The adiabatic Born-Oppenheimer approximation (ABO) has been the standard ansatz to describe the interaction between electrons and nuclei since the early days of quantum mechanics. ABO assumes that the lighter electrons adjust adiabatically to the motion of the heavier nuclei, remaining at any time in their instantaneous ground state. ABO is well justified when the energy gap between ground and excited electronic states is larger than the energy scale of the nuclear motion. In metals, the gap is zero and phenomena beyond ABO (such as phonon-mediated superconductivity or phonon-induced renormalization of the electronic properties) occur. The use of ABO to describe lattice motion in metals is, therefore, questionable. In spite of this, ABO has proved effective for the accurate determination of chemical reactions, molecular dynamics and phonon frequencies in a wide range of metallic systems. Here, we show that ABO fails in graphene. Graphene, recently discovered in the free state, is a zero-bandgap semiconductor that becomes a metal if the Fermi energy is tuned applying a gate voltage, Vg. This induces a stiffening of the Raman G peak that cannot be described within ABO.

/pdf/breakdown-of-the-adiabatic-born-oppenheimer-approximation-in-3ka4fhojkg.pdf

Breakdown of the adiabatic Born-Oppenheimer approximation in graphene.

The Born-Oppenheimer approximation (BO) has proven effective for the accurate determination of chemical reactions, molecular dynamics and phonon frequencies in a wide range of metallic systems. Graphene, recently discovered in the free state, is a zero band-gap semiconductor, which becomes a metal if the Fermi energy is tuned applying a gate-voltage Vg. Graphene electrons near the Fermi energy have twodimensional massless dispersions, described by Dirac cones. Here we show that a change in Vg induces a stiffening of the Raman G peak (i.e. the zone-center E2g optical phonon), which cannot be described within BO. Indeed, the E2g vibrations cause rigid oscillations of the Dirac-cones in the reciprocal space. If the electrons followed adiabatically the Dirac-cone oscillations, no change in the phonon frequency would be observed. Instead, since the electron-momentum relaxation near the Fermi level is much slower than the phonon motion, the electrons do not follow the Dirac-cone displacements. This invalidates BO and results in the observed phonon stiffening. This spectacular failure of BO is quite significant since BO has been the fundamental paradigm to determine crystal vibrations from the early days of quantum mechanics.

Born-Oppenheimer breakdown in graphene

Computational Materials Science X

It is essential to control the electronic structure of graphene in order to apply graphene films for use in electrodes. We have introduced chemical dopants that modulate the electronic properties of few-layer graphene films synthesized by chemical vapor deposition. The work function, sheet carrier density, mobility, and sheet resistance of these films were systematically modulated by the reduction potential values of dopants. We further demonstrated that the power generation of a nanogenerator was strongly influenced by the choice of a graphene electrode with a modified work function. The off-current was well quenched in graphene films with high work functions (Au-doped) due to the formation of high Schottky barrier heights, whereas leakage current was observed in graphene films with low work functions (viologen-doped), due to nearly ohmic contact.

http://ma.ecsdl.org/content/MA2011-02/40/2383.full.pdf

Control of Electronic Structure of Graphene by Various Dopants and Their Effects on a Nanogenerator

Physical vapour deposition (PVD) is a well-known technology that is widely used for the deposition of thin films regarding many demands, namely tribological behaviour improvement, optical enhancement, visual/esthetic upgrading, and many other fields, with a wide range of applications already being perfectly established. Machining tools are, probably, one of the most common applications of this deposition technique, sometimes used together with chemical vapour deposition (CVD) in order to increase their lifespan, decreasing friction, and improving thermal properties. However, the CVD process is carried out at higher temperatures, inducing higher stresses in the coatings and substrate, being used essentially only when the required coating needs to be deposited using this process. In order to improve this technique, several studies have been carried out optimizing the PVD technique by increasing plasma ionization, decreasing dark areas (zones where there is no deposition into the reactor), improving targets use, enhancing atomic bombardment efficiency, or even increasing the deposition rate and optimizing the selection of gases. These studies reveal a huge potential in changing parameters to improve thin film quality, increasing as well the adhesion to the substrate. However, the process of improving energy efficiency regarding the industrial context has not been studied as deeply as required. This study aims to proceed to a review regarding the improvements already studied in order to optimize the sputtering PVD process, trying to relate these improvements with the industrial requirements as a function of product development and market demand.

Sputtering Physical Vapour Deposition (PVD) Coatings: A Critical Review on Process Improvement and Market Trend Demands

The wave vector dependence of the E 2g optical phonon frequencies and linewidths in the hexagonal close-packed metal, osmium, was studied at 10 K. A strong anisotropic dispersion, as well as damping thresholds, were observed for two transverse branches in the high-symmetry directions of the Brillouin zone around a phonon wave vector magnitude of 10 6 cm -1 . Simultaneously, a continuum was found in the Raman spectra, presumably of electronic nature, whose q-dependence correlates with that the phonon self-energies. Numerical simulations of the electronic Raman response in terms of an intraband mechanism were performed with a model energy-band spectrum reproducing the main features of the Fermi surface anisotropy of Os. A large discrepancy was found between the calculated and the measured anisotropy of the electronic scattering with respect to momentum direction and polarization configuration. The possible assignment to a contribution of resonant terms to the Raman vertex is discussed.

/pdf/raman-scattering-in-os-nonadiabatic-renormalization-of-the-4h35k550yn.pdf

Raman Scattering in Os: Nonadiabatic Renormalization of the Optical Phonon Self‐Energies

2D-ordered kinked carbyne chains: DFT modeling and Raman characterization

We report the first systematic measurements of the Raman scattering by electrons in elemental metals of Al, Mo, Nb, Os, Pb, Re, Ta, Ti, V, W and metallic compound La$B_6$. Experimental spectra are modelled on the base of the band structures, calculated within the density functional theory, taking properly into account the effects of electron-phonon scattering. The agreement between our measured and calculated spectra is excellent for the variety of metals, thus providing estimates for the electron-phonon coupling constants and temperature-dependent relaxation rates. The method can be applied for other metallic materials to evaluate an electron-phonon coupling as an alternative to the transport and optical measurements.

/pdf/measurements-of-raman-scattering-by-electrons-in-metals-the-34pw3e1f7r.pdf

Measurements of Raman scattering by electrons in metals: The effects of electron-phonon coupling

Amorphous carbon (a-C) and titanium-carbon nanocomposite TiC x /a-C coatings were deposited using the PVD technique. TiC x /a-C were deposited by simultaneous sputtering of graphite and titanium targets using arc pulse (at the frequency of 3 to 25 Hz) and arc sputtering respectively. The composition, chemical structure and electrical properties of the surface have been studied by energy-dispersive X-ray spectroscopy, Raman spectroscopy and Kelvin probe force microscopy (KPFM). The increase in graphite sputtering arc pulse frequency is accompanied by the growth of carbon content in TiC x /a-C from 38 to 82 at.% and by the change of phase composition. The incorporation of the Ti + ions into the plasma flow of C + increases the number of sp 2 bonds in carbon matrix of TiC x /a-C, as compared to a-C deposited at the same frequency. The maps of contact potential difference ( CPD ) distribution over the surface of a-C and TiC x /a-C obtained by KPFM were used to calculate the root-mean-square ( RMS ) values of CPD . In a-C RMS of CPD decreases with scale up of arc pulse frequency. In TiC x /a-C RMS of CPD nonlinear depends on the carbon content and reaches maximum at C C  = 44 at.%. It has been suggested, that at the contacts of different phases an interface thin layer is formed. The electronic properties of this layer differ from the electronic properties of the bulk phases of nanocomposite. The increase in volume fraction of interface component leads to the drop of RMS of CPD .

The composition, structure and surface properties of the titanium-carbon coatings prepared by PVD technique

The crystal structure of the low-temperature form of K3PO4 has been determined for the first time using neutron diffraction (Rietveld method) and Raman spectroscopy: orthorhombic cell (sp. gr. Pnma, Z = 4), lattice parameters a = 1.12377(2) nm, b = 0.81046(1) nm, c = 0.59227(1) nm. The structure is made up of isolated [PO4] tetrahedra, with the potassium ions in between.

Yu. S. Ponosov

Papers

Raman Scattering in Os: Nonadiabatic Renormalization of the Optical Phonon Self‐Energies

2D-ordered kinked carbyne chains: DFT modeling and Raman characterization

Measurements of Raman scattering by electrons in metals: The effects of electron-phonon coupling

The composition, structure and surface properties of the titanium-carbon coatings prepared by PVD technique

Crystal structure of the low-temperature form of K3PO4