Self-consistent Electronic Structure Research Articles

Electronic structure of GaAs-AlGaAs heterojunctions in parallel magnetic fields

The authors' have performed self-consistent electronic structure calculations for a two-dimensional electron gas confined at a GaAs/AlxGa1-xAs interface in the presence of a parallel magnetic field. Their study has focused on the evolution of the subband states with increasing field strength and the depopulation of higher subbands as a result of diamagnetic energy shifts. Significant differences have been found between the self-consistent results and those obtained by the commonly used perturbative method. These differences can be attributed to two limitations of the perturbative formulation: (i) neglect of the magnetic field dependence of the self-consistent potentials and (ii) neglect of the modification of the density of states from the zero-field behaviour. A comparison of both theoretical formulations with data derived from magnetic depopulation experiments is also made.

Self-consistent electronic structure of parabolic semiconductor quantum wells: Inhomogeneous-effective-mass and magnetic-field effects.

We report the results of self-consistent density-functional electronic-structure calculations for parabolic-profile GaAs-Al x Ga 1-x As quantum wells in a magnetic field within the effective-mass approximation. We discuss in general the treatment of inhomogeneous effective mass in self-consistent electronic-structure calculations and, in particular, include the parabolic-well effective-mass variation across the well. We consider both in-plane and transverse-magnetic field. In the former case, the magnetic field couples the in-plane and transverse degrees of freedom

Electronic structure calculations for amorphous Co and Ni

Self-consistent electronic structure calculations were performed for amorphous Co and Ni metals using the most localized linear muffin-tin orbital method together with the recursion method. The calculated results for the paramagnetic states suggest that these metals exhibit ferromagnetism according to the Stoner criterion. Spin-polarized calculations were also performed. The resultant magnetic moments were 1.63 mu B for amorphous Co, and 0.61 mu B for amorphous Ni. These values agree well with the experimental values obtained by extrapolation to the pure amorphous limit in several Co or Ni-based amorphous alloys. The stability of ferromagnetism is discussed for both the amorphous metals on the basis of the electronic structures obtained.

Self-consistent pseudopotential calculations for sodium adsorption on GaAs(110).

We report self-consistent electronic structure, total-energy, and force calculations based on the density-functional theory for the Na adsorption on GaAs(110). In particular we studied three different coverages: Θ=1/4, 1/2, and 1 (Θ=1 means one adatom per substrate surface atom). We find that the Na core electrons play an important role for the exchange-correlation interaction and we therefore carry the atomic core-electron density of sodium along with the self-consistent calculations. Tests with our formulation of this core-valence exchange-correlation interaction are presented for the free sodium atom, the NaCl crystal, and Na metal

Magnetic instabilities in PtFe3 and in the fcc Ni-Fe system.

The self-consistent linear muffin-tin orbitals electronic structure calculations have been carried out for nonmagnetic and ferromagnetic phases of ${\mathrm{PtFe}}_{3}$ and for a few ordered phases of the Ni-Fe alloy. The fixed-spin-moment scheme has been employed. The results reveal the existence of high-spin (HS) and low-spin (LS) branches of the magnetization curve for both systems and allow for a direct calculation of the energy differences between the HS, LS, and nonmagnetic phases. For the Ni-Fe alloy, the total energy and the state equations are calculated for a number of concentrations. It is shown that a vanishing energy difference between the HS and the LS states and the magnetic instability close to equilibrium are correlated. For ${\mathrm{PtFe}}_{3}$ the pressure dependence of the total energy is discussed. It is shown that this information in conjunction with experimental values of the critical pressures at which the HS-LS transitions are observed allows for an independent estimation of the energy differences between these phases and that the estimate does not agree well with the results of local-density calculations. It is pointed out that this disagreement and the previously reported inability of the local-density approximation to account for observed high-field susceptibilities may be of common origin.

Local spin-density theory of itinerant magnetism in crystalline and amorphous transition metal alloys

The authors present self-consistent spin-polarized electronic structure calculations for realistic models of amorphous transition metal alloys. The atomic structure is prepared by a simulated molecular-dynamic quench, based on interatomic forces calculated using hybridized nearly-free-electron tight-binding-bond theory. The electronic structure is calculated in the local spin-density approximation, using a linear muffin-tin orbital (LMTO) supercell approach. Detailed results for crystalline and amorphous alloys of Ni, Co and Fe with Zr are presented. NixZr1-x alloys are predicted to be paramagnetic for x<or=0.85, both in the crystalline and in the amorphous state. In CoxZr1-x the onset of magnetic ordering occurs at x approximately=0.67 for crystalline and at x approximately=0.50 for amorphous alloys. The Co-rich alloys are predicted to be strong ferrimagnets. The formation of the negative Zr moments is related to a covalent coupling of Co- and Zr-d states, which is strongest for the Co minority-spin states. The enhancement of magnetism in the glassy alloys is related to an increase of the density of states at the Fermi level induced by structural disorder. In both crystalline and amorphous FexZr1-x alloys, the onset of ferrimagnetic ordering occurs at x approximately=0.33. In contrast to the Co-based alloys, FexZr1-x alloys are weak magnets. For x>or=0.75 the competition between ferromagnetic and antiferromagnetic exchange interactions leads to the formation of negative moments on isolated Fe sites. The number of negative Fe moments increases strongly for x>or=0.90, leading to a decrease of the average moment in the Fe-rich limit. The authors show that the formation of negative local Fe moments is related to a large number of contracted Fe-Fe pairs. The predictions of local spin-density theory are found to be in excellent agreement with experiment. For all crystalline and amorphous alloys the local exchange splitting of the 3d states is shown to be correlated linearly with the local magnetic moment, with a slope of approximately=1 eV mu B-1. This relation holds for all types of magnetic order and also for the crystalline and amorphous pure metals.

Hyperfine interactions in iron substituted high- Tc superconducting oxides

The hyperfine interactions in Fe substituted copper oxide ternary and quaternary compounds with perovskite-related structures are studied, using the local density theory in an embedded cluster approach. The self-consistent electronic structure is examined for Cu and Fe sites in a number of plausible local geometries representative of La 2CuO 4, YBa 2Cu 3O 7−δ and related materials. Mössbauer isomer shifts, electric field gradients, magnetic moments and contact hyperfine fields are presented for comparison with experiment and discussed in light of lattice structure data.

Theoretical electronic structure studies of diamond: surfaces, adsorbates, defects and heterointerfaces

The implementation of self-consistent electronic structure calculations has made a large impact in the past decade on our understanding of the origins of the properties of materials. In this paper we briefly review several applications and a few of the recent calculational results of our group for the material properties of diamond: defects, diamond-metal interfaces and chemical processes occuring at diamond surfaces.

Electronic structure of epitaxial interfaces

Metal-semiconductor (Schottky barrier) and semiconductor-semiconductor (heterojunction) interfaces show rectifying barrier heights and band offsets, which are two key quantities required to optimize the performance of a device. A large number of models and empirical theories have been put forward by various workers in the field during the last 50 years. But a proper understanding of the microscopic origin of these quantities is still missing. In this article, our focus is mainly to present a unified framework for first principles investigation of the electronic structure of epitaxial interfaces, in which one of the constituents is a semiconductor. LMTO method is now a well established tool for self-consistent electronic structure calculations of solids within LDA. Such calculations, when performed on supercell geometries, are quite successful in predicting a wide range of interface specific electronic properties accurately and efficiently. We describe here the basic formalism of this LMTO-supercell approach in its various levels of sophistication and apply it to investigate the electronic structure of A- and B-type NiSi2/Si(111) interface as a prototype metal-semiconductor system, and CaF2/Si(111) interface as a prototype insulator-semiconductor system. These are a few of the most ideal lattice matched epitaxial interfaces whose atomic and electronic structures have been extensively studied using a wide range of experimental probes. We give here a glimpse of these experimental results and discuss the success as well as limitations of LDA calculations to achieve accuracies useful for the device physicists.

Self-consistent electronic structure of a cylindrical quantum wire.

The self-consistent subband electronic structure, potential profile, and charge distribution of a GaAs quantum wire with cylindrical symmetry has been obtained by a numerical solution of the coupled Schro\ifmmode\ddot\else\textasciidieresis\fi{}dinger and Poisson equations (within the Hartree approximation). The potential profile is exactly (nearly) parabolic when there are no (few) free carriers inside the wire, but approximates a stretched harmonic oscillator as charge accumulates in the cylindrical quantum wire. The dependence of the results on physical parameters such as the wire radius and concentration of surface states is analyzed.

Formation of the β-SiC c(2×2) reconstructed surface

Self-consistent electronic structure, total energy MNDO/PM3 calculations modeling the β-SiC(100) surface are used to study energetics of the c(2×2) surface reconstruction. It is shown that although the resulting atomic configurations in the ordered reconstructed surfaces prepared by different methods are not strongly dependent on the method of surface preparation, the types and the densities of the prevailing surface defects most probably are. Implications of this result for the outcome of some surface structure sensitive experiments are then briefly discussed.

Numerical accuracy of the linear muffin-tin orbitals method for dilute metallic alloys

Some analytical aspects of the linear muffin-tin orbitals (LMTO) method for metals and alloys are considered, which are directly related to the accuracy of the numerical program giving the self-consistent electronic structure of dilute impurities. It is first shown that the existence of mathematical singularities at special energies within the valence energy range in the LMTO formalism induces no discontinuity in physical quantities such as the local density of states or the total displaced charge versus energy. Then, the numerical effects of the use of more than one energy panel for the valence states in the pure matrix and in the dilute alloy are compared, and the authors discuss the existence of a limit to the intrinsic accuracy of an impurity program for a given matrix of type X, by considering a fictitious alloy XX. Finally they give, as an example, our results for dilute Al alloys, which, in contrast to a recent publication, are very close to those obtained by the Green function method.

Self-consistent Green-function method for the calculation of electronic properties of localized defects at surfaces and in the bulk

The authors present a self-consistent Green-function method which enables parameter-free calculations of the charge density, the density of states, and related quantities in electronic systems where the three- or two-dimensional translational symmetry is broken by a perturbation which is localized in real space. In particular, the method is suited to study point defects in the interior of a metallic or semiconducting crystal or at a crystal surface. The self-consistent Green operator describes an infinitely extended system. The only restrictive assumption is that the self-consistent electronic structure of the unperturbed bulk material is well reproduced by a muffin-tin (pseudo) potential. In the perturbed region, however, no significant constraint is imposed either on the shape of the potential or on the charge density.

Electronic and magnetic structure of amorphous Fe-, Co-, NiZr alloys from band theory

We show that quantitative predictions of magnetic moments in amorphous transition metal alloys are possible by parameter-free, self-consistent electronic structure calculations for realistic structural models. Detailed results for alloys of Fe, Co and Ni with Zr are presented. The physical mechanism for moment formation and for the origin of complex spin-structures in Fe-rich alloys is discussed.

Charging energy and collective response of a quantum dot resonant tunneling device

Recent self-consistent electronic structure calculations and linear response calculations of the current have sought to clarify the role of Coulomb charging of a flat quantum dot connected via tunneling barriers to a two-dimensional electron gas collector and emitter. In particular, resonant tunneling through such a structure, which is observed to be periodic in a gate voltage applied to the dot, is currently explained in terms of resonant suppression of the Coulomb blockade by the gate. Nonetheless questions remain regarding the off resonance activation energy and the possible role of collective excitations of the dot. We calculate the self-consistent electronic structure of a two-dimensional quantum dot connected via point-contact tunneling barriers to two wide 2DEG regions, in the Hartree approximation. We include the effect of finite source-drain voltage and side-gate voltages. We compute the charging energy for a single electron entering the dot and discuss the dot capacitance. Finally, we compute the approximate collective resonance frequency for charge oscillations in the dot and discuss the possibility of plasmon coupling to single-electron tunneling.

Electronic properties of isostructural intermetallics of Ce

We present self-consistent FLAPW-LSD electronic structure calculations of ferromagnetic CeAg, CeCd and CeZn, including the spin-orbit interaction in the spin-polarised formalism. The incomplete Ce 4f-state polarisation indicates the presence of Anderson hybridisation with conduction bands. The calculated magnetic moments are compared to previous theoretical and experimental results, for the three compounds.

Anderson hybridisation in CeAg

We perform a tight-binding fit of the self-consistent LMTO-LDA electronic structure of CeAg. This allows us to calculate the width Δ(ε) of the Ce 4f-levels due to hybridisation with conduction states. The f-levels are shown to hybridise with p-and d-band states at the Fermi energy.

Electronic structure and thermodynamic properties of Fe-Pt alloys

We discuss the magnetic properties of the Pt-Fe alloys using a combined approach of the fixed-spin-moment, self-consistent electronic structure energy calculations for T = 0, and the improved spin- and density fluctuation theory for T > 0. For T = 0, we show the total energy and equations of state as functions of moment and volume. Special attention is paid to the issue of high- to low-moment transition for PtFe 3 Invar. Further, we discuss some improvements to the previously formulated spin- and density fluctuation theory. This theory, using the T = 0 total energy surfaces as a starting point, allows for a description of the thermal behavior of the magnetic and elastic properties of a magnetic material. We discuss the temperature dependence of the volume magnetostriction and bulk modulus, and the pressure dependence of the critical temperature for PtFe 3. We compare these quantities with the available experimental data.

Valence instability at the surface of rare-earth metals: samarium

Employing both LMTO and FLAPW band methods, we performed self-consistent electronic structure calculations on various Sm systems: Sm-atom, bulk-Sm, monolayer-Sm, 5-layer fcc(001)Sm and Sm/Al(001). It is found that the electronic properties, such as charge density, density of states, and the valence at the surface are not drastically different from the values in the bulk. Furthermore, for the ideal fcc Sm(001) surface we found that the trivalent state is energetically more stable than the divalent by about 0.2 Ry.

Self-consistent electronic structure and optical selection rules for tetrahedral quantum dots

Recently, Fukui et al. (22nd conference on solid state devices and materials, 1990, Japan) have employed a controlled faceting technique in metal organic chemical vapor deposition (MOCVD) crystal growth to construct tetrahedral quantum dots without resorting to ion-implantation or chemical-etching techniques. We investigate the electronic structure of these dots, in the two-dimensional flat triangular approximation, with Hartree and exchange-correlation interactions between electrons taken into account. We compute the self-consistent electronic structure of a triangular dot in a density functional formalism, taking into account finite barrier height and treating exchange and correlation in the local density approximation. We present the results of eigenvalues and density profiles as a function of charge number N, dot size, and (perpendicular)magnetic field strength. We show that the three-fold symmetry of the dot implies a form of the Bloch theorem that separates the eigenfunctions into ‘zones’. This allows us further to derive selection rules for long-wavelength electromageetic waves polarized arbitrarily within the plane of the dot. These selection rules are independent of self-consistent effects.