Self-consistent Electronic Structure Research Articles

The effect of lanthanide impurities on the physical properties of half-metallic ferromagnet Co 2MnSi

The paper reports some preliminary results on possible incorporation of Ho in Co 2MnSi lattice. In addition, the effect of holmium substitution on Mn sites is analyzed on the basis of self-consistent electronic structure calculations.

Two-dimensional electronic structure of the adsorbate systemN∕Cu(100): Photoelectron spectroscopy and one-step model calculations

In this paper, we present a detailed study of the electronic structure of the adsorbate system $\mathrm{Cu}(100)c(2\ifmmode\times\else\texttimes\fi{}2)\mathrm{N}$ by using angle-resolved ultraviolet photoelectron spectroscopy. The measured spectra reveal a variety of complex multipeak structures below the Fermi level. The adsorbate-derived states, which are of special interest, have been selected from the Cu-induced peaks by varying the polarization angle of the linear polarized light. The experimental analysis is supported by self-consistent electronic structure calculations within the density functional theory in order to determine the symmetry properties of the different adsorbate states. Due to the direct comparison with calculated photoemission spectra, a quantitative analysis of the surface geometry was possible. A structure model with outward relaxation of the first layer and twofold symmetry caused by combined rumpling of Cu and N atoms within this layer is favored by our investigation.

Spin-reorientation transitions in cobalt nanowires: Role of the microstructure and finite-size effects

We present self-consistent electronic structure calculations in order to analyze the magnetocrystalline anisotropy energy as well as the local spin ${S}_{\ensuremath{\delta}}(i)$ and orbital ${L}_{\ensuremath{\delta}}(i)$ magnetic moments for finite-length Co nanowires. We consider monatomic chains, as well as single-crystal and polycrystalline nanowires, assuming two directions of magnetization $\ensuremath{\delta}$ oriented along $(\ensuremath{\delta}=x)$ and perpendicular $(\ensuremath{\delta}=z)$ to the wire axis. The single-crystal wires are cylindrical fragments of the fcc lattice grown along the (111) direction, while the polycrystalline structures are characterized by the presence of various fcc and hcp grains. We obtain that the Co chains are defined by a highly stable magnetization direction oriented along the chain axis. However, when three-dimensional wires are considered, we found a strong influence of the local atomic environment on the determination of the easy axis. In fact, we observe strong oscillations of the low energy orientation of the magnetization between the parallel and perpendicular directions as the length of the wire increases or when the geometrical details of the wire caps are modified. In contrast, in the polycrystalline structures, we found that the electronic perturbation introduced by the presence of internal interfaces is more localized in nature. Actually, no spin reorientation transitions are found for different microstructures; however, reduced energy differences (compared to uniform single-crystal wires with approximately the same length) between the parallel and perpendicular directions are obtained. Finally, the existence of internal interfaces can induce appreciable perturbations in the density of states around the Fermi energy and, as a consequence, they are expected to modify the number of conduction channels as well as the transport properties of the wires.

Compton scatter profiles for warm dense matter

In this paper, we discuss the possibility of using x-ray Compton scattering as a probe of the outer electronic structure of ions immersed in warm dense matter. It is proposed that the x-ray free-electron lasers currently under construction will provide an ideal tool for this, with the main pulse being used to create a uniform well-defined sample and the third harmonic providing a clean monochromatic probe. We model the plasma photon scatter spectrum by combining self-consistent finite-temperature electronic structure calculations with molecular dynamics simulations of the ion-ion structure factor. In particular, we present bound-free Compton profiles that are more accurate that those obtained using form factor or impulse approximations.

Self-consistent electronic subband structure of undoped InAs/GaSb-based type II and broken-gap quantum well systems

Motivated by a very recent experimental work on investigating electronic properties of InAs/GaSb-based type II and broken-gap quantum well structures, in this article we present a simple and transparent theoretical approach to calculate electronic subband structure in such device systems. The theoretical model is developed on the basis of solving self-consistently the Schrödinger equation for the eigenfunctions and eigenvalues coupled with the Poisson equation for the confinement potentials, in which the effects such as charge distribution and depletion are considered. In particular, we examine the effect of a GaSb cap layer on electronic properties of the quantum well systems in conjunction with experiments and experimental findings. The results obtained from the proposed self-consistent calculation can be used to understand important experimental findings and are in line with those measured experimentally.

Linear scaling calculation of band edge states and doped semiconductors

Linear scaling methods provide total energy, but no energy levels and canonical wave functions. From the density matrix computed through the density matrix purification methods, we propose an order-N [O(N)] method for calculating both the energies and wave functions of band edge states, which are important for optical properties and chemical reactions. In addition, we also develop an O(N) algorithm to deal with doped semiconductors based on the O(N) method for band edge states calculation. We illustrate the O(N) behavior of the new method by applying it to boron nitride (BN) nanotubes and BN nanotubes with an adsorbed hydrogen atom. The band gap of various BN nanotubes are investigated systematically and the acceptor levels of BN nanotubes with an isolated adsorbed H atom are computed. Our methods are simple, robust, and especially suited for the application in self-consistent field electronic structure theory.

Orbital magnetism and magnetic anisotropy energy of Co nanoparticles: Role of polytetrahedral packing, polycrystallinity, and internal defects

We report extensive self-consistent electronic structure calculations in order to analyze the magnetic properties of Co nanoparticles with complex atomic configurations. Following recent experimental works, we consider polycrystalline structures in which a coexistence of bcc and compact (fcc) phases is present within the particles, polytetrahedral clusters, as well as fcc nanocrystals with ``stacking-fault-like'' defects. In all cases, the local spin moments ${S}_{\ensuremath{\delta}}(i)$ are found to be saturated $(\ensuremath{\sim}1.7{\ensuremath{\mu}}_{B})$, being almost independent of the direction of the magnetization $\ensuremath{\delta}$, the local atomic environment $i$, and the size of the particles, a behavior that is characteristic of magnetic materials in the large exchange regime. In contrast, the local-orbital moments ${L}_{\ensuremath{\delta}}(i)$ and the magnetic anisotropy energy (MAE) are found to be very sensitive to the size and structure of the systems. In our polycrystalline particles, we obtain considerably enhanced values for ${L}_{\ensuremath{\delta}}(i)$ at the internal bcc/fcc interfaces (even larger than at surfaces sites) and, interestingly, we obtain that by varying the fraction of bcc and fcc phases within the particles, several reorientations of the magnetization can be induced. Furthermore, our polytetrahedral structures are characterized by the existence of very isolated and low coordinated surface atoms, a fact that induces the formation of localized states with large local magnetic moments ${M}_{\ensuremath{\delta}}(i)$ $[{M}_{\ensuremath{\delta}}(i)=2{S}_{\ensuremath{\delta}}(i)+{L}_{\ensuremath{\delta}}(i)]$ of $3.7{\ensuremath{\mu}}_{B}$ and huge orbital contributions ${L}_{\ensuremath{\delta}}(i)$ as large as $2{\ensuremath{\mu}}_{B}$. Even if enhanced local MAE's are also found, negative and positive values are obtained in the different sites, resulting in nonadditive contributions that yield small energy barriers between our different considered $\ensuremath{\delta}$'s. Finally, we obtain that the existence of internal defects, created by locally interrupting the regular growth mode in fcc nanocrystals, considerably perturbs the electron spin-density distribution around the defect sites, leading to strong variations in ${L}_{\ensuremath{\delta}}(i)$ and to reorientations of the magnetization in the system.

Local electronic structure in the Peyrard–Bishop–Holstein model

There is increasing evidence for polaronic effects on charge localization and dynamics inDNA. The Peyrard–Bishop–Holstein model has been previously suggested as anappropriate model for the description of such effects. Here we report a self-consistent studyof local electronic structure within this model for both homopolymer and realistic viral P5promoter segments. Our results indicate that both the inter-base-pair stacking interactionand the electron filling can qualitatively influence the polaronic properties in a specificDNA sequence, including features of two distinct length scales and competition withsequence-disorder induced localization.

Substituting the main group element in cobalt–iron based Heusler alloys: Co2FeAl1−xSix

This work reports electronic structure calculations for the Heusler compound Co2FeAl1−xSix. Particular emphasis was put on the role of the main group element in this compound. The substitution of Al by Si leads to an increase in the number of valence electrons with increasing Si content and may be seen as electron-doping. Self-consistent electronic structure calculations were performed to investigate the consequences of the electron-doping on the magnetic properties. The series Co2FeAl1−xSix is found to exhibit half-metallic ferromagnetism and the magnetic moment follows the Slater–Pauling rule. It is shown that the electron-doping stabilizes the gap in the minority states for x = 0.5.

Carrier-mediated ferromagnetism in N co-doped (Zn, Mn)O-based diluted magnetic semiconductors

Mn-doped ZnO is anti-ferromagnetic spin glass state, however, it becomes half-metallic ferromagnets upon hole doping. In this Letter we report a theoretical study of (Zn, Mn)O system codoped with N, and show that this codoping can change the ground state from anti-ferromagnetic to ferromagnetic. We have carried out the first-principles electronic structure calculations and report total energy to estimate whether the ferromagnetic state was stable or not. Our approach is based on the spin-polarized relativistic Korringa–Kohn–Rostoker (SPR–KKR) density functional theoretical (DFT) method, within the coherent potential approximation (CPA). Self-consistent electronic structure calculations were performed within the local density approximation, using the Vosko–Wilk–Nusair parameterization of the exchange-correlation energy functional. Our results for energy difference between ferromagnetic sate and spin glass state as well as their dependence on concentrations were presented and discussed.

Order-Nfirst-principles calculation method for self-consistent ground-state electronic structures of semi-infinite systems

We present an efficient and highly accurate first-principles calculation method with linear system-size scaling to determine the self-consistent ground-state electron-charge densities of nanostructures suspended between semi-infinite bulks by directly minimizing the energy functional. By making efficient use of the advantages of the real-space finite-difference method, we can impose arbitrary boundary conditions on models and employ spatially localized orbitals. These advantages enable us to calculate the ground-state electron-charge densities in semi-infinite systems. Examples of electronic structure calculations for a one-dimensional case and a conductance calculation for sodium nanowires are presented. The calculated electronic structure of the one-dimensional system agrees well with the exact analytical solution, and the conduction properties of the sodium nanowires are consistent with experimental and other theoretical results. These results imply that our procedure enables us to accurately compute self-consistent electronic structures of semi-infinite systems.

Time-of-flight photoelectron spectromicroscopy of single MoS2 nanotubes

There is a recent interest in nanoscale materials, in particular, nanotubes based not only on carbon. In this study, photoemission spectra of single MoS2 nanotubes deposited on a Si surface were recorded in order to explain their electronic structure. The photoelectrons were excited by a femtosecond laser oscillator resulting in two-photon photoemission. A spectromicroscopic technique based on imaging time-of-flight detection was used to record the spatially resolved photoelectron spectra. Self-consistent electronic structure calculations for MoS2 slabs using the full potential linear augmented plane wave method are used to explain the peculiarities of the observed spectra. It turns out that the MoS2 nanotubes are semiconducting with a band gap of about 1eV. The two-photon transitions proceed through intermediate states in a region with high density of states; this gives rise to a high photoemission intensity.

Exchange interactions and Curie temperatures inNi2−xMnSballoys: First-principles study

We present a first-principles study of the physical properties of the disordered ${\mathrm{Ni}}_{2\ensuremath{-}x}\mathrm{Mn}\mathrm{Sb}$ alloys which form a continuous series connecting two typical members of the Heusler alloy family: namely, the half-metallic semi-Heusler alloy NiMnSb $(x=1)$ and related metallic Heusler alloy ${\mathrm{Ni}}_{2}\mathrm{Mn}\mathrm{Sb}$ $(x=0)$. Magnetic moments, exchange interactions, magnon spectra, and Curie temperatures at ambient and elevated pressures are determined and compared with available experimental data. The spin-spin correlation functions at the critical temperature are also calculated. The pair exchange interactions and corresponding classical Heisenberg Hamiltonian are derived from self-consistent electronic structure calculations using a magnetic force theorem. Heusler alloys NiMnSb and ${\mathrm{Ni}}_{2}\mathrm{Mn}\mathrm{Sb}$ exhibit strikingly different asymptotic behavior of exchange interactions with the distance between magnetic atoms. The Curie temperatures are estimated using multisublattice versions of the mean-field approximation, random-phase approximation, and Monte Carlo simulations. The robustness of the results with respect to the effect of correlations beyond the local density approximation, the selected reference state used for mapping to the Heisenberg Hamiltonian, and the applied mapping procedure are also discussed.

Exchange interactions, spin waves, and transition temperatures in itinerant magnets

This contribution reviews an ab initio two-step procedure to determine exchange interactions, spin-wave spectra, and thermodynamic properties of itinerant magnets. In the first step, the self-consistent electronic structure of a system is calculated for a collinear spin structure at zero temperature. In the second step, parameters of an effective classical Heisenberg Hamiltonian are determined using the magnetic force theorem and the one-electron Green functions. The Heisenberg Hamiltonian and methods of statistical physics are employed in subsequent evaluation of magnon dispersion laws, spin-wave stiffness constants, and Curie/Néel temperatures. The applicability of the developed scheme is illustrated by selected properties of various systems such as transition and rare-earth metals, disordered alloys including diluted magnetic semiconductors, ultrathin films, and surfaces. A comparison to other ab initio approaches is presented as well.

Electronic and optical properties ofRuO2andIrO2

We report first principles self-consistent electronic structure calculations of $\mathrm{Ru}{\mathrm{O}}_{2}$ and $\mathrm{Ir}{\mathrm{O}}_{2}$ using the full-potential linearized augmented plane wave method. Our electronic properties are in good agreement with x-ray photoelectron spectroscopy data regarding the bandwidths and peak positions. Additionally, we probe our electronic structures by calculating the dielectric functions and comparing them with optical measurements. Our calculations show that intraband transitions play an important role to describe properly the optical response of $\mathrm{Ru}{\mathrm{O}}_{2}$ and $\mathrm{Ir}{\mathrm{O}}_{2}$. We find that these materials are good absorbers at low energies where the dielectric functions exhibit a Drude like behavior. At higher energies, the optical features are due to electronic transitions from oxygen $2p$ to metal $d$ bands. Our results for the dielectric functions and energy loss spectra show a good agreement with optical measurements.

Density-based Globally Convergent Trust-region Methods for Self-consistent Field Electronic Structure Calculations

A theory of globally convergent trust-region methods for self-consistent field electronic structure calculations that use the density matrices as variables is developed. The optimization is performed by means of sequential global minimizations of a quadratic model of the true energy. The global minimization of this quadratic model, subject to the idempotency of the density matrix and the rank constraint, coincides with the fixed-point iteration. We prove that the global minimization of this quadratic model subject to the restrictions and smaller trust regions corresponds to the solution of level-shifted equations. The precise implementation of algorithms leading to global convergence is stated and a proof of global convergence is provided. Numerical experiments confirm theoretical predictions and practical convergence is obtained for difficult cases, even if their geometries are highly distorted. The reduction of the trust region is performed by a strategy that uses the structure of the energy function providing the algorithm with a nice practical behavior. This framework may be applied to any problem with idempotency constraints and for which the derivative of the objective function is a symmetric matrix. Therefore, application to calculations based both on Hartree–Fock or Kohn–Sham density functional theory are straightforward.

Reduced surface magnetization of NiMnSb(001)

The spin-dependent electronic structure at the (001) surface of the ferromagnetic half-Heusler alloy NiMnSb has been investigated by spin-resolved appearance potential spectroscopy. This surface-sensitive technique provides access to the spin- and element-resolved local density of unoccupied states and, therefore, probes the spin polarization above the Fermi level. In combination with low-energy electron diffraction and magneto-optical Kerr effect measurements, we characterized the sample with respect to its crystal structure, electronic structure, and magnetic properties. Our spectroscopic experiments were accompanied by a quantitative theoretical analysis. The theoretical appearance potential spectra are based on self-consistent electronic structure results for the bulk. They have been obtained in a fully relativistic framework as a self-convolution of the matrix element-weighted, orbitally resolved unoccupied density of states. The spin asymmetry in the experimental results, obtained from the surface region, is found to be significantly reduced compared with the calculations based on the bulk electronic structure of NiMnSb. Possible reasons for the reduced surface magnetization are discussed.

QUANTUM GENETIC ALGORITHM METHOD IN SELF-CONSISTENT ELECTRONIC STRUCTURE CALCULATIONS OF A QUANTUM DOT WITH MANY ELECTRONS

In this study, we have calculated energy levels of an N-electron quantum dot. For this purpose, we have used two different techniques, matrix diagonalization and quantum genetic algorithm, to obtain simultaneous solutions of the coupled Schrödinger and Poisson equation in the Hartree approximation. We have determined single particle energy levels, total energy, chemical potential and capacitive energy. We have also compared the results, demonstrated the applicability of QGA to many-electron quantum systems and evaluated its effectiveness.

Inelastic scattering from core electrons: A multiple scattering approach

The real-space multiple-scattering (RSMS) approach is applied to model non-resonant inelastic scattering from deep core electron levels over a broad energy spectrum. This approach is applicable to aperiodic or periodic systems alike and incorporates ab initio, self-consistent electronic structure and final state effects. The approach generalizes to finite momentum transfer a method used extensively to model x-ray absorption spectra (XAS), and includes both near edge spectra and extended fine structure. The calculations can be used to analyze experimental results of inelastic scattering from core-electrons using either x-ray photons (NRIXS) or electrons (EELS). In the low momentum transfer region (the dipole limit), these inelastic loss spectra are proportional to those from XAS. Thus their analysis can provide similar information about the electronic and structural properties of a system. Results for finite momentum transfer yield additional information concerning monopole, quadrupole, and higher couplings. Our results are compared both with experiment and with other theoretical calculations.

Random-phase approximation for critical temperatures of collinear magnets with multiple sublattices:GdXcompounds(X=Mg,Rh,Ni,Pd)

An efficient scheme for evaluating the critical temperatures of ferromagnetic, antiferromagnetic, and ferrimagnetic crystals with multiple sublattices is presented. The approach is based on a pairwise Heisenberg Hamiltonian and a random-phase approximation (Tyablikov's decoupling) for magnon Green's functions. The pair exchange interactions are derived from self-consistent electronic structure calculations using a magnetic force theorem. The developed technique is applied to hexagonal gadolinium and its selected intermetallic compounds $\mathrm{Gd}X$ $(X=\mathrm{Mg},\mathrm{Rh},\mathrm{Ni},\mathrm{Pd})$ with CsCl and CrB structures. The calculated critical temperatures are quite sensitive to a neglect of the nonmagnetic $(X)$ element; their values are in a fair agreement with experiment.