Showing papers by "Francesco Mauri published in 2010"
TL;DR: It is shown that Clar's theory of the aromatic sextet is a simple and powerful tool to predict the stability, the pi-electron distribution, the geometry, and the electronic/magnetic structure of graphene nanoribbons with different hydrogen edge terminations.
Abstract: We show that Clar's theory of the aromatic sextet is a simple and powerful tool to predict the stability, the \pi-electron distribution, the geometry, the electronic/magnetic structure of graphene nanoribbons with different hydrogen edge terminations. We use density functional theory to obtain the equilibrium atomic positions, simulated scanning tunneling microscopy (STM) images, edge energies, band gaps, and edge-induced strains of graphene ribbons that we analyze in terms of Clar formulas. Based on their Clar representation, we propose a classification scheme for graphene ribbons that groups configurations with similar bond length alternations, STM patterns, and Raman spectra. Our simulations show how STM images and Raman spectra can be used to identify the type of edge termination.
207 citations
TL;DR: In this paper, the authors use density functional theory to obtain the equilibrium atomic positions, simulated scanning tunneling microscopy (STM) images, edge energies, band gaps, and edge-induced strains of graphene ribbons that they analyze in terms of Clar formulas.
Abstract: We show that Clar’s theory of the aromatic sextet is a simple and powerful tool to predict the stability, the π-electron distribution, the geometry, and the electronic/magnetic structure of graphene nanoribbons with different hydrogen edge terminations. We use density functional theory to obtain the equilibrium atomic positions, simulated scanning tunneling microscopy (STM) images, edge energies, band gaps, and edge-induced strains of graphene ribbons that we analyze in terms of Clar formulas. On the basis of their Clar representation, we propose a classification scheme for graphene ribbons that groups configurations with similar bond length alternations, STM patterns, and Raman spectra. Our simulations show how STM images and Raman spectra can be used to identify the type of edge termination.
202 citations
TL;DR: In this article, the effect of local magnetic moments on the electron-phonon coupling in doped BaFe and As using the density-functional perturbation theory was investigated.
Abstract: We calculate the effect of local magnetic moments on the electron-phonon coupling in doped ${\text{BaFe}}_{2}{\text{As}}_{2}$ using the density-functional perturbation theory. We show that the magnetism enhances the total electron-phonon coupling by $\ensuremath{\sim}50\mathrm{%}$, up to $\ensuremath{\lambda}\ensuremath{\lesssim}0.35$, still not enough to explain the high critical temperature, but strong enough to have a non-negligible effect on superconductivity, for instance, by frustrating the coupling with spin fluctuations and inducing order-parameter nodes. The enhancement comes not from the phonon softening but mostly from a renormalization of the electron-phonon matrix elements. We also investigate, in the rigid band approximation, the effect of doping, and find that $\ensuremath{\lambda}$ versus doping does not mirror the behavior of the density of states; while the latter decreases upon electron doping, the former does not, and even increases slightly.
108 citations
Abstract: We develop a first-principles scheme to calculate adiabatic and nonadiabatic phonon frequencies in the full Brillouin zone. The method relies on the stationary properties of a force-constant functional with respect to the first-order perturbation of the electronic charge density and on the localization of the deformation potential in the Wannier function basis. This allows for calculation of phonon-dispersion curves free from convergence issues related to Brillouin-zone sampling. In addition our approach justifies the use of the static screened potential in the calculation of the phonon linewidth due to decay in electron-hole pairs. We apply the method to the calculation of the phonon dispersion and electron-phonon coupling in ${\text{MgB}}_{2}$ and ${\text{CaC}}_{6}$. In both compounds we demonstrate the occurrence of several Kohn anomalies, absent in previous calculations, that are manifest only after careful electron- and phonon-momentum integration. In ${\text{MgB}}_{2}$, the presence of Kohn anomalies on the ${\text{E}}_{2g}$ branches improves the agreement with measured phonon spectra and affects the position of the main peak in the Eliashberg function. In ${\text{CaC}}_{6}$ we show that the nonadiabatic effects on in-plane carbon vibrations are not localized at zone center but are sizable throughout the full Brillouin zone. Our method opens perspectives in large-scale first-principles calculations of dynamical properties and electron-phonon interaction.
103 citations
TL;DR: In this paper, a theoretical/experimental study of current-voltage characteristics of graphene devices near the Dirac point was performed. And the authors reported that the currentvoltage properties of these devices can be described by a power law.
Abstract: We report a theoretical/experimental study of current-voltage characteristics $(I\text{\ensuremath{-}}V)$ of graphene devices near the Dirac point. The $I\text{\ensuremath{-}}V$ can be described by a power law ($I\ensuremath{\propto}{V}^{\ensuremath{\alpha}}$ with $1l\ensuremath{\alpha}\ensuremath{\le}1.5$). The exponent is higher when the mobility is lower. This superlinear $I\text{\ensuremath{-}}V$ is interpreted in terms of the interplay between Zener-Klein transport, that is tunneling between different energy bands, and defect scattering. Surprisingly, the Zener-Klein tunneling is made visible by the presence of defects.
90 citations
TL;DR: In this paper, the stability and the structure of GNR edges in the presence of molecules such as oxygen, water, ammonia, and carbon dioxide were determined by means of density functional theory, and the most stable armchair and zigzag configurations were characterized by a non-metallic/non-magnetic nature.
Abstract: We determine, by means of density functional theory, the stability and the structure of graphene nanoribbon (GNR) edges in presence of molecules such as oxygen, water, ammonia, and carbon dioxide. As in the case of hydrogen-terminated nanoribbons, we find that the most stable armchair and zigzag configurations are characterized by a non-metallic/non-magnetic nature, and are compatible with Clar's sextet rules, well known in organic chemistry. In particular, we predict that, at thermodynamic equilibrium, neutral GNRs in oxygen-rich atmosphere should preferentially be along the armchair direction, while water-saturated GNRs should present zigzag edges. Our results promise to be particularly useful to GNRs synthesis, since the most recent and advanced experimental routes are most effective in water and/or ammonia-containing solutions.
87 citations
TL;DR: This is believed to be the first experimental solid-state NMR determination of the hydroxyl OH (1)H chemical shifts for a simple sugar.
Abstract: A disaccharide is a challenging case for high-resolution H-1 solid-state NMR because of the 24 distinct protons (14 aliphatic and 10 OH) having H-1 chemical shifts that all fall within a narrow range of approximately 3 to 7 ppm. High-resolution H-1 (500 MHz) double-quantum (DQ) combined rotation and multiple pulse sequence (CRAMPS) solid-state NMR spectra of beta-maltose monohydrate are presented. H-1-H-1 DQ-SQ CRAMPS spectra are presented together with H-1 (DQ)-C-13 correlation spectra obtained with a new pulse sequence that correlates a high-resolution H-1 DQ dimension with a C-13 single quantum (SQ) dimension using the refocused INEPT pulse-sequence element to transfer magnetization via one-bond C-13-H-1 J couplings. Compared to the observation of only a single broad peak in a H-1 DQ spectrum recorded at 30 kHz magic-angle spinning (MAS), the use of DUMBO H-1 homonuclear decoupling in the H-1 DQ CRAMPS experiment allows the resolution of distinct DQ correlation peaks which, in combination with first-principles chemical shift calculations based on the GIPAW (Gauge Including Projector Augmented Waves) plane-wave pseudopotential approach, enables the assignment of the H-1 resonances to the 24 distinct protons. We believe this to be the first experimental solid-state NMR determination of the hydroxyl OH H-1 chemical shifts for a simple sugar. Variable-temperature H-1-H-1 DQ CRAMPS spectra reveal small increases in the H-1 chemical shifts of the OH resonances upon decreasing the temperature from 348 K to 248 K.
86 citations
TL;DR: The EPC for highest optical branch at the high symmetry point K acquires a strong dependency on the doping level due to electron-electron correlation not accounted in mean-field approaches, which opens the possibility to construct tunable electronic devices through external control of the EPC.
Abstract: We present a new way to tune the electron-phonon coupling (EPC) in graphene by changing the deformation potential with electron/hole doping. We show the EPC for highest optical branch at the high symmetry point K acquires a strong dependency on the doping level due to electron-electron correlation not accounted in mean-field approaches. Such a dependency influences the dispersion (with respect to the laser energy) of the Raman D and 2D lines and the splitting of the 2D peak in multilayer graphene. Finally this doping dependence opens the possibility to construct tunable electronic devices through external control of the EPC.
76 citations
TL;DR: In this article, the structural properties of lithium tetrasilicate glass (LS4) obtained by combined classical and Car-Parrinello molecular dynamics simulations are compared with corresponding experimental results.
60 citations
TL;DR: In this article, the β -factor of each phase was calculated using the harmonic phonon dispersion curves obtained within density functional theory, and the resulting fractionation laws systematically overestimate measurements by 15 ǫ at low temperatures to 8 ð at ≈400 ǵ.
56 citations
TL;DR: Within density-functional theory, the orbital magnetization for periodic systems evaluating a recently discovered Berry-phase formula was computed in this article. But the contribution of the interstitial regions neglected so far in literature was not explicitly calculated.
Abstract: Within density-functional theory we compute the orbital magnetization for periodic systems evaluating a recently discovered Berry-phase formula. For the ferromagnetic metals Fe, Co, and Ni we explicitly calculate the contribution of the interstitial regions neglected so far in literature. We also use the orbital magnetization to compute the electron paramagnetic resonance $g$ tensor in paramagnetic systems. Here the method can also be applied in cases where linear-response theory fails, e.g., radicals and defects with an orbital-degenerate ground state or those containing heavy atoms.
TL;DR: Three potentially interesting perspectives in connection with PAW/GIPAW in solid‐state NMR and pure nuclear quadrupole resonance (NQR) are presented, including the calculation of J coupling tensors in inorganic solids; the calculations of the antisymmetric part of chemical shift tensors and the prediction of 14N and 35Cl pure NQR resonances including dynamics.
Abstract: In 2001, Pickard and Mauri implemented the gauge including projected augmented wave (GIPAW) protocol for first-principles calculations of NMR parameters using periodic boundary conditions (chemical shift anisotropy and electric field gradient tensors). In this paper, three potentially interesting perspectives in connection with PAW/GIPAW in solid-state NMR and pure nuclear quadrupole resonance (NQR) are presented: (i) the calculation of J coupling tensors in inorganic solids; (ii) the calculation of the antisymmetric part of chemical shift tensors and (iii) the prediction of (14)N and (35)Cl pure NQR resonances including dynamics. We believe that these topics should open new insights in the combination of GIPAW, NMR/NQR crystallography, temperature effects and dynamics. Points (i), (ii) and (iii) will be illustrated by selected examples: (i) chemical shift tensors and heteronuclear (2)J(P-O-Si) coupling constants in the case of silicophosphates and calcium phosphates [Si(5)O(PO(4))(6), SiP(2)O(7) polymorphs and α-Ca(PO(3))(2)]; (ii) antisymmetric chemical shift tensors in cyclopropene derivatives, C(3)X(4) (X = H, Cl, F) and (iii) (14)N and (35)Cl NQR predictions in the case of RDX (C(3)H(6)N(6)O(6)), β-HMX (C(4)H(8)N(8)O(8)), α-NTO (C(2)H(2)N(4)O(3)) and AlOPCl(6). RDX, β-HMX and α-NTO are explosive compounds.
Journal Article•
TL;DR: This short review discusses the ability to reproduce NMR parameters in the case of phosphates materials through electronic structure calculation within density functional theory linear response and emphasise on the crucial points that should be taken into account to perform such calculations.
Abstract: In this short review, we discuss the ability to reproduce NMR parameters in the case of phosphates materials through electronic structure calculation within density functional theory linear response. Indeed, the gauge-including projector-augmented wave is today largely used by the solid-state NMR community as a tool for structural determination and it has been applied to a large variety of materials. We emphasise on the crucial points that should be taken into account to perform such calculations. In particular, we discuss the influence of the electronic structure and of the geometry on the calculation of NMR parameters. To illustrate the review, we present experimental and theoretical comparison of (31)P, (1)H and (23)Na NMR data on a series of sodium phosphate systems.
TL;DR: In this article, an ab initio calculation of the electronic g-tensor of paramagnetic states at surfaces is presented taking the adsorption of hydrogen atoms at silicon surfaces as an example.
Abstract: Ab initio calculations of the electronic g -tensor of paramagnetic states at surfaces are presented taking the adsorption of hydrogen atoms at silicon surfaces as an example. We show that for silicon surfaces with different hydrogen coverages, the g -tensor is by far more characteristic than the hyperfine splitting of the Si dangling bonds or the adsorbed H atoms.
This holds also in the case of powder spectra (e.g. amorphous or microcrystalline material) where only the angular average of the spectra is available from experiments. Hence, the ab initio calculation of the g -tensor should be in general a basic key to a better understanding of the microscopic structure of paramagnetic surfaces or interfaces (© 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)
TL;DR: The (17)O NMR experimental and computing approach allowed for identification and quantification of the different crystalline phases involved in the weathering mechanism of a sodium phosphate glass, even in small amount.
Abstract: In the present study, we used a combination of 17O NMR methods at a high magnetic field with first-principles calculations in order to characterize the oxygen sites in a series of hydroxylated sodium phosphate compounds, namely the hydrogen pyrophosphate Na2H2P2O7 and the hydrogen orthophosphates NaH2PO4, NaH2PO4·H2O and NaH2PO4·2H2O. The chemical shifts and quadrupolar parameters of these compounds were interpreted in terms of local and semi-local environment, i.e., the chemical composition of the immediate surroundings and the nature of the bonds, e.g. hydrogen bonding. The magnitude of the quadrupolar interaction and its asymmetry were revealed to be a precise indicator of the local structure in sodium hydrogen phosphates. Our 17O NMR experimental and computing approach allowed for identification and quantification of the different crystalline phases involved in the weathering mechanism of a sodium phosphate glass, even in small amount.
TL;DR: In this article, the authors used electron paramagnetic resonance spectroscopy in combination with a nonperturbative ab initio scheme for the electronic g tensor to identify the nearest-neighbor antisite pair defects in 4H-SiC, 6H- SiC, and 3C-SiCs single crystals.
Abstract: The nearest-neighbor antisite pair defects in 4H-SiC, 6H-SiC, and 3C-SiC single crystals have been identified using electron paramagnetic resonance spectroscopy in combination with a nonperturbative ab initio scheme for the electronic g tensor. Based on the theoretical predictions, the positively charged defect has been found experimentally also in the cubic 3C-SiC polytype where it is characterized by spin 1/2 and highly anisotropic g values of g(xx)=2.0030, g(yy)=2.0241, and g(zz)=2.0390 within C-1h symmetry. The exceptional large g values are explained by details of the spin-orbit coupling causing a strongly anisotropic quenching of the orbital angular momentum of the p-like unpaired electron.
TL;DR: In this paper, the results of a neutron scattering study of the high-energy phonons in the superconducting graphite intercalation compound ${\text{CaC}}_{6}$ were presented.
Abstract: We present the results of a neutron scattering study of the high-energy phonons in the superconducting graphite intercalation compound ${\text{CaC}}_{6}$. The study was designed to address hitherto unexplored aspects of the lattice dynamics in ${\text{CaC}}_{6}$, and, in particular, any renormalization of the out-of-plane and in-plane graphitic phonon modes. We present a detailed comparison between the data and the results of density-functional theory (DFT). A description is given of the analysis methods developed to account for the highly textured nature of the samples. The DFT calculations are shown to provide a good description of the general features of the experimental data. This is significant in light of a number of striking disagreements in the literature between other experiments and DFT on ${\text{CaC}}_{6}$. The results presented here demonstrate that the disagreements are not due to any large inaccuracies in the calculated phonon frequencies.
TL;DR: In this article, the authors report measurements of phonon dispersion in CaC$_6$ using inelastic X-ray and neutron scattering and find good overall agreement, particularly in the 50 meV energy region, between experimental data and first-principles density functional theory calculations.
Abstract: We report measurements of phonon dispersion in CaC$_6$ using inelastic X-ray and neutron scattering. We find good overall agreement, particularly in the 50 meV energy region, between experimental data and first-principles density-functional-theory calculations. However, on the longitudinal dispersion along the $(1 1 1)$ axis of the rhombohedral representation, we find an unexpected anti-crossing with an additional longitudinal mode, at about 11 meV. At a comparable energy, we observe also unexpected intensity on the in-plane direction. These results resolve the previous incorrect assignment of a longitudinal phonon mode to a transverse mode in the same energy range. By calculating the electron susceptibility from first principles we show that this longitudinal excitation is unlikely to be due to a plasmon and consequently can probably be due to defects or vacancies present in the sample.
01 Feb 2010
TL;DR: Within density-functional theory, the orbital magnetization for periodic systems evaluating a recently discovered Berry-phase formula was computed in this paper. But the contribution of the interstitial regions neglected so far in literature was not explicitly calculated.
Abstract: Within density-functional theory we compute the orbital magnetization for periodic systems evaluating a recently discovered Berry-phase formula. For the ferromagnetic metals Fe, Co, and Ni we explicitly calculate the contribution of the interstitial regions neglected so far in literature. We also use the orbital magnetization to compute the electron paramagnetic resonance $g$ tensor in paramagnetic systems. Here the method can also be applied in cases where linear-response theory fails, e.g., radicals and defects with an orbital-degenerate ground state or those containing heavy atoms.
01 Jan 2010
TL;DR: In this article, a combined electrical resistivity and x-ray diffraction study under high pressures provides a comprehensive account of both phenomena within the frame of BCS theory in terms of a P-induced softening of the in-plane Ca mode relevant to the electron-phonon coupling.
Abstract: Superconducting CaC6 is found to exhibit two important pressure effects: (i) a large P-induced T c enhancement up to 15.1 K at 7.5 GPa, the highest T c value hitherto reported for graphite intercalated compounds; and (ii) a dramatic T c drop down to ~3 K at a critical pressure of ~9 GPa suggestive of a structural instability. We show that a combined electrical resistivity and x-ray diffraction study under high pressures provides a comprehensive account of both phenomena within the frame of the BCS theory in terms of a P-induced softening of the in-plane Ca mode relevant to the electron–phonon coupling. Our data analysis indicates that, below ~8 GPa, the softening contributes to the T c enhancement whilst, at higher pressures, it drives the system to a disordered phase presumably characterized by a disordering of the Ca sublattice. Thus, pressure induces a simultaneous order-disorder and lattice-softening phase transition from a good metal phase with high T c to a bad metal phase with low T c.
01 May 2010
TL;DR: In this paper, the authors present an overview of the state-of-the-art research work in the area of Nanosciences at the University of Paris 6, 140 rue de Lourmel, F-75015 Paris, France.
Abstract: U. Gerstmann,1,2 A. P. Seitsonen,1 D. Ceresoli,3 F. Mauri,1 H. J. von Bardeleben,4 J. L. Cantin,4 and J. Garcia Lopez5 1Institut de Minéralogie et de Physique des Milieux Condensés, Université Paris 6, 140 rue de Lourmel, F-75015 Paris, France 2Lehrstuhl für Theoretische Physik, Department für Naturwissenschaften, Universität Paderborn, Warburger Str. 100, D-33098 Paderborn, Germany 3Department of Materials Science and Engineering, Massachusetts Institute of Technology (MIT), 77 Massachusetts Avenue, Cambridge, Massachusetts 02139-4307, USA 4Institut des Nanosciences de Paris, Université Paris 6, 140 rue de Lourmel, F-75015 Paris, France 5Centro Nacional de Aceleradores, Thomas A. Edison, Isla de La Cartuja, E-41092 Sevilla, Spain Received 8 April 2010; published 17 May 2010