1,721,003 research outputs found
Effects of doping on the vibrational properties of C-60 from first principles: K6C60
No full textAb initio calculations of the phonon spectrum of K6C60 are presented, based on the local-density approximation of density-functional theory and on linear response theory. The effects of doping on frequencies and infrared intensities are identified and their physical origin discussed in detail for optically allowed modes. Whereas structural relaxation is primarily responsible for the frequency changes, and especially for the softening of tangential modes, the change in infrared relative intensities is a direct consequence of the electron transfer. The potassium vibrations are found to lie within 60-100 cm(-1) and are well decoupled from C-60 intramolecular modes
SURFACE PHONONS AND DIPOLE ACTIVITY OF SI(111)2X1 FROM ABINITIO CALCULATIONS
No full textWe present a first-principles linear-response study of the phonon spectrum and dipole activity of the Si(111)2 x 1 surface. Our results are in good agreement with the experimental He-scattering results for the acoustic modes, as well as the electron-energy-loss data for the optical modes. We clarify a current controversy on the assignment of the electron-energy-loss peak at approximately 56 meV. We find that the dominant contribution to the loss peak is anisotropic and arises from a longitudinal-optical vibration of the surface chains. However, a small but non-negligible isotropic contribution comes from subsurface modes polarized perpendicular to the surface
SI(111), (2 X-1) RECONSTRUCTION AND SURFACE PHONONS FROM ABINITIO MOLECULAR-DYNAMICS
No full textWe present results of ab-initio Molecular Dynamics simulations of the Si(III) surface. Pandey's 2 x 1 chain-geometry is obtained by simulated annealing, starting directly from the ideal bulk-terminated surface. The dynamical path followed during the reconstruction is described. The resulting 2 x 1 structure agrees well with Low Energy Electron Diffraction and Medium Energy Ion Scattering results. The phonon spectrum which is calculated ab-initio for the first time shows well defined surface modes at frequencies close to the measured values. The atomic motions and the dynamic effective charges of these modes are also analyzed
DENSITY MATRIX AND WANNIER FUNCTIONS IN TERMS OF LOCALIZED ORBITALS
No full textWe discuss how to obtain the density matrix and Wannier functions in crystals, when localized functions are used as basis set. As a specific example we consider π bands in graphite and hexagonal BN and determine density matrix and Wannier functions. We also show that the cluster approach, together with some overall information on the band structure, leads to equivalent results
ABSENCE OF METAL-CLUSTERS AND APPEARANCE OF NEW ELECTRON-STATES IN NA6C60
No full textWe calculate the structural and electronic properties of the Na6C60 fulleride using the Car-Parrinello method as well as standard local density approximation band structure calculations. In contrast to previous claims we find that metal clusters do not form but, like the potassium and rubidium fullerides, sodium intercalation gives rise to a conventional ionic compound. Unlike the case of heavier alkali fullerides, a characteristic new kind of low-lying electron state appears that is not the C60 state. Effects of temperature on the electronic nature of the compound are discussed as well as consequences of these findings for higher fullerides
13-ATOM CLUSTERS - EQUILIBRIUM GEOMETRIES, STRUCTURAL TRANSFORMATIONS, AND TRENDS IN NA, MG, AL, AND SI
No full textWe report the results of an extensive structural study of Na13, Mg13, Al13, and Si13 carried out with the Car-Parrinello method. Several and mostly unforeseen noncrystalline structures are discovered to characterize the low portion of the potential energy surface. Crystalline structures are shown either to correspond to high-energy local minima or to be highly unstable. The low-energy structural pattern appears to change significantly from one element to the other. Specific characteristics as well as trends are discussed
LOW-TEMPERATURE STRUCTURES OF C4 AND C10 FROM THE CAR-PARRINELLO METHOD - SINGLET-STATES
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