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ABSTRACT: We use first-principles methods based on density functional perturbation theory to characterize the lifetimes of the acoustic phonon modes and their consequences on the thermal transport properties of graphene. We show that using a standard perturbative approach, the transverse and longitudinal acoustic phonons in free-standing graphene display finite lifetimes in the long-wavelength limit, making them ill-defined as elementary excitations in samples of dimensions larger than ∼1 μm. This behavior is entirely due to the presence of the quadratic dispersions for the out-of-plane phonon (ZA) flexural modes that appear in free-standing low-dimensional systems. Mechanical strain lifts this anomaly, and all phonons remain well-defined at any wavelength. Thermal transport is dominated by ZA modes, and the thermal conductivity is predicted to diverge with system size for any amount of strain. These findings highlight strain and sample size as key parameters in characterizing or engineering heat transport in graphene.
Nano Letters 05/2012; 12(6):2673-8. · 13.20 Impact Factor
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Nano Letters. 05/2012;
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ABSTRACT: The thermal conductivity of ideal short-period superlattices is computed using harmonic and anharmonic force constants derived from density-functional perturbation theory and by solving the Boltzmann transport equation in the single-mode relaxation time approximation, using silicon-germanium as a paradigmatic case. We show that in the limit of small superlattice period the computed thermal conductivity of the superlattice can exceed that of both the constituent materials. This is found to be due to a dramatic reduction in the scattering of acoustic phonons by optical phonons, leading to very long phonon lifetimes. By variation of the mass mismatch between the constituent materials in the superlattice, it is found that this enhancement in thermal conductivity can be engineered, providing avenues to achieve high thermal conductivities in nanostructured materials.
Nano Letters 12/2011; 11(12):5135-41. · 13.20 Impact Factor
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ABSTRACT: We investigate the dominant phonon wavevectors q* and the associated dominant phonon-assisted electronic transitions implied by the 2D Raman mode of graphene by combining ab initio calculations with a full two-dimensional integration over the graphene Brillouin zone. We find that q* are highly anisotropic and rotate with the polarizer:analyzer condition, providing access to the entire angular extent around K. The resonant electronic transitions do not lie along the line and can be transformed from being apparently “inner” to “outer” with the addition of a reciprocal lattice vector, showing that both are equivalent. We thus invalidate the notion of “inner” and “outer” processes completely.
physica status solidi (b) 10/2011; 248(11):2635 - 2638. · 1.32 Impact Factor
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ABSTRACT: The dominant phonon wavevectors $q^{*}$ probed by the 2D Raman mode of
graphene are highly anisotropic and rotate with the orientation of the
polarizer:analyzer direction relative to the lattice. The corresponding
electronic transitions connect the electronic equibandgap contours where the
product of the ingoing and outgoing optical matrix elements is strongest,
showing a finite component along $\bm{K}-\bm{\Gamma}$ that sensitively
determines $q^{*}$. We revoke the notion of 'inner' and 'outer' processes. Our
findings explain the splitting of the 2D mode of graphene under uniaxial
tensile strain. The splitting originates from a strain-induced distortion of
the phonon dispersion; changes in the electronic band structure and resonance
conditions are negligeable for the 2D Raman spectrum.
02/2011;
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ABSTRACT: The thermal conductivity of disordered silicon-germanium alloys is computed from density-functional perturbation theory and with relaxation times that include both harmonic and anharmonic scattering terms. We show that this approach yields an excellent agreement at all compositions with experimental results and provides clear design rules for the engineering of nanostructured thermoelectrics. For Si(x)Ge(1-x), more than 50% of the heat is carried at room temperature by phonons of mean free path greater than 1 μm, and an addition of as little as 12% Ge is sufficient to reduce the thermal conductivity to the minimum value achievable through alloying. Intriguingly, mass disorder is found to increase the anharmonic scattering of phonons through a modification of their vibration eigenmodes, resulting in an increase of 15% in thermal resistivity.
Physical Review Letters 01/2011; 106(4):045901. · 7.37 Impact Factor
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Paolo Giannozzi,
Stefano Baroni, Nicola Bonini,
Matteo Calandra,
Roberto Car,
Carlo Cavazzoni,
Davide Ceresoli,
Guido L Chiarotti,
Matteo Cococcioni,
Ismaila Dabo, [......],
Stefano Paolini,
Alfredo Pasquarello,
Lorenzo Paulatto,
Carlo Sbraccia,
Sandro Scandolo,
Gabriele Sclauzero,
Ari P Seitsonen,
Alexander Smogunov,
Paolo Umari,
Renata M Wentzcovitch
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ABSTRACT: QUANTUM ESPRESSO is an integrated suite of computer codes for electronic-structure calculations and materials modeling, based on density-functional theory, plane waves, and pseudopotentials (norm-conserving, ultrasoft, and projector-augmented wave). The acronym ESPRESSO stands for opEn Source Package for Research in Electronic Structure, Simulation, and Optimization. It is freely available to researchers around the world under the terms of the GNU General Public License. QUANTUM ESPRESSO builds upon newly-restructured electronic-structure codes that have been developed and tested by some of the original authors of novel electronic-structure algorithms and applied in the last twenty years by some of the leading materials modeling groups worldwide. Innovation and efficiency are still its main focus, with special attention paid to massively parallel architectures, and a great effort being devoted to user friendliness. QUANTUM ESPRESSO is evolving towards a distribution of independent and interoperable codes in the spirit of an open-source project, where researchers active in the field of electronic-structure calculations are encouraged to participate in the project by contributing their own codes or by implementing their own ideas into existing codes.
Journal of Physics Condensed Matter 08/2009; 21(39):395502. · 2.55 Impact Factor
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ABSTRACT: The anharmonic properties of low-dimensional carbon crystal lattices are reviewed. The energy and crystal momentum conservation rules in two- and one-dimensional crystals lead to a drastic reduction of the phase space available for anharmonic phonon decay. This is illustrated with first principles calculations of the anharmonic properties of graphite and graphene. Experimental Raman linewidth data for the Radial Breathing Mode (RBM) in suspended single-walled carbon nanotubes are also interpreted in terms of a simple model in which a phonon decay bottleneck induced by the low dimensionality leads to a population time dependence in which a fast initial decay is followed by a slow decay determined by the decay rate of a large population of secondary phonons. These results are key to understanding the combined dynamics of electrons and phonons that determines the electrical transport properties in low-dimensional carbon nanostructures. In the case of the RBM in carbon nanotubes, they raise the intriguing possibility of using the linewidth of the Raman peak to determine the chirality of the nanotube. (© 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)
physica status solidi (b) 09/2008; 245(10):2149 - 2154. · 1.32 Impact Factor
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ABSTRACT: We determine from first principles the finite-temperature properties-linewidths, line shifts, and lifetimes-of the key vibrational modes that dominate inelastic losses in graphitic materials. In graphite, the phonon linewidth of the Raman-active E(2g) mode is found to decrease with temperature; such anomalous behavior is driven entirely by electron-phonon interactions, and does not appear in the nearly degenerate infrared-active E(1u) mode. In graphene, the phonon anharmonic lifetimes and decay channels of the A(1)' mode at K dominate over E(2g) at Gamma and couple strongly with acoustic phonons, highlighting how ballistic transport in carbon-based interconnects requires careful engineering of phonon decays and thermalization.
Physical Review Letters 11/2007; 99(17):176802. · 7.37 Impact Factor
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ABSTRACT: The behavior of metals at high pressure is of great importance to the fields of shock physics, geophysics, astrophysics, and nuclear materials. In order to further understand the properties of metals at high pressures we studied the equation of state of aluminum using first-principles techniques up to 2500 GPa, pressures within reach of the planned L.L.N.L. National Ignition Facility. Our simulations use density-functional theory and density-functional perturbation theory in the generalized gradient approximation at 0K. We found core overlaps to become relevant beyond pressures of 1200 GPa. The equations of state for three phases (fcc, bcc, and hcp) were calculated predicting the fcc-hcp, fcc-bcc, and hcp-bcc transitions to occur at 215 GPa, 307 GPa, and 435 GPa respectively. From the phonon dispersions at increasing pressure, we predict a softening of the lowest transverse acoustic vibrational mode along the [110] direction, which corresponds to a Born instability of the fcc phase at 725 GPa. Comment: 4 pages, 5 figures, accepted to Phys. Rev. B as a Brief Report. This version has update many figures. Moreover we provided updated and more accurate numbers based on further in-depth analyses of potential computational errors
10/2007;
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ABSTRACT: The issue of tuning the relative height of the first two dehydrogenation barriers of methane (CH(4) --> CH(3) + H and CH(3) --> CH(2) + H) is addressed using density-functional theory. It is shown that the combination of a very active reaction center-such as Rh-with a more inert substrate-such as Cu(111)-may hinder the second dehydrogenation step with respect to the first, thus resulting in the reverse of the natural order of the two barriers' heights.
Journal of the American Chemical Society 10/2006; 128(38):12448-54. · 9.91 Impact Factor
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C Grazioli,
Dario Alfè,
S R Krishnakumar,
Subhra Sen Gupta,
M Veronese,
S Turchini, Nicola Bonini,
Andrea Dal Corso,
D D Sarma,
Stefano Baroni,
C Carbone
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ABSTRACT: The occurrence of a noncollinear magnetic structure at a Mn monolayer grown epitaxially on Fe(100) is predicted theoretically, using spinor density-functional theory, and observed experimentally, using x-ray magnetic circular dichroism (XMCD) and linear dichroism (XMLD) spectroscopies. The combined use of XMCD and XMLD at the Mn-absorption edge allows us to assess the existence of ferromagnetic and antiferromagnetic order at the interface, and also to determine the moment orientations with element specificity. The experimental results thus obtained are in excellent agreement with the magnetic structure determined theoretically.
Physical Review Letters 10/2005; 95(11):117201. · 7.37 Impact Factor
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ABSTRACT: The first two steps of methane dissociation on Rh(111) have been investigated using density-functional theory, focusing on the dependence of the catalyst's reactivity on the atomic coordination of the active metal site. We find that, although the barrier for the dehydrogenation of methane (CH4 --> CH3 + H) decreases as expected with the coordination of the binding site, the dehydrogenation of methyl (CH3 --> CH2 + H) is hindered at an ad-atom defect, where the first reaction is instead most favored. Our findings indicate that, if it were possible to let the dissociation occur selectively at ad-atom defects, the reaction could be blocked after the first dehydrogenation step, a result of high potential interest for many dream reactions such as, for example, the direct conversion of methane to methanol.
Journal of the American Chemical Society 01/2005; 126(51):16732-3. · 9.91 Impact Factor
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ABSTRACT: The dominant phonon wave vectors q* probed by the 2D Raman mode of pristine and uniaxially strained graphene are determined via a combination of ab initio calculations and a full two-dimensional integration of the transition matrix. We show that q* are highly anisotropic and rotate about K with the polarizer and analyzer condition relative to the lattice. The corresponding phonon-mediated electronic transitions show a finite component along K-Γ that sensitively determines q*. We invalidate the notion of “inner” and “outer” processes. The characteristic splitting of the 2D mode of graphene under uniaxial tensile strain and given polarizer and analyzer setting is correctly predicted only if the strain-induced distortion and red-shift of the in-plane transverse optical (iTO) phonon dispersion as well as the changes in the electronic band structure are taken into account.
Phys. Rev. B. 85(11).
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T. M. G. Mohiuddin,
A. Lombardo,
R. R. Nair,
A. Bonetti,
G. Savini,
R. Jalil, Nicola Bonini,
D. M. Basko,
C. Galiotis,
Nicola Marzari,
K. S. Novoselov,
A. K. Geim,
A. C. Ferrari
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ABSTRACT: We uncover the constitutive relation of graphene and probe the physics of its optical phonons by studying its Raman spectrum as a function of uniaxial strain. We find that the doubly degenerate E[subscript 2g] optical mode splits in two components: one polarized along the strain and the other perpendicular. This splits the G peak into two bands, which we call G+ and G−, by analogy with the effect of curvature on the nanotube G peak. Both peaks redshift with increasing strain and their splitting increases, in excellent agreement with first-principles calculations. Their relative intensities are found to depend on light polarization, which provides a useful tool to probe the graphene crystallographic orientation with respect to the strain. The 2D and 2D′ bands also redshift but do not split for small strains. We study the Grüneisen parameters for the phonons responsible for the G, D, and D′ peaks. These can be used to measure the amount of uniaxial or biaxial strain, providing a fundamental tool for nanoelectronics, where strain monitoring is of paramount importance University of Palermo Sultan Qaboos University MITRE Interconnect Focus Center European Research Council Royal Society
APS.
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ABSTRACT: In this work we investigate the vibrational properties of atomic oxygen on Ag(2 1 0) in a mixed on-surface + sub-surface adsorption configuration using density-functional perturbation theory. In particular, we consider a geometry in which oxygen decorates the steps and an additional O species is located in an octahedral sub-surface site just below an Ag step atom. This structure presents a mode at 55 meV due to the coupled vibration of on-surface and sub-surface O species, which is close to that observed in HREEL spectra.
Surface Science. 587:50-54.
