Full-potential Linear Muffin-tin Orbital Method Research Articles

First-principles study of solid iodine and bromine under high pressure

Experiments on solid iodine under pressure are reviewed. Theoretical results obtained on solid iodine by the full-potential linear muffin-tin orbital method are reviewed. The pressure dependences of the Raman-active modes of molecular solid bromine have been studied theoretically by the same method. The calculated results reproduce recent experimental results very well. After metallization, the softening of frequencies that is seen is not the same as in experiments. This contrasts with the case for solid iodine.

A real-space full-potential localized LMTO method for non-collinear magnetism

Abstract We have implemented the localized full-potential linear muffin-tin orbital (LMTO) method in real-space representation suitable for the study of noncollinear magnetism. In particular. we show (1) how to obtain a full potential localized basis-set, (2) how to construct the Hamiltonian and the overlap matrix elements, (3) the use of the recursion method to obtain the Greens function, (4) the calculation of the charge density, and (5) the solution of the Poisson equation to compute the Coulomb potential.

Electronic structure of crystalline FeSi and FeGe

X-Ray photoelectron valence band spectra were recorded for stoichiometric compounds FeSi and FeGe; their electronic structure was calculated by the ab initio full-potential linear muffintin orbital method. For nonmagnetic FeSi, good agreement was obtained between the calculated densities of states and the X-ray photoelectron spectrum in both peak positions and forbidden gap. For FeGe, which is an antiferromagnet, the nonmagnetic calculation yields worse agreement with experiment and explicitly indicates that the paramagnetic phase is unstable. In both compounds, the calculation gives a high degree of d—p hybridization and covalence, which is estimated quantitatively. In FeGe, the degree of covalence of the Fe-Fe bond is higher than that of the Fe-Ge bond.

Ab initio LMTO calculation of surface localized and resonance states of the SiC(110) zinc-blende surface

The surface localized and resonance states of unrelaxed and relaxed SiC(110) zinc-blende surfaces have been investigated within the local density approximation of density functional theory (DFT), employing a first-principles full-potential self-consistent linear muffin-tin orbital (LMTO) method using a thicker (thirteen layer) slab. Intrinsic surface states appear in the fundamental energy gap for the unrelaxed surface. We have allowed non-bond-length-conserving relaxation of the surface atoms to obtain the minimum energy configuration. The shift in the surface states arising from the relaxation of atoms is small in comparison to other studied heteropolar covalent semiconductors because of the smaller value of the relaxation angle of the SiC(110) surface. New surface states have been predicted in the valence band region. The present atomic geometry is somewhat different from other theoretical results. We observe an inward displacement of the Si cation and an outward displacement of the C anion both parallel and perpendicular to the surface, different from the results of Sabisch et al. No experimental data are available for comparison.

Effect of generalized gradient corrections on lanthanide cohesive properties

We have calculated ab initio, using a full-potential linear muffin-tin orbital method, the equilibrium volumes, the bulk moduli, and the generalized cohesive energies for the entire lanthanide series and barium. Two different approximations to the density functional (DF) were compared: the local (spin) density approximation in the Hedin–Lundqvist parameterization (LDA) and the recently developed Perdew–Wang generalized gradient corrected functional, called the generalized gradient approximation (GGA). We find that GGA corrects most of the overbonding tendency of LDA. An interesting consequence of this is that, for the early lanthanides, the LDA results appear to agree better with experimental results than do the GGA results. In this case, we conclude that the 4f states probably have a significant impact on the equilibrium volumes of the early lanthanides. Both for the bulk moduli and for the generalized cohesive energies, we find that GGA generally, in comparison to LDA, gives results in closer agreement with experiment over the entire lanthanide series.

Calculated optical properties of a solar energy material: CuGaS 2

The full-potential linear muffin-tin orbital method is used to study the electronic structure of the solar energy material CuGaS 2. From this the optical reflectivity is calculated and compared with experiment, both for the case when the polarization vector of the light, E is parallel to the c-axis and when E is perpendicular to the c -axis. An encouraging agreement with experiment is obtained.

Total energy differences between SiC polytypes revisited

The total energy differences between various SiC polytypes (3C, 6H, 4H, 2H, 15R and 9R) were calculated using the full-potential linear muffin-tin orbital method using the Perdew-Wang-(91) generalized gradient approximation to the exchange-correlation functional in the density functional method. Numerical convergence versus k-point sampling and basis set completeness are demonstrated to be better than 1 meV/atom. The parameters of several generalized anisotropic next-nearest-neighbor Ising models are extracted and their significance and consequences for epitaxial growth are discussed.

First-principles calculation of electron surface states of the zinc-blende GaN(110) surface

The surface electronic structure for an unrelaxed as well as relaxed zinc-blende GaN(110) surface has been investigated within the local density approximation of density functional theory, employing a first-principles full-potential self-consistent linear muffin-tin orbital (LMTO) method and a supercell approach. Intrinsic surface states appear in the fundamental energy gap for an unrelaxed surface. These intrinsic gap surface states shift towards the bulk valence and the conduction band region on considering the relaxation of the surface atoms. Orbitals characters of surface states at Γ, X, M, X″ symmetry points have been identified. Several localized and resonance states are predicted for the first time in different energy regions.

Scaling of theL2,3circular magnetic x-ray dichroism of Fe nitrides

We have implemented the calculation of the x-ray-absorption cross section for left- and right-circularly polarized x-ray beams within the local-density approximation by means of our all-electron full-relativistic and spin-polarized full-potential linear muffin-tin orbital method. We show that the ${L}_{2,3}$ circular magnetic x-ray dichroism of Fe, ${\mathrm{Fe}}_{3}\mathrm{N},$ and ${\mathrm{Fe}}_{4}\mathrm{N}$ compounds scales to a single curve when divided by the local magnetic moment. Sum rules determine the spin and orbital magnetic moment of iron atoms in these ordered iron nitrides.

Valence degeneracy in cerium systems

We have calculated the generalized cohesive energies for Ce-systems, using a full-potential linear muffin-tin orbital method and a generalized gradient-corrected density functional. Together with atomic coupling energies for cerium, the generalized cohesive energies are used to study the valence configuration degeneracy in CeA1 2 and CeX ( X = N, P, As, Sb, Bi). We find that in all these systems, the f 1 and f 0 configurations are far from being degenerate.

Full-potential optical calculations of lead chalcogenides

We report on ab initio calculations of the optical properties of the lead chalcogenides PbS, PbSe, and PbTe performed with a relativistic full-potential linear muffin-tin orbital method within the local density approximation. Our calculated spectra are in excellent agreement with recent ellipsometry measurements. The origin of the peaks in the spectra is discussed, as well as the effects of increasing the chalcogen atomic number. Q 1998 John Wiley & Sons, Inc. Int J Quant Chem 69: 349)358, 1998

Relativistic effects on the structural phase stability of molybdenum

The body-centered cubic-face-centered cubic (bcc-fcc) structural phase stability of molybdenum (Mo) is studied as a function of volume with both nonrelativistic and scalar-relativistic linear combinations of Gaussian-type orbitals-fitting functions (LCGTO-FF) calculations. It is demonstrated that relativity has a significant, albeit small effect, on the bcc-fcc structural energy difference, which increases with pressure. The scalar-relativistic structural energy difference curve is shown to be in excellent agreement with an earlier scalar-relativistic calculation using the full-potential linear muffin-tin orbital (FP-LMTO) method, clearly demonstrating the ability of the scalar-relativistic LCGTO-FF method to resolve an extremely subtle relativistic effect. It is argued that relativity will tend to delay pressure-induced structural phase transitions that are triggered by electron band reordering.

Phase stability and grain boundary structure in the Cu-Bi system

Cu-Bi system is a model system for studies of interfacial phenomena, such as segregation and segregation induced faceting. In previous studies it was found that there is a strong preference for Σ = 3}111{-}111{ type facets, and their atomic structure was successfully resolved by combining high-resolution electron microscopy and computer simulation using Finnis-Sinclair type interatomic potential. The resolved grain-boundary structure was examined usingab initio full-potential linear muffin-tin orbital method by calculating formation enthalpies of several (hypothetical) Cu-Bi compounds under pressure. It was found that there is no driving force for the ordered alloy formed at the boundary to grow into a three-dimensional phase and thus specific interfacial phases are formed in this system. The range of applicability of Finnis-Sinclair potential used in the previous studies was also investigated by comparison withab initio calculations, and it was shown that the potential is entirely appropriate when Cu concentration is higher than about 66 at. %. In those cases the Cu-Bi system exhibits metallic behavior.

Electronic and optical properties of InP

We have calculated the band structure and dielectric function of InP by means of an accurate first-principles method using the full potential linear muffin-tin orbital method (FPLMTO). Our calculated dielectric functions is compared with the recent data of Herzinger et al. [ J. Appl. Phys., 77, 1995, 1715]. The calculated dielectric function is in good agreement with the recent data. The calculated band gap is smaller by about 0.7 eV in comparison with experimental gap.

LDA simulations of pressure-induced anomalies inc/aand electric-field gradients for Zn and Cd

We present results of ab initio simulations of the effect of hydrostatic pressure on the electronic structure, lattice parameters, and electric-field gradients (EFG) for hcp Zn and Cd using the full-potential linear muffin-tin orbital method in conjunction with the new Perdew-Burke-Ernzerhof generalized gradient approximation (GGA) to the density functional for exchange correlation. Theoretical equilibrium volumes for Zn and Cd are found to be in excellent agreement with experiment (whereas non-GGA corrected local density approximation underestimates them by as much as 10%). We find an anomaly in the pressure dependence of $c/a$ at reduced unit cell volumes (at ${V/V}_{0}\ensuremath{\simeq}0.89$ for Zn and in a broad region from ${V/V}_{0}=0.92$ to 0.85 for Cd) and a similar anomaly in the EFG tensor. At the same time we do not find the electronic topological transition due to the destruction of a giant Kohn anomaly which was previously thought to be responsible for the lattice anomalies in Zn.

Optical properties of monoclinic SnI2from relativistic first-principles theory

Within the local-density approximation, using the relativistic full-potential linear muffin-tin orbital method, the electronic structure is calculated for the anisotropic, layered material SnI${}_{2}$. The direct interband transitions are calculated using the full electric-dipole matrix elements between the Kohn-Sham eigenvalues in the ground state of the system. The inclusion of spin-orbit coupling was found to change the optical properties of this material considerably. Polarized absorption and reflection spectra are calculated and compared with recent experimental results. The experimentally suggested cationic excitation for the lowest-energy transition is confirmed. From the site and angular momentum decomposed electronic structure studies and the detailed analysis of the optical spectra it is found that the lowest-energy transition is taking place between Sn $5s$ (atom type 2a) $\ensuremath{\rightarrow}$ Sn $5p$ (atom type 4i) states. The ground state calculation was repeated using the tight-binding linear muffin-tin orbital--atomic sphere approximation method, and the resulting band structure agrees very well with the one calculated with the full-potential method. In contrast to recent experimental expectations, our calculations show an indirect band gap, which is in agreement with earlier semiempirical tight-binding calculations as well as with absorption and reflection spectra.

Behavior of p-type dopants in HgCdTe

Obtaining high concentrations of active p-type dopants in HgCdTe is an issue of much current interest. We discuss the results of our calculations on column IB and VA dopants. The full-potential linear muffin-tin orbital method, based on the local density approximation is used to calculate electronic total energies and localized levels in the band gap. Free energies are predicted and incorporated into a thermodynamical model to calculate impurity and native defect concentrations as a function of temperature, stoichiometry, and total impurity density. Copper, silver, and gold are found to be incorporated nearly exclusively on the metal sublattice and to be 100% active for all near-equilibrium growth and processing conditions. The density of interstitial copper is high enough to impact copper diffusion. In contrast, significant concentrations of phosphorus, arsenic, and antimony are found on the metal sublattice where they behave as n-type dopants, accounting for highly compensated, or even n-type material, depending on the equilibration temperature and equivalent mercury partial pressure.

Directional bonding and asymmetry of interfacial structure in intermetallic TiAl: Combined theoretical and electron microscopy study

Abstract An ordered twin boundary in the L10 TiAl alloy has been investigated by computer modelling and high-resolution electron microscopy (HREM). This combined study demonstrates the significance of directional covalent-type bonding for the interfacial structure. When using the ab-initio full-potential linear muffin-tin orbital method the stable structure exhibits a pronounced asymmetry. This structure is in an excellent agreement with HREM observations. In contrast, calculations employing central-force many-body potentials predict a structure the asymmetry of which is so small that it is not discernible in the HREM image. The preference for the strongly asymmetric structure can be understood in terms of the covalent d-type bonding between Ti atoms in this alloy.

First-principles investigation ofReO3sand related oxides

Electronic-structure calculations are performed, within the local-density approximation to the density-functional theory, using the full-potential linear muffin-tin orbital method to understand the relation between structural and electronic properties of ${\mathrm{ReO}}_{3}$ , ${\mathrm{WO}}_{3}$ , and the stoichiometric tungsten bronze ${\mathrm{NaWO}}_{3}$ . Energy changes associated with small deformations from the cubic phase indicate that ${\mathrm{ReO}}_{3}$ and the tungsten bronze are stable when cubic while the W ion in ${\mathrm{WO}}_{3}$ shows a tendency to off-center displacements. The different behavior is explained by examining the band structure of the compounds. Calculated frequencies and eigenvectors of \ensuremath{\Gamma} phonons in ${\mathrm{ReO}}_{3}$ corroborate the existence of high-frequency modes in this crystal which supports a recent theoretical proposal for the interpretation of its electrical resistivity.

Optical properties of graphite from first-principles calculations

We present theoretical results for the frequency-dependent dielectric response, both for the electric field parallel and perpendicular to the c axis of graphite. The calculations are performed using a full-potential linear muffin-tin orbital method. Our calculations show fair agreement with experimental data and the different features observed are identified from interband transitions in various regions of the Brillouin zone. The anisotropy of the dielectric function is discussed in detail and shown to be due to the difference in the optical matrix elements for the two different polarizations, which is a result of the anisotropic crystallographic and electronic properties of graphite.