Spontaneous exciton dissociation in carbon nanotubes

Simultaneous photoluminescence and photocurrent measurements on individual single-walled carbon nanotubes reveal spontaneous dissociation of excitons into free electron-hole pairs. Correlation of luminescence intensity and photocurrent shows that a significant fraction of excitons are dissociating during their relaxation into the lowest exciton state. Furthermore, the combination of optical and electrical signals also allows for extraction of the absorption cross section and the oscillator strength. Our observations explain the reasons for photoconductivity measurements in single-walled carbon nanotubes being straightforward despite the large exciton binding energies.Comment: 4 pages, 3 figure

Gate-controlled generation of optical pulse trains using individual carbon nanotubes

We report on optical pulse-train generation from individual air-suspended carbon nanotubes under an application of square-wave gate voltages. Electrostatically-induced carrier accummulation quenches photoluminescence, while a voltage sign reversal purges those carriers, resetting the nanotubes to become luminescent temporarily. Frequency domain measurements reveal photoluminescence recovery with characteristic frequencies that increase with excitation laser power, showing that photoexcited carriers quench the emission in a self-limiting manner. Time-resolved measurements directly confirm the presence of an optical pulse train sychronized to the gate voltage signal, and flexible control over pulse timing and duration is demonstrated.Comment: 4 pages, 4 figure

Exciton diffusion in air-suspended single-walled carbon nanotubes

Direct measurements of the diffusion length of excitons in air-suspended single-walled carbon nanotubes are reported. Photoluminescence microscopy is used to identify individual nanotubes and to determine their lengths and chiral indices. Exciton diffusion length is obtained by comparing the dependence of photoluminescence intensity on the nanotube length to numerical solutions of diffusion equations. We find that the diffusion length in these clean, as-grown nanotubes is significantly longer than those reported for micelle-encapsulated nanotubes.Comment: 4 pages, 4 figure

EFFECT OF SOLVENT ON PROTON TUNNELING IN THE REACTIONS OF 4-NITROPHENYLNITROMETHANE WITH 1-ETHYLPIPERIDINE AND 1,8-DIAZABICYCLO [5.4.0] UNDEC-7-ENE

A kinetic study has been carried out on the proton-transfer reactions from 4-nitrophenylnitromethane (4NPNM) to 1-ethylpiperidine (EP) and to 1,8-diazabicyclo[5.4.0]undec-7-ene (DBU) in acetonitrile and dichloromethane by means of a stopped-flow method, the attention being focused to solvent effect on kinetic isotope effect (k_f^H/k_f^D) and degree of tunneling. For the 4NPNM-EP system, k_f^H/k_f^D at 25℃ was 17.2 and the activation energy difference, E_af^D-E_af^H, was as large as 10.8 kJmol^-, which suggested that the degree of proton-tunneling was significant. On the contrary, for the 4NPNM-DBU system the degree of proton-tunneling in both acetonitrile and dichloromethane was not so significant as 4NPNM-EP reaction. The semi-empirical analysis on the basis of the Bell\u27s treatment showed that the barrier width for 4NPNM-EP was larger and barrier height became much higher in comparison to the reaction of 4NPNM with the bases such as DBU or tetramethylguanidine (TMG). Those results were considered from the viewpoints of solvation effect on the transition state

Orbital selectivity of the kink in the dispersion of Sr2RuO4

We present detailed energy dispersions near the Fermi level on the monolayer perovskite ruthenate Sr2RuO4, determined by high-resolution angle-resolved photoemission spectroscopy. An orbital selectivity of the kink in the dispersion of Sr2RuO4 has been found: A kink for the Ru 4d_xy orbital is clearly observed, but not for the Ru 4d_yz and 4d_zx ones. The result provides insight into the origin of the kink.Comment: 5 pages, 4 figures. Accepted for publication in Phys. Rev.

A Universal Lifetime Distribution for Multi-Species Systems

Lifetime distributions of social entities, such as enterprises, products, and media contents, are one of the fundamental statistics characterizing the social dynamics. To investigate the lifetime distribution of mutually interacting systems, simple models having a rule for additions and deletions of entities are investigated. We found a quite universal lifetime distribution for various kinds of inter-entity interactions, and it is well fitted by a stretched-exponential function with an exponent close to 1/2. We propose a "modified Red-Queen" hypothesis to explain this distribution. We also review empirical studies on the lifetime distribution of social entities, and discussed the applicability of the model.Comment: 10 pages, 6 figures, Proceedings of Social Modeling and Simulations + Econophysics Colloquium 201