Showing papers by "Francesco Mauri published in 1999"
TL;DR: Two-dimensional supramolecular clusters and chains are observed upon submonolayer deposition of 1-nitronaphthalene (NN) onto reconstructed Au(111) as discussed by the authors.
Abstract: Two-dimensional supramolecular clusters and chains are observed upon submonolayer deposition of 1-nitronaphthalene (NN) onto reconstructed Au(111) The molecules become pseudochiral upon adsorption Their handedness is determined from high-resolution scanning tunneling microscope images and local-density calculations Modeling shows that hydrogen bonds cause the observed self-assembly Clusters and chains mutually interact via electrostatic repulsion
373 citations
TL;DR: In this article, the MPL approach was applied to calculate the chemical shifts in condensed matter systems using periodic boundary conditions using density functional theory, where the wave functions are expanded in plane waves.
Abstract: A theory (MPL) to compute the NMR chemical shifts in condensed matter systems using periodic boundary conditions was presented by F. Mauri, B. Pfrommer, and S. G. Louie [Phys. Rev. Lett. 77, 5300 (1996)]. The MPL method has been implemented so far within a pseudopotential formulation in which the wave functions are expanded in plane waves. In this paper, we compare analytically the MPL approach within the density functional theory to existing methods for the calculation of the chemical shifts such as GIAO (gauge-including atomic orbitals), CSGT (continuous set of gauge transformations), and IGAIM (individual gauges for atoms in molecules). To this end we apply the MPL approach to molecules since the latter methods are conceived only for finite systems. We show theoretically the equivalence between a variant of the CSGT and the MPL method applied to finite systems. Moreover, we analyze numerically the efficiency of the different methods when atomic orbital basis sets are employed, by comparing the basis-se...
116 citations
TL;DR: In this paper, it was shown that the atomic layers close to the surface experience a variation of the chemical shift, which is proportional to the density of empty surface states, and this effect could be used as a direct probe of the surface state density profile, by measuring experimentally the NMR chemical shift resolved for each atomic layer.
Abstract: We show with an {ital ab initio} calculation that electronic surface states have a strong effect on NMR chemical-shift spectra. For the hydrogen-chemisorbed diamond (111) surface, we find that the atomic layers close to the surface experience a variation of the chemical shift, which is proportional to the density of empty surface states. This effect could be used as a direct probe of the surface-state density profile, by measuring experimentally the NMR chemical shift resolved for each atomic layer. {copyright} {ital 1999} {ital The American Physical Society}
22 citations
TL;DR: In this paper, a detailed ab initio study of the BT8 and BC32 phases in silicon was performed in a density-functional theory in the local density approximation, and it was shown that the energy barrier for phason defect formation in the BC8 phase is about 0.12 eV per jumping atom and the energy of the defect is very small (less then 0.02eV per hopping atom).
Abstract: We show that special structural changes (phason atomic jumps) can transform the BC8 and R8 phases, known to occur in Si and Ge, into a phase with tetrahedral coordination. These structural changes are analogous to the BC8-R8 transformation. The phase has the body-centered-tetragonal symmetry (the space group ${C}_{4h}^{6}\ensuremath{-}{I4}_{1}/a)$ with 16 atoms per unit cell or 8 atoms per primitive cell and will be referred to as BT8. All the atoms occupy equivalent general positions $x,y,z$ and there are three different tetrahedral bonds. It is shown that all three phases, BT8, BC8, and R8 may be considered as a result of phason-ordering transitions from a disordered phase with $\mathrm{Ia}3\ifmmode\bar\else\textasciimacron\fi{}d$ symmetry. An infinite number of other structures with tetrahedral bonding (ordered and disordered) can be constructed this way. The phason jumps are typical of these phases because they are related to hypothetical icosahedral quasicrystals. In particular, the BC8 and the recently suggested BC32 structure may be considered as $1/0$ and $1/1$ cubic approximants of the same quasicrystal. We present a detailed ab initio study of BT8 and BC32 phases in silicon within density-functional theory in the local-density approximation. Our results show that the energetics and diffraction patterns of the BT8 phase are very similar to those of the BC8 and R8 phases. We conclude that the available diffraction data do not exclude the existence of the BT8 phase. In contrast, the energy of the BC32 structure is significantly larger (for positive pressure), and the existence of this phase is therefore questionable. The pressure dependence of the bond lengths, angles, and the lattice parameters of the BT8 phase is investigated. Our estimation of the energy barrier for phason defect formation in the BC8 phase is about 0.12 eV per jumping atom and the energy of the defect is very small (less then 0.02 eV per jumping atom), therefore this type of defect could be present in real samples. Finally, we show that the computed elastic constants of the BC8 phase are almost isotropic as expected for the approximants of icosahedral quasicrystals.
14 citations
TL;DR: In this paper, the authors investigate atomic and molecular nanostructures on metal surfaces by variable low-temperature scanning tunnelling microscopy and achieve a detailed understanding of the stability of these structures.
Abstract: We investigate atomic and molecular nanostructures on metal surfaces by variable low-temperature scanning tunnelling microscopy. In combination with molecular dynamics calculations we achieve a detailed understanding of the stability of these structures.¶Atomic nanostructures in homoepitaxial metallic systems are thermodynamically only metastable. Two-dimensional islands on Ag(110) decay above a threshold temperature of Tl=175 K. Caused by the anisotropy of the surface, distinct decay behaviours exist above and below a critical temperature of Tc=220 K. Calculations based on effective medium potentials of the underlying rate limiting atomic processes allow us to identify the one-dimensional decay below Tc as well as the two-dimensional decay above Tc.¶In contrast to atoms, the intermolecular electrostatic interaction of polar molecules leads to thermodynamically stable structures. On the reconstructed Au(111) surface, the pseudo-chiral 1-nitronaphthalin forms two-dimensional supermolecular clusters consisting predominantly of ten molecules. Comparison of images with submolecular resolution to local density calculations elucidates the thermodynamical stability as well as the internal structure of the decamers.
12 citations
Abstract: We study the vibrational properties of hydrogenated tetrahedral amorphous carbon from first principles. Our results reproduce the C density of states (DOS) measured by electron energy loss. We decompose the theoretical DOS in terms of vibrational modes of microscopic units. These partial DOS have large overlaps in frequency. Only the region between 1350 and 1600 cm−1 is covered by a single type of stretching (sp2–sp2), whose DOS reproduces the experimental visible Raman spectra. We find a linear correlation between CH stretch frequency and CH bond length, and we show that the spreading of CH distances due to the constraints imposed by the C network is an important source of broadening of the CH stretching spectrum.
7 citations