Quantum Theory of Advanced Materials

This paper gives a brief overview of the capability of modern electron-structure calculations, the widely used density-functional theory, and the challenge to search for the exact nonlocal exchange-correlation functional. The study of the thermal properties of silicon is used as an example to illustrate the accuracy accomplished by the state-of-the-art first-principles calculations. A recent attempt to extract quantities of central importance in density function theory via computational many-body techniques is also discussed.

A model interaction is introduced for quantum many-body simulations of Coulomb systems using periodic I boundary conditions. The interaction gives much smaller finite size effects than the standard Ewald interaction and is also much faster to compute. Variational quantum Monte Carlo simulations of diamond-structure silicon with up to 1000 electrons demonstrate the effectiveness of our method.

A combination of the coupling constant integration technique and the quantum Monte Carlo method is used to investigate the most relevant quantities in Kohn-Sham density-functional theory. Variational quantum Monte Carlo is used to construct realistic many-body wave functions for diamond-structure silicon at different values of the Coulomb coupling constant. The exchange-correlation energy density along with the coupling constant dependence and the coupling-constant-integrated form of the pair-correlation function, the exchange-correlation hole, and the exchange-correlation energy are presented. Comparisons of these functions an mode with results obtained from the local-density approximation, the average density approximation, the weighted density approximation, and the generalized gradient approximation. We discuss reasons for the success of the local-density approximation. The insights provided by this approach will make it possible to carry out stringent tests of the effectiveness of exchange-correlation functionals and in the long term aid in the search for better functionals. [S0163-1829(98)02115-8].

We have evaluated the surface free energies of hydrogen-covered (100), (111), and (110) surfaces of diamond and silicon as a function of the hydrogen chemical potential using first-principles methods. The change in surface-energy anisotropy and equilibrium crystal shape due to hydrogen adsorption is examined. The three low-index facets are affected differently by the presence of hydrogen and unexpected differences are found between diamond and silicon. Taking into account possible formation of local facets on the hydrogen-covered (100) surfaces, we find that the dihydride phase is not stable on both C(100) and Si(100). nor is the 3x1 phase on C(100).

Using classical mechanical and quantum Monte Carlo methods we trace the ground-state behavior with an applied magnetic field of localized electron pair states in a quantum dot. By developing a method to treat nonconserved paramagnetic interactions using variational and diffusion quantum Monte Carlo techniques we find (i) a single-triplet transition at very small magnetic field strengths, (ii) enhanced localization of the two electrons with increasing magnetic field, and (iii) a mechanism for pair breakup that is different from that proposed recently by Wan et al. [Phys. Rev. Lett. 75, 2879 (1995)]. [S0163-1829(98)04016-8].

When disilane (Si2H6) is used in the homoepitaxial growth of Si by chemical vapor deposition (CVD), the fragment SiH2 is believed to be the basic unit adsorbed on the surface. The bonding site of SiH2 on Si(100) has been proposed in the literature to be either on top of a dimer (the on-dimer site) or between two dimers in the same row (the intrarow site). Since the pathway of SiH2 combination is dependent on the adsorption site, a first-principles calculation will shed light on the underlying process. We have performed self-consistent pseudopotential density-functional calculations within the local-density approximation. On the bare Si(100) surface, the on-dimer site is found to be more stable than the intrarow site, even though the former has unfavorable Si-Si bond angles. This is ascribed to the extra dangling bond created in the latter geometry when the weak dimer a bonds are broken. However, the presence of hydrogen adatoms eliminates this difference and makes the intrarow site more favorable than the on-dimer site. It is therefore revealed in this theoretical study that hydrogen, an impurity unavoidable in the CVD process, plays an important role in determining the stable configuration of adsorbed SiH2 on Si(100) and hence affects the growth mechanism. [S0163-1829(98)52544-1].

Angle-resolved photoemission has been utilized to study the surface electronic structure of 1/3 monolayer of Sn on Ge(lll) in both the room-temperature (root3 x root3)R30 degrees phase and the low-temperature (3 x 3) charge-density-wave phase. The results reveal a gap opening around the (3 x 3) Brillouin zone boundary, suggesting a Peierls-like transition despite the well-documented lack of Fermi nesting, a highly sensitive electronic response to doping by intrinsic surface defects is the cause for this unusual behavior, and a detailed calculation illustrates the origin of the (3 x 3) symmetry.

The charge-density-wave transition in TiSe2, which results in a commensurate (2 X 2 X 2) superlattice at temperatures below similar to 200 K, presumably involves softening of a zone-boundary phonon mode. For the first time, this phonon-softening behavior has been examined over a wide temperature range by synchroton x-ray thermal diffuse scattering.

We have studied the pair-correlation function, the exchange-correlation hole, and the exchange-correlation energy density of the valence electrons in the Si atom using the Coulomb-coupling constant integration technique with the variational quantum Monte Carlo method. These quantities are compared to those derived from various approximate models within the Kohn-Sham density functional theory. We find that the charge density prefactor in the expression for the exchange-correlation hole dominates the errors found in the local spin density approximation (LSDA), that the generalized gradient approximation improves energy calculations by improving the LSDA at long ranges, and that the weighted spin density approximation, which uses the correct charge density prefactor, gives the lowest root mean square error for the exchange-correlation energy density.

We have studied the structural stability of thin silver films with thicknesses of N = 1 to 15 monolayers, deposited on an Fe(100) substrate. Photoemission spectroscopy results show that films of N = 1, 2, and 5 monolayer thicknesses are structurally stable for temperatures above 800 kelvin, whereas films of other thicknesses are unstable and bifurcate into a film with N +/- 1 monolayer thicknesses at temperatures around 400 kelvin, The results are in agreement with theoretical predictions that consider the electronic energy of the quantum well associated with a particular film thickness as a significant contribution-to the film stability.

We present a detailed study of the exchange-correlation hole and exchange-correlation energy per particle in the Si crystal as calculated by the variational Monte Carlo method and predicted by various density-functional models. Nonlocal density-averaging methods prove to be successful in correcting severe errors in the local-density approximation (LDA) at low densities where the density changes dramatically over the correlation length of the LDA hole. but fail to provide systematic improvements at higher densities where the effects of density inhomogeneity are more subtle. Exchange and correlation considered separately show a sensitivity to the nonlocal semiconductor-crystal environment, particularly within the Si bond. which is not predicted by the nonlocal approaches based on density averaging. The exchange hole is well described by a bonding-orbital picture, while the correlation hole has a significant component due to the polarization of the nearby bonds, which partially screens out the anisotropy in the exchange hole.

The work function of atomically uniform Ag films grown on Fe(100) is measured as a function of film thickness. It shows layer-resolved variations as a result of quantum confinement of the valence electrons. A first-principles calculation reproduces the observed variations except for very thin films (one and two monolayers), and the differences can be attributed, in part, to strain effects caused by the lattice mismatch between Ag and Fe. These results illustrate the close interaction between interface effects and surface properties.

We have carried out first-principles calculations of Pb (111) films up to 25 monolayers to study the oscillatory quantum size effects exhibited in the surface energy and work function. These oscillations are correlated with the thickness dependence of the energies of confined electrons, which can be properly modeled by an energy-dependent phase shift of the electronic wave function upon reflection at the interface. It is found that a quantitative description of these quantum size effects requires a full consideration of the crystal band structure.

One third of a monolayer of Sn adsorbed on Ge(111) undergoes a broad phase transition upon cooling from a (root3 x root3)R30degrees normal phase at room temperature to a (3 x 3) phase at low temperatures. Since band-structure calculations for the ideal (root3 x root3)R30degrees phase show no Fermi-surface nesting, the underlying mechanism for this transition has been a subject of much debate. Evidently, defects formed by Ge substitution for Sn in the adlayer, at a concentration of just a few percent, play a key role in this complex phase transition. Surface areas near these defects are pinned to form (3 x 3) patches above the transition temperature. Angle-resolved photoemission is employed to examine the temperature-dependent band structure, and the results show an extended gap forming in k-space as a result of band splitting at low temperatures. On account of the fact that the room temperature phase is actually a mixture of (root3 x root3)R30degrees areas and defect-pinned (3 x 3) areas, the band structure for the pure (root3 x root3)R30degrees phase is extracted by a difference-spectrum method. The results are in excellent agreement with band calculations. The mechanism for the (3 x 3) transition is discussed in terms of a response function and a tight-binding cluster calculation. A narrow bandwidth and a small group velocity near the Fermi surface render the system highly sensitive to surface perturbations, and formation of the (3 x 3) phase is shown to involve a Peierls-like lattice distortion mediated by defect doping. Included in the discussion, where appropriate, are dynamic effects and many-body effects that have been previously proposed as possible mechanisms for the phase transition.

Angle-resolved photoemission is employed to measure the band structure of TiSe2 in order to clarify the nature of the (2x2x2 ) charge density wave transition. The results show a very small indirect gap in the normal phase transforming into a larger indirect gap at a different location in the Brillouin zone. Fermi surface topology is irrelevant in this case. Instead, electron-hole coupling together with a novel indirect Jahn-Teller effect drives the transition.

Real-time in situ x-ray studies of continuous Pb deposition on Si(111)-(7x7) at 180 K reveal an unusual growth behavior. A wetting layer forms first to cover the entire surface. Then islands of a fairly uniform height of about five monolayers form on top of the wetting layer and grow to fill the surface. The growth then switches to a layer-by-layer mode upon further deposition. This behavior of alternating layer and island growth can be attributed to spontaneous quantum phase separation based on a first-principles calculation of the system energy.

Quasiparticle band structures for the cubic trihydrides YH3 and LaH3 have been calculated by evaluating the self-energy in the GW approximation using ab initio pseudopotentials and plane waves. These are the prototype metal hydrides that exhibit switchable optical properties. For both materials, the local-density approximation (LDA) yields semimetallic energy bands with a direct overlap of about 1 eV. We find the self-energy correction to the LDA energies opens a gap at Gamma of 0.8-0.9 eV for LaH3 and 0.2-0.3 eV for YH3, where the latter is in sharp contrast to a previous study using linear-muffin-tin orbitals. The quasiparticle band gaps are analyzed as a function of an initial shift in the LDA bands used to evaluate the random-phase approximation screening in constructing the self-energy. We also make a comparison of results obtained by using two different pseudopotentials, each designed to better approximate exchange and correlation between the semicore states and valence states of Y and La.

We have studied the properties of quantum well states in supported Ag(100) films on the Fe substrate by first-principles density-functional calculations. The energies of these quantum well states as a function of thickness N are examined in terms of the characteristic phase shift of the electronic wave function at the interface. These energy-dependent phase shifts are determined numerically for both the film-substrate and film-vacuum interfaces. It is also found that the substrate has a major effect on film stability, enhancing the stability of the N=5 film and reversing that of the N=2 film.

Atomically uniform Pb films are successfully prepared on Si(111), despite a large lattice mismatch. Angle-resolved photoemission measurements of the electronic structure show layer-resolved quantum well states which can be correlated with dramatic variations in thermal stability. The odd film thicknesses N=5, 7, and 9 monolayers show sharp quantum well states. The even film thicknesses N=6 and 8 do not, but are much more stable than the odd film thicknesses. This correlation is discussed in terms of a total energy calculation and Friedel-like oscillations in properties.

We investigate the structural, electronic, and optical properties of hydrogen-passivated silicon nanowires along [110] and [111] directions with diameter d up to 4.2 nm from first principles. The size and orientation dependence of the band gap is investigated and the local-density gap is corrected with the GW approximation. Quantum confinement becomes significant for d<2.2 nm, where the dielectric function exhibits strong anisotropy and new low-energy absorption peaks start to appear in the imaginary part of the dielectric function for polarization along the wire axis.

We present a first-principles investigation of the structural properties, electronic structure, and the chemical stability of the complex hydrides NaAlH(4) and Na(3)AlH(6). The calculations are performed within the density functional framework employing norm conserving pseudopotentials. The structural properties of both hydrides compare well with experimental data. A detailed study of the electronic structure and the charge-density redistribution reveal the features of an ionic covalent bonding between Al and H in the (AlH(4))(-) and (AlH(6))(-3) anionic complexes embedded in the matrix of Na(+) cations. The orbital hybridization and the characteristics of bonding orbitals within the complexes are identified. The calculated reaction energies of these complex hydrides are in good agreement with the experimentally determined values.

The atomic geometry, electronic structure, and magnetic moment of 4d transition-metal clusters with 13 atoms are studied by pseudopotential density-functional calculations. We find a new buckled biplanar structure with a C-2v symmetry stabilized by enhanced s-d hybridization. It has a lower energy than the close-packed icosahedral or cuboctahedral structure for elements with more than half-filled d shells. The magnetic moments of this buckled biplanar structure are found to be smaller than those of the icosahedral structure and closer to available experimental results.

Hydrogenation-induced metal-semiconductor transitions usually occur in simple systems based on rare earths and/or magnesium, accompanied by major reconstructions of the metal host (atom shifts >2 Angstrom). We report on the first such transition in a quaternary system based on a transition element. Metallic LaMg2Ni absorbs hydrogen near ambient conditions, forming the nonmetallic hydride LaMg2NiH7 which has a nearly unchanged metal host structure (atom shifts <0.7 Angstrom). The transition is induced by a charge transfer of conduction electrons into tetrahedral [NiH4](4-) complexes having closed-shell electron configurations.