Ultrafast far-infrared optics of carbon nanotubes

The optical properties of single-wall carbon nanotube sheets in the far-infrared (FIR) spectral range from few THz to several tens of THz have been investigated with terahertz spectroscopy both with static measurements elucidating the absorption mechanism in the FIR and with time-resolved experiments yielding information on the charge carrier dynamics after optical excitation of the nanotubes. We observe an overall depletion of the dominating broad absorption peak at around 4THz when the nanotubes are excited by a short visible laser pulse. This finding excludes particle-plasmon resonances as absorption mechanism and instead shows that interband transitions in tubes with an energy gap of ~10meV govern the far-infrared conductivity. A simple model based on an ensemble of two-level systems naturally explains the weak temperature dependence of the far-infrared conductivity by the tube-to-tube variation of the chemical potential. Furthermore, the time-resolved measurements do not show any evidence of a distinct free-carrier response which is attributed to the photogeneration of strongly bound excitons in the tubes with large energy gaps. The rapid decay of a featureless background with pronounced dichroism is associated with the increased absorption of spatially localized charge carriers before thermalization is completed

Temperature dependence of ultrafast phonon dynamics in graphite

Nonequilibrium optical phonons are generated in graphite following the excitation of electron-hole pairs with a femtosecond laser pulse. Their energy relaxation is probed by means of terahertz pulses. We find that the hot-phonon lifetime increases by a factor of 2 when the sample temperature decreases from 300 to 5 K. These results suggest that the energy relaxation in graphite at room temperature and above is dominated by the anharmonic decay of hot A′1phonons at the K point into acoustic phonons with energies of about 10 meV

Individual differences in updating are not related to reasoning ability and working memory capacity

Previous research assumes that executive functions such as inhibition, shifting, and updating explain individual differences in cognitive abilities. Of these three executive functions, updating was previously found to relate most strongly to fluid intelligence. However, this relationship could be a methodological artifact: Measures of inhibition and shifting usually isolate the contribution of this executive function to performance by contrasting conditions with high and low demands on these processes, whereas updating is measured by overall accuracy in working memory tasks involving updating. This updating measure conflates updating-specific individual differences (e.g., removal of outdated information) with variance in working memory maintenance. Reanalyzing data (N = 111) from von Bastian et al. (2016), we separated updating-specific variance from working memory maintenance variance. Updating contributed only 15% to individual differences in performance in the updating tasks, and it correlated neither with fluid intelligence nor with independent working memory measures reflecting storage and processing or relational integration. In contrast, the working memory maintenance component of the updating task correlated with both abilities. These findings challenge the view that updating contributes to variance in higher cognitive abilities

Ultrafast changes in the far-infrared conductivity of carbon nanotubes

The ultrafast charge-carrier dynamics in single-wall carbon nanotubes (NTs) have been investigated by time-resolved THz spectroscopy. Both the equilibrium and non-equilibrium conductivity data of the NTs in the far-infrared (FIR) spectral range from 1 to 40 THz are dominated by optical transitions across the band gap of tubes with gap energies of ~ 10 meV. A simple model based on an ensemble of two-level systems excellently explains all experimental findings. In particular, the surprisingly weak temperature dependence of the FIR conductivity has been shown to arise from tube-to-tube variation of the chemical potential which is ~ 100 meV in our sample. The results strongly suggest to use the temperature dependence of the FIR conductivity as a very sensitive and contact-free probe of the NT sample purity. Finally, the relaxation of the photo-excited NT sheet on a picosecond time scale mainly reflects the cooling of hot phonons which is about five times faster than in graphite. This points to much stronger lattice anharmonicities in NTs

Ultrafast dynamics of coherent optical phonons in α-quartz

Femtosecond laser excitation of alpha-quartz launches coherent optical phonons modulating the refractive index of the sample. The observed oscillations in the transmission and ellipticity of probe light decays due to phonon-phonon scattering. With decreasing temperature, the vibrations shift towards higher energies and are accompanied by a rise of the phonon lifetime caused by lattice stiffening and freezing of phonon modes, respectively