Quantum Theory of Advanced Materials

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.

We propose a simple method to evaluate the energies of ideal metal surfaces as a function of orientation based on cluster energy expansion. By symmetry only clusters with even-number corners will be present. It is found that the energy expansion converges rapidly and in most cases can be truncated at the pair interaction level. The parameters can be determined from a limited number of low-index surface energies obtained from first-principles calculations. The equilibrium crystal shape at T = O is then predicted and the step energy on major facets is derived for some fee metals.

We present a first-principles investigation to study the possible alloy phases of sodium and lithium alanates. Structural and energetics properties of alloy systems Na(1-x)Li(x)AlH(4) and Na(3(1-x))Li(3x)AlH(6) are studied via phase interpolation. Alloy system Na(1-x)Li(x)AlH(4) is found to have a small mixing energy (<5 kj/mol). The equilibrium structure undergoes a transition from a tetragonal structure to a monoclinic structure between x = 0.25 and 0.5. Within each structure the cell volume decreases with increasing x, which can be explained by Li having a smaller ion size than Na. Alloy system Na(3(1-x))Li(3x)AlH(6) is also studied, and one intermediate composition Na(2)LiAlH(6) is found to be stable in agreement with experimental findings. (C) 2009 Elsevier B.V. All rights reserved.

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.

Recently it was discovered that a total of 5.6 wt. % H-2 could be released from the 1:2 mixture of lithium amide and magnesium hydride at temperatures as low as 150 degrees C. With a reaction enthalpy of 44 KJ/mol H-2, this system has high potential for on-board hydrogen storage applications. The fully desorbed product is believed to be a mixed lithium and magnesium imide Li2Mg(NH)(2). In this work, the crystal structure of this mixed imide is studied from total-energy density-functional calculations. Based on a recent experimentally established space group, possible ordered configurations are examined. Important local orderings are identified for the experimentally observed disordered phase at room temperature. These unique local arrangements are also connected with the observed structural transitions above room temperature. In addition, the local ordering in Mg(NH2)(2) is analyzed. The similarity and difference of local arrangements among hydrogen, cations, and vacancies are discussed for the three amide (imide) systems: LiNH2, Mg(NH2)(2), and Li2Mg(NH)(2). The identification of the cation and hydrogen local orderings are expected to facilitate the design of new mixed imides and amides as hydrogen storage materials with desired physical properties.

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.

We studied the hydrogen-induced reconstruction of the W(110) surface using the pseudopotential plane wave approach. The calculations for a full monolayer of hydrogen coverage showed that the quasithreefold hollow site (distorted bridge) has the lowest energy, and that for this geometry a surface reconstruction, consisting of a small uniform shift of the W top layer in the [1(1) over bar0$] direction, is energetically favorable. We also studied the surface states for clean and H-covered W(110) and investigated the effect of the reconstruction on electronic structure.

The Hofstadter butterfly spectrum for Landau levels in a two-dimensional periodic lattice is a rare example exhibiting fractal properties in a truly quantum system. However, the observation of this physical phenomenon in a conventional material will require a magnetic field strength several orders of magnitude larger than what can be produced in a modern laboratory. It turns out that for a specific range of rotational angles twisted bilayer graphene serves as a special system with a fractal energy spectrum under laboratory accessible magnetic field strengths. This unique feature arises from an intriguing electronic structure induced by the interlayer coupling. Using a recursive tight-binding method, we systematically map out the spectra of these Landau levels as a function of the rotational angle. Our results give a complete description of LLs in twisted bilayer graphene for both commensurate and incommensurate rotational angles and provide quantitative predictions of magnetic field strengths for observing the fractal spectra in these graphene systems.