Showing papers by "Francesco Mauri published in 2006"

Raman spectrum of graphene and graphene layers.

Abstract: Graphene is the two-dimensional building block for carbon allotropes of every other dimensionality We show that its electronic structure is captured in its Raman spectrum that clearly evolves with the number of layers The D peak second order changes in shape, width, and position for an increasing number of layers, reflecting the change in the electron bands via a double resonant Raman process The G peak slightly down-shifts This allows unambiguous, high-throughput, nondestructive identification of graphene layers, which is critically lacking in this emerging research area

Nonadiabatic kohn anomaly in a doped graphene monolayer

Abstract: We compute, from first principles, the frequency of the E(2g), Gamma phonon (Raman G band) of graphene, as a function of the charge doping. Calculations are done using (i) the adiabatic Born-Oppenheimer approximation and (ii) time-dependent perturbation theory to explore dynamic effects beyond this approximation. The two approaches provide very different results. While the adiabatic phonon frequency weakly depends on the doping, the dynamic one rapidly varies because of a Kohn anomaly. The adiabatic approximation is considered valid in most materials. Here, we show that doped graphene is a spectacular example where this approximation miserably fails.

Phonon linewidths and electron-phonon coupling in graphite and nanotubes

Abstract: We show that electron-phonon coupling (EPC) is the major source of broadening for the Raman $G$ and ${G}^{\ensuremath{-}}$ peaks in graphite and metallic nanotubes. This allows us to directly measure the optical-phonon EPCs from the $G$ and ${G}^{\ensuremath{-}}$ linewidths. The experimental EPCs compare extremely well with those from the density functional theory. We show that the EPC explains the difference in the Raman spectra of metallic and semiconducting nanotubes and their dependence on tube diameter. We dismiss the common assignment of the ${G}^{\ensuremath{-}}$ peak in metallic nanotubes to a resonance between phonons and plasmons and we attribute it to a resonance between phonons and electron-hole pairs. For metallic tubes, we assign the ${G}^{+}$ and ${G}^{\ensuremath{-}}$ peaks to TO (circumferential) and LO (axial) modes, the opposite of what is commonly done in literature.

First-principles study of the OH-stretching modes of gibbsite

Abstract: The theoretical infrared (IR) and Raman spectra of gibbsite [α-Al(OH) 3 ] were computed using ab initio quantum mechanical calculations. The low-frequency dielectric tensor and the Raman tensors of gibbsite were determined using linear response theory. The transmission powder IR spectrum was found to strongly depend on the shape of the gibbsite particles. In the region of the OH-stretching bands, an excellent agreement between theory and experiment was obtained, providing an unambiguous interpretation of the OH bands in terms of vibrational modes. In contrast, the assignment of the bands observed at lower frequency is complicated by the significant overlap between neighboring bands together with their sensitivity to particle shape.

Coupled dynamics of electrons and phonons in metallic nanotubes: Current saturation from hot-phonon generation

Abstract: We show that the self-consistent dynamics of both phonons and electrons is the necessary ingredient for the reliable description of the hot phonons generation during electron transport in metallic single-wall carbon nanotubes (SWNTs). We solve the coupled Boltzmann transport equations to determine in a consistent way the current vs voltage (I-V) curve and the phonon occupation in metallic SWNTs which are lying on a substrate. We find a good agreement with measured I-V curves and we determine an optical phonon occupation which corresponds to an effective temperature of several thousands K (hot phonons), for the voltages typically used in experiments. We show that the high-bias resistivity strongly depends on the optical phonon thermalization time. This implies that a drastic improvement of metallic nanotubes performances can be achieved by increasing the coupling of the optical phonons with a thermalization source.

Magnetic response and NMR spectra of carbon nanotubes from ab initio calculations

Abstract: We present ab initio calculations of the magnetic susceptibility and of the $^{13}\mathrm{C}$ chemical shift for carbon nanotubes, both isolated and in bundles. These calculations are performed using the recently proposed gauge-including projector augmented-wave approach for the calculation of magnetic response in periodic insulating systems. We have focused on the semiconducting zigzag nanotubes with diameters ranging from 0.6 to $1.6\phantom{\rule{0.3em}{0ex}}\mathrm{nm}$ Both the susceptibility and the isotropic shift exhibit a dependence with the diameter $(D)$ and the chirality of the tube (although this dependence is stronger for the susceptibility). The isotropic shift behaves asymptotically as $\ensuremath{\alpha}∕D+116.0$, where $\ensuremath{\alpha}$ is a different constant for each family of nanotubes. For tubes with diameter around $1.2\phantom{\rule{0.3em}{0ex}}\mathrm{nm}$, a value normally found experimentally, our results are in excellent agreement with experiments. Moreover, we calculated the chemical shift of a double-wall tube. We found a diamagnetic shift of the isotropic lines corresponding to the atoms of the inner tube due to the effect of the outer tube. This shift is in good agreement with recent experiments, and can be easily explained by demagnetizing currents circulating the outer tube.

Possibility of superconductivity in graphite intercalated with alkaline earths investigated with density functional theory

Abstract: Using density functional theory we investigate the occurrence of superconductivity in $A{\mathrm{C}}_{6}$ with $A=\mathrm{Mg},\mathrm{Ca},\mathrm{Sr},\mathrm{Ba}$. We predict that at zero pressure, Ba and Sr should be superconducting with critical temperatures $({T}_{c})$ 0.2 and $3.0\phantom{\rule{0.3em}{0ex}}\mathrm{K}$, respectively. We study the pressure dependence of ${T}_{c}$ assuming the same symmetry for the crystal structures at zero and finite pressures. We find that the $\mathrm{Sr}{\mathrm{C}}_{6}$ and $\mathrm{Ba}{\mathrm{C}}_{6}$ critical temperatures should be substantially enhanced by pressure. On the contrary, for $\mathrm{Ca}{\mathrm{C}}_{6}$ we find that in the $0--5\phantom{\rule{0.3em}{0ex}}\mathrm{GPa}$ region, ${T}_{c}$ weakly increases with pressure. The increase is much smaller than that shown in several recent experiments. Thus we suggest that in $\mathrm{Ca}{\mathrm{C}}_{6}$ a continous phase transformation, such as an increase in staging, occurs at finite pressure. Finally we argue that, although $\mathrm{Mg}{\mathrm{C}}_{6}$ is unstable, the synthesis of intercalated systems of the kind ${\mathrm{Mg}}_{x}{\mathrm{Ca}}_{1\ensuremath{-}x}{\mathrm{C}}_{y}$ could lead to higher critical temperatures.

EPR g-tensor of paramagnetic centers in yttria-stabilized zirconia from first-principles calculations

Abstract: In order to assign the defect responsible for the experimental electron paramagnetic resonance (EPR) signal with trigonal symmetry (T center), we have studied the properties of different paramagnetic centers in yttria-stabilized cubic zirconia by computing the EPR g-tensor from density functional perturbation theory. We have considered reduced vacancy-zirconium complexes and reduced Ti impurities. These first-principles calculations allow us to discard the experimental assignment of the T center to an extrinsic Ti3+ ion nearest neighbor to a single vacancy. Instead, the calculated EPR g tensors of both a Zr3+ or a Ti3+ ion at the center of a divacancy aligned along the directions are compatible with the experimental EPR signal. However, since the EPR signal of the T center is correlated experimentally with an optical absorption band at 370 nm, calculated optical excitations allow us to decide in favor of the Ti3+ divacancy complex.

Origin of superconductivity of CaC6 and of other intercalated graphites

Abstract: By intercalation of donor the low conductivity of graphite can be enhanced and a superconducting state can occur at low temperature. The first discovered superconductors were alkali-intercalated compounds, with a critical temperatures (T c ) of the order of 1 K. In 2005 we learned with surprise that Ca intercalated graphite (CaC 6 ) is a superconductor with the sizable T c of 11.5 K. Using density functional theory we demonstrate that superconductivity in CaC 6 is phonon-mediated with an electron-phonon coupling λ equal to 0.83 and a phonon-frequency logarithmic-average equal to 25 meV. Superconductivity is mostly due C vibrations perpendicular and Ca vibrations parallel to the graphite layers. A non zero electron-phonon coupling for these modes can not be associated to the Fermi surface of the graphite pi bands but requires the presence of a second Fermi surface associated to the intercalant atoms. This result suggests a general mechanism for the occurrence of superconductivity in intercalated graphite. In order to stabilize a superconducting state it is necessary to have an intercalant Fermi surface since the simple doping of the π bands in graphite does not lead to a sizeable electron-phonon coupling. This condition occurs if the intercalant band is partially occupied, i.e. when the intercalant is not fully ionized.

Ab-Initio Calculations on Borate Systems

Abstract: This paper is part of an ongoing effort aimed at modeling the srtucture of liquid and vitreous B2O3 within a first-principles framework. Using density-functional theory, pseudoatomic orbitals basis sets and pseudopotentials, we carried out a series of calculations on known borate systems, namely boric acid B(OH)3 and the two polymorphic B2O3 crstals. The obtained results show that rather small basis sizes ccan compete with the much more expensive planewave ones. This paves the way for an efficient description of the disordered phases using ab-initio molecular dynamics simulations.

Anharmonic effects in carbon nanotubes: from thermal expansion to phonon lifetimes

Abstract: Submitted for the MAR06 Meeting of The American Physical Society Anharmonic effects in carbon nanotubes: from thermal expansion to phonon lifetimes N. BONINI, N. MOUNET, N. MARZARI, Department of Materials Science and Engineering, MIT, Cambridge, MA, USA, M. LAZZERI, F. MAURI, Institut de Minéralogie et Physique des Milieux Condensés, Paris, France — We study anharmonic effects in carbon nanotubes using a combination of densityfunctional theory and density-functional perturbation theory. In particular, we investigate thermal expansion and phonon lifetimes, which are key quantities that govern mechanical and transport properties in these systems. The thermal expansion coefficients are calculated from a minimization of the vibrational free energy in the quasi-harmonic approximation. Our results show that carbon nanotubes contract both in the axial and radial directions at low and room temperature and expand at higher temperatures. The role of different phonon modes in the thermal contraction is discussed together with their Grüneisen parameters. Anharmonic phonon lifetimes are evaluated from the cubic terms in the interatomic potential, using density-functional perturbation theory and the 2n+1 theorem. Finally, we discuss the possibility of estimating anharmonic effects using downfolding from graphene. Nicola Bonini Department of Materials Science and Engineering, MIT, Cambridge, MA, USA Date submitted: 01 Dec 2005 Electronic form version 1.4

Dipolar and J- Derived Solid State NMR Techniques and First Principles Calculations Applied to the Structure of Silicophosphates and to the Characterization of Phosphonate Grafting on Silica Nanoparticles

Abstract: J-derived (HMQC, INEPT) and D-derived (double and triple resonance) experiments were applied to the detailed characterization of crystalline and amorphous silicophosphate derivatives. 31P/29Si and 1H/31P/29Si CP MAS experiments were suitable for the description of complex silicophosphate gels, which can act as precursors for biocompatible materials. First principles calculations involving the GIPAW approach (first developed by Mauri and Pickard) were applied for the determination of CSA (29Si, 31P, 17O) and quadrupolar (17O) parameters. Excellent agreement between experimental and calculated data was obtained.