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

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

Research Methods in Cognition and Emotion

In this chapter we critically survey research methods used in the field of cognition and emotion. Research on cognition and emotion addresses a great variety of topics, which include the ways in which emotional states influence cognitive processes, the role of cognition in producing emotion, and folk categories and knowledge of emotion. So great is this variety that a brief chapter cannot address all the research methods that have contributed to the expansion of knowledge that has occurred in recent years; there are too many methods, and many are relevant only to particular specialized topics. Specialized research methods are discussed throughout this volume in the chapters devoted to the relevant topics. In this chapter we restrict our attention to methodological issues that span the field of cognition and emotion, yet are in some way unique to it. Not surprisingly, these are the issues and methods that have to do with emotion itself

Temperature Dependence of Electron to Lattice Energy-Transfer in Single-Wall Carbon Nanotube Bundles

The electron-phonon coupling strength in single-wall carbon nanotube (SWNT) bundles has been studied directly in the time-domain by femtosecond time-resolved photoelectron spectroscopy. We have measured the dependence of H(T_e,T_l), the rate of energy-transfer between the electronic system and the lattice as a function of electron and lattice temperatures T_e and T_l. The experiments are consistent with a T^5 dependence of H on the electron- and lattice-temperatures, respectively. The results can be related to the e-ph mass enhancement parameter lambda. The experimentally obtained value for lambda/theta_D^2, where theta_D is the Debye temperature, suggests that e-ph scattering times at the Fermi level of SWNT bundles can be exceptionally long, exceeding 1.5 ps at room temperature.Comment: 5 pages, 4 figures, submitted to the Journal of Nanoscience and Nanotechnologiy, special issue on nanotube

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

Ultrafast Electron Dynamics at Cu(111): Response of an Electron Gas to Optical Excitation

Time-resolved two-photon photoemission is used to directly investigate the electron dynamics at a Cu(111) surface with 60 fs laser pulses. We find that the time evolution of the photoexcited electron population in the first image state can be described only by solving the optical Bloch equations to properly account for coherence in the excitation process. Our experiments also provide evidence that the dynamics of photoexcited bulk electrons is strongly influenced by hot electron cascades and that the initial relaxation rates are in agreement with Fermi liquid theory

The magnetoelectrochemical switch

In the field of spintronics, the archetype solid-state two-terminal device is the spin valve, where the resistance is controlled by the magnetization configuration. We show here how this concept of spin-dependent switch can be extended to magnetic electrodes in solution, by magnetic control of their chemical environment. Appropriate nanoscale design allows a huge enhancement of the magnetic force field experienced by paramagnetic molecular species in solutions, which changes between repulsive and attractive on changing the electrodes' magnetic orientations. Specifically, the field gradient force created within a sub-100-nm-sized nanogap separating two magnetic electrodes can be reversed by changing the orientation of the electrodes' magnetization relative to the current flowing between the electrodes. This can result in a breaking or making of an electric nanocontact, with a change of resistance by a factor of up to 103. The results reveal how an external field can impact chemical equilibrium in the vicinity of nanoscale magnetic circuits