6,837 research outputs found
Existence of an upper limit on the density of excitons in carbon nanotubes by diffusion-limited exciton-exciton annihilation: Experiment and theory
Through an investigation of photoemission properties of highly-photoexcited
single-walled carbon nanotubes, we demonstrate that there is an upper limit on
the achievable excitonic density. As the intensity of optical excitation
increases, all photoluminescence emission peaks arising from different
chirality single-walled carbon nanotubes showed clear saturation in intensity.
Each peak exhibited a saturation value that was independent of the excitation
wavelength, indicating that there is an upper limit on the excitonic density
for each nanotube species. We propose that this saturation behavior is a result
of efficient exciton-exciton annihilation through which excitons decay
non-radiatively. In order to explain the experimental results and obtain
excitonic densities in the saturation regime, we have developed a model, taking
into account the generation, diffusion-limited exciton-exciton annihilation,
and spontaneous decays of one-dimensional excitons. Using the model, we were
able to reproduce the experimentally obtained saturation curves under certain
approximations, from which the excitonic densities were estimated. The validity
of the model was confirmed through comparison with Monte Carlo simulations.
Finally, we show that the conventional rate equation for exciton-exciton
annihilation without taking into account exciton diffusion fails to fit the
experimentally observed saturation behavior, especially at high excitonic
densities.Comment: 5 figures, 1 tabl
Anisotropic Decay Dynamics of Photoexcited Aligned Carbon Nanotube Bundles
We have performed polarization-dependent ultrafast pump-probe spectroscopy of
a film of aligned single-walled carbon nanotube bundles. By taking into account
imperfect nanotube alignment as well as anisotropic absorption cross sections,
we quantitatively determined distinctly different photo-bleaching dynamics for
polarizations parallel and perpendicular to the tube axis. For perpendicular
polarization, we observe a slow (1.0-1.5 ps) relaxation process, previously
unobserved in randomly-oriented nanotube bundles. We attribute this slower
dynamics to the excitation and relaxation of surface plasmons in the radial
direction of the nanotube bundles.Comment: 4 pages, 3 figure
Loop space homology associated with the mod 2 Dickson invariants
Peer reviewedPublisher PD
Foreign aid encourages trade liberalization when given topolitically insecure leaders
Can foreign aid be a useful tool to promote trade liberalization? In new research which studies 66 autocracies over two decades, Daniel Yuichi Kono and Gabriella R. Montinola find that foreign aid does encourage trade liberalization – but only in countries where leaders are politically insecure. They write that the less chance that an autocratic leader has of staying in power, the greater the effect of aid on tariff reductions
Foreign Aid and Climate Change Policy: What Can(’t) the Data Tell Us?
Climate-related foreign aid is on the rise, with signatories to the Paris Climate agreement pledging $100 billion annually to promote mitigation and adaptation in recipient countries. While this seems like a welcome development, we have little evidence that climate aid actually encourages recipients to adopt climate legislation. In this article, we examine the relationship between climate aid and recipient climate policy. Using multiple measures of each, we find no evidence that the former is systematically related to the latter. Although this suggests that climate aid is ineffective, this conclusion must be qualified due to the poor quality of both climate aid and climate policy data. More definitive conclusions will require more accurate coding of climate aid as well as better climate policy measures that distinguish truly consequential policies from less consequential ones
Spin Relaxation Times of Single-Wall Carbon Nanotubes
We have measured temperature ()- and power-dependent electron spin
resonance in bulk single-wall carbon nanotubes to determine both the
spin-lattice and spin-spin relaxation times, and . We observe that
increases linearly with from 4 to 100 K, whereas {\em
decreases} by over a factor of two when is increased from 3 to 300 K. We
interpret the trend as spin-lattice relaxation via
interaction with conduction electrons (Korringa law) and the decreasing
dependence of as motional narrowing. By analyzing the latter, we
find the spin hopping frequency to be 285 GHz. Last, we show that the Dysonian
lineshape asymmetry follows a three-dimensional variable-range hopping behavior
from 3 to 20 K; from this scaling relation, we extract a localization length of
the hopping spins to be 100 nm.Comment: 6 pages, 3 figure
Stability of 1-D Excitons in Carbon Nanotubes under High Laser Excitations
Through ultrafast pump-probe spectroscopy with intense pump pulses and a wide
continuum probe, we show that interband exciton peaks in single-walled carbon
nanotubes (SWNTs) are extremely stable under high laser excitations. Estimates
of the initial densities of excitons from the excitation conditions, combined
with recent theoretical calculations of exciton Bohr radii for SWNTs, suggest
that their positions do not change at all even near the Mott density. In
addition, we found that the presence of lowest-subband excitons broadens all
absorption peaks, including those in the second-subband range, which provides a
consistent explanation for the complex spectral dependence of pump-probe
signals reported for SWNTs.Comment: 4 pages, 4 figure
Evaluation of Hepatocellular Carcinoma Transarterial Chemoembolization using Quantitative Analysis of 2D and 3D Real-time Contrast Enhanced Ultrasound.
Quantitative 2D and 3D contrast-enhanced ultrasound (CEUS) was assessed to evaluate early transarterial chemoembolization (TACE) treatment response. Seventeen patients scheduled for TACE for the treatment of hepatocellular carcinoma participated in the study. 2D and 3D CEUS were performed for each patient at three time points: Prior to TACE, 1-2 weeks post TACE, and 1 month post TACE. Peak-intensities of the tumor and surrounding liver tissue were calculated from 2D and 3D data before and after TACE and used to evaluate tumor treatment response. Residual tumor percentages were calculated from 2D and 3D CEUS acquired 1-2 weeks and 1 month post TACE and compared with results from MRI 1 month post TACE. Nine subjects had complete response while 8 had incomplete response. Peak-intensities of the tumor from 3D CEUS prior to TACE were similar between the complete and incomplete treatment groups (p = 0.70), while 1-2 weeks (p \u3c 0.01) and 1 month post treatment (p \u3c 0.01) were significantly lower in the complete treatment group than in the incomplete treatment group. For 2D CEUS, only the peak-intensity values of the tumor from 1 month post TACE were significantly different (p \u3c 0.01). The correlation coefficients between 2D and 3D residual tumor estimates 1-2 weeks post TACE and the estimates from MRI were 0.73 and 0.94, respectively, while those from 2D and 3D CEUS 1 month post TACE were 0.66 and 0.91, respectively. Quantitative analysis on 2D and 3D CEUS shows potential to differentiate patients with complete versus incomplete response to TACE as early as 1-2 weeks post treatment
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