Showing papers by "Francesco Mauri published in 2011"

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.

Variations in the work function of doped single- and few-layer graphene assessed by Kelvin probe force microscopy and density functional theory

Abstract: substrates. We compare the layer thickness dependency of the measured surface potential with ab initio density functional theory calculations of the work function for substrate-doped graphene. The ab initio calculations show that the work function of single- and bilayer graphene is mainly given by a variation of the Fermi energy with respect to the Dirac point energy as a function of doping, and that electrostatic interlayer screening only becomes relevant for thicker multilayer graphene. From the Raman G-line shift and the comparison of the Kelvin probe data with the ab initio calculations, we independently find an interlayer screening length in the order of four to five layers. Furthermore, we describe in-plane variations of the work function, which can be attributed to partial screening of charge impurities in the substrate, and result in a nonuniform charge density in single-layer graphene.

Charge-Density Wave and Superconducting Dome in TiSe 2 from Electron-Phonon Interaction

Abstract: At low temperature ${\mathrm{TiSe}}_{2}$ undergoes a charge-density wave instability. Superconductivity is stabilized either by pressure or by Cu intercalation. We show that the pressure phase diagram of ${\mathrm{TiSe}}_{2}$ is well described by first-principles calculations. At pressures smaller than 4 GPa charge-density wave ordering occurs, in agreement with experiments. At larger pressures the disappearing of the charge-density wave is due to a stiffening of the short-range force constants and not to the variation of nesting with pressure. Finally, we show that the behavior of ${T}_{c}$ as a function of pressure is entirely determined by the electron-phonon interaction without need of invoking excitonic mechanisms. Our work demonstrates that phase diagrams with competing orders and a superconducting dome are also obtained in the framework of the electron-phonon interaction.

Investigation of the Interface in Silica-Encapsulated Liposomes by Combining Solid State NMR and First Principles Calculations

Abstract: In the context of nanomedicine, liposils (liposomes and silica) have a strong potential for drug storage and release schemes: such materials combine the intrinsic properties of liposome (encapsulation) and silica (increased rigidity, protective coating, pH degradability). In this work, an original approach combining solid state NMR, molecular dynamics, first principles geometry optimization, and NMR parameters calculation allows the building of a precise representation of the organic/inorganic interface in liposils. {1H–29Si}1H and {1H–31P}1H Double Cross-Polarization (CP) MAS NMR experiments were implemented in order to explore the proton chemical environments around the silica and the phospholipids, respectively. Using VASP (Vienna Ab Initio Simulation Package), DFT calculations including molecular dynamics, and geometry optimization lead to the determination of energetically favorable configurations of a DPPC (dipalmitoylphosphatidylcholine) headgroup adsorbed onto a hydroxylated silica surface that co...

Intercalant and Intermolecular Phonon Assisted Superconductivity in K-Doped Picene

Abstract: ${\mathrm{K}}_{3}$ picene is a superconducting molecular crystal with a critical temperature of ${T}_{c}=7$ or 18 K, depending on the preparation conditions. Using density functional theory we show that electron-phonon interaction accounts for ${T}_{c}$ 3--8 K. The average electron-phonon coupling, calculated by including the phonon energy scale in the electron-phonon scattering, is $\ensuremath{\lambda}=0.73$ and ${\ensuremath{\omega}}_{\mathrm{log} }=18.0\text{ }\text{ }\mathrm{meV}$. Intercalant and intermolecular phonon modes contribute substantially (40%) to $\ensuremath{\lambda}$ as also shown by the isotope exponents of potassium (0.19) and carbon (0.31). The relevance of these modes makes superconductivity in K-doped picene peculiar and different from that of fullerenes.

Structural properties of carbon nanotubes derived from 13 C NMR

Abstract: We present a detailed experimental and theoretical study on how structural properties of carbon nanotubes can be derived from 13C NMR investigations. Magic angle spinning solid state NMR experiments have been performed on single-and multiwalled carbon nanotubes with diameters in the range from 0.7 to 100 nm and with number of walls from 1 to 90. We provide models on how diameter and the number of nanotube walls influence NMR linewidth and line position. Both models are supported by theoretical calculations. Increasing the diameter D, from the smallest investigated nanotube, which in our study corresponds to the inner nanotube of a double-walled tube to the largest studied diameter, corresponding to large multiwalled nanotubes, leads to a 23.5 ppm diamagnetic shift of the isotropic NMR line position d. We show that the isotropic line follows the relation d = 18.3/D + 102.5 ppm, where D is the diameter of the tube and NMR line position d is relative to tetramethylsilane. The relation asymptotically tends to approach the line position expected in graphene. A characteristic broadening of the line shape is observed with the increasing number of walls. This feature can be rationalized by an isotropic shift distribution originating from different diamagnetic shielding of the encapsulated nanotubes together with a heterogeneity of the samples. Based on our results, NMR is shown to be a nondestructive spectroscopic method that can be used as a complementary method to, for example, transmission electron microscopy to obtain structural information for carbon nanotubes, especially bulk samples.

High-resolution solid state NMR experiments for the characterization of calcium phosphate biomaterials and biominerals

Abstract: Calcium phosphates form a vast family of biominerals, which have attracted much attention in fields like biology, medicine, and materials science, to name a few. Solid state Nuclear Magnetic Resonance (NMR) is one of the few techniques capable of providing information about their structure at the atomic level. Here, examples of recent advances of solid state NMR techniques are given to demonstrate their suitability to characterize in detail synthetic and biological calcium phosphates. Examples of high-resolution 31P, 1H (and 17O), solid state NMR experiments of a 17O-enriched monocalcium phosphate monohydrate-monetite mixture and of a mouse tooth are presented. In both cases, the advantage of performing fast Magic Angle Spinning NMR experiments at high magnetic fields is emphasized, notably because it allows very small volumes of sample to be analyzed.

Comparative study of the phonons in nonsuperconducting BaC 6 and superconducting CaC 6 using inelastic x-ray scattering

Abstract: The low-energy phonons of two different graphite intercalation compounds (GICs) have been measured as a function of temperature using inelastic x-ray scattering (IXS). In the case of the non-superconductor BaC6, the phonons observed are significantly higher (up to 20%) in energy than those predicted by theory, in contrast to the reasonable agreement found in superconducting CaC6. Additional IXS intensity is observed below 15 meV in both BaC6 and CaC6. It has been previously suggested that this additional inelastic intensity may arise from defect or vacancy modes not predicted by theory [d'Astuto et al., Phys. Rev. B 81, 104519 (2010)]. Here it is shown that this additional intensity can arise directly from the polycrystalline nature of the available samples. Our results show that future theoretical work is required to understand the relationship between the crystal structure, the phonons, and the superconductivity in GICs.

How to make graphene superconducting

Abstract: Graphene [1] is the physical realization of many funda- mental concepts and phenomena in solid state-physics[2], but in the long list of graphene remarkable proper- ties [3-6], a fundamental block is missing: supercon- ductivity. Making graphene superconducting is relevant as the easy manipulation of this material by nanolyto- graphic techniques paves the way to nanosquids, one- electron superconductor-quantum dot devices[7, 8], su- perconducting transistors at the nano-scale[9] and cryo- genic solid-state coolers[10]. Here we explore the doping of graphene by adatoms coverage. We show that the occurrence of superconduc- tivity depends on the adatom in analogy with graphite intercalated compounds (GICs). However, most surpris- ingly, and contrary to the GIC case[11, 12], Li covered graphene is superconducting at much higher temperature with respect to Ca covered graphene. As graphene itself is not superconducting, phonon- mediated superconductivity must be induced by an en- hancement of the electron-phonon coupling ( ), = N(0)D2 M!2 ph (1) In Eq. 1 N(0) is the electronic density of states (DOS) at the Fermi level, D is the deformation potential, while M and !ph are effective atom mass and phonon frequency that in metallic alloys reflect the role of the different atomic species and phonon vibrations involved in su- perconductivity. In undoped graphene is small and phonon-mediated superconductivity does not occur as the small number of carriers, intrinsic in a semimetal, leads to a vanishingly small N(0). In this respect the sit- uation is similar to the bulk graphite case, where, with- out intercalation of foreign atoms superconductivity is not stabilized.

Tuning the Kohn Anomaly in the Phonon Dispersion of Graphene by Interaction with the Substrate and by Doping

Abstract: Submitted for the MAR11 Meeting of The American Physical Society Tuning the Kohn Anomaly in the Phonon Dispersion of Graphene by Interaction with the Substrate and by Doping LUDGER WIRTZ, CNRS IEMN, Lille, France, ADRIEN ALLARD, CNRS IEMN, Lille, CLAUDIO ATTACCALITE, CNRS, Institut Neel, Grenoble, MICHELE LAZZERI, CNRS IMPMC, Paris, FRANCESCO MAURI, ANGEL RUBIO, ETSF/Univ. Basque Country, San Sebastian, Spain — The phonon dispersion of graphene displays two strong Kohn Anomalies (kinks) in the highest optical branch (HOB) at the high-symmetry points G and K. The slope of the HOB around K is a measure of the electron-phonon coupling (EPC) and determines the dispersion of the Raman D and 2D lines as a function of the laser energy. We show that the EPC can be strongly modified both due to interaction with a metallic substrate and due to doping. For graphene grown on a Ni(111) surface, a total suppression of the Kohn anomaly occurs: the HOB around K becomes completely flat. This is due to the strong hybridization of the graphene p-bands with the Nickel d-bands which lifts the linear crossing of the p-bands at K. From experimental phonon dispersions one can therefore draw conclusions about the interaction strength between graphene and its different substrates. Furthermore, we present a new way to tune the EPC in graphene through electron/hole doping. We show that for the highest optical branch at K, the EPC is strongly dependent on the doping level. This dependency influences the dispersion of the Raman D and 2D lines and makes it possible to measure the charge state of graphene via resonant Raman spectroscopy. Ludger Wirtz CNRS IEMN, Lille, France Date submitted: 06 Dec 2010 Electronic form version 1.4