Local Spin Density Approximation Research Articles

Magnetic properties of the low-dimensional BaM2Si2O7 system (M=Cu, Co, Mn)

We investigated the magnetic properties of the low-dimensional $\mathrm{Ba}{M}_{2}{\mathrm{Si}}_{2}{\mathrm{O}}_{7}$ ($M$ = Cu, Co, and Mn) system using both experimental measurements and theoretical calculations. Magnetization, specific heat, and single crystal neutron diffraction measurements have been performed on single crystal ${\mathrm{BaMn}}_{2}{\mathrm{Si}}_{2}{\mathrm{O}}_{7}$. The spin structure was determined and a magnetic phase diagram with applied field along the $b$ axis was constructed, which contains a spin flop transition around 6 T. Magnetization and specific heat measurements confirmed the presence of weak ferromagnetism in ${\mathrm{BaCo}}_{2}{\mathrm{Si}}_{2}{\mathrm{O}}_{7}$. Furthermore, we performed local-spin density approximation with on-site Coulomb energy ($\mathrm{LSDA}+U$) calculations for the $\mathrm{Ba}{M}_{2}{\mathrm{Si}}_{2}{\mathrm{O}}_{7}$ ($M$ = Cu, Co, and Mn) system. Based on the first-principles calculations, the origin of the magnetic differences of the three materials is discussed.

First-principles investigation of structural, electronic and mechanical properties of some Dysprosium chalcogenides, DyX (X = S, Se and Te)

The structural, electronic and mechanical properties of heavy rare earth (DyX, X = S, Se and Te) chalcogenide compounds, which crystallize in B1 phase, have been investigated for the first time using FP-LAPW method within the framework of density functional theory. The local spin density approximation with Hubbard parameter (LSDA + U) has been used for exchange–correlation potentials. The calculated ground-state properties such as lattice constants, bulk moduli and pressure derivatives are in good agreement with previously reported results. Further, magnetic moments for these compounds have also been calculated. The metallic nature of these compounds was confirmed by electronic band structures and density of states. Furthermore, to meet the applications of such materials, elastic constants and mechanical properties such as Young modulus, shear modulus, Poisson ratio, anisotropic factor, B/G ratio and Cauchy’s pressure have also been calculated. DyS shows brittle nature, while DySe and DyTe show ductile nature under ambient conditions.

Anisotropy and orbital moment in Sm-Co permanent magnets

Structural and magnetic properties of iron-free and iron-substituted $\mathrm{SmC}{\mathrm{o}}_{5}$ have been investigated theoretically and experimentally. The nanocrystalline ribbons of ${\mathrm{SmCo}}_{5\ensuremath{-}x}{\mathrm{Fe}}_{x}\phantom{\rule{4pt}{0ex}}(0\ensuremath{\le}x\ensuremath{\le}2)$, which were produced by rapid solidification, crystallize in the hexagonal $\mathrm{CaC}{\mathrm{u}}_{5}$ structure for $x\ensuremath{\le}0.75$. Small Fe additions $(x=0.25)$ substantially improve the coercivity, from 0.45 to 2.70 T, which we interpret as combined intrinsic and extrinsic effect. Most of our findings are consistent with past samarium-cobalt research, but some are at odds with findings that have seemingly been well established through decades of rare-earth transition-metal research. In particular, our local spin-density approximation with Hubbard parameter calculations indicate that the electronic structure of the Sm atoms violates Hund's rules and that the orbital moment is strongly quenched. Possible reasons for the apparent disagreement between theory and experiment are discussed. We explicitly determine the dependence of the Sm $4f$ charge distribution, arguing that an accurate density-functional description of $\mathrm{SmC}{\mathrm{o}}_{5}$ is a challenge to future research.

Self-interaction-free electric dipole polarizabilities for atoms and their ions using the Fermi-Löwdin self-interaction correction

The static electric dipole polarizability of a system is a measure of the binding of its electrons. In density functional theory calculations, this binding is weakened by the presence of unphysical self-interaction in the density functional approximation (DFA), leading to overestimates of polarizabilities. To investigate this systematically we compare polarizabilities for the atoms from H to Ar and their anions and cations calculated in several DFAs and the corresponding self-interaction-corrected (SIC) DFAs with experiment and with high-level quantum chemistry reference values. The SIC results are obtained using the Fermi-L\"owdin orbital self-interaction correction (FLO-SIC) method. Removing self-interaction generally leads to smaller polarizabilities that agree significantly better with reference values. We find that FLO-SIC improves the performance of the local spin density approximation and the generalized gradient approximation (GGA) for polarizabilities to a quality that is comparable to so-called rung 4 functionals, but slightly degrades the performance of the strongly constrained and appropriately normed meta-GGA functional.

Ab initio study of GdCo5 magnetic and magneto-optical properties

Abstract The full potential linearized augmented plane wave method (FLAPW) including the spin-orbit coupling has been used to study the structural, electronic and magnetic properties of GdCo5 compound. The calculations were performed within the local spin density approximation (LSDA) as well as Coulomb corrected LSDA + U approach. The study revealed that the LSDA + U method gave a better representation of the band structure, density of states and magnetic moments than LSDA. It was found that the spin magnetic moment of Co (2c) and Co (3g) atoms in the studied compound is smaller compared to the one in bulk Co. The optical and magneto-optical properties and the magneto-optical Kerr effect have also been investigated.

Perdew-Zunger self-interaction correction: How wrong for uniform densities and large-Z atoms?

Semilocal density functionals for the exchange-correlation energy of a many-electron system cannot be exact for all one-electron densities. In 1981, Perdew and Zunger (PZ) subtracted the fully nonlocal self-interaction error orbital-by-orbital, making the corrected functional exact for all collections of separated one-electron densities and making no correction to the exact functional. Although the PZ self-interaction correction (SIC) eliminates many errors of semilocal functionals, it is often worse for equilibrium properties of sp-bonded molecules and solids. Nonempirical semilocal functionals are usually designed to be exact for electron gases of uniform density and, thus, also make 0% error for neutral atoms in the limit of large atomic number Z, but PZ SIC is not so designed. For localized SIC orbitals, we show analytically that the local spin density approximation (LSDA)-SIC correlation energy per electron of the uniform gas in the high-density limit makes an error of -50% in the spin-unpolarized case and -100% in the fully spin-polarized case. Then we extrapolate from the Ne, Ar, Kr, and Xe atoms to estimate the relative errors of the PZ SIC exchange-correlation energies (with localized SIC orbitals) in the limit of large atomic number: about +5.5% for the LSDA-SIC and about -3.5% for nonempirical generalized gradient [Perdew-Burke-Ernzerhof (PBE)-SIC] and meta-generalized gradient strongly constrained and appropriately normed (SCAN)-SIC approximations. The SIC errors are considerably larger than those that have been estimated for LSDA-SIC by approximating the localized SIC orbitals for the uniform gas and may explain the errors of PZ SIC for equilibrium properties, opening the door to a generalized SIC that is more widely accurate.

An ab initio study of filled skutterudites EuOs4Sb12

The study of structural, elastic, thermodynamic and electronic properties of filled skutterudites EuOs4Sb12 is presented using the full-potential linear muffin-tin orbital method based on the density functional theory. As exchange–correlation potential, we used the local density and local spin density approximation. The equilibrium lattice parameters, bulk modulus, elastic constants and band structure energy of filled skutterudites EuOs4Sb12 are calculated and systematically compared to the available theoretical and experimental data. The total energy variation as a function of strain is used to determine the independent elastic constant and its pressure dependence. The effect of pressure P and temperature T on the lattice parameter, bulk modulus, Debye temperature and the heat capacity of EuOs4Sb12 is investigated for the first time. The electronic band structure shows that EuOs4Sb12 presents a metallic character.

Fully numerical Hartree‐Fock and density functional calculations. II. Diatomic molecules

AbstractWe present the implementation of a variational finite element solver in the HelFEM program for benchmark calculations on diatomic systems. A basis set of the form is used, where (μ, ν, φ) are transformed prolate spheroidal coordinates, B n(μ) are finite element shape functions, and are spherical harmonics. The basis set allows for an arbitrary level of accuracy in calculations on diatomic molecules, which can be performed at present with either nonrelativistic Hartree‐Fock (HF) or density functional (DF) theory. Hundreds of DFs at the local spin density approximation (LDA), generalized gradient approximation (GGA), and the meta‐GGA level can be used through an interface with the Libxc library; meta‐GGA and hybrid DFs are not available in other fully numerical diatomic program packages. Finite electric fields are also supported in HelFEM, enabling access to electric properties. We introduce a powerful tool for adaptively choosing the basis set by using the core Hamiltonian as a proxy for its completeness. HelFEM and the novel basis set procedure are demonstrated by reproducing the restricted open‐shell HF limit energies of 68 diatomic molecules from the first to the fourth period with excellent agreement with literature values, despite requiring orders of magnitude fewer parameters for the wave function. Then, the electric properties of the BH and N2 molecules under finite field are studied, again yielding excellent agreement with previous HF limit values for energies, dipole moments, and dipole polarizabilities, again with much more compact wave functions than what were needed for the literature references. Finally, HF, LDA, GGA, and meta‐GGA calculations of the atomization energy of N2 are performed, demonstrating the superb accuracy of the present approach.

Fully numerical Hartree‐Fock and density functional calculations. I. Atoms

AbstractAlthough many programs have been published for fully numerical Hartree‐Fock (HF) or density functional (DF) calculations on atoms, we are not aware of any programs that support hybrid DFs, which are popular within the quantum chemistry community due to their better accuracy for many applications, or that can be used to calculate electric properties. Here, we present a variational atomic finite element solver in the HelFEM program suite that overcomes these limitations. A basis set of the type is used, where B n(r) are finite element shape functions and are spherical harmonics, which allows for an arbitrary level of accuracy. HelFEM supports nonrelativistic HF and DF calculations even with hybrid functionals, which are not available in other commonly available program packages. Hundreds of functionals at the local spin‐density approximation (LDA), generalized‐gradient approximation (GGA), as well as the meta‐GGA levels of theory are included through an interface with the Libxc library. Electric response properties are achievable via finite field calculations. We introduce an alternative grid that yields faster convergence to the complete basis set limit than commonly used alternatives. We also show that high‐order Lagrange interpolating polynomials yield the best convergence, and that excellent agreement with literature HF limit values for electric properties, such as static dipole polarizabilities, can be achieved with the present approach. Dipole moments and dipole polarizabilities at finite field are also reported with the PBE, PBEh, TPSS, and TPSSh functionals. Finally, we show that a recently published Gaussian basis set is able to reproduce absolute HF and DF energies of neutral atoms, cations, as well as anions within a few dozen microhartrees.

Longitudinal Spin Fluctuations and Magnetic Ordering Temperature in Metals: First-Principle Modeling and Phase Space Integration Measure

In magnetically ordered metals the magnitude of the local atomic moment become temperature dependent. To deal with this problem on the ab-initio level one need to employ a specific methodology for calculation of the electronic structure that takes into the account the magnetic disorder effects. In addition one needs to setup a special statistical models allowing simultaneously for ab-initio mapping and for the variation of the local spin magnitude. To this end here we discuss and employ methodology that is based on the Disordered Local Moment (DLM) formalism, spin-constraint Local Spin Density Approximation (LSDA) and Lichtenstein theorem for calculation of the inter-site exchange interactions. An extended classical Heisenberg Hamiltonian used for mapping allows for the variation of the lattice site spin magnitude. We consider here three representative canonical transition metals ferromagnets hcp Gd, bcc Fe and fcc Ni with quite a different character of the magnetic moment localization and illustrate the relative importance of the longitudinal spin fluctuations and the magnetic disorder induced electronic structure reconstruction. We use recently introduced linear measure [1] for integration over the longitudinal spin component in the classical configurational spin space.

Analysis of Optical, Electronic and Spectroscopic properties of (Biopolymer-SiC) Nanocomposites For Electronics Applications

In this paper, studying the effect of increase the number of SiC nanoparticles atoms on the optimized geometrical parameters, electronic and spectroscopic properties of polyvinyl alcohol. The studied structures are (PVA)(43Atom),(PVA-SiC)(35 Atom) and (PVA-SiC)(51Atom) nanocomposites. The optimization parameters included both bonds and angles. The electronic and structural properties included the (energy gap, cohesive energy, ionization potential, electron affinity, chemical hardness, chemical softness, electronegativity, electrophilicity and density of states) as well as spectral properties, which included IR and UV-Visible. This study uses Gaussian 0.9 program with help of Gaussian View 0.5 using density function theory (DFT) with local spin density approximation B3LYP level and6-31Gbasis sets. The results showed that the number of atoms has a direct impact on all the properties of the molecules studied. The increase of number of atoms are caused changes in spectral of (PVA)which include shift in some bonds and change in the intensities. These changes attributed to interactions of nanoparticles (SiC) with (PVA). Also, from Ultra Violet and Visible spectrum observed that absorption increases by increasing the number of atoms, this is due to the excitations of donor level electrons to the conduction band at these energies. The energy gap of the nanocomposites reduce from 6.8568 eV for (PVA) to5.0715 eV for (PVA-SiC)(35Atom) and 4.5330 eV for (PVA-SiC)(51Atom). The total energies decreases with the increase the number of atoms forming the nanocomposites. The results showed that the nanocomposites can be used for different applications such as: solar cells, diodes, transistors, sensors, electronics gates, electronics devices.

First Principle Mechanical and Thermodynamic Properties of Some TbX (X = S, Se) Compounds

The mechanical and thermodynamic results of some terbium chalcogenides, TbX (X = S, Se) compounds have been examined using full-potential linear augmented plane wave (FP-LAPW) method based on density functional theory. The local spin density approximation with Hubbard potential (LSDA+U) has been used for proper treatment of 4f-electrons of Tb. The magnetic moment of TbS and TbSe was calculated to be 5.44μB and 5.57μB, respectively. The ground state properties such as lattice parameter, bulk modulus, and pressure derivative of bulk modulus were obtained and found in good agreement with already available theory and experiment results. The mechanical properties such as Young modulus (E), shear modulus (G), Poisson ratio (υ), Cauchy pressure and B/G ratio were also predicted. Further, in thermodynamic properties, Debye temperatures (θD), specific heats at constant volume(CV), thermal expansion co-efficient (α), and entropies (S) and their behavior with high temperature(0–1200 K) and pressure (0–50 GPa) were also predicted.

Ab initio study of the effect of thickness and epitaxial strain on the magnetic structure of NiMn freestanding films

The magnetic properties of tetragonal structure of stoichiometric NiMn alloy is investigated using density functional theory within the local spin density approximation. The system studied here, is a free standing film. The effect of thickness and epitaxial strain on the magnetic and structural properties is examined. It is found that while the magnetic moments of Mn surface atoms vary depending on the number of layers being odd (3.60 μB) or even (3.55 μB) the magnitude of the magnetic moment for surface Ni atoms is constant (0.11 μB). By applying epitaxial strain on the slabs, it was observed, for the first time, that the magnetic phase of NiMn films changes from “A-type-like” ferrimagnetic for compressive strains to “G-type-like” ferrimagnetic for tensile strains.

(Fe,Sn)4N alloy as a model spin-glass system with short-range competing interactions on a nonfrustrated simple cubic lattice

The origin of the spin-glass state in (Fe,Sn)4N alloys is studied on the basis of a Heisenberg Hamiltonian with parameters derived from first principles within the magnetic force theorem applied in the framework of the disordered local moments method and local spin-density approximation. We show that in the alloy concentration range where the spin-glass state is stable only one Fe sublattice is intrinsically magnetic and the interatomic exchange magnetic interactions are essentially short ranged due to effects of chemical and magnetic disorder. The magnetic Fe atoms with well-localized spin moments are randomly distributed over the nongeometrically frustrated simple cubic lattice. The magnetic frustration, which generally is believed to be an essential ingredient of the spin-glass state formation condition, may occur only due to the competition of the two nearest-neighbor interactions. We thus argue that (Fe,Sn)4N is a rare example of a spin-glass system where the mechanism of spin-glass state formation might be studied in the framework of the minimal random-site model on a simple cubic lattice with competing interactions, while the effects of the geometrical frustration can be excluded.

Charge density functional plus U calculation of lacunar spinel GaM4Se8(M = Nb, Mo, Ta, and W)

Charge density functional plus U calculations are carried out to examine the validity of molecular Jeff = 1/2 and 3/2 states in lacunar spinel GaM4Se8 (M = Nb, Mo, Ta, and W). With LDA (spin-unpolarized local density approximation)+U, which has recently been suggested as a more desirable choice than LSDA (local spin density approximation)+U, we examine the band structure in comparison with the previous prediction based on the spin-polarized version of functional and with the prototypical Jeff = 1/2 material Sr2IrO4. It is found that the previously suggested Jeff = 1/2 and 3/2 band characters remain valid still in LDA+U calculations while the use of charge-only density causes some minor differences. Our result provides further support for the novel molecular Jeff state in this series of materials, which can hopefully motivate the future exploration toward its verification and the further search for new functionalities.

Роль 3d-электронной подсистемы в эволюции зонной структуры, магнитных и оптических свойств соединений ErNi-=SUB=-5-x-=/SUB=-Co-=SUB=-x-=/SUB=- (x=0-4)

AbstractThe evolution of the electronic structure and optical and magnetic properties of ErNi_5 – _ x Co_ x compounds ( x = 0–4), occurring upon substitution of nickel by cobalt atoms, has been investigated. Spin-polarization calculations of the band spectrum of these intermetallic compounds have been performed within the local spin density approximation with a correction for strong electron correlations in the 4 f shell of a rare-earth ion (method LSDA+U). The values of magnetic moments for erbium, nickel, and cobalt ions (located at different crystallographic sites) are obtained. The exchange-interaction parameters for the 3 d sublattice of transition metals are determined. The spectral properties of the compounds have been investigated in the wavelength range of 0.22–15 μm by optical ellipsometry. The experimental frequency dependences of the optical conductivity in the interband absorption region are compared with the corresponding characteristics calculated based on the density of electronic states.

Energetics and electronic structure of native point defects in antiferromagnetic CrN

We present a detailed analysis of the role of native point defects in the antiferromagnetic (AFM) phases of bulk chromium nitride (CrN). We perform first-principles calculations by using the local spin-density approximation, including local interaction effects $(\text{LSDA}+U)$, to study the two lowest energy AFM models expected to describe the low-temperature phase of the material. We study the formation energies, lattice deformations, and electronic and magnetic structure introduced by native point defects. We find that, as expected, nitrogen vacancies are the most likely defect present in the material at low temperatures. Nitrogen vacancies present different charged states in the cubic AFM model, exhibiting two transition energies, which could be measured by thermometry experiments and could help identify the AFM structure in a sample. These vacancies also result in partial spin polarization of the induced impurity band, which would have interesting consequences in experiments. Other point defects have also signature electronic and magnetic structure that could be identified in scanning probe experiments.

Comparative study of magnetic and electronic properties of room‐temperature polar magnets ScFeO3 and InFeO3

We performed calculations based on collinear and non‐collinear spin density functional theory to investigate magnetic and electronic properties of new room‐temperature polar magnetic compounds AFeO3 (A = Sc, In). Exchange and correlation effects were simulated by standard local spin density approximation and the generalized gradient approximation. We determined that the canted G‐type antiferromagnetic (G‐AFM) structure of the Fe spin moments is the most stable magnetic structure for both compounds, in accordance with experimental facts. The calculated Fe spin magnetic moments and resulting weak ferromagnetic components perpendicular to the c‐axis also agree well with the experiment. The magnitude of the Fe spin moment does not depend on the fact if the G‐AFM structure is collinear or canted. The energy difference between these two structures is found to be much larger for InFeO3, so as the exchange interaction constant Jnn between nearest‐neighbor magnetic Fe ions. The ScFeO3 and InFeO3 electronic structures, which reproduce well their magnetic properties are presented and discussed. By comparing them with the band structures of some typical multiferroics, we conclude that different electronic mechanisms should be responsible for their ferroelectric distortion. In ScFeO3, the cause of this distortion should be attributed to the hybridization between almost empty Sc 3d and occupied O 2p states along the hexagonal c‐axis. For InFeO3, however, the stereochemical activity of the In 5s localized electrons should be the principal driving force for ferroelectric distortion.

First-Principles Calculations to Investigate the Structural, Electronic, and Half-Metallic Properties of Ti2RhSn1-xSix, Ti2RhSn1-xGex, and Ti2RhGe1-xSix (x = 0, 0.25, 0.5, 0.75, and 1) Quaternary Heusler Alloys

The structural, electronic, and magnetic properties of Ti2RhSn1-xSix, Ti2RhSn1-xGex, and Ti2RhGe1-xSix (x = 0, 0.25, 0.5, 0.75, and 1) quaternary Heusler alloys have been investigated using first-principles density functional theory within generalized gradient approximation (GGA) and local spin-density approximation with Hubbard-U corrections (LSDA+U). The calculations have been performed using the full-potential linearized augmented Plane wave plus local orbitals method. The crystal structures of Ti2RhSn1-xSix, Ti2RhSn1-xGex, and Ti2RhGe1-xSix were found to be cubic in (F-43m) symmetry for x = 0, 1 and cubic in (P-43m) symmetry for x = 0.25, 0.75, while tetragonal in (P-4m2) symmetry for x = 0.5. The results show that all Ti2RhSn1-xSix, Ti2RhGe0.75Si0.25, and Ti2RhGe alloys are half-metallic ferrimagnetic. These new materials are good candidates for potential applications in spintronic.

Spin-orbital coupling and magnetic properties of Ir-based double perovskites with different 5dn (n = 3, 4, 5) states

We have investigated the spin and orbital moments of Ir-based double perovskites with 5dn (n = 3, 4, 5) states by local spin-density approximation with spin-orbital coupling and Hubbard correlation (LSDA+SOC+U). Our calculations reveal that the ratio of orbital to spin momentum Lz/Sz approaches to certain values for the double perovskites with different 5dn (n = 3, 4, 5) shell fillings. Based on d orbits, a spin-orbital coupling model with exchange splitting is proposed and it can well describe the ratio of angular momentums for the compounds. Our model calculations reveal that Lz/Sz is determined by the exchange splitting, spin-orbital coupling as well as the state of shell filling. Our model is well corroborated by the experiments and density-functional calculations.