8,201 research outputs found
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
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