416 research outputs found
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
The economic impact of workplace wellness programmes in Canada
Background The economic benefits of workplace wellness programmes (WWPs) are commonly cited as a reason for employers to implement such programmes; however, there is limited evidence outside of the US context exploring their economic impact. US evidence is less relevant in countries such as Canada with universal publicly funded health systems because of the lower potential employer savings from WWPs. Aims To conduct a systematic review of the Canadian literature investigating the economic impact of WWPs from an employer perspective. The quality of that evidence was also assessed. Methods We reviewed literature which included analyses of four economic outcomes: return on investment calculations; cost-effectiveness or cost-benefit analyses; valuations of productivity, turnover, absenteeism and/or presenteeism costs; and valuations of health care utilization costs. We applied the British Medical Journal (BMJ) Economic Evaluation Working Party Checklist to evaluate the quality of this evidence. Results Eight studies met the inclusion criteria. Although the studies showed that WWPs generated economic benefits from an employer perspective (largely from productivity changes), none of the reviewed studies were in the high-quality category (i.e. fulfilled at least 75% of the checklist criteria) and most had severe methodological issues. Conclusions Though the Canadian literature pertaining to the economic impact of WWPs spans over three decades, robust evidence on this topic remains sparse. Future research should include a comparable control group, a time horizon of over a year, both direct and indirect costs, and researchers should apply analytical techniques that account for potential selection bias
Quantum Particles Constrained on Cylindrical Surfaces with Non-constant Diameter
We present a theoretical formulation of the one-electron problem constrained
on the surface of a cylindrical tubule with varying diameter. Because of the
cylindrical symmetry, we may reduce the problem to a one-dimensional equation
for each angular momentum quantum number along the cylindrical axis. The
geometrical properties of the surface determine the electronic structures
through the geometry dependent term in the equation. Magnetic fields parallel
to the axis can readily be incorporated. Our formulation is applied to simple
examples such as the catenoid and the sinusoidal tubules. The existence of
bound states as well as the band structures, which are induced geometrically,
for these surfaces are shown. To show that the electronic structures can be
altered significantly by applying a magnetic field, Aharonov-Bohm effects in
these examples are demonstrated.Comment: 7 pages, 7 figures, submitted to J. Phys. Soc. Jp
Gauge field for edge state in graphene
By considering the continuous model for graphene, we analytically study a
special gauge field for the edge state. The gauge field explains the properties
of the edge state such as the existence only on the zigzag edge, the partial
appearance in the -space, and the energy position around the Fermi energy.
It is demonstrated utilizing the gauge field that the edge state is robust for
surface reconstruction, and the next nearest-neighbor interaction which breaks
the particle-hole symmetry stabilizes the edge state.Comment: 9 pages, 5 figure
Ultrafast Optical Spectroscopy of Micelle-Suspended Single-Walled Carbon Nanotubes
We present results of wavelength-dependent ultrafast pump-probe experiments
on micelle-suspended single-walled carbon nanotubes. The linear absorption and
photoluminescence spectra of the samples show a number of chirality-dependent
peaks, and consequently, the pump-probe results sensitively depend on the
wavelength. In the wavelength range corresponding to the second van Hove
singularities (VHSs), we observe sub-picosecond decays, as has been seen in
previous pump-probe studies. We ascribe these ultrafast decays to intraband
carrier relaxation. On the other hand, in the wavelength range corresponding to
the first VHSs, we observe two distinct regimes in ultrafast carrier
relaxation: fast (0.3-1.2 ps) and slow (5-20 ps). The slow component, which has
not been observed previously, is resonantly enhanced whenever the pump photon
energy resonates with an interband absorption peak, and we attribute it to
radiative carrier recombination. Finally, the slow component is dependent on
the pH of the solution, which suggests an important role played by H ions
surrounding the nanotubes.Comment: 6 pages, 8 figures, changed title, revised, to be published in
Applied Physics
Anomalous Aharonov--Bohm gap oscillations in carbon nanotubes
The gap oscillations caused by a magnetic flux penetrating a carbon nanotube
represent one of the most spectacular observation of the Aharonov-Bohm effect
at the nano--scale. Our understanding of this effect is, however, based on the
assumption that the electrons are strictly confined on the tube surface, on
trajectories that are not modified by curvature effects. Using an ab-initio
approach based on Density Functional Theory we show that this assumption fails
at the nano-scale inducing important corrections to the physics of the
Aharonov-Bohm effect. Curvature effects and electronic density spilled out of
the nanotube surface are shown to break the periodicity of the gap
oscillations. We predict the key phenomenological features of this anomalous
Aharonov-Bohm effect in semi-conductive and metallic tubes and the existence of
a large metallic phase in the low flux regime of Multi-walled nanotubes, also
suggesting possible experiments to validate our results.Comment: 7 figure
Effective polyethylenimine-mediated gene transfer into human endothelial cells
Background
The major advantage in choosing nonâviral vectors such as cationic polymers for in vitro and in vivo transfection is their higher biosafety than viral ones. Among the cationic polymers, polyethylenimines (PEIs) are promising molecules for gene delivery to a variety of cells. Efficient transfection of primary endothelial cells using PEIs could be regarded as an interesting strategy of treatment in some ischemic cardiovascular diseases.
Methods
Efficacies of a 22âkDa linear PEI (LâPEI) and its glucoseâgrafted derivative (LâPEIâGlc4) were compared for gene transfer into human umbilical vein endothelial cells (HUVEC) using the reporter gene luciferase. Cells were incubated for 2, 4 and 24 h with PEI/DNA complexes made in 150 mM sodium chloride (NaCl) or in 5% glucose solution. Luciferase activity was measured 24 h after the onset of transfection. The effects of low (2%) and high (30%) concentrations of serum on transfection efficacy were assessed as well. We then studied the intracellular fate of the PEI/DNA complexes labelled with the DNA intercalator YOYOâ1 using flow cytometry analysis (FACS) and confocal microscopy.
Results
PEI/DNA complexes formed in NaCl led to a higher transfection efficacy than those made in glucose. The optimal formulation, depending on the incubation time and the presence of serum in the medium, was obtained using DNA complexed to LâPEIâGlc4 and incubated for 4 h with the cells. This condition led to 50% fluorescent cells after GFP transfection. A high serum concentration diminished the LâPEI associated toxicity but decreased LâPEIâGlc4 transfection efficiency. FACS analysis using both vectors showed that almost 90% of the cells had internalized the DNA complexes. Confocal microscopic observations showed a fast attachment of the complexes to the cell surface followed by inclusion into vesicles that migrated to the perinuclear region.
Conclusions
In this work, we defined the optimal conditions for gene delivery in HUVEC. These conditions were obtained when using derivatives LâPEI and LâPEIâGlc4 complexed with DNA in 150 mM NaCl and added to cells for 2 and 4 h, respectively. Cellular trafficking of the complexes suggested that cell entry was not a limiting factor for gene delivery using PEI. This study underlined the interest in PEIs as efficient vectors for gene transfer into human endothelial cells
Cost-effectiveness of using a gene expression profiling test to aid in identifying the primary tumour in patients with cancer of unknown primary.
We aimed to investigate the cost-effectiveness of a 2000-gene-expression profiling (GEP) test to help identify the primary tumor site when clinicopathological diagnostic evaluation was inconclusive in patients with cancer of unknown primary (CUP). We built a decision-analytic-model to project the lifetime clinical and economic consequences of different clinical management strategies for CUP. The model was parameterized using follow-up data from the Manitoba Cancer Registry, cost data from Manitoba Health administrative databases and secondary sources. The 2000-GEP-based strategy compared to current clinical practice resulted in an incremental cost-effectiveness ratio (ICER) of 36.2 million per year. A value-of-information analysis revealed that the expected value of perfect information about the test\u27s clinical impact was $4.2 million per year. The 2000-GEP test should be considered for adoption in CUP. Field evaluations of the test are associated with a large societal benefit.The Pharmacogenomics Journal advance online publication, 29 March 2016; doi:10.1038/tpj.2015.94
Orbital Kondo effect in carbon nanotubes
Progress in the fabrication of nanometer-scale electronic devices is opening
new opportunities to uncover the deepest aspects of the Kondo effect, one of
the paradigmatic phenomena in the physics of strongly correlated electrons.
Artificial single-impurity Kondo systems have been realized in various
nanostructures, including semiconductor quantum dots, carbon nanotubes and
individual molecules. The Kondo effect is usually regarded as a spin-related
phenomenon, namely the coherent exchange of the spin between a localized state
and a Fermi sea of electrons. In principle, however, the role of the spin could
be replaced by other degrees of freedom, such as an orbital quantum number.
Here we demonstrate that the unique electronic structure of carbon nanotubes
enables the observation of a purely orbital Kondo effect. We use a magnetic
field to tune spin-polarized states into orbital degeneracy and conclude that
the orbital quantum number is conserved during tunneling. When orbital and spin
degeneracies are simultaneously present, we observe a strongly enhanced Kondo
effect, with a multiple splitting of the Kondo resonance at finite field and
predicted to obey a so-called SU(4) symmetry.Comment: 26 pages, including 4+2 figure
Carbon Nanotube Terahertz Polarizer
We describe a film of highly-aligned single-walled carbon nanotubes that acts
as an excellent terahertz linear polarizer. There is virtually no attenuation
(strong absorption) when the terahertz polarization is perpendicular (parallel)
to the nanotube axis. From the data we calculated the reduced linear dichrosim
to be 3, corresponding to a nematic order parameter of 1, which demonstrates
nearly perfect alignment as well as intrinsically anisotropic terahertz
response of single-walled carbon nanotubes in the film.Comment: 13 pages, 3 figure
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