Showing papers by "Francesco Mauri published in 2012"

First-principles calculation of NMR parameters using the gauge including projector augmented wave method:a chemist's point of view

Abstract: Including Projector Augmented Wave Method: A Chemist’s Point of View Christian Bonhomme,*,† Christel Gervais,*,† Florence Babonneau,† Cristina Coelho,‡ Fred́eŕique Pourpoint,† Thierry Azaïs,† Sharon E. Ashbrook,* John M. Griffin, Jonathan R. Yates,* Francesco Mauri, and Chris J. Pickard †Laboratoire de Chimie de la Matier̀e Condenseé de Paris, Universite ́ Pierre et Marie Curie, Paris 06, CNRS UMR 7574, Colleg̀e de France, 75005 Paris, France ‡IMPC, Institut des Mateŕiaux de Paris Centre, FR2482, UPMC Universite ́ Pierre et Marie Curie Paris 06, Colleg̀e de France, 11 place Marcelin Berthelot, 75231 Paris Cedex 05, France School of Chemistry and EaStCHEM, University of St. Andrews, North Haugh, St. Andrews KY16 9ST, United Kingdom Department of Materials, University of Oxford, Oxford OX1 3PH, United Kingdom Laboratoire de Mineŕalogie Crystallographie, UMR CNRS 7590, Universite ́ Pierre et Marie Curie, UPMC, 75015 Paris, France Department of Physics and Astronomy, University College London, London WC1E 6BT, United Kingdom

Phonon-mediated superconductivity in graphene by lithium deposition

Abstract: Graphene exhibits many extraordinary properties. But, despite many attempts to find ways to induce it, superconductivity is not one of them. First-principles calculations suggest that by decorating the surface of graphene with lithium atoms, it could yet be made to superconduct.

Theory of double-resonant Raman spectra in graphene: intensity and line shape of defect-induced and two-phonon bands

Abstract: We calculate the double resonant (DR) Raman spectrum of graphene, and determine the lines associated to both phonon-defect processes, and two-phonons ones. Phonon and electronic dispersions reproduce calculations based on density functional theory corrected with GW. Electron-light, -phonon, and -defect scattering matrix elements and the electronic linewidth are explicitly calculated. Defect-induced processes are simulated by considering different kind of idealized defects. For an excitation energy of $\epsilon_L=2.4$ eV, the agreement with measurements is very good and calculations reproduce: the relative intensities among phonon-defect or among two-phonon lines; the measured small widths of the D, $D'$, 2D and $2D'$ lines; the line shapes; the presence of small intensity lines in the 1800, 2000 cm$^{-1}$ range. We determine how the spectra depend on the excitation energy, on the light polarization, on the electronic linewidth, on the kind of defects and on their concentration. According to the present findings, the intensity ratio between the $2D'$ and 2D lines can be used to determine experimentally the electronic linewidth. The intensity ratio between the $D$ and $D'$ lines depends on the kind of model defect, suggesting that this ratio could possibly be used to identify the kind of defects present in actual samples. Charged impurities outside the graphene plane provide an almost undetectable contribution to the Raman signal.

Hidden polymorphs drive vitrification in B2O3

Abstract: Whether a liquid forms a crystal or a glass on solidification depends on many factors. The finding now that a disordered structure is favoured in B2O3 because the system cannot choose between several crystalline polymorphs of similar energy highlights a link between glass formation and crystallization.

Hidden polymorphs drive the vitrification in B2O3

Abstract: Physikalisch-Chemisches Institut der Universita¨t Zu¨rich, Winterthurerstrasse 190, CH-8057, Switzerland.(Dated: September 18, 2012)Understanding the conditions which favor crystallisation or vitrification of liquids has been a long-standingscientific problem [1–3]. Another connected, and not yet wel l understood question is the relationship betweenthe glassy and the various possible crystalline forms a system may adopt [4, 5]. In this context, B

Local and nonlocal electron-phonon couplings in K 3 picene and the effect of metallic screening

Abstract: We analyze the properties of electron-phonon couplings in K${}_{3}$ picene by exploiting a molecular-orbital representation derived in the maximally localized Wannier function formalism. This allows us to go beyond the analysis done in Phys. Rev. Lett. 107, 137006 (2011), and separate not only the intra- and intermolecular phonon contributions but also the local and nonlocal electronic states in the electron-phonon matrix elements. Despite the molecular nature of the crystal, we find that the purely molecular contributions (Holstein-like couplings where the local deformation potential is coupled to intramolecular phonons) account for only $20%$ of the total electron-phonon interaction $\ensuremath{\lambda}$. In particular, the Holstein-like contributions to $\ensuremath{\lambda}$ in K${}_{3}$ picene are four times smaller than those computed for an isolated neutral molecule, as they are strongly screened by the metallic bands of the doped crystal. Our findings invalidate the use of molecular electron-phonon calculations to estimate the total electron-phonon coupling in metallic picene, and possibly in other doped metallic molecular crystals. The major contribution ($80%$) to $\ensuremath{\lambda}$ in K${}_{3}$ picene comes from nonlocal couplings due to phonon-modulated hoppings. We show that the crystal geometry together with the molecular picene structure leads to a strong one-dimensional spatial anisotropy of the nonlocal couplings. Finally, based on the parameters derived from our density-functional theory calculations, we propose a lattice modelization of the electron-phonon couplings in K${}_{3}$ picene which gives $90%$ of ab initio $\ensuremath{\lambda}$.

Zener tunneling in the electrical transport of quasimetallic carbon nanotubes

Abstract: We study theoretically the impact of Zener tunneling on the charge-transport properties of quasimetallic (Qm) carbon nanotubes (characterized by forbidden band gaps of a few tens of meV). We also analyze the interplay between Zener tunneling and elastic scattering on defects. To this purpose we use a model based on the master equation for the density matrix, which takes into account the interband Zener transitions induced by the electric field (a quantum mechanical effect), the electron-defect scattering, and the electron-phonon scattering. In the presence of Zener tunneling the Qm tubes support an electrical current even when the Fermi energy lies in the forbidden band gap. In the absence of elastic scattering (in high-quality samples), the small size of the band gap of Qm tubes enables Zener tunneling for realistic values of the the electric field (above $\ensuremath{\sim}$1 V/$\ensuremath{\mu}$m). The presence of a strong elastic scattering (in low-quality samples) further decreases the values of the field required to observe Zener tunneling. Indeed, for elastic-scattering lengths of the order of 50 nm, Zener tunneling affects the current-voltage characteristic already in the linear regime. In other words, in quasimetallic tubes, Zener tunneling is made more visible by defects.

Superconductivity in metal‐coated graphene

Abstract: In this work we explore, by first-principles density functional theory (DFT) calculations, the possibility of inducing electron–phonon mediated superconductivity in a graphene sheet by doping its surface with alkaline metal adatoms. We demonstrate that, contrary to what could be naively believed, simple exfoliation to one layer of superconducting graphite intercalated compounds (GICs) does not necessarily lead to superconducting graphene, as it is the case in CaC6. On the contrary, it is meaningful to look for superconductivity in monolayers obtained by exfoliating non-superconducting GICs. In particular, we demonstrate that Li coating and double-coating of graphene leads to superconductivity in graphene with Tc that could be as large as 18 K.

Comment on "Electronic structure of superconducting KC8 and nonsuperconducting LiC6 graphite intercalation compounds: evidence for a graphene-sheet-driven superconducting state".

Abstract: A Comment on the Letter by Z.-H. Pan et al., Phys. Rev. Lett. 106, 187002 (2011). The authors of the Letter offer a Reply.