Multiple-scattering Method Research Articles

The resonance-inducedin-gap modes in photonic crystals composed of metal-coateddielectric spheres

Using the vector wave multiple-scattering method we show that asizable photonic band gap and resonance-induced in-gap modes can berealized simultaneously in crystals composed of metal-coateddielectric spheres. The position and width of the in-gap modes canbe controlled by making appropriate choices of the dielectricconstant of the inner dielectric sphere and thickness of the metal-coating layer, respectively. Such properties can be very useful inmaking optical band filters as well as microcavity lasers if theyare sustained in the presence of dissipation. The calculatedtransmission spectra suggest that the resonance-induced in-gap modesdecrease in intensity with dissipation and exist only for goodmetals.

Screened real-space Korringa-Kohn-Rostoker description ofthe relativistic and magnetic properties of transition metals

The use of the relativistic and spin-polarized real-spaceKorringa-Kohn-Rostoker (KKR) method is limited to small systems(less than 100 atoms). This is due to the prohibitively largeCPU times needed for the inversion of the KKR matrix. To studysystems of more than a thousand atoms, we have implemented theconcept of a screened reference medium, within the fullyrelativistic spin-polarized version of the real-space locallyself-consistent multiple-scattering method (LSMS). The LSMS methodmakes use of a local interaction zone (LIZ) for solving the quantummechanical problem, while the Poisson equation is solved in thewhole space. The screened reference medium gives rise to sparseKKR matrices and using state-of-the-art sparse matrix technology asubstantial reduction in the CPU times is obtained, enablingapplications of the method to systems whose LIZ consists of morethan a thousand atoms. The method is benchmarked by application tothe elemental transition metals, the fcc (face-centred-cubic) Co andNi, and the bcc (body-centred cubic) Fe, and compared to the resultsof the conventional k-space methods. The convergence inreal space of the magnetic moments, the magnetocrystallineanisotropy energy and the orbital moment anisotropy is discussed indetail.

Adsorption and Thermal Decomposition of SiH4 on Cu (111) by Multiple-Scattering Cluster Method

Adsorption and thermal decomposition of SiH4 on Cu (111) at 110 K has been studied by the calculation of silicon K-edge near edge x-ray absorption fine structure spectra using multiple-scattering cluster method. It is found that the cleavage of Si-H bond yields SiH3 species adsorbed on the fcc 3-fold hollow sites on Cu (111) surface at 110 K.

Calculating properties with the polymorphous coherent-potential approximation

The formulas for calculating properties of an alloy such as the density of states, the charge density, and the Bloch spectral density function are derived from multiple-scattering theory for the polymorphous coherent-potential approximation (PCPA). The chemical shifts obtained for three alloy systems using the PCPA, the Korringa-Kohn-Rostoker CPA, and the locally self-consistent multiple-scattering method are compared with experiment. A significant improvement in the treatment of Coulomb effects is achieved using the PCPA with only a little more computational effort than for the older isomorphous CPA's.

Theoretical studies of near-edge X-ray absorption fine structure for SO2/Ag(110)

The theoretical analysis for sulfur K-edge near-edge X-ray adsorption fine structure of SO2 adsorbed on Ag(110) at half monolayer has been performed by using the multiple-scattering cluster method. The resonance located between π* and σ* is proved to arise from transitions into an antibonding orbital induced by the interaction between SO2 and the substrate and its unclear polarisation dependence is found to be caused by the slight tilt of SO2 molecular plane away from the surface normal. Compared to gas phase SO2, the O–S intramolecular bond length is elongated by 0.17±0.03 A and the OSO bond angle is reduced by 15°±5° after adsorption. It is concluded that SO2 is adsorbed on Ag(110) with the S atom sitting in a hollow site and the two O atoms being in asymmetric geometry, that the adsorption height is 1.8±0.1 A and that the molecular plane rotates about 52° from the [001] azimuth and tilts slightly away from the surface normal.

The near-edge X-ray absorption fine structure investigation of c(2×2)-(K+CO) on the Co{101̄0} surface

Theoretical studies of O 1s near-edge X-ray absorption fine structure of c(2×2)-(K+CO) adsorption on Co{101̄0} surface have been performed by the multiple-scattering cluster method (MSC). The results show that the bond-length of the adsorbed CO molecule is longer than that of the one with no K by ca 0.1 Å. The comparison between the calculated spectra and the experimental data demonstrates that a new weak π-like structure appears at the higher energy side of the original π resonance. This structure comes from the combined interaction of the CO molecule with both the K atoms and the Co substrate.

Transmission and absorption properties of two-dimensional metallic photonic-band-gap materials

Using the multiple-scattering method, we study systematically the transmission and absorption properties of two-dimensional (2D) metallic photonic-band-gap (PBG) materials for a wide range of frequency and structure scales by adopting a realistic dielectric constant for metal cylinders. For both S and P waves, analytic expressions for the absorption power of a single metal cylinder under an arbitrary external field have been derived to explain the absorption behaviors of metallic PBG materials in a coherent way. The discussions and conclusions presented here can be generalized to 3D systems as well.

Noncollinear magnetism of Fe-rich Fe-Ni alloys

The magnetic structure of Fe-rich FeNi alloys has long been a subject of great scientific interest and controversy. In this study, the authors attempt to understand an interesting phenomenon that the average magnetic moment of the alloys in the γ-phase (fcc) decreases dramatically in the composition range near 70% Fe. Although the observation was made more than 30 years ago, the nature of the mechanism for this moment collapse is still controversial. In this approach, the noncollinear locally self-consistent multiple-scattering (LSMS) method is applied to the magnetic structure calculation of large unit cell samples consisting of Fe and Ni atoms. The Fe and Ni atoms are randomly distributed on a fcc lattice. The moment directions are initialized to be randomly oriented and then, as the self-consistent iterations proceed, are allowed to rotate to minimize the total energy. Astable magnetic structure of the alloy is determined by the final moment configuration. Results are compared with experiments and the possible existence of noncollinear magnetic structures are discussed.

Structure of the Cl-passivated GaAs(111) surface

Cl $1s$ (K-shell) near-edge and extended x-ray-absorption fine-structure spectroscopy has been used to study the chemistry and structure of the Cl passivated GaAs(111) surface. The Cl passivation is achieved in ambient atmosphere by wet chemical methods. It is found that Cl forms a stable monochloride structure with Cl bonded to Ga. Strong polarization dependence was observed both in the Cl $1\stackrel{\ensuremath{\rightarrow}}{s}3p$ \ensuremath{\sigma}-resonance and in the extended fine-structure region. Quantitative analysis, including numerical calculations using the multiple-scattering method on an 85-atom cluster, show that the Ga-Cl bond is oriented along the 〈111〉 surface normal with a Ga-Cl bond length of $2.17\ifmmode\pm\else\textpm\fi{}0.02\AA{}.$

Core Level Chemical Shifts in Metallic Alloys

The average chemical shifts and the distribution of the shifts about the average are obtained for CuPd, CuZn, and AgPd alloys using supercells that contain hundreds of atoms. Density functional theory and local density approximation (DFT-LDA) calculations are carried out using the order- $N$ locally self-consistent multiple-scattering method. The DFT-LDA calculations provide reliable predictions for the chemical shifts in the alloys, but the electron spectroscopy for chemical analysis potential model does not.

Real-space approach to the calculation of magnetocrystalline anisotropy in metals

We have implemented the fully relativistic and spin-polarized extension of the locally self-consistent multiple-scattering method. We have calculated the spin and orbital magnetic moments and magnetocrystalline anisotropy energy of Fe, Ni, and Co in the face-centered-cubic (fcc) and hexagonal-close-packed (hcp) crystal structures. We have obtained fast convergence of these quantities in real space. Moreover, these results compare favorably with the results of conventional k-space methods.

Near-Edge X-Ray Absorption Fine Structure Analyses of the Chain Model of Solid CS2

The S K-shell near-edge x-ray absorption fine structure spectra of solid CS2 have been calculated by means of the multiple-scattering cluster method. The results show the existence of two kinds of absorption spectra which correspond to single-bond (C-S) and double-bond (C=S), respectively, and their superposition of the two spectra fits the experimental spectrum well. Our calculation supports the zigzag chain model of solid CS2 with C-C distance of 3.60±0.01Å.

Double ionization of 2-butyne, 2-pentyne, 2-hexyne and 3-hexyne to singlet and triplet electronic states of their dications

The double-ionization energies (DIEs) of 2-butyne, 2-pentyne, 2-hexyne and 3-hexyne molecules to singlet and triplet electronic states of their dications have been measured using double-charge-transfer spectroscopy. Values of DIEs to the lowest singlet and triplet states of the four dications were calculated using an ab initio method and the split-valence 3-21G and 6-31G basis sets. The pattern of those DIEs for the four molecules (and the previously studied propyne molecule) agree with that observed in the experimental values, thus giving an insight into the double-ionization processes of part of the alkyne family of molecules. The spectra showed many other peaks which correspond to double ionization to electronically excited states. For 2-butyne, the DIE values to those states calculated using a semi-empirical form of the multiple-scattering Xα computational method allowed the relevant electronic transition to be identified. © 1997 John Wiley & Sons, Ltd.

The local environmental effects on the magnetism of short-range clustered CuNi alloys

The locally self-consistent multiple-scattering method has been applied to spin-polarized LDA calculations for ferromagnetic Cu0.2Ni0.8alloys. The sample used to simulate the alloys involved up to 256 atoms per cell and was constructed to be short-range clustering with the experimentally measured short-range order parameters. The magnetic cross section for neutron scattering was also calculated and compared with the neutron scattering data. The magnetic moments of Ni atoms were found to be sensitive to local atomic configurations.

Identification of the electronic transitions in the double ionization of six amines to singlet electronic states of their dications

The double-ionization energies of methylamine, dimethylamine, trimethylamine, ethylamine, propylamine and butylamine were measured and calculated. Transitions to singlet states of the ions were considered in order to complement the previously obtained information on transitions to triplet states. The double-ionization energies were determined experimentally from the positions of peaks observed in double-charge-transfer spectra. Information on the electronic transitions giving rise to the peaks was obtained by comparing the measured energies with those calculated using a semi-empirical form of the multiple-scattering Xα computational method. It was shown that for some of the dications the density of states is high. An ab-initio computational method, used with the 3-21G and 6-31G ∗ basis sets, predicted quite accurately the trend in the lowest double-ionization energies measured.

Short-range structure in solid and liquid CuBr probed by multiple-edge x-ray-absorption spectroscopy

X-ray-absorption measurements of solid and liquid CuBr in the $T=295--793\mathrm{K}$ range of temperature are presented. Accurate short-range structural parameters are extracted using simultaneous Cu and Br $K$-edge extended x-ray-absorption fine-structure (EXAFS) refinement in the framework of the multiple-scattering GNXAS method for data analysis. In solid CuBr we find an increasing asymmetry of the first-neighbor ${g}_{\mathrm{CuBr}}(r)$ distribution function as a function of temperature. The foot and the most probable value of the first-neighbor distribution are found to shift up to about 0.09 \AA{} toward shorter distances from 295 to 670 K, while the long-range part of distribution broadens at high temperatures. Results are discussed taking into account previous neutron diffraction and EXAFS results. A complete account of our EXAFS experiment on liquid CuBr is also reported. The short-range Cu-Br distribution is extracted showing evidence for nearly covalent bonding and improving previous neutron diffraction results. The short-range CuBr liquid structure is found to be quite similar to that of the high-temperature solid. The excellent short-range EXAFS sensitivity and its complementarity with usual diffraction techniques for studying disordered systems are discussed.

Multiple-Edge EXAFS Study of Solid and Liquid CuBr

X-ray absorption measurements of solid and liquid CuBr have been carried out around the Cu and Br K-edge at 295, 400, 600, 670, 793 K. EXAFS data of both edges have been analyzed simultaneously, for the first time on a binary compound, by using the ab-initio multiple-scattering GNXAS method. Multiple-edge EXAFS refinement allowed us to obtain an accurate determination of the first-neighbor distribution as a function of temperature. Mean bond distance, variance and skewness of the distribution have been measured and their statistical errors evaluated. A slight contraction of the most probable Cu-Br bond-length is observed at high temperatures approaching the superionic and the liquid phases (T ≥ 670 K). The asymmetry of the first-neighbor distribution is found to increase at high-temperature. Evidence for nearly-covalent bonding is found in liquid CuBr.

Short-Range Structure in Solid and Liquid Matter Using Multiple-Edge EXAFS Refinement

A novel data-analysis approach for simultaneous analysis of EXAFS (Extended X-ray Absorption Fine Structure) spectra recorded at different core-level edges in monatomic and multiatomic systems is presented. This approach is based on the ab initio multiple-scattering GNXAS method and allows to perform a multiple-edge refinement of the local structure. The usefulness of the method for structural analysis of monatomic and binary liquids is discussed. Applications on K and L edges of solid and liquid Sn and RbBr are reported. The short-range pair distribution g(r) is accurately determined and compared with previous diffraction and molecular-dynamics results.

Partial Summation Method for XANES Calculation

A new efficient multiple-scattering method is proposed, which is based on partitioning method developed by one of the authors. This method includes important multiple-scatterings in infinite order, which is similar to the finite path-by-path approach. We also obtain some symmetric relations and recurrence formulas of the free electron propagators G LL , to reduce computation time. Some illustrative examples are shown. For small systems convergence is easily attained, however the efficiency is not so remarkable. For large systems the convergence is rather slow, however it is accelerated by including the photoelectron wave damping. The computation time is about 5% of that for the full multiple scattering calculation for a cluster with 27 Fe atoms. The convergence is dependent on the system, whether it is closely packed or not.

Radial Distribution Function in Solid and Liquid KBr Probed by EXAFS

Accurate EXAFS spectra of solid and molten KBr are analysed using the GNXAS multiple-scattering method. Short-range radial distribution functions derived by EXAFS are compared with results of molecular dynamics (MD) simulations. Important anharmonic effects are found in solid KBr, in agreement with MD calculations. Local structure of liquid KBr is measured and compared with MD results for the first time.