromagnetic quantum critical point, making it an intriguing candidate for the category of quantum materials.

A study on pseudo-potential effect, electronic structure, aquatic toxicity, and optical properties of perovskites solar cell of Cs2NiCl6, Cs2NiBr6, and Cs2PtBr6: Through DFT methods

The main impediment to practical application is the toxicity of lead ions in halide perovskite absorbing materials. Computing tools based on density functional theory (DFT) were used to predict the intrinsic properties of potential for double perovskites to be effective and suitable for optoelectronic applications, replacing the conventional lead halide perovskites with environmentally friendly elements. The Generalized Gradient Approximation (GGA) with Perdew-Burke-Ernzerhof (PBE) functional was used to screen homovalent alternatives for B and X-site ions in vacancy-ordered double perovskite Cs2BX6 (B=Pt, Ni, X= Cl, Br) for solar cell applications. Using the GGA with PBE functional, the band gap was calculated to be 1.411 eV, 0.482 eV, and 0.378 eV for the Cs2PtBr6, Cs2NiCl6, and Cs2NiBr6, respectively. The experimental band gap value of mother crystal's (Cs2PtBr6) was at 1.42 eV. Next, the DOS, PDOS and optical properties were computed using GGA with PBE functional. Then, the local density approximation (LDA) with Ceperley and Alder with Perdew and Zunger (CA-PZ) was executed to compare the GGA with PBE for electronic band structure. In addition, the OTFG ultra soft, OTFG norm conserving, ultra soft and norm conserving methods of pseudopotential were used for both GGA with PBE and LDA with CA-PZ to make and ensure the right or accurate DFT functional for those crystals. At last, the optical properties and their toxicity have been evaluated for their rational design of potential double perovskite materials with improved optoelectronic properties.

Structural, electronic and thermoelectric properties of LiAlX2 (X=S and Se) chalcopyrites: promising for thermoelectric power generators

Herein, we have inquired the structural, electronic and thermoelectric properties of the couple of chalcopyrite structured solids LiAlX2 (X=S and Se) with the help of density functional theory (DFT), which is tracked by resolution of the Boltzmann transport equation with the constant relaxation time calculations. The LDA (Localized Density Approximation), PBE (Perdew-Burke-Ernzerhof), PBEsol (PBE functional revised for solids) and WC (Wu-Cohen) exchange correlation potentials have been used. The calculated lattice constants a = 5.271 Å; c = 10.178 Å and a = 6.226 Å; c = 12.165 Å for LiAlS2 and LiAlSe2 respectively and the band gap of the mentioned compounds are found in range from 1.74 eV to 3.13 eV. The dependency of thermoelectric parameters are calculated with different temperature (300-800K) and carrier concentration 1018 1019 cm-3 . From the study of ZT (figure of merit’s ZT= S2 σT/κ the dimensionless parameter) and it is found that it’s value for both the compounds in n-type as well as in p-type region is ‘unity’. Since these compounds can be the promising candidate for thermoelectric devices also these compounds are non-toxic, eco-friendly and good alternative for the green and renewable source of electric power generators.

Theoretical study of structural and optical properties of ZnO in wurtzite phase

Our calculations are done with the help of density functional theory (DFT). Actually, we could find the structural and optical properties of the wurtzite-type ZnO compound. The pseudo-potential linearised augmented plane wave (PP-LAPW) method is applied to solve the Kuhn-Sham equations. The results are obtained using Both Generalized Gradient Approximation according to the scheme described by Perdew-Burke-Ernzerhof(GGAPBE) and Local Density Approximation according to the scheme described by CeperlyAlder (LDA-CA) approximations as two types of exchange-correlation. The convergence of energy and charge has been checked. This is in order to study the properties of the ground state. It was found that the primary cell constants calculated in the equilibrium state are very close to the previous theoretical works. The general results of optical properties including the imaginary part of the dielectric constant, reflectivity, absorption coefficient, refractive index, optical conductivity, and extinction coefficient of wurtzitephase ZnO under the imposed conditions are discussed and compared with previous works. Our results show new and important optical properties. Besides, we predicted the behavior of transparent conductive oxides in the direction of light

First principle study on the Structural and Electronic Properties of Lead-Free Double Halide Perovskite Cs2InSbCl6

The structural and electronic characteristics of Cs2InSbCl6 were investigated using first principle density functional theory (DFT) within local density approximation (LDA) and generalized gradient approximation (GGA) as implemented in the Quantum ESPRESSO package. A supercell with 40 atoms was used in carrying out the calculations, using the optimized lattice parameters of the Cs2InSbCl6 (a = b = c =11.342 Ǻ). The findings revealed that Cs2InSbCl6 is a direct band gap material with gaps of 0.74 eV and 0.99 eV for LDA/PZ and GGA/PBE respectively, in which the GGA/PBE gap shows consistency within the literature range. The calculated densities of states highlight the contributions of the constituent atoms within the valence and the conduction bands.

An ab initio DFT perspective on experimentally synthesized CuBi2O4.

Here we present a comprehensive density functional theory (DFT) based ab initio study of copper bismuth oxide CuBi2O4 (CBO) in combination with experimental observations. The CBO samples were prepared following both solid-state reaction (SCBO) and hydrothermal (HCBO) methods. The P4/ncc phase purity of the as-synthesized samples was corroborated by Rietveld refinement of the powdered X-ray diffraction measurements along with Generalized Gradient Approximation of Perdew-Burke-Ernzerhof (GGA-PBE) and the Hubbard interaction U corrected GGA-PBE+U relaxed crystallographic parameters. Scanning and field emission scanning electron micrographs confirmed the particle size of the SCBO and HCBO samples to be ∼250 and ∼60 nm respectively. The GGA-PBE and GGA-PBE+U derived Raman peaks are in better agreement with that of the experimentally observed ones when compared to local density approximation based results. The DFT derived phonon density of states conforms with the absorption bands in Fourier transform infrared spectra. Both structural and dynamic stability criteria of the CBO are confirmed by elastic tensor and density functional perturbation theory-based phonon band structure simulations respectively. The CBO band gap underestimation of GGA-PBE as compared to UV-vis diffuse reflectance derived 1.8 eV was eliminated by tuning the U and the Hartree-Fock exact-exchange mixing parameter αHF in GGA-PBE+U and Heyd-Scuseria-Ernzerhof (HSE06) hybrid functionals respectively. The HSE06 with αHF = 14% yields the optimum linear optical properties of CBO in terms of the dielectric function, absorption, and their derivatives as compared to that of GGA-PBE and GGA-PBE+U functionals. Our as-synthesized HCBO shows ∼70% photocatalytic efficiency in degrading methylene blue dye under 3 h optical illumination. This DFT-guided experimental approach to CBO may help to gain a better understanding of its functional properties.

Structural and Electronic Properties of Hexagonal Y1−xEuxMnO3

Magnetoelectric materials attract interest due to coupling between the magnetic and dipol moments, which provides additional degrees of freedom in magnetoelectric device design and nanotechnological applications. Despite intensive theoretical and experimental studies already carried out in magnetoelectric materials, some issues deserve more attention, specifically their structural and electronic properties. Here, density functional theory (DFT) was used to investigate the structural and electronic properties of hexagonal Y1-xEuxMnO3 (x = 0.0, 0.1 and 0.2) compounds. Our approach is based on the local spin density approximation (LSDA+U). The magnetic moment carried out by Mn atoms is very sensitive to the LSDA+U. We obtain the lattice parameters that compare well with experimental X-ray measurements, showing a difference between calculated values and experiment less than 2%. The calculated PDOS shows important contributions from the rare earth and the oxygen atoms in these systems, in which main contributions comes from the manganese atom. In addition, the electronic partial density of states (PDOS) shows a dominant contribution from the Mn and rare earth atoms near the Fermi level.

First-principles calculations for comparative band structure study of SrTiO3 perovskite on bulk and layered phases for efficient optoelectronic conversion

A computational investigation over structural, electronic, optical, thermal, thermoelectric and elastic properties of the bulk and layered phases of perovskite compound namely SrTiO3 have been computed using WIEN2K code by means of Full-potential (FP) Linearized Augmented Plane Wave (LAPW) method implemented from DFT (Density Functional Theory) by applying Generalized Gradient Approximations (GGA), Local Density Approximations (LDA), modified Becke Johnson (mBJ) and Spin Orbit Coupling (SOC) computations. Further on this study we made surface structure of this compound by creating a supercell with various distances between the layers and compared their properties with bulk SrTiO3. The layered surfaces of SrTiO3 with varying distances are (i) single top layer (ii) double SrTiO3 layers with 5 a.u. distance (iii) double SrTiO3 layers with 18 a.u. distance have been constructed using the Struct editor program implemented in the WIEN2k code. The structural parameters viz., lattice constant, bond length, bond angles, formation energy, the electronic parameters such as band gap, Density of States (DoS), band structure and optical fundamental constants like dielectric function, optical conductivity, refraction, transmittance, absorbance, resistivity, reflectivity, Phonon dispersion curves and thermal properties such as Debye temperature, entropy, Gibbs free optimized energy (stabled / optimized state), thermoelectric properties such as Seebeck coefficient, Power factor, thermoelectric figure of merit, Pugh's ratio, elastic constants, modulus of elasticity and stiffness matrix have been computed and available results are compared and agreed well with the literature. The SrTiO3 bulk and layered surface exhibits direct band gap located at the Γ symmetry point of the Brillouin zone. From all the above analysis the layered materials with optimum variation of distances (double layers with 5 a.u) are predominantly favorable and efficient than bulk SrTiO3 for optoelectronic devices.

Symmetry Breaking with the SCAN Density Functional Describes Strong Correlation in the Singlet Carbon Dimer.

The SCAN (strongly constrained and appropriately normed) meta-generalized gradient approximation (meta-GGA), which satisfies all 17 exact constraints that a meta-GGA can satisfy, accurately describes equilibrium bonds that are normally correlated. With symmetry breaking, it also accurately describes some sd equilibrium bonds that are strongly correlated. While sp equilibrium bonds are nearly always normally correlated, the C2 singlet ground state is known from correlated wave function theory to be a rare case of strong correlation in an sp equilibrium bond. Earlier work that calculated atomization energies of the molecular sequence B2, C2, O2, and F2 in the local spin density approximation (LSDA), the Perdew-Burke-Ernzerhof (PBE) GGA, and the SCAN meta-GGA, without symmetry breaking in the molecule, found that only SCAN was accurate enough to reveal an anomalous under-binding for C2. This work shows that spin symmetry breaking in singlet C2, which involves the appearance of net up- and down-spin densities on opposite sides (not ends) of the bond, corrects that underbinding, with a small SCAN atomization-energy error more like that of the other three molecules, suggesting that symmetry breaking with an advanced density functional might reliably describe strong correlation. This article also discusses some general aspects of symmetry breaking and the insights into strong correlation that symmetry breaking can bring. The normally correlated low-lying triplet excited state has the right vertical excitation energy in SCAN but not in LSDA or PBE, where the triplet is a false ground state. Fractional occupation numbers are found only for the symmetry-unbroken singlet and only in LSDA and PBE GGA.

Structure, magnetic, opto-electronic and thermoelectric properties of A3In2As4 and A5In2As6 (A = Sr and Eu) Zintl phase compounds

Structural, electronic, optical, thermoelectric and magnetic properties of A3In2As4 and A5In2As6 (A = Eu and Sr) Zintl phase compounds in orthorhombic (Pnma and Pbam) phases are examined using density functional theory. Structure properties are an agreement with the experiment. Magnetic properties disclose the Non-magnetic nature of Sr3In2As4 and Sr5In2As6 and anti-ferromagnetic nature of Eu3In2As4 and Eu5In2As6 compounds.Electronic properties of Sr3In2As4, Sr5In2As6 and Eu3In2As4, Eu5In2As6 shows that all these compounds have narrow direct bandgap semiconductors at ᴦ and M symmetry. Electrical conductivity also gives the information about the semiconducting characteristic of these compounds. These compounds show that reduction in bandgap occurs through the substitution of Sr by Eu while increasing by going from local density approximation to modified Becke-Johnson potential and further reduce by spin orbit coupling effect. The outcome demonstrates that all compounds are optically dynamic in the infrared region, anisotropic and obstruct ultraviolet waves so these compounds are capable to use as shield for ultraviolet radiation and can be used as paramount applicant for optoelectronic devices. Owing to low thermal conductivity, high Seebeck coefficient, power factor and Figure of merit ranges from 0.39 to 1.2 at 800 K indicating that all compounds are active aspirants for thermoelectric applications.

Characterization of adsorption properties inherent to zirconia dioxide for different positions of yttrium in the ZrO2–Y2O3 lattice

Presented in this paper is theoretical studying redistribution of electric charges in the layer of a tetragonal plate of yttrium-stabilized zirconia based on the position of yttrium atom in the crystal lattice for both dry and humid ambient atmosphere. The density functional theory with local density approximation (DFT-LDA) has been employed for this modelling. Calculations have been performed for layer-by-layer electron density distribution over the thickness of an infinite plate 001 of yttrium-stabilized tetragonal zirconium dioxide, which show that a change in the position of stabilizing yttrium atom and its symmetry in the layer leads to changing the total energy of zirconium dioxide both for the dry 001 surface and for the hydrated one. It has been ascertained that the surface charge density for the 001-surface of an infinite tetragonal zirconia plate increases in proportion to the degree of hydration.

High Curie Temperature and Half‐Metallic Ferromagnetism in ZnSe:Co,Ni with Wurtzite Structure: First‐Principles Study

The aim is to examine the spin‐polarized electronic band structures, the density of states (DOS) as well as magnetism of CoxZn1−xSe and NixZn1−xSe diluted magnetic semiconductors (DMSs) in the ferromagnetic (FM) phase, and with 12.5% and 6.25% concentrations of impurity. The calculations are implemented by the recent ab initio norm‐conserving FHI pseudopotential method within the local spin density approximation and Hubbard U (LSDA + U) method. The analysis of the total DOS (TDOS) curves shows the half‐metallic FM character for Ni‐doped and (Co, Ni)‐co‐doped ZnSe with a half‐metallic bandgap of 1 ÷ 1.6 eV. The exchange splittings produced by dopant d states are determined: for the CoxZn1−xSe, it is obtained that the effective potential for the majority spin is more attractive than for the minority spin. The total magnetic moments of Co‐doped, Ni‐doped, and co‐doped ZnSe DMSs are found to be equal to 3, 4, and 5.0 μB, respectively. The Curie temperature of NixZn1−xSe is higher than room temperature and this is half‐metallic ferromagnetic (HMFM) material for spintronics application. Co‐doped ZnSe is an excellent choice for electronic devices owing to its below Curie temperature.

Study of electrical transport properties for central-distorted graphene nanoribbon using density functional theory

The electrical transport properties of zigzag-graphene nanoribbons (zGNR) and central-distorted graphene nanoribbons (cdGNR) are determined by density functional theory (DFT) with the attachment of 12 carbon atoms in each case. The cdGNR was modelled with the Stone-Wales defect in the middle of the matrix which shows the enhanced current–voltage (I-V) curve as compared to zGNR. The cdGNR exhibits the Ohmic linear characteristic enhanced I-V plot by the presence of disorder. The current was calculated by the Landauer–Büttiker formula using Green’s function. The exchange–correlation potential is characterized by the local density approximation (LDA). The conductance is exponentially damped in both GNRs; however, it has higher values in cdGNR as compared to zGNR. The transmission spectrum shows the decreasing peak shift pattern from 0 to 1 V in the vicinity of 0 eV Fermi energy, Ef. The exponentially increasing and decreasing peaks are observed at the negative and positive sides of the Ef.

Modeling of Gold Adsorption by the Surface of Defect Graphene

The results of the theoretical investigation of the local structural changes and adsorption characteristics of the graphene (GP) surface in the presence of a vacancy + Auads adatom complex are presented. Based on the density functional theory (DFT), the adsorption properties of Auads at the surface of GP supercells containing 50 carbon atoms with vacancies ({text{GP}}leftlangle {{text{A}}{{{text{u}}}_{{{text{ads}}}}}} rightrangle ,{text{ G}}{{{text{P}}}_{{text{V}}}}leftlangle {{text{A}}{{{text{u}}}_{{{text{ads}}}}}} rightrangle ) are calculated. The most stable configuration of {text{G}}{{{text{P}}}_{{text{V}}}}leftlangle {{text{A}}{{{text{u}}}_{{{text{ads}}}}}} rightrangle supercells with a vacancy + Auads adatom complex is determined. The effect that an Auads adatom has on the band structure and local magnetic moment in {text{G}}{{{text{P}}}_{{text{V}}}}leftlangle {{text{A}}{{{text{u}}}_{{{text{ads}}}}}} rightrangle is calculated. The data are analyzed based on the equilibrium atomic configuration {text{G}}{{{text{P}}}_{{text{V}}}}leftlangle {{text{A}}{{{text{u}}}_{{{text{ads}}}}}} rightrangle , local density of electronic states, and spin polarization. The calculations are made using the exchange-correlation functional in a local electron-spin density approximation (LSDA).

Prediction of electronic and optical properties of monoclinic 1T’-phase OsSe2 monolayer using DFT principles

Following the discovery and characterization of transition metal dichalcogenides (TMDs) monolayers, which has created interest in last few years, it would seem natural to explore new possibilities of monolayers. Thus, using the density functional theory (DFT), we investigated the structural, electronic, vibrational and thermodynamic properties, and optical absorption of osmium selenide monolayer (monoclinic 1T’-phase OsSe 2 monolayer). We perform the calculations using the approaches based on the generalized gradient (GGA) and the local density (LDA) approximations for the optimized structure with the minimum energy, as well as we use the hybrid exchange–correlation functional HSE06 recommended in the literature for bandgap energy estimation. An indirect bandgap E g = 0 . 85 eV, E g = 0 . 95 eV and E g = 1 . 80 eV was obtained within the LDA-CAPZ, GGA-PBE and HSE06 level of calculation, respectively. The vibrational normal modes, infrared and Raman spectra were obtained and assigned in the frequency range of 0 − 400 c m − 1 together with the phonon dispersion relation. In addition, the optical absorption was shown to be sensitive to the plane of polarization of the incident light mainly in the range of UV radiation. The thermodynamic potentials and specific heat at constant volume were calculated and analyzed. These results indicated that the monoclinic 1T’-phase OsSe 2 structure could be potentially synthesized and bringing new potential technological applications.

Theoretical Investigation of Structural, Electronic, and Optical Properties of ZnSnP2 Semiconductor

Abstract The structural, electronic, and optical properties of ZnSnP2 compound were determined using the first principles calculations. We applied the full-potential enhanced plane wave method (FP-LAPW) within the framework of density functional theory (DFT) as implemented in the Wien2k package. The exchange-correlation potential term was treated using the local density approximation (LDA), the generalized gradient approximation (GGA), the Engel–Vosko generalized gradient approximation (EV–GGA) and GGA plus modified Becke– Johnson (mBJ). The lattice parameters of the ZnSnP2 obtained by minimizing the total energy are consistent well with the existing theoretical and experimental results. The Dugdale and MacDonald Grüneisen parameter was found to be 1.43 from the GGA and 1.44 from the LDA, respectively. According to the electronic properties, the band structure analysis of ZnSnP2 shows that it has a direct band gap in the (Γ-Γ) direction with a value of 1.43 eV. We have investigated the optical properties of ZnSnP2 semiconducting compound. The data of the dielectric functions shown that the peaks are positioned at around 2.41, 3.21, 3.83 and 4.09 eV, respectively.

Elastic Constants of Tetragonal Cu2ZnSnS4 Semiconductor: AB-Initio Calculation

Abstract In this work, an ab-initio calculation is used to investigate the elastic constants and some other mechanical and thermal parameters of tetragonal Cu2ZnSnS4 (CZTS) quaternary semiconducting bulk material in Kesterite (KS) and Stannite (ST) phases. The Quantum Espresso code within the Ultra Soft pseudo potentials (USPP) and the local density approximation (LDA) approach were used in the calculation. Firstly,, studies are started with the prediction of the elastic stiffness constants Cij and the normal and shear anisotropy factors. Then some other mechanical moduli, especially the isotropic bulk modulus B, the shear modulus G, the Young modulus E, the Poisson’s ratio ν, and the Pugh’s criteria (G/B) are delivered. The analysis of the mechanical stability criteria at equilibrium shows that our elastic stiffness constants Cij of CZTS material obey all the stability conditions. Additionally, some other parameters of the CZTS semiconductor, especially: the Vickers hardness HV , the sound velocity, the Debye temperature θD and the melting temperature Tm were also calculated. The obtained values of the elastic constants Cij and other mechanical and thermal parameters agree well with experimental and other theoretical results of the literature. The Debye temperature θD of the KS phase was found at around 332.7 K, and that of the stannite phase was found equal to 329.1 K, respectively.

Satellites in the Ti 1s core level spectra of SrTiO3 and TiO2

Satellites in core level spectra of photoelectron spectroscopy (PES) can provide crucial information on the electronic structure and chemical bonding in materials, particular in transition metal oxides. This paper explores satellites of the Ti 1$s$ and 2$p$ core level spectra of SrTiO$_3$ and TiO$_2$. Conventionally, soft x-ray PES (SXPS) probes the Ti 2$p$ core level; however, it is not ideal to fully capture satellite features due to its inherent spin-orbit-splitting (SOS). Here, hard x-ray PES(HAXPES) provides access to the Ti 1$s$ spectrum instead, which allows us to study intrinsic charge responses upon core-hole creation without the complication from SOS and with favorable intrinsic linewidths. The experimental spectra are theoretically analyzed by two impurity models, including an Anderson impurity model (AIM) built on local density approximation (LDA) and dynamical mean-field theory (DMFT), and a conventional TiO$_6$ cluster model. The theoretical results emphasize the importance of explicit inclusion of higher-order Ti-O charge-transfer processes beyond the nearest-neighboring Ti-O bond to simulate the core level spectra of SrTiO$_3$ and TiO$_2$. The AIM approach with continuous bath orbitals provided by LDA+DMFT represents the experimental spectra well. Crucially, with the aid of the LDA+DMFT method, this paper provides a robust prescription of how to use the computationally cheap cluster model in fitting analyses of core level spectra.