307 research outputs found
Pattern Formation of Ion Channels with State Dependent Electrophoretic Charges and Diffusion Constants in Fluid Membranes
A model of mobile, charged ion channels in a fluid membrane is studied. The
channels may switch between an open and a closed state according to a simple
two-state kinetics with constant rates. The effective electrophoretic charge
and the diffusion constant of the channels may be different in the closed and
in the open state. The system is modeled by densities of channel species,
obeying simple equations of electro-diffusion. The lateral transmembrane
voltage profile is determined from a cable-type equation. Bifurcations from the
homogeneous, stationary state appear as hard-mode, soft-mode or hard-mode
oscillatory transitions within physiologically reasonable ranges of model
parameters. We study the dynamics beyond linear stability analysis and derive
non-linear evolution equations near the transitions to stationary patterns.Comment: 10 pages, 7 figures, will be submitted to Phys. Rev.
Inversion of the exciton built-in dipole moment in In(Ga)As quantum dots via nonlinear piezoelectric effect
We show that anisotropic biaxial stress can be used to tune the built-in
dipole moment of excitons confined in In(Ga)As quantum dots up to complete
erasure of its magnitude and inversion of its sign. We demonstrate that this
phenomenon is due to piezoelectricity. We present a model to calculate the
applied stress, taking advantage of the so-called piezotronic effect, which
produces significant changes in the current-voltage characteristics of the
strained diode-membranes containing the quantum dots. Finally, self-consistent
k.p calculations reveal that the experimental findings can be only accounted
for by the nonlinear piezoelectric effect, whose importance in quantum dot
physics has been theoretically recognized although it has proven difficult to
single out experimentally.Comment: 6 pages, 4 figure
Fabrication of large area photovoltaic devices containing various blends of polymer and fullerene derivatives by using the doctor blade technique
Fabrication of large area photovoltaic devices containing various blends of polymer and fullerene derivatives by using the doctor blade technique
Formation of complex Langmuir and Langmuir-Blodgett films of water soluble rosebengal
This communication reports the formation of complex Langmuir monolayer at the
air-water interface by charge transfer types of interaction with the water
soluble N- cetyl N, N, N trimethyl ammonium bromide (CTAB) molecules doped with
rosebengal (RB), with the stearic acid (SA) molecules of a preformed SA
Langmuir monolayer. The reaction kinetics of the formation of RB-CTAB-SA
complex monolayer was monitored by observing the increase in surface pressure
with time while the barrier was kept fixed. Completion of interaction kinetics
was confirmed by FTIR study. This complex Langmuir films at the air-water
interface was transferred onto solid substrates at a desired surface pressure
to form multilayered Langmuir-Blodgett films. Spectroscopic characterizations
reveal some molecular level interactions as well as formation of
microcrystalline aggregates depending upon the molar ratios of CTAB and RB
within the complex LB films. Presence of two types of species in the complex LB
films was confirmed by fluorescence spectroscopy.Comment: 13 pages, figures
Forces on Dust Grains Exposed to Anisotropic Interstellar Radiation Fields
Grains exposed to anisotropic radiation fields are subjected to forces due to
the asymmetric photon-stimulated ejection of particles. These forces act in
addition to the ``radiation pressure'' due to absorption and scattering. Here
we model the forces due to photoelectron emission and the photodesorption of
adatoms. The ``photoelectric'' force depends on the ambient conditions relevant
to grain charging. We find that it is comparable to the radiation pressure when
the grain potential is relatively low and the radiation spectrum is relatively
hard. The calculation of the ``photodesorption'' force is highly uncertain,
since the surface physics and chemsitry of grain materials are poorly
understood at present. For our simple yet plausible model, the photodesorption
force dominates the radiation pressure for grains with size >~0.1 micron
exposed to starlight from OB stars. We find that the anisotropy of the
interstellar radiation field is ~10% in the visible and ultraviolet. We
estimate size-dependent drift speeds for grains in the cold and warm neutral
media and find that micron-sized grains could potentially be moved across a
diffuse cloud during its lifetime.Comment: LaTeX(41 pages, 19 figures), submitted to Ap
Molecular hydrogen formation on grain surfaces
We reconsider H2 formation on grain surfaces. We develop a rate equation
model which takes into account the presence of both physisorbed and chemisorbed
sites on the surface, including quantum mechanical tunnelling and thermal
diffusion. In this study, we took into consideration the uncertainties on the
characteristics of graphitic surfaces. We calculate the H2 formation efficiency
with the Langmuir Hinshelwood and Eley Rideal mechanisms, and discuss the
importance of these mechanisms for a wide range of grain and gas temperatures.
We also develop a Monte Carlo simulation to calculate the H2 formation
efficiency and compare the results to our rate equation models. Our results are
the following: (1) Depending on the barrier against chemisorption, we predict
the efficiency of H2 formation for a wide range of grain and gas temperatures.
(2) The Eley-Rideal mechanism has an impact on the H2 formation efficiency at
high grain and gas temperatures. (3) The fact that we consider chemisorption in
our model makes the rate equation and Monte Carlo approaches equivalent.Comment: in "Light, dust and chemical evolution", Journal of Physics:
Conference Serie
Resolving the temporal evolution of line broadening in single quantum emitters
Light emission from solid-state quantum emitters is inherently prone to environmental decoherence, which results in a line broadening and in the deterioration of photon indistinguishability. Here we employ photon correlation Fourier spectroscopy (PCFS) to study the temporal evolution of such a broadening in two prominent systems: GaAs and In(Ga)As quantum dots. Differently from previous experiments, the emitters are driven with short laser pulses as required for the generation of high-purity single photons, the time scales we probe range from a few nanoseconds to milliseconds and, simultaneously, the spectral resolution we achieve can be as small as ∼ 2µeV. We find pronounced differences in the temporal evolution of different optical transition lines, which we attribute to differences in their homogeneous linewidth and sensitivity to charge noise. We analyze the effect of irradiation with additional white light, which reduces blinking at the cost of enhanced charge noise. Due to its robustness against experimental imperfections and its high temporal resolution and bandwidth, PCFS outperforms established spectroscopy techniques, such as Michelson interferometry. We discuss its practical implementation and the possibility to use it to estimate the indistinguishability of consecutively emitted single photons for applications in quantum communication and photonic-based quantum information processing
Strong-field terahertz-optical mixing in excitons
Driving a double-quantum-well excitonic intersubband resonance with a
terahertz (THz) electric field of frequency \omega_{THz} generated terahertz
optical sidebands \omega=\omega_{THz}+\omega_{NIR} on a weak NIR probe. At high
THz intensities, the intersubband dipole energy which coupled two excitons was
comparable to the THz photon energy. In this strong-field regime the sideband
intensity displayed a non-monotonic dependence on the THz field strength. The
oscillating refractive index which gives rise to the sidebands may be
understood by the formation of Floquet states, which oscillate with the same
periodicity as the driving THz field.Comment: 4 pages, 6 figure
Optical Properties of Vanadium in 4H Silicon Carbide for Quantum Technology
We study the optical properties of tetravalent vanadium impurities in 4H
silicon carbide (4H SiC). Emission from two crystalline sites is observed at
wavelengths of 1.28 \mum and 1.33 \mum, with optical lifetimes of 163 ns and 43
ns. Group theory and ab initio density functional supercell calculations enable
unequivocal site assignment and shed light on the spectral features of the
defects. We conclude with a brief outlook on applications in quantum photonics
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