Local Spin Density Approximation Research Articles

Photoferroic prospective of multiferroic HoMnO3 compound, evaluated on the base of the DFT study of its magnetic, electronic, and optical properties

In this work, non-collinear spin density functional theory based all-electron full-potential linearized augmented plane wave electronic structure method have been employed to study magnetic, electronic, and optical properties of multiferroic HoMnO3 compound in order to evaluate its potential to be used in photovoltaic applications. Exchange and correlation effects of the Ho 4f- and Mn 3d-electrons were treated on the base of the local spin density approximation including effective Hubbard (Ueff) correction. We concluded that the ground state magnetic structure should be described by Γ3 irreducible representation of the P63cm space group, for both the Mn and the Ho magnetic sub-lattices. The calculated indirect and direct band gaps, corresponding to the transition from the O 2p-to the Mn 3d-states, are found to be 1.25 eV and 1.4 eV, respectively, in very good agreement with the experimental findings. Calculated dielectric tensor spectra are found to be in satisfactory agreement with the experiment as well. On the base of these results, we investigated photoferroic properties of the HoMnO3 by calculating its absorption coefficient spectrum and the effective mass tensor of the photogenerated electrons and holes. By comparing results with those of isostructural LuMnO3 and the YMnO3 multiferroics, we concluded that the HoMnO3 exhibits similar photoferroic properties, a fact that classifies it as a material suitable for photovoltaic applications.

First-principles study of structural and electronic properties of ZnSe with wurtzite structure

Using Density Functional Theory (DFT) and Local Spin Density Approximation (LSDA) the structural and electronic properties of pure ZnSe wurtzite compound were theoretically investigated. The Hubbard U semiempirical correction has also been used to improve the underestimated DFT calculated band gap. From the DFT+LSDA+U method obtained that the wide of band gap is 2.7 eV and this result is closer to the available experimental results. Current studies reveal that the maximum of valence band and the minimum of conduction band located at the Γ (0, 0, 0) symmetry point on the Brillouin zone. Accurate investigation of the electronic structure of studied material is helpful for the technical applications.

Spin-state gaps and self-interaction-corrected density functional approximations: Octahedral Fe(II) complexes as case study.

Accurate prediction of a spin-state energy difference is crucial for understanding the spin crossover phenomena and is very challenging for density functional approximations, especially for local and semi-local approximations due to delocalization errors. Here, we investigate the effect of the self-interaction error removal from the local spin density approximation (LSDA) and Perdew-Burke-Ernzerhof generalized gradient approximation on the spin-state gaps of Fe(II) complexes with various ligands using recently developed locally scaled self-interaction correction (LSIC) by Zope et al. [J. Chem. Phys. 151, 214108 (2019)]. The LSIC method is exact for one-electron density, recovers the uniform electron gas limit of the underlying functional, and approaches the well-known Perdew-Zunger self-interaction correction (PZSIC) as a particular case when the scaling factor is set to unity. Our results, when compared with reference diffusion Monte Carlo results, show that the PZSIC method significantly overestimates spin-state gaps favoring low spin states for all ligands and does not improve upon density functional approximations. The perturbative LSIC-LSDA using PZSIC densities significantly improves the gaps with a mean absolute error of 0.51eV but slightly overcorrects for the stronger CO ligands. The quasi-self-consistent LSIC-LSDA, such as coupled-cluster single double and perturbative triple [CCSD(T)], gives a correct signof spin-state gaps for all ligands with a mean absolute error of 0.56eV, comparable to that of CCSD(T) (0.49eV).

First principles study on electronic, magnetic and optical properties of Gd2NiMnO6

The double perovskite oxide materials have various applications in light absorber, solid oxide fuel cells, electrode materials for electrochemical energy devices, solid state thermoelectric devices, etc. The calculations of electronic, optical and magnetic properties of Gd2NiMnO6 (GNMO) double perovskite are aimed in the present work. GNMO crystallizes in monoclinic structure with P21/n (14) space group. Implementing first principles density functional theory, we have reported the spin-polarised electronic band structure, density of states (DOS), optical absorption property and magnetic moment of GNMO double perovskite. Furthermore, the effect of on-site d-d Coulomb interaction energy (Ueff) on the electronic and optical properties were investigated by applying a range of Hubbard parameters from 0 to 6 eV on Ni-3d and Mn-3d orbitals within the local spin density approximation (LSDA) and generalised gradient approximation (GGA). Interestingly, on applying Ueff in this range, band gap of GNMO enhanced progressively in both majority and minority spins and also for Ueff = 4 and 6 eV implemented on Ni-3d and Mn-3d orbits, respectively, we observe the band gap value 1.16 and 2.12 eV for the spin-up and spin-down states which is in good agreement with the previously reported experimental band gap value (1.5 eV). In this regard, the electronic structure and light absorption of GNMO have been analysed for Ueff = 4 and 6 eV implemented on Ni-3d and Mn-3d orbits, respectively. The calculations revealed suitable narrow band gap and large visible light absorption coefficient in GNMO, which are extremely desired for the high photovoltaic performance.

Theoretical validation of experimental properties of TiO2 prepared through organometallic precursors

Our TiO2 thin films were elaborated using spray pyrolysis technique. The spray solutions were prepared by two titanium precursors: titanium (IV) propoxide and titanium chloride. The elaborated thin films were analyzed with different characterization techniques such as X‐ray diffraction (XRD), Raman spectroscopy, scanning electron microscopy (SEM), and UV–visible spectroscopy. These characterizations showed that the change of precursors from titanium chloride to titanium propoxide has a significant impact on all TiO2 properties. The structural characterizations showed the formation of anatase TiO2 and the crystallization improvement when the precursor changed from titanium chloride to titanium (IV) propoxide, which proves the superiority of our TiO2 thin layers made without any type of annealing in comparison with other previous studies. The SEM images showed an improvement of the surface quality of the TiO2 thin layers with the change of TiO2 precursors' solution. The obtained optical properties confirmed the semiconducting properties of TiO2 prepared by titanium propoxide with a large band gap equal to 3.28 eV and shifted to 3.18 eV. The refractive index increases from ~1.75 to ~2.14, while the porosity decreases. To confirm the experimental results, the Wien2K code, founded on the full‐potential linearized augmented plane wave (FP‐LAPW), has been used. The theoretical study has been done by applying the generalized gradient approximation (GGA), the modified Beck‐Johnson (MBJ) exchange potential, and the local spin density approximation + Hubbard's parameter U (LSDA + U). The theoretical results varied with the variation of approximations. The obtained properties were improved with LSDA + U and acquired the real values of Eg, n, and k for anatase TiO2 (like Eg = 3.214 eV). The obtained experimental and theoretical results could be considered superposable or better than other ones recently published.

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.

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.

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.

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.

Structural properties of novel potential spintronic material and Full-Heusler alloy Co2NbGe: First Principles investigation

The main aim of the paper is to explore the structure of Co2NbGe, which is a full-Heusler alloy and is could be of great importance in future spintronic applications. We hereby performed the first-principle calculations which are based on the full-potential linearized augmented plane wave method using WIEN2k. By this we attempt to investigate the structural properties of Co2NbGe. We have utilized different generalized gradient approximations (GGA), and local spin density approximations (LSDA) of density functional theory (DFT) as manifested in WIEN2k.

Impact of electron correlation on the light-induced demagnetization of elemental ferromagnetic metals

The local spin-density approximation (LSDA) is known to describe poorly the electronic structure of 3d transition metals, yet most density-functional-based ab-initio studies of ultra-fast demagnetization rely on it. One way to account for Coulomb correlations among the localized d electrons and go beyond LSDA is to include the effective correlation energy (or Hubbard) U. By doing so, we show here that electronic correlations lead to sizable changes of the laser-induced demagnetization of iron, cobalt, and nickel. We study how the various laser parameters, such as pulse duration or intensity, change the magnetization dynamics. It turns out that the total laser fluence is not suitable to quantify how much a laser pulse demagnetizes a material, as changes in pulse duration and shape influence significantly the outcome. The findings are traced back to the electronic structure of the material, and explained based on phase space for optical transitions.Graphic abstract

First principles calculations of structural, electronic and optical properties of Sn-doped ZnS

Using the full potential linearized augmented plane wave approach within the density functional theory (DFT), the structural, electronic and optical properties of Zn1−xSnxS (x= 6.25, 12.5 and 25%) solid solutions in most stable cubic phase of ZnS are computed. The bandgap values are calculated by using Tran–Blaha approach of modified Becke and Johnson (TB-mBJ) local spin density approximation. For Sn-doped ZnS, the compounds are non-magnetic indirect bandgap semiconductors with narrow bandgaps. The contribution of the Sn-5p state near Fermi level in the valance band reduces the bandgap of the doped systems. Optical characteristics are estimated using the dielectric function spectra as well as other associated optical properties such as absorption coefficient, reflectivity, refractive index, extinction coefficient and conductivity. The investigation shows that this compounds might be a good choice for optoelectronic devices.

First-Principles Study of Half Metallic Ferromagnetic and Optical Properties of Nb Doped Cubic ZnS using TB-mBJ Approximation

Structural, electronic and optical properties of niobium doped ZnS are studied by using full-potential linearized augmented plane wave plus local orbital (FPLAPW+lo) method within the density functional theory (DFT). Computed results of Nb doped ZnS are compared with that of the pristine zinc blende. Tran-Blaha approach of modified Becke and Johnson local spin density approximation (TB-mBJ) is used to study electronic and optical properties. Estimated result shows that Nb reduces the bandgap of ZnS due to hybridization of Nb-4d orbital with S-3p orbital near the Fermi level. Niobium dopant provides half metallic nature to ZnS with 100% spin polarization. Maximum photo-response is noticed in the ultraviolet range for Zn1-xNbxS (x = 25, 12.5, 6.25 %). Highest peaks are shifted toward the lower energy range for higher dopant percentage. All these suggest that Nb doped ZnS solid solutions are suitable candidate for both energy filter of UV spectrum and spintronic device. Dhaka Univ. J. Sci. 69(3): 194-201, 2022 (June)

The structure, electronic, and magnetic properties of PbRuO3 under hydrostatic pressure: A first-principles study

Density functional theory in the local spin density approximation is used to investigate the structural, electronic, and magnetic properties of PbRuO3 under hydrostatic pressure. The calculated results show that under hydrostatic pressure, the ground state of Pm3¯m PbRuO3 is a ferromagnetic metal. The covalent bond of Pb–O strengthens as hydrostatic pressure increases, the exchange splitting of up- and down-spin channels of Ru 4d t2g weakens, and the magnetic properties decrease. The calculated ground state of Pbnm PbRuO3 is a paramagnetic metal, which is consistent with the experimental results at P = 0 GPa. The ground state of Imma PbRuO3 has weak ferromagnetism, which is close to the experimental results at P = 0 GPa. The Pbnm–Imma phase transition occurs when passing the critical hydrostatic pressure. The main reason is the distortion of Pb2+ at the A-site, which changed the bond length of Pb–O and the covalent strength of Pb–O, affecting the electron occupation of Ru 4d and leading to the Pbnm–Imma phase transition.

Complex Fermi-Löwdin orbital self-interaction correction.

This paper introduces the use of complex Fermi orbital descriptors (FODs) in the Fermi-Löwdin self-interaction-corrected density functional theory (FLOSIC). With complex FODs, the Fermi-Löwdin orbitals (FLOs) that are used to evaluate the SIC correction to the total energy become complex. Complex FLO-SIC (cFLOSIC) calculations based on the local spin density approximation produce total energies that are generally lower than the corresponding energies found with FLOSIC restricted to real orbitals (rFLOSIC). The cFLOSIC results are qualitatively similar to earlier Perdew-Zunger SIC (PZ-SIC) calculations using complex orbitals [J. Chem. Phys. 80, 1972 (1984); Phys. Rev. A 84, 050501(R) (2011); and J. Chem. Phys. 137, 124102 (2012)]. The energy lowering stems from the exchange-correlation part of the self-interaction correction. The Hartree part of the correction is more negative in rFLOSIC. The energy difference between real and complex solutions is greater for more strongly hybridized FLOs in atoms and for FLOs corresponding to double and triple bonds in molecules. The case of N2 is examined in detail to show the differences between the real and complex FLOs. We show that the complex triple-bond orbitals are simple, and physically appealing combinations of π and σg orbitals that have not been discussed before. Consideration of complex FODs, and resulting unitary transformations, underscores the fact that FLO centroids are not necessarily good guesses for FOD positions in a FLOSIC calculation.

Longitudinal fluctuations of Co spin moments and their impact on the Curie temperature of the Heusler alloy Co2FeSi

The magnetism of the full Heusler alloy Co2FeSi with its high magnetic ordering temperature is studied on a first-principles basis employing the disordered local moment approximation, the magnetic force theorem and single-site spin fluctuation theory as formulated recently in the framework of the Local Spin Density Approximation. We find that the magnetic moments of Fe and Co in Co2FeSi exhibit a quite distinctive behavior at high temperatures. The Fe moments are well localized and keep their magnitude unchanged with temperature, whereas the Co moments are itinerant and show a temperature dependence. We find that the effects of magnetic disorder strongly renormalize the Fe-Co inter-atomic exchange interactions. Our results suggest a deficiency of the classical Heisenberg model with rigid localized atomic spin moments for the description of magnetism in Co2FeSi. An accurate estimation of the Curie temperature is obtained by taking into the account the thermal longitudinal fluctuations the Co moments.

The density functional theory study of substitution effect in antiferromagnetic USn0.5Sb1.5

The electronic structure of antiferromagnetic (AF) USn0.5Sb1.5 crystallizing in the tetragonal Cu2Sb-type structure is investigated by means of ab-initio calculations using the full-potential linearized augmented plane wave method (FP-LAPW) method. Using Local-spin density approximation (LSDA) with the inclusion of spin-orbit coupling (LSDA ​+ ​SO) and Hubbard corrections (LSDA ​+ ​SO ​+ ​U-J) in the calculations, the most stable magnetic ground with an ordered magnetic moment of ∼ 1.7 μB is predicted for the system. Meaningful changes in the lattice parameters, magnetic moments, and magnetic ground state instabilities of USn0.5Sb1.5 are discussed in terms of effects brought by the Coulomb interaction, and the Hund coupling. The calculated electronic properties involving Densities of States (DOS), Electronic Band Structures (EBS), Electron Localization Functions (ELF), and Fermi Surfaces (FS) are presented and compared to those of the parent USb2 compound. The electronic properties, particularly FS and ELF, are found to be strongly modified as a result of the Sn-substitution. The reformation of anisotropy in the FS and ELF upon changing magnetic states (paramagnetic → antiferromagnetic) is examined.

Fermi-Löwdin orbital self-interaction correction of adsorption energies on transition metal ions.

Density functional theory (DFT)-based descriptions of the adsorption of small molecules on transition metal ions are prone to self-interaction errors. Here, we show that such errors lead to a large over-estimation of adsorption energies of small molecules on Cu+, Zn+, Zn2+, and Mn+ in local spin density approximation (LSDA) and Perdew, Burke, Ernzerhof (PBE) generalized gradient approximation calculations compared to reference values computed using the coupled-cluster with single, doubles, and perturbative triple excitations method. These errors are significantly reduced by removing self-interaction using the Perdew-Zunger self-interaction correction (PZ-SIC) in the Fermi-Löwdin Orbital (FLO) SIC framework. In the case of FLO-PBE, typical errors are reduced to less than 0.1eV. Analysis of the results using DFT energies evaluated on self-interaction-corrected densities [DFT(@FLO)] indicates that the density-driven contributions to the FLO-DFT adsorption energy corrections are roughly the same size in DFT = LSDA and PBE, but the total corrections due to removing self-interaction are larger in LSDA.

DFT+Σ2 method for electron correlation effects at transition metal surfaces

We present a computational approach for electronically correlated metallic surfaces and interfaces, which combines Density Functional and Dynamical Mean Field Theory using a multi-orbital perturbative solver for the many-body problem. Our implementation is designed to describe ferromagnetic metallic thin films on a substrate. The performances are assessed in detail for a Fe monolayer on a W(110) substrate, a prototypical nanoscale magnetic system. Comparing our results to photoemission data, we find qualitative and quantitative improvements in the calculated spectral function with respect to the results of Density Functional Theory within the local spin density approximation. In particular, the spin-splitting of the $d$ states is drastically reduced and, at the same time, their spectral width becomes narrower. The method is therefore able to account for the main correlation effects in the system.

Study of Self-Interaction Errors in Density Functional Calculations of Magnetic Exchange Coupling Constants Using Three Self-Interaction Correction Methods.

We examine the role of self-interaction error (SIE) removal on the evaluation of magnetic exchange coupling constants. In particular, we analyze the effect of scaling down the self-interaction correction (SIC) for three nonempirical density functional approximations (DFAs) namely, the local spin density approximation, the Perdew-Burke-Ernzerhof generalized gradient approximation, and the recent SCAN family of meta-GGA functionals. To this end, we employ three one-electron SIC methods: Perdew-Zunger SIC [Perdew, J. P.; Zunger, A. Phys. Rev. B, 1981, 23, 5048.], the orbitalwise scaled SIC method [Vydrov, O. A. et al. J. Chem. Phys. 2006, 124, 094108.], and the recent local scaling method [Zope, R. R. et al. J. Chem. Phys. 2019, 151, 214108.]. We compute the magnetic exchange coupling constants using the spin projection and nonprojection approaches for sets of molecules composed of dinuclear and polynuclear H···He models, organic radical molecules, and chlorocuprate and compare these results against accurate theories and experiment. Our results show that for the systems that mainly consist of single-electron regions, PZSIC performs well, but for more complex organic systems and the chlorocuprates, an overcorrecting tendency of PZSIC combined with the DFAs utilized in this work is more pronounced, and in such cases, LSIC with kinetic energy density ratio performs better than PZSIC. Analysis of the results in terms of SIC corrections to the density and to the total energy shows that both density and energy correction are required to obtain an improved prediction of magnetic exchange couplings.