193 research outputs found
Charge Carrier Extraction by Linearly Increasing Voltage:Analytic framework and ambipolar transients
Up to now the basic theoretical description of charge extraction by linearly
increasing voltage (CELIV) is solved for a low conductivity approximation only.
Here we present the full analytical solution, thus generalize the theoretical
framework for this method. We compare the analytical solution and the
approximated theory, showing that especially for typical organic solar cell
materials the latter approach has a very limited validity. Photo-CELIV
measurements on poly(3-hexyl thiophene-2,5-diyl):[6,6]-phenyl-C61 butyric acid
methyl ester based solar cells were then evaluated by fitting the current
transients to the analytical solution. We found that the fit results are in a
very good agreement with the experimental observations, if ambipolar transport
is taken into account, the origin of which we will discuss. Furthermore we
present parametric equations for the mobility and the charge carrier density,
which can be applied over the entire experimental range of parameters.Comment: 8 pages, 5 figure
Anomalous conductance oscillations and half-metallicity in atomic Ag-O chains
Using spin density functional theory we study the electronic and magnetic
properties of atomically thin, suspended chains containing silver and oxygen
atoms in an alternating sequence. Chains longer than 4 atoms develop a
half-metallic ground state implying fully spin polarized charge carriers. The
conductances of the chains exhibit weak even-odd oscillations around an
anomalously low value of 0.1G_0 (G_0 = 2e^2h) which coincide with the averaged
experimental conductance in the long chain limit. The unusual conductance
properties are explained in terms of a resonating-chain model which takes the
reflection probability and phase-shift of a single bulk-chain interface as the
only input. The model also explains the conductance oscillations for other
metallic chains.Comment: 5 pages, 4 figure
Establishing Visible Interferometer System Responses: Resolved and Unresolved Calibrators
The propagation of errors through the uniform disk visibility function is
examined. Implications of those errors upon measures of absolute visibility
through optical and near-infrared interferometers are considered within the
context of using calibration stars to establish system visibilities for these
instruments. We suggest a simple ratio test to establish empirically whether or
not the measured visibilities produced by such an instrument are relative
(errors dominated by calibrator angular size prediction error) or absolute
(errors dominated by measurement error).Comment: 20 pages, 7 figures, to be published in the PAS
Effect of spatial resolution on the estimates of the coherence length of excitons in quantum wells
We evaluate the effect of diffraction-limited resolution of the optical
system on the estimates of the coherence length of 2D excitons deduced from the
interferometric study of the exciton emission. The results are applied for
refining our earlier estimates of the coherence length of a cold gas of
indirect excitons in coupled quantum wells [S. Yang et al., Phys. Rev. Lett.
97, 187402(2006)]. We show that the apparent coherence length is well
approximated by the quadratic sum of the actual exciton coherence length and
the diffraction correction given by the conventional Abbe limit divided by
3.14. In practice, accounting for diffraction is necessary only when the
coherence length is smaller than about one wavelength. The earlier conclusions
regarding the strong enhancement of the exciton coherence length at low
temperatures remain intact.Comment: 6 pages, 5 figure
Thin film interference in the optomechanical response of micromechanical silicon cantilevers
The mechanical response of uncoated silicon microcantilevers is shown to modulate as a function of incident wavelength. Cantilever motion is measured interferometrically, using phase sensitive detection in response to a mechanically chopped excitation source. Thin film interference modeling shows that the fraction of absorbed light within the cantilever varies periodically over the range of 450-1000 nm, in excellent agreement with the measurements. The results show that the optomechanical responsivity of these cantilevers can be tuned due to the effect via an appropriate selection of incident wavelength, incidence angle, lever thickness, and optical constants of the lever. (c) 2006 American Institute of Physics
Rainbow scattering in the gravitational field of a compact object
We study the elastic scattering of a planar wave in the curved spacetime of a compact object such as a
neutron star, via a heuristic model: a scalar field impinging upon a spherically symmetric uniform density
star of radius R and mass M. For R<rc, there is a divergence in the deflection function at the light-ring
radius rc ¼ 3GM=c2, which leads to spiral scattering (orbiting) and a backward glory; whereas for R>rc,
there instead arises a stationary point in the deflection function which creates a caustic and rainbow
scattering. As in nuclear rainbow scattering, there is an Airy-type oscillation on a Rutherford-like cross
section, followed by a shadow zone. We show that, for R ∼ 3.5GM=c2, the rainbow angle lies close to 180°,
and thus there arises enhanced backscattering and glory. We explore possible implications for gravitational
wave astronomy and dark matter models
Arago (1810): the first experimental result against the ether
95 years before Special Relativity was born, Arago attempted to detect the
absolute motion of the Earth by measuring the deflection of starlight passing
through a prism fixed to the Earth. The null result of this experiment gave
rise to the Fresnel's hypothesis of an ether partly dragged by a moving
substance. In the context of Einstein's Relativity, the sole frame which is
privileged in Arago's experiment is the proper frame of the prism, and the null
result only says that Snell's law is valid in that frame. We revisit the
history of this premature first evidence against the ether theory and calculate
the Fresnel's dragging coefficient by applying the Huygens' construction in the
frame of the prism. We expose the dissimilar treatment received by the ray and
the wave front as an unavoidable consequence of the classical notions of space
and time.Comment: 16 pages. To appear in European Journal of Physic
Slack Dynamics on an Unfurling String
An arch will grow on a rapidly deployed thin string in contact with a rigid
plane. We present a qualitative model for the growing structure involving the
amplification, rectification, and advection of slack in the presence of a
steady stress field, validate our assumptions with numerical experiments, and
pose new questions about the spatially developing motions of thin objects.Comment: significant changes. removed one figur
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