Linear Muffin-tin Orbital Atomic-sphere Approximation Research Articles

The study of the electronic structure of [formula omitted

In this study, through theoretical and experimental analyses, we demonstrated that unlike most semiconductors, RuS2 has different indirect bandgaps, which makes it a distinct energy conversion and storage device. In our experimental work, we used the chemical vapor transport and solvent evaporation methods. We obtained RuS2 at a low temperature of 800°C with different stoichiometric shifts of sulfur, such as RuS2.00,RuS1.96, andRuS1.90. Moreover, we studied the correlation between the band structure of RuS2 calculated using the linear muffin-tin orbitals-atomic sphere approximation (LMTO-ASA) method and the growth parameters for each sample. We obtained some noteworthy values of indirect bandgap, such as, 1.84,1.72,1.42,and1.25eV,and direct transition, such as, 2.04,1.93,1.77,and1.49eV. The bandgap values obtained by LMTO-ASA are.1.80727,1.69614,1.46743,and1.23522eV.We obtained indirect bandgaps and a direct transition at the gamma point; their values are 2.04,1.94,1.77, and 1.49eV. We found that RuS2 has valence and conduction bands. The gap energy evaluated by LMTO-ASA was close to the value of that obtained through experimental measurements. We showed that band energy is insensitive to the lattice constant, a. Bandgaps depend on the method of preparation because they change with the temperature and structure (ν). Similar results were obtained for the effective mass. We found two phonons at X and M points, as well as the probability of the existence of two indirect transitions to the bandgap. To the best of our knowledge, this is the first study to confirm that the position of sulfur and S–S distance significantly affect the bandgap.

Magnetic properties of FexCo1−x nanochains on Pt(1 1 1) surfaces

The magnetic properties of FexCo1−x nanochains on Pt(1 1 1) were studied using the first-principles real-space linear muffin-tin orbital-atomic sphere approximation (RS-LMTO-ASA) method within the density functional theory. The relative amounts of Fe and Co atoms in a chosen nanochain were varied and several possible arrangements of the atomic species were taken into account. The results of the exchange interaction demonstrates ferromagnetic coupling for the nanowires. Our calculations of Fe and Co average magnetic moments reveal a large enhancement of both spin and orbital moments compared to Fe–Co films deposited on a Pt(1 1 1) surface. The trend for the orbital moments with respect to stoichiometry differs from all previous higher-dimensional Fe–Co alloys on Pt(1 1 1) studies.

V1+xNb1–xIrB2 (x ≈ 0.1), The First Quaternary Metal‐Rich ­Boride Adopting The Mo2IrB2‐Type Structure: Synthesis, Crystal and Electronic Structure and Bonding Analysis

AbstractPolycrystalline samples and single crystals of the new metal‐rich boride V1+xNb1–xIrB2 (x ≈ 0.1), were synthesized by arc‐melting the elements in a water‐cooled copper crucible under an argon atmosphere and characterized by X‐ray diffraction and EDX measurements. The crystal structure was refined on the basis of single crystal data. The new phase adopts the Mo2IrB2‐type structure (space group Pnnm, no. 58) with the lattice parameters a = 7.301(7) Å, b = 9.388(9) Å and c = 3.206(5) Å. It is the first quaternary representative of Mo2IrB2‐type structure. The structure contains zigzag B4‐fragments with boron–boron distances of 1.83–1.85 Å. The electronic density of states and crystal orbital Hamilton population (for bonding analysis) were calculated, using the linear muffin‐tin orbital atomic sphere approximation (LMTO‐ASA) method. According to these calculations, this metal‐rich compound should be metallic, as expected. Furthermore, very strong boron–boron interactions are observed in the zigzag B4‐fragment and two significantly different Ir–B interactions are observed in the new phase and the prototype Mo2IrB2.

The complex metal-rich boride Ti1+xRh2−x+yIr3−yB3 (x=0.68, y=1.06) with a new structure type containing B4 zigzag fragments: Synthesis, crystal chemistry and theoretical calculations

Polycrystalline samples and single crystals of the new complex boride Ti1+xRh2−x+yIr3−yB3 (x=0.68; y=1.06) were synthesized by arc-melting the elements in a water-cooled copper crucible under an argon atmosphere and characterized by X-Ray diffraction as well as EDX measurements. The crystal structure was refined on the basis of single crystal data. The new phase, which represents a new structure type containing trans zigzag B4 fragments as well as isolated boron atoms crystallizes in the orthorhombic space group Pbam (Nr. 55) with the lattice parameters a=8.620(1)Å, b=14.995(2)Å and c=3.234(1)Å. First-principles density functional theory calculations using the Vienna ab-initio simulation package (VASP) were performed on an appropriate structural model (using a supercell approach) and the experimental crystallographic data could be reproduced accurately. Based on this model, the density of states and crystal orbital Hamilton population (for bonding analysis) were calculated, using the linear muffin-tin orbital atomic sphere approximation (LMTO-ASA) method. According to these calculations, this metal-rich compound should be metallic, as expected. Furthermore, very strong boron–boron interactions are observed in the trans zigzag B4 fragment, which induce a clear differentiation of two types of metal–boron contacts with different strength. The observed three-dimensional metal–metal interaction is in good agreement with the predicted metallic behavior.

Anisotropy analysis of surface energy and prediction of surface segregation for fcc metals

In the atomic scale, the surface energy anisotropy analysis of 38 surface planes of 10 fcc metals Cu, Ag, Au, Ni, Pd, Pt, Rh, Al, Ir and Pb have been simulated by using the elemental variables φ* and nWS and modified analytical embedded-atom method (MAEAM). The results show that the close-packed surface (111) of fcc metals which have the lowest surface energies will grow preferentially, the surface energies for all the other surface planes increase linearly with cosθ(hkl), where cosθ(hkl) are the angles between the surface planes (hkl) and (111), which is consistent with the experimental and the linear-muffin-tin-orbital atomic-sphere approximation (LMTO－ASA) results. A graphical approach which correctly explains the relation of the surface segregation energy and surface energy is employed. We conclude that the surface segregation takes place or not is mainly determined by the rule that an impurity (solute) with lower surface energy will segregate to the surface of the host (solution) with higher surface energy.

Self-interaction correction in multiple scattering theory: application to transition metaloxides

We apply to transition metal monoxides the self-interaction corrected (SIC) local spindensity approximation, implemented locally in the multiple scattering theory within theKorringa–Kohn–Rostoker (KKR) band structure method. The calculated electronicstructure and in particular magnetic moments and energy gaps are discussed in reference tothe earlier SIC results obtained within the linear muffin-tin orbital atomic sphereapproximation band structure method, involving transformations between Bloch andWannier representations, in order to solve the eigenvalue problem and calculate the SICcharge and potential. Since the KKR method can be easily extended to treat disorderedalloys, by invoking the coherent potential approximation (CPA), in this paper we comparethe CPA approach and supercell calculations to study the electronic structure of NiO withcation vacancies.

LSDA+U APPROXIMATION-BASED ANALYSIS OF THE ELECTRONIC STRUCTURE OF CeFeGe3

We perform ab initio electronic structure calculations of the intermetallic compound CeFeGe 3 by means of the Tight Binding Linear Muffin-Tin Orbitals-Atomic Sphere Approximation (TB-LMTO-ASA) within the Local Spin Density Approximation containing the so-called Hubbard correction term ( LSDA+U SIC), using the Stuttgart's TB (Tight Binding)-LMTO-ASA code in the framework of the Density Functional Theory (DFT). At the LSDA-vBH level, our electronic structure calculations indicate an intermediate valence character compound whereas a DOS (density of states) analysis favors its metallic nature. We also study electronic correlation effects via the introduction of the U parameter on Ce and Fe , where a metallic and magnetic characters for this compound are found, along with certain features that may possibly be associated with a heavy fermion system.

Universal tight-binding model for transition metals: from bulk to cluster

A universal orthogonal tight-binding (TB) model is developed for transition metals, withspecial emphasis on spin-polarized self-consistent calculations. The parameters of the TBmodel are directly obtained from the ab initio bulk calculations within the linear muffin-tinorbital atomic sphere approximation method, without any fitting. With the environmentaldependence of the Hamiltonian included in the localized structure constants, the TB modelcan be reliably applied to various situations, from periodic bulk structures to smallclusters, for both pure metals and alloys. We have tested the model in spin-polarizedcalculations against ab initio results for small computational cell systems, andapplied the model to study the spin and orbital magnetism of some large clusters.

Investigation of the Hume–Rothery stabilization mechanism from ab initio band calculations for different electron compounds: Cu 5Zn 8 and Al–Mg–Zn, Al–Cu–Ru–Si approximants

The origin of the pseudogap formed across the Fermi level has been studied in relation to the Fermi surface–Brillouin zone interaction and the sp–d hybridization effect by performing the LMTO–ASA (Linear Muffin–Tin Orbital–Atomic Sphere Approximation) band calculations for the three electron compounds: the γ-phase Cu 5Zn 8 alloy, the free-electron-like Al 30Mg 40Zn 30 Frank–Kasper-type 1/1-1/1-1/1 approximant and the Al 68Cu 7Ru 17Si 8 Mackay–Icosahedral-type 1/1-1/1-1/1 approximant. We revealed that, in the free-electron-like Al–Mg–Zn approximant, the Fermi surface–Brillouin zone interaction is solely responsible for the formation of the pseudogap at the Fermi level. In the case of the γ-phase and the Al–Cu–Ru–Si approximant, where d-states are involved across the Fermi level, we have demonstrated that the Fermi surface–Brillouin zone interaction is still strong enough to produce the pseudogap near the Fermi level but that its depth and width are substantially enhanced by the sp–d hybridization effect which splits the d-states into the bonding and antibonding states across the Fermi level.

All-electron and pseudopotential study of the spin-polarization of the V(001) surface: LDA versus GGA

The spin-polarization at the V(001) surface has been studied by using different local [local spin-density approximation (LSDA)] and semilocal [generalized gradient approximation (GGA]) approximations to the exchange-correlation potential of DFT within two ab initio methods: the all-electron tight-binding linear muffin-tin orbital atomic-sphere approximation and the pseudopotential linear combination of atomic orbitals code SIESTA (Spanish initiative for electronic simulations with thousands of atoms). A comparative analysis is performed first for the bulk and then for a N-layer V(001) film $(7<~N<~15).$ The LSDA approximation leads to a nonmagnetic V(001) surface with both theoretical models in agreement (disagreement) with magneto-optical Kerr (electron-capture spectroscopy) experiments. The GGA within the pseudopotential method needs thicker slabs than the LSDA to yield zero moment at the central layer, giving a high surface magnetization (1.70 Bohr magnetons), in contrast with the nonmagnetic solution obtained by means of the all-electron code.

Density functional theory analysis of the local chemical bonds in the periodic tantalum dichalcogenides TaX2 (X=S, Se, Te)

The electronic structure of layered tantalum dichalcogenides 1T-TaX2 (X=S, Se, Te) have been studied both with the linear muffin tin orbitals-atomic sphere approximation (LMTO-ASA) and the Amsterdam density functional for band (ADF-band) programs. The first code (LMTO) provides band structures, density of states (DOS), and crystal orbitals Hamiltonian populations (COHP) while the second one allows accurate atomic charge calculations by means of a powerful electron density numerical integration. All those analyses were used to rationalize the electronic structures of the three 1T-TaX2 phases, in particular to enlighten the 13×13 structural modulations observed in TaS2 and TaSe2, and to put forward the influence of the local chemical Ta–Te bonds on the relative stability of the 1T-TaTe2 phase vs the distorted monoclinical one. The indirect overlap between the two bands responsible for the metallic properties of TaS2 and TaSe2 has been shown to significantly increase the tantalum d electron count compared to its formal value (d1) leading to a more realistic occupation of the threefolded t2g-like bands involved in the 13×13 instability. Owing to the low electronegative character of Te compared to S and Se, the direct overlap occurring at the Fermi level results in an electron transfer from local Ta–Te bonding states to local Ta–Te antibonding ones yielding a destabilization of the metal–chalcogen bonds.

Screened exact exchange functional calculations of the spin-wave dispersion in transition metals

The screened “exact” nonlocal exchange approach is applied to the calculation of magnetic susceptibility of transition metals, particularly to the calculation of spin-wave dispersion. We discuss the choice of an appropriate approach and aspects of linear muffin-tin orbital-atomic sphere approximation (ASA) calculations, in particular calculation of the Green function in the three-center approximation. We calculated transverse spin-wave frequencies in Fe with the local density approximation and with the nonlocal functional using both the “frozen” magnon method and calculation of bound state energies (corresponding to spin-wave excitations).

Influence of spin-dependent scattering at rough interface on the giant magnetoresistance in magnetic multilayers

Spin-dependent scattering in Co/Cu and Co/Ru multilayers with interface roughness is estimated. The electronic band structures of Co/Cu and Co/Ru multilayers with smooth interfaces for parallel and antiparallel spin configurations are calculated self-consistently using linear muffin-tin orbital atomic sphere approximation (LMTO-ASA) band structure calculations based on density functional theory. The spin-dependent scattering caused by interface roughness is calculated from the transition probabilities obtained from self-consistent wave functions and potential differences found from these band structure calculations. Our results indicate that spin-dependent scattering caused by interface roughness plays an important role in the giant magnetoresistance (GMR) effect for magnetic multilayers, as predicted by calculations using a simple model. Furthermore, it is concluded that the scattering depends on differences in the band structure for different spin configurations, as well as for interface roughness. The GMR difference between Co/Cu and Co/Ru can be explained qualitatively by difference in spin-dependent scattering due to interface roughness, which is caused by the band structure with different spin configuration.

Band structure and electronic properties of the incommensurate misfit compound

The electronic properties of the incommensurate misfit compound are investigated by different methods. Linear muffin-tin orbital atomic-sphere approximation band-structure calculation shows that this material should be a metal with a Fermi level located near a minimum of the density of states. Experimentally, the electrical resistivity is hopping-like while the magnetic susceptibility is metallic-like with a strong enhancement. We suggest that these paradoxical properties can be reconciled by taking into account the effects of incommensurability and electronic correlations. A comparison with commensurate and with Ti or Cr incommensurate misfit derivatives is discussed.

Full-potential linear-muffin-tin orbital atomic-sphere approximation calculations of the electric field gradient in HCP metals

In this paper we suggest a new scheme for obtaining electronic contribution to the electric field gradient (EFG) at the nucleus. The EFG for all HCP metals from Be to Cd is obtained from band structure calculations using the recently developed scheme of full-potential (FP) linear-muffin-tin-orbital (LMTO) formalism in the atomic-sphere approximation (ASA). This method is computationally very effective. A comparison with the theoretically most accurate full-potential linear augmented plane wave calculations and experimental values shows very good agreement. According to the investigations presented in this paper, FP–LMTO–ASA method is probably the best choice presently for the calculation of the EFG in more complicated compounds.

Electronic properties of the yttriumdicarbide superconductorsYC2,Y1−xThxC2,Y1−xCaxC2(0<x<~0.3)

We characterize the superconducting state of the carbides YC${}_{2},$ ${\mathrm{Y}}_{1\ensuremath{-}x}{\mathrm{Th}}_{x}{\mathrm{C}}_{2},$ and ${\mathrm{Y}}_{1\ensuremath{-}x}{\mathrm{Ca}}_{x}{\mathrm{C}}_{2}$ $(0lxl~0.3)$ by means of magnetization and specific-heat measurements. ${\mathrm{YC}}_{2}$ is a superconductor with ${T}_{c}=4.02(5)$ K. Partial substitution with Ca and Th, as well as doping with the strongly pair breaking Gd, reduces the critical temperature. Isothermal magnetization measurements on ${\mathrm{YC}}_{2}$ indicate a superconducting behavior close to the type-I limit with ${B}_{c2}(0)=59(2)$ mT. Specific-heat data of ${\mathrm{YC}}_{2},$ ${\mathrm{Y}}_{0.8}{\mathrm{Th}}_{0.2}{\mathrm{C}}_{2}$, and ${\mathrm{Y}}_{0.9}{\mathrm{Ca}}_{0.1}{\mathrm{C}}_{2}$ are analyzed in terms of weak-coupling BCS theory and the $\ensuremath{\alpha}$ model. The comparison with the model predictions as well as the ${}^{12}\mathrm{C}{/}^{13}$C-isotope effect on ${T}_{c}$ indicate excellent agreement with weak-coupling BCS theory for YC${}_{2}$. A strong dependence of the superconducting properties on the carbon deficiency is observed. We describe high-temperature annealing procedures to optimize the superconducting properties of the samples. Ab initio calculations of the electronic band structure using the tight-binding linear muffin-tin orbital atomic-sphere approximation method are presented and the density of states at the Fermi energy is discussed in view of the experimental Pauli susceptibilities and heat-capacity results.

Calculation of theoretical strength of solids by linear muffin-tin orbitals (LMTO) method

Ab initio calculation of theoretical strength of Si, Na, W, Cu and Ir cubic crystals under three-axial tension is performed using the linear muffin-tin orbitals-atomic-sphere approximation (LMTO-ASA) method. This method is particularly effective in case of isotropic deformation modes and can be applied by means of advanced personal computers. Computed values are compared with those obtained previously by other methods. The results obtained verify some semi-empirical pair and many-body potential approximations.

Correlation energy functionals for ab initio calculations: Application to transition metals.

The method for including strong correlation effects in self-consistent electronic structure calculations in solids is presented. On the basis of the mean-field-type (Gutzwiller-type) approximation the correlation energy functional is obtained, which depends on the partial electron density. In turn, the variation of this functional yields the nonlocal effective potential. Two possible variational procedures are tested: variation of the functional over occupation numbers only and over both occupation numbers and linear-muffin-tin-orbital--atomic-sphere-approximation (LMTO-ASA) wave function. The LMTO-ASA calculations for a number of 3d and 4d transition metals are carried out. The results are compared with those for density-functional calculations. Quantitative estimates of the correlation renormalization factor for the bandwidth ``dynamical'' narrowing in these metals are presented.

The electronic structures of the pyrite-type disulphides (MS2, where M = Mn, Fe, Co, Ni, Cu, Zn) and the bulk properties of pyrite from local density approximation (LDA) band structure calculations

A local density approximation (LDA) band structure method, the Linear Muffin-Tin Orbital Atomic Sphere Approximation (LMTO-ASA) method has been used to calculate the electronic structures of the pyrite-type disulphides (MS2, where M = Mn, Fe, Co, Ni, Cu, Zn). The total density of states has been calculated for 10 eV above and below the Fermi Level, along with the separate contributions from metal and sulphur and shows that the metal d band occurs above the sulphur p bands in MnS2, FeS2, CoS2 and NiS2, whereas in CuS2, the d band passes through the sulphur p band and in ZnS2, it lies below the sulphur p band. Substantial hybridization of the metal d states with the sulphur states occurs. FeS2 is calculated to be a semiconductor with a direct band gap of 0.64 eV in good agreement with experiment. The calculated local densities of states have been used in turn to calculate X-ray photoelectron spectra and Bremsstrahlung Isochromat spectra for this series of compounds, and these also show reasonable agreement with experimental data. A particular strength of the LMTO-ASA method is the ability to calculate and predict certain bulk properties of solids of interest in mineral physics. This has enabled the first reasonably accurate calculations of the total energy of the valence electrons of the system for pyrite (FeS2), given as — 345.885 rydbergs per unit cell, and the equilibrium unit cell volume which is within 3.3% of that determined experimentally. A theoretical pressure vs. volume curve for pyrite was also calculated along with values for the bulk modulus. However, our calculations predict a bulk modulus of 6.75 Mbar which is too high by a factor of 4.6 due to the simplifying assumption of a uniform scaling of interatomic distances on compression.

First-principles linear muffin-tin orbital atomic-sphere approximation calculations in real space.

We have developed an approach, based on the linear muffin-tin orbital atomic-sphere approximation (LMTO-ASA) formalism and the recursion method, that allows us to perform first-principles density-functional calculations of electronic structure in real space. Using ${\mathrm{Zr}}_{2}$Fe as a test case, we compare our results with those obtained by using the standard reciprocal-space LMTO-ASA method. The scheme described here can be applied to nonperiodic systems and is also very useful in the study of complex metallic systems with a large number of atoms per unit cell. To illustrate the application of the first-principles real-space approach to a complex system, we calculate the electronic structure for a cluster of amorphous Zr. We use our results to evaluate the distribution of charge transfer among the sites of this randomly packed system. As a first guess we take the potential parameters to be the same for all atoms. The final self-consistent results show charge transfers that are almost an order of magnitude smaller than the ones obtained in the initial run. The major effect of the self-consistent process in this case is to rearrange the center of the bands in order to screen large charge fluctuations. This explains why the parametrized LMTO-ASA approach, where the relative position of the bands is fixed using approximate charge neutrality, works so well when applied to transition-metal alloys.