Quantum Theory of Advanced Materials

n/a

n/a

n/a

n/a

n/a

Two-dimensional (2D) topological insulators (TIs) are promising platforms for low-dissipation spintronic devices based on the quantum-spin-Hall (QSH) effect, but experimental realization of such systems with a large band gap suitable for room-temperature applications has proven difficult. Here, we report the successful growth on bilayer graphene of a quasi-freestanding WSe2 single layer with the 1T′ structure that does not exist in the bulk form of WSe2. Using angle-resolved photoemission spectroscopy (ARPES) and scanning tunneling microscopy/spectroscopy (STM/STS), we observe a gap of 129 meV in the 1T′ layer and an in-gap edge state located near the layer boundary. The system′s 2D TI characters are confirmed by first-principles calculations. The observed gap diminishes with doping by Rb adsorption, ultimately leading to an insulator–semimetal transition. The discovery of this large-gap 2D TI with a tunable band gap opens up opportunities for developing advanced nanoscale systems and quantum devices.

Two-dimensional materials constitute a promising platform for developing nanoscale devices and systems. Their physical properties can be very different from those of the corresponding three-dimensional materials because of extreme quantum confinement and dimensional reduction. Here we report a study of TiTe2 from the single-layer to the bulk limit. Using angle-resolved photoemission spectroscopy and scanning tunneling microscopy and spectroscopy, we observed the emergence of a (2 × 2) charge density wave order in single-layer TiTe2 with a transition temperature of 92 ± 3 K. Also observed was a pseudogap of about 28 meV at the Fermi level at 4.2 K. Surprisingly, no charge density wave transitions were observed in two-layer and multi-layer TiTe2, despite the quasi-two-dimensional nature of the material in the bulk. The unique charge density wave phenomenon in the single layer raises intriguing questions that challenge the prevailing thinking about the mechanisms of charge density wave formation.

Hydrogenation of the inten-netallic compound LaMg2Pd at 200 degrees C and 10 bar leads to a complex metal hydride of composition LaMg2PdH7. Its structure has orthorhombic symmetry and displays tetrahedral [PdH4](4-) anions. The Pd-H bond distances as measured on the deuteride range from 1.71 to 1.78 angstrom and the H-Pd-H bond angles from 95 degrees to 122 degrees. Three additional hydride anions H- occupy La2Mg2-type interstices having tetrahedral metal configurations. Band structure calculations suggest the hydride to be non-metallic and to have a band gap of similar to 1.0ev. The compound desorbs hydrogen at 125 degrees C yielding a pressure of more than I bar absolute. (C) 2006 Elsevier B.V. All rights reserved.

We present a first-principles investigation of the lattice dynamics and thermodynamical properties of a complex hydride NaAlH(4), a promising material for hydrogen storage. The calculations are performed within the density-functional-theory framework and using a linear response theory. Calculations of the phonon spectrum, Born effective charges Z(*), and dielectric constants in high and low frequency limits are reported. The mode characters of the zone-center phonons, including the LO-TO splitting, are identified and compared to the experiment. The quasiharmonic approach is used to study thermal expansion as well as the mean square displacement of each atom as a function of temperature. A connection is established between the latter and the melting point. The inclusion of the zero-point motion leads to an expanded lattice compared to the static lattice, while the low frequency oscillations are found to play an important role in the melting and decomposition of NaAlH(4).

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 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 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 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.

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].