Lorin X. Benedict

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 ab initio many-electron Greens 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 approximately 1 eV in the semiconducting (8,0) tube and by approximately 100 meV in the metallic (3,3) tube. These large many-electron effects explain the discrepancies between previous theories and experiments.

Microscopic determination of the interlayer binding energy in graphite

Abstract High-tensile-strength carbon nanotubes are nonetheless susceptible to large radial deformations. In particular, tubes may collapse so that opposing tube walls attain the graphitic interlayer spacing. A simple elastic model shows that the ratio of mean curvature modulus to the interwall attraction of graphite determines the cross-section of a collapsed tube. Transmission electron microscopy of collapsed tubes confirms the elastic model and affords the first microscopic measurement of the strength of the intersheet attraction, a quantity otherwise difficult to assess.

Heat capacity of carbon nanotubes

Abstract The low-temperature behavior of the heat capacity, CV, of carbon nanotubes is estimated in order to address the question: How small must nanotubes be for CV to be measureably different from that of bulk graphite? We predict that all single-walled tubes should have CV ∝ T at sufficiently low temperature, even if the tubes are semiconducting. The range of temperature and tube radii for which this is expected to be observable is shown to be well within current experimental constraints. Multi-walled tubes are also discussed.

Molecular dynamics simulations of temperature equilibration in dense hydrogen.

The temperature equilibration rate between electrons and protons in dense hydrogen has been calculated with molecular dynamics simulations for temperatures between 10 and 600eV and densities between 10;{20}cm;{-3}to10;{24}cm;{-3} . Careful attention has been devoted to convergence of the simulations, including the role of semiclassical potentials. We find that for Coulomb logarithms L greater, similar1 , a model by Gericke-Murillo-Schlanges (GMS) [D. O. Gericke, Phys. Rev. E 65, 036418 (2002)] based on a T -matrix method and the approach by Brown-Preston-Singleton [L. S. Brown, Phys. Rep. 410, 237 (2005)] agrees with the simulation data to within the error bars of the simulation. For smaller Coulomb logarithms, the GMS model is consistent with the simulation results. Landau-Spitzer models are consistent with the simulation data for L>4 .

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 100000K(bothfor ρ between3and12g/cc,withselecthigher-ρ DFTcalculationsas 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.

Large enhancement of boron solubility in silicon due to biaxial stress

One of the important challenges to the semiconductor industry today is to enhance the solid solubility of several dopants, boron in particular, in silicon. We calculate the equilibrium solid solubility of boron in Si from first principles and examine the effect of biaxial stress. We find an unexpectedly large enhancement, on the order of 150%, for only 1% strain primarily due to the charge of the substitutional boron impurity in Si. We point out that this effect is an intrinsic property of Si and is expected to be important for other dopants as well.

Anomalous Quasiparticle Lifetime in Graphite: Band Structure Effects

We report ab initio calculations of quasiparticle lifetimes in graphite, as determined from the imaginary part of the self-energy operator within the GW approximation. The inverse lifetime in the energy range from 0.5 to 3.5 eV above the Fermi level presents significant deviations from the quadratic behavior naively expected from Fermi liquid theory. The deviations are explained in terms of the unique features of the band structure of this material. We also discuss the experimental results from different groups and make some predictions for future experiments.

Ab initio calculation of band-gap renormalization in highly excited GaAs

We present ab initio quasiparticle self-energy calculations in crystalline GaAs for cases of intense electronic excitation

Density-functional calculations of transport properties in the nondegenerate limit and the role of electron-electron scattering

(\ensuremath{\sim}10%

Origin of the spin reorientation transitions in (Fe1–xCox)2B alloys

of valence electrons excited into conduction band), relevant for high-intensity ultrashort pulsed laser experiments. Calculations are performed using an out-of-equilibrium generalization of the