33 research outputs found
Excitonic Effects and Optical Spectra of Single-Walled Carbon Nanotubes
Many-electron effects often dramatically modify the properties of reduced
dimensional systems. We report calculations, based on an many-electron Green's
function approach, of electron-hole interaction effects on the optical spectra
of small-diameter single-walled carbon nanotubes. Excitonic effects
qualitatively alter the optical spectra of both semiconducting and metallic
tubes. Excitons are bound by ~ 1 eV in the semiconducting (8,0) tube and by ~
100 meV in the metallic (3,3) tube. These large many-electron effects explain
the discrepancies between previous theories and experiments.Comment: 6 pages, 3 figures, 2 table
Origin of the spin reorientation transitions in (FeCo)B alloys
Low-temperature measurements of the magnetocrystalline anisotropy energy
in (FeCo)B alloys are reported, and the origin of this
anisotropy is elucidated using a first-principles electronic structure
analysis. The calculated concentration dependence with a maximum near
and a minimum near is in excellent agreement with experiment.
This dependence is traced down to spin-orbital selection rules and the filling
of electronic bands with increasing electronic concentration. At the optimal Co
concentration, depends strongly on the tetragonality and doubles under a
modest 3% increase of the ratio, suggesting that the magnetocrystalline
anisotropy can be further enhanced using epitaxial or chemical strain.Comment: 4 pages + supplementary material, 6 figures. Accepted in Applied
Physics Letter
Multiphase equation of state for carbon addressing high pressures and temperatures
We present a 5-phase equation of state for elemental carbon which addresses a wide range of density and temperature conditions: 3g/cc 100 000K (both for ρ between 3 and 12 g/cc, with select higher-ρ DFT calculations as well). The liquid free energy model includes an atom-in-jellium approach to account for the effects of ionization due to temperature and pressure in the plasma state, and an ion-thermal model which includes the approach to the ideal gas limit. The precise manner in which the ideal gas limit is reached is greatly constrained by both the highest-temperature DFT data and the path integral data, forcing us to discard an ion-thermal model we had used previously in favor of a new one. Predictions are made for the principal Hugoniot and the room-temperature isotherm, and comparisons are made to recent experimental results.United States. Dept. of Energy (Contract DE-AC52-07NA27344