Self-consistent Electronic Structure Calculations Research Articles

Orthonormal Wavelet Bases for Quantum Molecular Dynamics

We report on the use of compactly supported, orthonormal wavelet bases for quantum molecular-dynamics (Car-Parrinello) algorithms. A wavelet selection scheme is developed and tested for prototypical problems, such as the three-dimensional harmonic oscillator, the hydrogen atom, and the local density approximation to atomic and molecular systems. Our method shows systematic convergence with increased grid size, along with improvement on compression rates, thereby yielding an optimal grid for self-consistent electronic structure calculations.

Realistic calculations of correlated incompressible electronic states in GaAs-AlxGa1−xAs heterostructures and quantum wells

We perform an exact spherical geometry finite-size diagonalization calculation for the fractional quantum Hall ground state in three different experimentally relevant GaAs-Al_{x}Ga_{1-x}As systems: a wide parabolic quantum well, a narrow square quantum well, and a heterostructure. For each system we obtain the Coulomb pseudopotential parameters entering the exact diagonalization calculation by using the realistic subband wave function from a self-consistent electronic structure calculation within the local density approximation (LDA) for a range of electron densities. We compare our realistic LDA pseudopotential parameters with those from widely used simpler model approximations in order to estimate the accuracies of the latter. We also calculate the overlap between the exact numerical ground state and the analytical Laughlin state as well as the excitation gap as a function of density. For the three physical systems we consider the calculated overlap is found to be large in the experimental electron density range. We compare our calculated excitation gap energy to the experimentally obtained activated transport energy gaps after subtracting out the effect of level broadening due to collisions. The agreement between our calculated excitation gaps and the experimental measurements is excellent.

Calculated Transport and Magnetic Properties of some Perovskite Metallic Oxides AMO3

ABSTRACTWe study some compounds of the perovskite (or pseudo-cubic perovskite) series AMO3, where M is a transition metal and A is Ca, Sr, or Nd, by LSDA self-consistent electronic structure calculations with the LMTO method. Transport and magnetic properties, as well as Fermi surfaces are calculated. These materials exhibit sharp density of states features in the vicinity of the Fermi level that strongly affect their transport and magnetic properties and make them very sensitive to structural deformation and stoichiometry. Calculated total energies are very close for anti-ferromagnetic and ferromagnetic solutions. This explains qualitatively the magnetoresistive anomalies shown by this family of compounds.

Electronic Structure Calculations for YBa2Cu3O7 within the Slave Boson Formalism.

A novel method for self-consistent electronic structure calculations for correlated metals is presented. This approach takes into account on-site Coulomb repulsion among localized electrons and is based on the slave boson formalism. The proposed method was applied to the study of the electronic structure of YBa{sub 2}Cu{sub 3}O{sub 7}: the on-site Coulomb repulsion of Cu3{ital d} electrons gives origin to a flat band at the Fermi level, as observed in experiments. A sharp Van Hove singularity has been found 25meV below the Fermi energy. {copyright} {ital 1996 The American Physical Society.}

Determination of the electronic density of states near buried interfaces: Application to Co/Cu multilayers.

High-resolution ${\mathit{L}}_{3}$ x-ray absorption and emission spectra of Co and Cu in Co/Cu multilayers are shown to provide unique information on the occupied and unoccupied density of d states near buried interfaces. The d bands of both Co and Cu interfacial layers are shown to be considerably narrowed relative to the bulk metals, and for Cu interface layers the d density of states is found to be enhanced near the Fermi level. The experimental results are confirmed by self-consistent electronic structure calculations. \textcopyright{} 1996 The American Physical Society.

Theoretical interpretation of optical conductivity of YbCu4Ag,Au

We focus on YbCu4Ag and YbCu4Au and interpret the optical conductivity, measured from 2 meV to 6 eV at 10 and 300 K, in terms of self-consistent electronic structure calculations. Most of the relevant spectral structures are interpreted as interband transitions from hybridized f–d states below and at the Fermi level at high symmetry points. In particular, the large shoulder at about 2.4 and 2 eV in the spectrum of YbCu4Au and YbCu4Ag, respectively, can be interpreted in terms of a transition Γ4→Γ3 involving mostly d states. We also compare the electronic structure of YbCu4Ag with that of LuCu4Ag, to ascertain the influence of the f states on the conduction bands and carrier density at EF. This allows us to draw some qualitative considerations on the f-band hybridization in these compounds.

Wavelets in self-consistent electronic structure calculations.

We report the first implementation of orthonormal wavelet bases in self-consistent electronic structure calculations within the local-density approximation. These local bases of different scales efficiently describe localized orbitals of interest. As an example, we studied two molecules, ${\mathrm{H}}_{2}$ and ${\mathrm{O}}_{2}$, using pseudopotentials and supercells. Considerably fewer bases are needed compared with conventional plane-wave approaches, yet calculated binding properties are similar. Our implementation employs fast wavelet and Fourier transforms, avoiding evaluating any three-dimensional integral numerically.

Ground state structural stability of ordered fcc- and bcc-based LiAl compounds under first and second nearest-neighbour pair approximation

Self-consistent local density electronic structure calculations have been performed on a series of ground state ordered superstructures of lithium-aluminium alloys spanning the entire concentration range. These structures are based on both fcc and bcc lattices under the first and second nearest neighbour pair approximation which is adequate to stabilize all the stable and metastable phases of the LiAl system. Using the efficient tight-binding linear muffin-tin orbital (TB-LMTO) method, we have calculated the volume dependent total ground state energies and the systematic trends in various cohesive and electronic properties at zero temperature, as a function of Li concentration. The predicted heats of formation for all the different ground state superstructures result in a representative stability profile, which shows that the L1 2 , B32 and DO 3 structures are the most stable amongst various phases having Al 3 Li, AlLi and AlLi 3 compositions, respectively. Moreover, we have parameterized the cohesive energies using the Connolly-Williams cluster expansion method and estimated the effective many-body interactions for the fcc lattice in an octahedron-tetrahedron cluster approximation, and for the bcc lattice in an irregular tetrahedron cluster approximation. These volume dependent but configuration (as well as concentration) independent interactions coming out of the TB-LMTO-CWM approach are not only important for first principles calculation of phase diagram but are also useful for predicting the evolutionary path of ordering processes.

Behavior of carriers in ?-doped quantum wells under in-plane magnetic fields

Self-consistent electronic structure calculations of δ-doped quantum wells (QW) in the presence of in-plane magnetic fields B up to 20 Tesla are carried out within the frameworks of the effective mass and the local density approximations. QWs composed of two layers of Ga1-xA1xAs, separated by a layer of GaAs with a donor δ-doped sheet in the center, are considered. The width of the GaAs layer was varied from 100 to 400 A. It is shown that the diamagnetic shift increases with the increasing of the GaAs QW width. The magnetic field induces remarkable changes in the energy dispersions of electrons and holes, along an in-plane direction perpendicular to B. The most striking effect occurs in the nature of the band gap of these systems. We found that the valence band displays a double-maximum character instead of a single maximum at the center of the Brillouin zone. © 1996 John Wiley & Sons, Inc.

Valence band electronic redistribution in ion-beam-mixed PdAg alloys

The core-level binding energy shift and L 2,3 absorption-edges for pure Pd, Ag, and ion-beam-mixed Pd 1− x Ag x ( x = 0.5-0.9) alloys are measured to investigate the charge redistribution in ion-beam-mixed PdAg alloys. It is found that, in ion-beam-mixed alloys, there is a substantial decrease in the area of the Pd L 2,3 white lines compared with that of pure Pd. The observed decrease in white line area was attributed to an increase in the number of d-electrons at the Pd site upon alloy formation. Using the observed core-level binding energy shifts and white line area changes, the net charge transfer at the Pd site of ion-beam-mixed PdAg alloys has been estimated on the basis of a charge compensation model. We find that the net charge transfer is very small owing to back donation of sp-like conduction electron from the Pd site. These results are compared with the self-consistent electronic structure calculation.

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.

Electronic structure and phase stability of three series of B2 Ti-transition-metal compounds

Self-consistent electronic structure calculations have been performed for the 12 B2 TiM compounds (M=Fe, Co, Ni, Cu, Ru, Rh, Pd, Ag, Os, Ir, Pt, Au) using the linear muffin-tin orbital (LMTO) method. The calculated lattice constants and bulk moduli are in good agreement with measurements. An analysis of the possible correlations between the instability of the B2 structure and the electronic structures of the 12 TiM compounds has been carried out. The sequence of the B2 phase instabilities and the cubic to orthorhombic (or monoclinic) to orthorhombic (or monoclinic) to tetragonal tendency in the three TiM series have been discussed in terms of a band filling effect. The possible driving mechanisms of the structural phase transitions in the TiM series have been analysed based on the results of generalized susceptibility X0(q) calculations.

Theoretical investigation of hydroxylated oxide surfaces

A self-consistent electronic structure calculation, in the slab geometry, is performed to model the dissociative adsorption of water on several oxide surfaces in the limit of complete saturation. We discuss the adsorption characteristics along a series of oxides presenting a growing acidity: BaO, SrO, CaO, MgO, TiO 2, and SiO 2, and on three MgO surfaces: (100), (110), and (211) on which the atoms have different environments. Special emphasis is borne on the charge transfers, the densities of states and the oxide gap modification upon hydroxylation. We show that these quantities are dependent upon the coverage and that unstable MgO surfaces are more reactive towards water dissociation. Finally, by optimizing the geometry on the three MgO surfaces, we discuss the link between the electronic and structural degrees of freedom on hydroxylated surfaces.

Enhanced magnetism in amorphous Co-Y alloys: An ab initio approach.

The magnetic properties of Co-Y crystalline intermetallic compounds and amorphous alloys have been investigated using molecular-dynamics simulations of the amorphous structure (based on effective tight-binding-bond forces) and self-consistent spin-polarized electronic-structure calculations (using the supercell approximation for the amorphous phases). We find that the amorphous structure is characterized by a rather strong chemical short-range order (stronger than in amorphous Fe-Y, but weaker than in Ni-Y allows). As a consequence, the total electronic density of states (DOS) is also similar in the crystalline and amorphous phases, apart from a smearing of the fine-structure characteristic for the long-range order in the intermetallic compounds. All crystalline ${\mathrm{Co}}_{\mathit{x}}$${\mathrm{Y}}_{100\mathrm{\ensuremath{-}}\mathit{x}}$ alloys with x\ensuremath{\ge}75 and all amorphous alloys with x\ensuremath{\ge}45 are ferrimagnetic. The Laves phase ${\mathrm{Co}}_{2}$Y shows metamagnetism. The disorder-induced smearing of the electronic DOS eliminates the metamagnetic instability and is responsible for the increase of the paramagnetic DOS at the Fermi level and for the enhancement of magnetism. We find that the ferrimagnetic coupling, together with the strong tendency to heterocoordination is important for the persistence of magnetic ordering in the Y-rich regime.

Use of the DX center as a probe to study the profile of Si impurities in planar-doped GaAs

Photo Hall concentration and mobility were measured for two molecular beam epitaxy-grown samples having a silicon planar-doped structure in the GaAs layer of a GaAs/AlGaAs heterojunction. The nominal silicon concentration for both samples was 1.5×1013 cm−2 and the distance between the ideal localization of the doped plane and the interface was adjusted to be 15 Å. The difference between the two samples is the growth direction. The Hall measurements were carried out at 77 K both in darkness and under illumination using an infrared light emitting diode as light source. Photoexcited effects indicate the presence of silicon atoms inside the undoped AlGaAs layer and that the silicon profile spreads mainly in the growth direction. Self-consistent electronic structure calculations, in the effective-mass approximations, were performed assuming doping profiles that simulate both samples. The calculations show that parallel conduction occurs when the growth direction is from GaAs to AlGaAs. This is consistent with the Hall measurements.

Real space approach to electronic-structure calculations

We have applied the Finite Element Method to the self-consistent electronic-structure calculations of molecules and solids for the first time. In this approach all the calculations are performed in “real space” and the use of non-uniform mesh is made possible, thus enabling us to deal with localized systems with ease. To illustrate the utility of this method, we perform an all-electron calculation of hydrogen molecule in a supercell with LDA approximation. Our method is also applicable to mesoscopic systems.

Is ThInPd2 magnetic?

Self-consistent spin-polarized electronic structure calculations predict that TiInPd 2 cannot undergo a magnetic phase transition. This agrees with the neutron scattering data but disagrees with the magnetization result.

Electronic structures and Curie temperatures of iron-based rare-earth permanent-magnet compounds.

The modification of the electronic structures of Sm[sub 2]Fe[sub 17[minus][ital x]]Al[sub [ital x]]N[sub [ital y]], NdFe[sub 11]TiN[sub [ital y]], and YFe[sub 12[minus][ital x]]Mo[sub [ital x]] upon alloying and nitriding are examined with self-consistent spin-polarized calculations and soft-x-ray photoemission measurements between 18 and 135 eV. The changes in the Curie temperature [ital T][sub [ital c]] with substitutional modifications and nitrogen addition are modeled with self-consistent spin-polarized electronic structure calculations and the spin-fluctuation theory of Mohn and Wohlfarth which relates the electronic structure to [ital T][sub [ital c]]. The calculations show that the spin-summed density of states at the Fermi energy is related to [ital T][sub [ital c]]. The photoemission spectra are dominated by the Fe 3[ital d] electrons within 3 eV of the Fermi energy in agreement with calculations. Changes in the density of states at the Fermi energy for interstitial and and substitutional modification compare well with calculations. Using photoemission results with experimental magnetic moments for the substitutional modification of the compound Sm[sub 2]Fe[sub 17[minus][ital x]]Al[sub [ital x]], the spin-fluctuation theory predicts a change in [ital T][sub [ital c]] in agreement with the measured change in [ital T][sub [ital c]]. Spin-resolved photoemission spectra for [ital c]-axis oriented Sm[sub 2]Fe[sub 17]N[submore » 2.6] with magnetization perpendicular to the surface are presented and compared to theoretical calculations.« less

Relaxation and rumpling mechanisms on oxide surfaces

We associate a self-consistent electronic structure calculation with a conjugate gradient technique for geometry optimization, to study structural distortions on stoichiometric oxide surfaces. We discuss the relaxation and rumpling effects and their consequences on the surface electronic structure in a series of three MgO surfaces: (100), (110), (211) characterized by an increasing number of broken bonds, on the (110) faces of rocksalt oxides presenting various ionic characters: MgO, CaO, SrO and BaO, and on two non-polar rutile TiO 2 faces: (110) and (001). The numerical results are interpreted in the framework of an analytical model and the competition between covalent and electrostatic effects is investigated. A new mechanism of rumpling on oxide surfaces is proposed.

Self-consistent electronic-structure calculation of rectangular modulation-doped GaAs/Ga1-xAlxAs quantum wires.

A method for the self-consistent calculation of rectangular quantum wires is presented and used for the analysis of the classical (two-dimensional well) quantum wire embedded in an infinitely extended bulk. The method relies on the Fourier expansion of the wave functions and on using the structural symmetry properties for computation time savings. Furthermore, L\"owdin's perturbation method is also employed to increase efficiency. Calculations are performed at room temperature and at a number of dopant densities, and the influence of structure parameters is analyzed.