Supermetallic conductivity in bromine-intercalated graphite

Exposure of highly oriented pyrolytic graphite to bromine vapor gives rise to in-plane charge conductivities which increase monotonically with intercalation time toward values (for ~6 at% Br) that are significantly higher than Cu at temperatures down to 5 K. Magnetotransport, optical reflectivity and magnetic susceptibility measurements confirm that the Br dopes the graphene sheets with holes while simultaneously increasing the interplanar separation. The increase of mobility (~ 5E4 cm^2/Vs at T=300 K) and resistance anisotropy together with the reduced diamagnetic susceptibility of the intercalated samples suggests that the observed supermetallic conductivity derives from a parallel combination of weakly-coupled hole-doped graphene sheets.Comment: 5 pages, 4 figure

Electronic and structural properties of alkali doped SWNT

Comprehensive experiments on structural and transport properties of alkali intercalated single walled carbon nanotubes (SWNT) are presented. The increasing electron density was measured as a shift of the Drude-edge in optical reflectivity in-situ with progressive doping. In saturation-doped samples the Drude-edge shifts into the visible (to 25,000 - 30,000 cm— 1 for potassium and rubidium doped samples) and the samples have a golden-brown color, similar to stage I graphite. X-ray diffraction reveals a crystalline rope structure with expanded lattice constant, similar to results of Duclaux et al. The change in the low temperature divergence of the resistivity after degassing at high temperature and high vacuum and after K-doping is studied in-situ

Wide range optical studies on transparent SWNT films

We present transmission spectra from the far infrared through the ultraviolet region on freestanding SWNT films at temperatures between 40 and 300 K. Several interesting features are observed in the low-frequency part of the spectrum: the Drude-like frequency dependence of the metallic tubes as well as a (sample-dependent) peak in the conductivity around 0.01 eV. We also studied the accidental nitrate doping of the SWNT samples during purification by nitric acid. Asprepared purified samples exhibit increased metallic absorption and decreased interband transitions; these features disappear on heating in vacuum

Optical conductivity for a dimer in the Dynamic Hubbard model

The Dynamic Hubbard Model represents the physics of a multi-band Hubbard model by using a pseudo-spin degree of freedom to dynamically modify the on-site Coulomb interaction. Here we use a dimer system to obtain analytical results for this model. The spectral function and the optical conductivity are calculated analytically for any number of electrons, and the distribution of optical spectral weight is analyzed in great detail. The impact of polaron-like effects due to overlaps between pseudo-spin states on the optical spectral weight distribution is derived analytically. Our conclusions support results obtained previously with different models and techniques: holes are less mobile than electrons.Comment: 11 pages, 4 figure

Optical and electron-energy-loss studies of the monomeric and dimeric phases of decamethylferrocenium tetracyanoquinodimethanide, (DMeFc)(TCNQ)

Journal ArticleThe optical properties of the two crystallographic phases of 1:l decaniethylferrocenium tetracyanoauinodimethanide, (DMeFc)(TCNQ), have been measured from 0.1 to 10 eV. One phase consists of isolated paramagnetic TCNQ anion monomers while the other contains isolated diamagnetic dimers. The spectrum of the monomeric phase exhibits a strong localized monomer exciton which is not normally observed in solid TCNQ salts, while the dimeric phase shows a charge-transfer excitation as well as a shifted local exciton. From the frequency- dependent conductivity of the dimeric phase the effective on-site Coulomb interaction and the transfer matrix element are measured to be 1.0 and 0.27 eV, respectively. The infrared absorption spectrum of the dimeric phase shows an unusual activity of the symmetric phonon modes due to the interaction of these modes with the radical electron, whereas in the monomeric phase only normally infrared active phonons are observed. In electron-energy-loss measurements an anomalous momentum dependence of the line shape of the monomeric exciton was observed. This result is attributed to a dielectric effect caused by the decrease in strength of local excitons with increasing momentum