3,974 research outputs found
Flow-induced voltage and current generation in carbon nanotubes
New experimental results, and a plausible theoretical understanding thereof,
are presented for the flow-induced currents and voltages observed in
single-walled carbon nanotube samples. In our experiments, the electrical
response was found to be strongly sublinear -- nearly logarithmic -- in the
flow speed over a wide range, and its direction could be controlled by an
electrochemical biasing of the nanotubes. These experimental findings are
inconsistent with the conventional idea of a streaming potential as the
efficient cause. Here we present a new, physically appealing, Langevin-equation
based treatment of the nanotube charge carriers, assumed to be moving under
coulombic forcing by the correlated ionic fluctuations, advected by the liquid
in flow. The resulting 'Doppler-shifted' force-force correlation, as seen by
the charge carriers drifting in the nanotube, is shown to give a strongly
sublinear response, broadly in agreement with experiments.Comment: 11 pages including 3 figures. To appear in Phys. Rev B (2004
Modelling and Simulation of Multi-target Multi-sensor Data Fusion for Trajectory Tracking
An implementation of track fusion using various algorthims has been demonstrated . The sensor measurements of these targets are modelled using Kalman filter (KF) and interacting multiple models (IMM) filter. The joint probabilistic data association filter (JPDAF) and neural network fusion (NNF) algorithms were used for tracking multiple man-euvring targets. Track association and fusion algorithm are executed to get the fused track data for various scenarios, two sensors tracking a single target to three sensors tracking three targets, to evaluate the effects of multiple and dispersed sensors for single target, two targets, and multiple targets. The targets chosen were distantly spaced, closely spaced and crossing. Performance of different filters was compared and fused trajectory is found to be closer to the true target trajectory as compared to that for any of the sensor measurements of that target.Defence Science Journal, 2009, 59(3), pp.205-214, DOI:http://dx.doi.org/10.14429/dsj.59.151
Femtosecond Photoexcited Carrier Dynamics in Reduced Graphene Oxide Suspensions and Films
We report ultrafast response of femtosecond photoexcited carriers in single
layer reduced graphene oxide flakes suspended in water as well as few layer
thick film deposited on indium tin oxide coated glass plate using pump-probe
differential transmission spectroscopy at 790 nm. The carrier relaxation
dynamics has three components: ~200 fs, 1 to 2 ps, and ~25 ps, all of them
independent of pump fluence. It is seen that the second component (1 to 2 ps)
assigned to the lifetime of hot optical phonons is larger for graphene in
suspensions whereas other two time constants are the same for both the
suspension and the film. The value of third order nonlinear susceptibility
estimated from the pump-probe experiments is compared with that obtained from
the open aperture Z-scan results for the suspension.Comment: 4 pages, 4 figures, to appear in International Journal of Nanoscience
(IJN), 201
Large nonlinear absorption and refraction coefficients of carbon nanotubes estimated from femtosecond Z-scan measurements
Nonlinear transmission of 80 and 140 femtosecond pulsed light with wavelength through single walled carbon nanotubes suspended in water
containing sodium dodecyl sulphate is studied. Pulse-width independent
saturation absorption and negative cubic nonlinearity are observed,
respectively, in open and closed aperture Z-scan experiments. The theoretical
expressions derived to analyze the z-dependent transmission in the saturable
limit require two photon absorption coefficient and a
nonlinear index to fit the data.Comment: 10 pages, 2 figures. Accepted and to appear in Applied Physics
Letter
Femtosecond carrier dynamics and saturable absorption in graphene suspensions
Nonlinear optical properties and carrier relaxation dynamics in graphene,
suspended in three different solvents, are investigated using femtosecond (80
fs pulses) Z-scan and degenerate pumpprobe spectroscopy at 790 nm. The results
demonstrate saturable absorption property of graphene with a nonlinear
absorption coefficient, , of ~2 to 9x10^-8 cm/W. Two distinct time scales
associated with the relaxation of photoexcited carriers, a fast one in the
range of 130-330 fs (related to carrier-carrier scattering) followed by a
slower one in 3.5-4.9 ps range (associated with carrier-phonon scattering) are
observed.Comment: 3 pages, 2 figures, 2 table
Large-amplitude chirped coherent phonons in tellurium mediated by ultrafast photoexcited carrier diffusion
We report femtosecond time-resolved reflectivity measurements of coherent
phonons in tellurium performed over a wide range of temperatures (3K to 296K)
and pump laser intensities. A totally symmetric A coherent phonon at 3.6
THz responsible for the oscillations in the reflectivity data is observed to be
strongly positively chirped (i.e, phonon time period decreases at longer
pump-probe delay times) with increasing photoexcited carrier density, more so
at lower temperatures. We show for the first time that the temperature
dependence of the coherent phonon frequency is anomalous (i.e, increasing with
increasing temperature) at high photoexcited carrier density due to
electron-phonon interaction. At the highest photoexcited carrier density of
1.4 10cm and the sample temperature of 3K, the
lattice displacement of the coherent phonon mode is estimated to be as high as
0.24 \AA. Numerical simulations based on coupled effects of optical
absorption and carrier diffusion reveal that the diffusion of carriers
dominates the non-oscillatory electronic part of the time-resolved
reflectivity. Finally, using the pump-probe experiments at low carrier density
of 6 10 cm, we separate the phonon anharmonicity to
obtain the electron-phonon coupling contribution to the phonon frequency and
linewidth.Comment: 22 pages, 6 figures, submitted to PR
The Impact of Repeat HIV Testing on Risky Sexual Behavior: Evidence from a Randomized Controlled Trial in Malawi.
A significant proportion of HIV-positive adults in sub-Saharan Africa are in serodiscordant relationships. Identification of such serodiscordant couples through couple HIV testing and counseling (HTC) is thought to promote safe sexual behavior and reduce the probability of within couple seroconversion. However, it is possible HTC benefits are not sustained over time and therefore repeated HTC may be more effective at preventing seroconversion than one time HTC. We tested this theory in Zomba, Malawi by randomly assigning 170 serodiscordant couples to receive repeated HTC and 167 serodiscordant couples to receive one time HTC upon study enrollment (control group). We used linear probability models and probit model with couple fixed effects to assess the impact of the intervention on risky sexual behavior. At one-year follow-up, we found that couples that received repeated HTC reported significantly more condom use. However, we found no difference in rate of seroconversion between groups, nor did we find differences in subjective expectations about seroconversion or false beliefs about HIV, two expected pathways of behavior change. We conclude that repeated HTC may promote safe sexual behavior, but this result should be interpreted with caution, as it is inconsistent with the result from biological and subjective outcomes
Brillouin Scattering Studies of La_{0.77}Ca_{0.23}MnO_3 Across Metal-Insulator Transition
Temperature-dependent Brillouin scattering studies have been carried out on
La_{0.77}Ca_{0.23}MnO_3 across the paramagnetic insulator - ferromagnetic metal
(I-M) transition. The spectra show a surface Rayleigh wave (SRW) and a high
velocity pseudo surface acoustic wave (HVPSAW) besides bulk acoustic waves
(BAW). The Brillouin shifts associated with SRW and HVPSAW show blue-shifts,
where as the frequencies of the BAW decrease below the I-M transition
temperature (T_C) of 230 K. These results can be understood based on the
temperature dependence of the elastic constants. We also observe a central peak
whose width is maximum at T_C.Comment: 7 pages, 8 figure
Ultrafast switching time and third order nonlinear coefficients of microwave treated single walled carbon nanotube suspensions
Microwave treated water soluble and amide functionalized single walled carbon
nanotubes have been investigated using femtosecond degenerate pump-probe and
nonlinear transmission experiments. The time resolved differential transmission
using 75 femtosecond pulse with the central wavelength of 790 nm shows a
bi-exponential ultrafast photo-bleaching with time constants of of 160 fs (130
fs) and 920 fs (300 fs) for water soluble (amide functionalized) nanotubes.
Open and closed aperture z-scans show saturation absorption and positive
(negative) nonlinear refraction for water soluble (amide functionalized)
nanotubes. Two photon absorption coefficient,beata ~250 cm/GW (650 cm/GW) and
nonlinear index, gamma ~ 15 cm^2/pW (-30 cm^2/pW) are obtained from the
theoretical fit in the saturation limit to the data for two types of nanotubes.Comment: 15 pages, 4 figure
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