Electronic structure and dynamics of optically excited single-wall carbon nanotubes

We have studied the electronic structure and charge-carrier dynamics of individual single-wall carbon nanotubes (SWNTs) and nanotube ropes using optical and electron-spectroscopic techniques. The electronic structure of semiconducting SWNTs in the band-gap region is analyzed using near-infrared absorption spectroscopy. A semi-empirical expression for $E_{11}^{\rm S}$ transition energies, based on tight-binding calculations is found to give striking agreement with experimental data. Time-resolved PL from dispersed SWNT-micelles shows a decay with a time constant of about 15 ps. Using time-resolved photoemission we also find that the electron-phonon ({\it e-ph}) coupling in metallic tubes is characterized by a very small {\it e-ph} mass-enhancement of 0.0004. Ultrafast electron-electron scattering of photo-excited carriers in nanotube ropes is finally found to lead to internal thermalization of the electronic system within about 200 fs.Comment: 10 pages, 10 figures, submitted to Applied Physics

Magnetization structure of a Bloch point singularity

Switching of magnetic vortex cores involves a topological transition characterized by the presence of a magnetization singularity, a point where the magnetization vanishes (Bloch point). We analytically derive the shape of the Bloch point that is an extremum of the free energy with exchange, dipole and the Landau terms for the determination of the local value of the magnetization modulus.Comment: 4 pages, 2 figure

Femtosecond time-resolved photoemission as a probe of electronic transport in single wall carbon nanotubes

We have performed the first time-domain measurements of the electron-electron (e-e) and electron-phonon (e-ph) dynamics in single-wall carbon nanotube samples (bucky paper) using time-resolved two-photon photoemission. In these room temperature experiments the absorption of a visible femtosecond pump pulse creates a non-equilibrium electron distribution whose evolution in time can be probed by a second UV-pulse. The decay of the excited electron distribution is characterized by a fast channel on the subpicosecond time-scale—associated with thermalization of the non-equilibrium distribution—and a slower channel which can be attributed to e-ph interaction. Once thermalized the electron distribution cools down to the lattice temperature as determined by the electron-phonon coupling constant g which was found to be 1×1015 Wm−3 K−1

Electron-Phonon Interaction in Single-Wall Carbon Nanotubes: A Time Domain Study

We investigate the electron-phonon (e-ph) interaction in single-wall carbon nanotube samples at room temperature using femtosecond time-resolved photoemission. By probing electrons from the vicinity of the Fermi level we are able to study the e-ph interaction in the metallic nanotube species only. The observed electron dynamics can be used to calculate e-ph scattering matrix elements for two likely scattering scenarios: forward scattering from twistons and backscattering by longitudinal acoustic phonons. The corresponding matrix elements reveal an intrinsically weak e-ph interaction approximately 50% smaller than predicted by tight-binding calculations

Intentional Forgetting Benefits From Thought Substitution

This study provides both experimental and correlational evidence that forgetting in the think/no-think paradigm (Anderson & Green, 2001) is sensitive to the substitution of thoughts about new events forthoughts that are to be suppressed. All the participants learned a list of adjective-noun pairs. Then the adjectives were presented as cues for recalling half of the nouns and as cues for suppressing the other half, 0, 2, or 12 times. Aided participants were provided with substitute nouns, to use during suppression. On a final test that requested recall of all initially learned nouns, aided participants showed evidence of below-baseline forgetting of suppressed nouns. Unaided participants produced below-baseline forgetting only if their later self-reports indicated that they had complied relatively well with instructions for suppression. Independently, forgetting in the unaided condition was more successful when the participants reportedly thought about something else during suppression trials. In general, the use of self-initiated strategies seems to affect the degree of forgetting in the think/no-think paradigm

Social environment shapes female settlement decisions in a solitary carnivore

How and where a female selects an area to settle and breed is of central importance in dispersal and population ecology as it governs range expansion and gene flow. Social structure and organization have been shown to influence settlement decisions, but its importance in the settlement of large, solitary mammals is largely unknown. We investigate how the identity of overlapping conspecifics on the landscape, acquired during the maternal care period, influences the selection of settlement home ranges in a non-territorial, solitary mammal using location data of 56 female brown bears (Ursus arctos). We used a resource selection function to determine whether females' settlement behavior was influenced by the presence of their mother, related females, familiar females, and female population density. Hunting may remove mothers and result in socio-spatial changes before settlement. We compared overlap between settling females and their mother's concurrent or most recent home ranges to examine the settling female's response to the absence or presence of her mother on the landscape. We found that females selected settlement home ranges that overlapped their mother's home range, familiar females, that is, those they had previously overlapped with, and areas with higher density than their natal ranges. However, they did not select areas overlapping related females. We also found that when mothers were removed from the landscape, female offspring selected settlement home ranges with greater overlap of their mother's range, compared with mothers who were alive. Our results suggest that females are acquiring and using information about their social environment when making settlement decisions.Information about the social environment may help female brown bears to select a settlement home range for breeding. We found that a female uses the identity of other females that overlapped her natal home range and female density when making settlement decisions. Specifically, females select settlement home ranges that overlap with home ranges of their mothers and familiar females known from their natal period. Relatedness does not appear to influence settlement decisions in this population

Collisionless energy absorption in the short-pulse intense laser-cluster interaction

In a previous Letter [Phys. Rev. Lett. 96, 123401 (2006)] we have shown by means of three-dimensional particle-in-cell simulations and a simple rigid-sphere model that nonlinear resonance absorption is the dominant collisionless absorption mechanism in the intense, short-pulse laser cluster interaction. In this paper we present a more detailed account of the matter. In particular we show that the absorption efficiency is almost independent of the laser polarization. In the rigid-sphere model, the absorbed energy increases by many orders of magnitude at a certain threshold laser intensity. The particle-in-cell results display maximum fractional absorption around the same intensity. We calculate the threshold intensity and show that it is underestimated by the common over-barrier ionization estimate.Comment: 12 pages, 13 figures, RevTeX