Showing papers by "Francesco Mauri published in 2005"
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TL;DR: The Born-Oppenheimer approximation (BO) has proven effective for the accurate determination of chemical reactions, molecular dynamics and phonon frequencies in a wide range of metallic systems as discussed by the authors.
Abstract: The Born-Oppenheimer approximation (BO) has proven effective for the accurate determination of chemical reactions, molecular dynamics and phonon frequencies in a wide range of metallic systems. Graphene, recently discovered in the free state, is a zero band-gap semiconductor, which becomes a metal if the Fermi energy is tuned applying a gate-voltage Vg. Graphene electrons near the Fermi energy have twodimensional massless dispersions, described by Dirac cones. Here we show that a change in Vg induces a stiffening of the Raman G peak (i.e. the zone-center E2g optical phonon), which cannot be described within BO. Indeed, the E2g vibrations cause rigid oscillations of the Dirac-cones in the reciprocal space. If the electrons followed adiabatically the Dirac-cone oscillations, no change in the phonon frequency would be observed. Instead, since the electron-momentum relaxation near the Fermi level is much slower than the phonon motion, the electrons do not follow the Dirac-cone displacements. This invalidates BO and results in the observed phonon stiffening. This spectacular failure of BO is quite significant since BO has been the fundamental paradigm to determine crystal vibrations from the early days of quantum mechanics.
971 citations
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TL;DR: The comparison of the results with the scattering lengths fitted from experimental I-V curves indicates the presence of a nonequilibrium optical phonon heating induced by electron transport, and predicts an effective temperature for optical phonons of thousands Kelvin.
Abstract: We demonstrate the key role of phonon occupation in limiting the high-field ballistic transport in metallic carbon nanotubes. In particular, we provide a simple analytic formula for the electron transport scattering length, which we validate by accurate first principles calculations on (6, 6) and (11, 11) nanotubes. The comparison of our results with the scattering lengths fitted from experimental I-V curves indicates the presence of a nonequilibrium optical phonon heating induced by electron transport. We predict an effective temperature for optical phonons of thousands Kelvin.
259 citations
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TL;DR: Using density functional theory, it is demonstrated that superconductivity in C6Ca is phonon mediated with lambda = 0.83 and phonon frequency (omega)log = 24.7 meV.
Abstract: Using density functional theory, we demonstrate that superconductivity in ${\mathrm{C}}_{6}\mathrm{Ca}$ is phonon mediated with $\ensuremath{\lambda}=0.83$ and phonon frequency $⟨\ensuremath{\omega}{⟩}_{\mathrm{log} }=24.7\text{ }\text{ }\mathrm{meV}$. The calculated isotope exponents are $\ensuremath{\alpha}(\mathrm{Ca})=0.24$ and $\ensuremath{\alpha}(\mathrm{C})=0.26$. Superconductivity is due mostly to C vibrations perpendicular and Ca vibrations parallel to the graphite layers. Since the electron-phonon couplings of these modes are activated by the presence of an intercalant Fermi surface, the occurrence of superconductivity in graphite intercalated compounds requires a noncomplete ionization of the intercalant.
185 citations
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TL;DR: A clear correlation between a large chemical shift change and both a short H...O distance and a CHO bond angle greater than 130 degrees is observed, thus showing that directionality is important in C-H...O hydrogen bonding.
Abstract: Two-dimensional 1H−13C MAS-J-HMQC solid-state NMR spectra of the two anomeric forms of maltose at natural abundance are presented. The experimental 1H chemical shifts of the CH and CH2 protons are assigned using first-principles chemical shift calculations that employ a plane-wave pseudopotential approach. Further calculations show that the calculated change in the 1H chemical shift when comparing the full crystal and an isolated molecule is a quantitative measure of intermolecular C−H···O weak hydrogen bonding. Notably, a clear correlation between a large chemical shift change (up to 2 ppm) and both a short H···O distance (<2.7 A) and a CHO bond angle greater than 130° is observed, thus showing that directionality is important in C−H···O hydrogen bonding.
179 citations
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TL;DR: Calculations using the two experimental structures for L-alanine have shown that, while the calculated isotropic chemical shift values of 13C and 15N are relatively insensitive to small differences in the experimental structure, the 17O shift is markedly affected.
Abstract: C-13, N-14, N-15, O-17, and Cl-35 NMR parameters, including chemical shift tensors and quadrupolar tensors for N-14, O-17, and Cl-35, are calculated for the crystalline forms of various amino acids under periodic boundary conditions and complemented by experiment where necessary. The C-13 shift tensors and N-14 electric field gradient (EFG) tensors are in excellent agreement with experiment. Similarly, static O-17 NMR spectra could be precisely simulated using the calculation of the full chemical shift (CS) tensors and their relative orientation with the EFG tensors. This study allows correlations to be found between hydrogen bonding in the crystal structures and the O-17 NMR shielding parameters and the Cl-35 quadrupolar parameters, respectively. Calculations using the two experimental structures for L-alanine have shown that, while the calculated isotropic chemical shift values of C-13 and N-15 are relatively insensitive to small differences in the experimental structure, the O-17 shift is markedly affected.
166 citations
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TL;DR: Shielding parameters for all three nuclei have been calculated using Density Functional Theory (DFT) together with the Gauge Including Projector Augmented Wave (GIPAW) method which takes full allowance for the repetition inherent in crystalline structures.
Abstract: The 1H, 13C and 19F magic-angle spinning NMR spectra have been recorded for Form 1 of flurbiprofen. In the case of 19F, spinning sideband analysis has produced data for the components of the shielding tensor. The chemical shift of the hydrogen-bonded proton was found to be 14.0 ppm. Shielding parameters for all three nuclei have been calculated using Density Functional Theory (DFT) together with the Gauge Including Projector Augmented Wave (GIPAW) method which takes full allowance for the repetition inherent in crystalline structures. Such computations were made for the reported geometry, for a structure with all the atomic positions relaxed using DFT, and with only the hydrogen positions relaxed. The relationships of the computed shifts to those observed are discussed. In general, the correlations are good.
135 citations
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TL;DR: The simulated 2D-3QMAS NMR spectrum of the CAS glass is in very good agreement with the available experimental data, notably because it takes into account the disorder present in the glass.
Abstract: We have computed the 17O NMR parameters of an amorphous calcium aluminosilicate (CAS) from firstprinciples. The atomic coordinates of a CAS glass of composition (CaO)0.21(Al2O 3)0.12(SiO 2)0.67 were obtained by quenching a liquid to room temperature by the means of ab initio molecular-dynamics simulations of the Car-Parrinello type. The structure of the glass is found to be overall in good agreement with diffraction experiments. Some excess nonbridging O (NBO) atoms are found and are compensated by tricluster O atoms, i.e., by 3-fold coordinated O atoms to 4-fold coordinated Al or Si atoms. The glass coordinates were used to compute the 17O NMR parameters using GGA-DFT and a correction of the Ca 3d orbital energy. The chemical shifts and the electric field gradients were obtained with the gauge including projector augmented-wave (GIPAW) and the projector augmented-wave (PAW) methods, respectively. The simulated 2D-3QMAS NMR spectrum of the CAS glass is in very good agreement with the available experimental data, notably because it takes into account the disorder present in the glass. This agreement further validates our CAS glass model. We show that the oxygen triclusters are not visible in a 2D-3QMAS NMR 17O spectrum since their NMR parameters overlap with those of the Al-O-Si, Si-O-Si, or Al-O-Al sites. Finally, correlations between the structural characteristics and the values of the NMR parameters are extracted from the calculation with the aim of helping the interpretation of NMR spectra of glasses of similar compositions.
83 citations
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TL;DR: This work presents a self-interaction-corrected density-functional-theory approach for the description of systems with an unpaired electron or hole such as spin-$1∕2$ defect centers in solids or radicals and shows that it corrects for the well-known failures of standard DFT functionals in this system.
Abstract: We present a self-interaction-corrected (SIC) density-functional-theory (DFT) approach for the description of systems with an unpaired electron or hole such as spin-$1∕2$ defect centers in solids or radicals. Our functional is easy to implement and its minimization does not require additional computational effort with respect to ordinary DFT functionals. In particular it does not present multiminima, as do the conventional SIC functionals. We successfully validate the method studying the hole self-trapping in quartz associated with the Al substitutional impurity. We show that our approach corrects for the well-known failures of standard DFT functionals in this system.
82 citations
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TL;DR: In this paper, the authors investigated the OH-stretching modes of kaolinite group minerals, namely, dickite, and nacrite, within the density functional theory framework and obtained good agreement between theoretical and experimentally derived structures.
Abstract: The OH-stretching modes of kaolinite group minerals, namely kaolinite, dickite, and nacrite, are investigated within the density functional theory framework. Good agreement between theoretical and experimentally derived structures is obtained. The total energy of the three polymorphs is found to be similar within 1 kJ/mol. After a review of existing experimental data, infrared and Raman spectra of the three polymorphs are computed. While interpreting the spectra, special attention is given to the role played by the macroscopic geometry of the system, explaining the differences experimentally observed between infrared and Raman spectra. In dickite and nacrite, a significant interlayer coupling is observed for the modes polarized along a direction nearly parallel to c *. The assignment of OH-stretching bands is given in a consistent way and is compared with previous suggestions.
81 citations
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TL;DR: The temperature dependence of (17)O and (25)Mg NMR chemical shifts in solid MgO have been calculated using a first-principles approach and it is shown that the chemical shift behavior with temperature cannot be explained by thermal expansion alone.
Abstract: The temperature dependence of 17O and 25Mg NMR chemical shifts in solid MgO have been calculated using a first-principles approach. Density functional theory, pseudopotentials, a plane-wave basis set, and periodic boundary conditions were used both to describe the motion of the nuclei and to compute the NMR chemical shifts. The chemical shifts were obtained using the gauge-including projector augmented wave method. In a crystalline solid, the temperature dependence is due to both (i) the variation of the averaged equilibrium structure and (ii) the fluctuation of the atoms around this structure. In MgO, the equilibrium structure at each temperature is uniquely defined by the cubic lattice parameters, which we take from experiment. We evaluate the effect of the fluctuations within a quasiharmonic approximation. In particular, the dynamical matrix, defining the harmonic Hamiltonian, has been computed for each equilibrium volume. This harmonic Hamiltonian was used to generate nuclear configurations that obey ...
60 citations
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TL;DR: In this paper, the authors calculate the resonant Raman spectra of tetrahedral amorphous carbon using an approach different from Placzek's approximation, which allows calculation of Raman intensities also in resonant conditions.
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TL;DR: In this paper, the structural relaxations around paramagnetic impurities (Ti, Cr, Fe) in corundum (alpha-Al2O3) were determined by combining x-ray absorption near edge structure (XANES) experiments and ab initio calculations.
Abstract: We determine the structural relaxations around paramagnetic impurities (Ti, Cr, Fe) in corundum (alpha-Al2O3), by combining x-ray absorption near edge structure (XANES) experiments and ab initio calculations. The structural relaxations are found to be very local. We then show that XANES is sensitive to small variations in interatomic distances within the coordination shell of the absorbing atom. The experiments were carried out on single crystals of ruby and sapphires. Linear dichroic signals are essential to characterize the geometry of the impurity site. The calculations were performed within a self-consistent ''non muffin-tin'' framework, that uses pseudopotentials, plane-wave basis set, and the continued fraction for the absorption cross section.
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TL;DR: This work applies the notion of electronic enthalpy for first-principles structural and dynamical calculations of finite systems under pressure to the study of group-IV nanoparticles during a shock wave, highlighting the significant differences in the plastic or elastic response of the diamond cage under load.
Abstract: We introduce the notion of electronic enthalpy for first-principles structural and dynamical calculations of finite systems under pressure. An external pressure field is allowed to act directly on the electronic structure of the system studied via the ground-state minimization of the functional E+PV(q), where V(q) is the quantum volume enclosed by a charge isosurface. The Hellmann-Feynman theorem applies, and assures that the ionic equations of motion follow an isoenthalpic dynamics. No pressurizing medium is explicitly required, while coatings of environmental ions or ligands can be introduced if chemically relevant. We apply this novel approach to the study of group-IV nanoparticles during a shock wave, highlighting the significant differences in the plastic or elastic response of the diamond cage under load, and their potential use as novel nanostructured impact-absorbing materials.
09 Dec 2005
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TL;DR: In this article, a reciprocal-space "non muffin-tin" scheme for calculating X-ray absorption near-edge structure (XANES) is presented.
Abstract: We present a reciprocal-space “non muffin-tin” scheme for calculating X-ray Absorption Near-Edge Structure (XANES). The method uses pseudopotentials and reconstructs all electronwave functions within the ProjectorAugmentedWave framework. The method incorporates a recursive method to compute absorption cross section as a continued fraction. The continued fraction formulation of absorption is advantageous in that it permits the treatment of core-hole-electron interaction through large supercells (hundreds of atoms). This opens new fields of applications like surfaces, molecules, small aggregates or amorphous materials, for which large supercells are required. The method is applied to the natural linear dichroism at theAl K-edge in corundum (-Al2O3). Details about the convergence process of the calculation are given. The influence of the core-hole effects is emphasized and comparison with “muffin-tin” multiple scattering calculation is made.
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TL;DR: In this paper, the authors used IDRIS sgrant 051202d and CEPBA supercomputer centers to perform calculations at the same time and the same day.
Abstract: This work was supported by EU Network of Excellence NANOQUANTA, Grant No. NMP4-CT-2004-500198 and Spanish-MCyT. Calculations were performed at IDRIS sgrant 051202d and CEPBA supercomputer centers.
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TL;DR: In this article, the authors consider a model Hamiltonian fitted on the ab initio band structure to describe the electron-phonon coupling between the electronic $\ensuremath{\sigma}$ bands and the phonon ${E}_{2g}$ mode in the Raman spectra.
Abstract: We consider a model Hamiltonian fitted on the ab initio band structure to describe the electron-phonon coupling between the electronic $\ensuremath{\sigma}$ bands and the phonon ${E}_{2g}$ mode in ${\mathrm{MgB}}_{2}$. The model allows for analytical calculations and numerical treatments using very large $k$-point grids. We calculate the phonon self-energy of the ${E}_{2g}$ mode along two high symmetry directions in the Brillouin zone. We demonstrate that the contribution of the $\ensuremath{\sigma}$ bands to the Raman linewidth (Landau damping) of the ${E}_{2g}$ mode via the electron-phonon coupling is zero. As a consequence the large resonance seen in Raman experiments cannot be interpreted as originated from the ${E}_{2g}$ mode at $\ensuremath{\Gamma}$. We examine in details the effects of Fermi surface singularities in the phonon spectrum and linewidth and we determine the magnitude of finite temperature effects in the phonon self-energy. From our findings we suggest several possible effects which might be responsible for the ${\mathrm{MgB}}_{2}$ Raman spectra.
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TL;DR: In this article, the structural relaxations around paramagnetic impurities (Ti, Cr, Fe) in corundum (alpha-Al2O3) were determined by combining x-ray absorption near edge structure (XANES) experiments and ab initio calculations.
Abstract: We determine the structural relaxations around paramagnetic impurities (Ti, Cr, Fe) in corundum (alpha-Al2O3), by combining x-ray absorption near edge structure (XANES) experiments and ab initio calculations. The structural relaxations are found to be very local. We then show that XANES is sensitive to small variations in interatomic distances within the coordination shell of the absorbing atom. The experiments were carried out on single crystals of ruby and sapphires. Linear dichroic signals are essential to characterize the geometry of the impurity site. The calculations were performed within a self-consistent ``non muffin-tin'' framework, that uses pseudopotentials, plane-wave basis set, and the continued fraction for the absorption cross section.
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TL;DR: The structure and properties of two different modifications of moganite have been studied using density functional theory, and the results have been compared to quartz as mentioned in this paper, and it is shown that the enthalpy difference between the two is negligible.
Abstract: The structure and properties of two different modifications of moganite have been studied using density functional theory, and the results have been compared to quartz. It is shown that the enthalpy difference between quartz and moganite, whose structure can be understood as Brazil twinning of quartz on a unit cell length scale, is negligible. This explains the significant amount of moganite in fine-grained quartz samples, as well as the frequent occurrence of Brazilian twinning in quartz. The compression mechanism of moganite has been elucidated, and it is argued that moganite is significantly more compressible than quartz. Observed and calculated NMR spectra are compared, and it is found that the bonding in quartz and moganite is very similar, consistent with the results of a Mulliken population analysis. The elastic stiffness coefficients of moganite have been predicted, and it is shown that formal-charge shell model interatomic potentials appear to be more transferable from quartz to moganite than partial-charge rigid ion equivalents.
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TL;DR: In this article, the Cr K-edge in -Al2O3:Cr3+ with a reciprocal space approach was analyzed for the computation of self-consistent spin polarized charge density for large unit cells, beyond the traditional muffin-tin approximation.
Abstract: The red color of ruby (-Al2O3:Cr3+) is assigned to transitions between 3d states of the chromium ion. A precise description of the structural and electronic environment of the Cr3+ ion in its slightly distorted octahedral site (C3 symmetry) is needed to understand the cause of the color. We have analysed the Cr K-edge in -Al2O3:Cr3+with a reciprocal space approach that allows the computation of self-consistent spin polarized charge density for large unit cells, beyond the traditional “muffin-tin” approximation. Structural and electronic information is deduced from the calculation.
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TL;DR: In this paper, the experimental H-1 chemical shifts of the CH and CH2 protons are assigned using first-principles chemical shift calculations that employ a plane-wave pseudo-potential approach.
Abstract: Two-dimensional H-1-C-13 MAS-J-HMQC solid-state NMR spectra of the two anomeric forms of maltose at natural abundance are presented. The experimental H-1 chemical shifts of the CH and CH2 protons are assigned using first-principles chemical shift calculations that employ a plane-wave pseudo-potential approach. Further calculations show that the calculated change in the 1H chemical shift when comparing the full crystal and an isolated molecule is a quantitative measure of intermolecular C-(HO)-O-... weak hydrogen bonding. Notably, a clear correlation between a large chemical shift change (up to 2 ppm) and both a short (HO)-O-... distance (< 2.7 angstrom) and a CHO bond angle greater than 1301 is observed, thus showing that directionality is important in C-(HO)-O-... hydrogen bonding.
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TL;DR: The local environments around substitutional Cr3+ or Fe3+ ions in doped -Al2O3 have been studied experimentally by dichroic X-ray absorption at the Cr and Fe Kedges in single crystals as mentioned in this paper.
Abstract: The local environments around substitutional Cr3+ or Fe3+ ions in doped -Al2O3 have been studied experimentally by dichroic X-ray absorption at the Cr and Fe Kedges in single crystals.A reliable method has been applied to remove diffraction peaks. This paper shows that the dichroic EXAFS analysis leads to the precise determination of the environment around impurity ions in -Al2O3. The Cr or Fe coordination shell in doped -Al2O3 is similar to the one of Cr in -Cr2O3 or of Fe in -Fe2O3. Aluminum or oxygen atoms at a farther distance are weakly affected by the presence of the impurity.
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12 Oct 2005
TL;DR: In this paper, two Kohn anomalies are present in the phonon dispersion of graphene and their slope is proportional to the square of the electron-phonon coupling, and they are enhanced in metallic nanotubes due to their reduced dimensionality.
Abstract: Kohn anomalies are distinct features of the phonon dispersion in metallic systems, associated to the presence of a Fermi surface. Graphene is a model system to understand the physical properties of carbon nanotubes. Two Kohn anomalies are present in the phonon dispersion of graphene and their slope is proportional to the square of the electron‐phonon coupling. Kohn anomalies are enhanced in metallic nanotubes due to their reduced dimensionality and absent for semiconducting. The electron‐phonon coupling for nanotubes of arbitrary chirality and diameter can be obtained from graphite.