229 research outputs found
Quantum transport in weakly coupled superlattices at low temperature
We report on the study of the electrical current flowing in weakly coupled
superlattice (SL) structures under an applied electric field at very low
temperature, i.e. in the tunneling regime. This low temperature transport is
characterized by an extremely low tunneling probability between adjacent wells.
Experimentally, I(V) curves at low temperature display a striking feature, i.e
a plateau or null differential conductance. A theoretical model based on the
evaluation of scattering rates is developed in order to understand this
behaviour, exploring the different scattering mechanisms in AlGaAs alloys. The
dominant interaction in usual experimental conditions such as ours is found to
be the electron-ionized donors scattering. The existence of the plateau in the
I(V) characteristics is physically explained by a competition between the
electric field localization of the Wannier-Stark electron states in the weakly
coupled quantum wells and the electric field assisted tunneling between
adjacent wells. The influence of the doping concentration and profile as well
as the presence of impurities inside the barrier are discussed
Long Exciton Dephasing Time and Coherent Phonon Coupling in CsPbBrCl Perovskite Nanocrystals
Fully-inorganic cesium lead halide perovskite nanocrystals (NCs) have shown
to exhibit outstanding optical properties such as wide spectral tunability,
high quantum yield, high oscillator strength as well as blinking-free single
photon emission and low spectral diffusion. Here, we report measurements of the
coherent and incoherent exciton dynamics on the 100 fs to 10 ns timescale,
determining dephasing and density decay rates in these NCs. The experiments are
performed on CsPbBrCl NCs using transient resonant three-pulse four-wave
mixing (FWM) in heterodyne detection at temperatures ranging from 5 K to 50 K.
We found a low-temperature exciton dephasing time of 24.51.0 ps, inferred
from the decay of the photon-echo amplitude at 5 K, corresponding to a
homogeneous linewidth (FWHM) of 545 {\mu}eV. Furthermore, oscillations in
the photon-echo signal on a picosecond timescale are observed and attributed to
coherent coupling of the exciton to a quantized phonon mode with 3.45 meV
energy
Quantum and classical criticalities in the frustrated two-leg Heisenberg ladder
This talk was about the frustration-induced criticality in the
antiferromagnetic Heisenberg model on the two-leg ladder with exchange
interactions along the chains, rungs, and diagonals, and also about the effect
of thermal fluctuations on this criticlity. The method used is the bond
mean-field theory, which is based on the Jordan-Wigner transformation in
dimensions higher than one. In this paper, we will summarize the main results
presented in this talk, and report on new results about the couplings and
temperature dependences of the spin susceptibility.Comment: 6 pages, 4 figures, talk presented at the Theory Canada 3 conference
in 2007, submitted to the Canadian Journal of Physic
Ultimate performance of Quantum Well Infrared Photodetectors in the tunneling regime
Thanks to their wavelength diversity and to their excellent uniformity,
Quantum Well Infrared Photodetectors (QWIP) emerge as potential candidates for
astronomical or defense applications in the very long wavelength infrared
(VLWIR) spectral domain. However, these applications deal with very low
backgrounds and are very stringent on dark current requirements. In this paper,
we present the full electro-optical characterization of a 15 micrometer QWIP,
with emphasis on the dark current measurements. Data exhibit striking features,
such as a plateau regime in the IV curves at low temperature (4 to 25 K). We
show that present theories fail to describe this phenomenon and establish the
need for a fully microscopic approach
THEORETICAL ASPECTS OF THE AERATION DRYING PROCESS WITH APPLICATION IN THE HAY TECHNOLOGY
In the current context of the development of mechanized agriculture in several directions in the field of feed storage needs appear in the effective implementation of the drying method used. In terms of maintaining higher production achieved by reducing losses of fodder harvesting and secondly by increasing the number of cycles of harvest due to reduced time of harvest (representing the time elapsed mowing fodder plant and to store hay), outline the need for a more careful study of the drying process. This paper presents some theoretical aspects that determine and control the drying process used in technology of hay
Localized holes and delocalized electrons in photoexcited inorganic perovskites: Watching each atomic actor by picosecond X-ray absorption spectroscopy
We report on an element-selective study of the fate of charge carriers in
photoexcited inorganic CsPbBr3 and CsPb(ClBr)3 perovskite nanocrystals (NCs) in
toluene solutions using time-resolved X-ray absorption spectroscopy with 80 ps
time resolution. Probing the Br K-edge, the Pb L3-edge and the Cs L2-edge, we
find that holes in the valence band are localized at Br atoms, forming small
polarons, while electrons appear as delocalized in the conduction band. No
signature of either electronic or structural changes are observed at the Cs
L2-edge. The results at the Br and Pb edges suggest the existence of a weakly
localized exciton, while the absence of signatures at the Cs edge indicates
that the Cs+ cation plays no role in the charge transport, at least beyond 80
ps. These results can explain the rather modest charge carrier mobilities in
these materials.Comment: 19 pages, 3 figure
Optimizing end-labeled free-solution electrophoresis by increasing the hydrodynamic friction of the drag-tag
We study the electrophoretic separation of polyelectrolytes of varying
lengths by means of end-labeled free-solution electrophoresis (ELFSE). A
coarse-grained molecular dynamics simulation model, using full electrostatic
interactions and a mesoscopic Lattice Boltzmann fluid to account for
hydrodynamic interactions, is used to characterize the drag coefficients of
different label types: linear and branched polymeric labels, as well as
transiently bound micelles.
It is specifically shown that the label's drag coefficient is determined by
its hydrodynamic size, and that the drag per label monomer is largest for
linear labels. However, the addition of side chains to a linear label offers
the possibility to increase the hydrodynamic size, and therefore the label
efficiency, without having to increase the linear length of the label, thereby
simplifying synthesis. The third class of labels investigated, transiently
bound micelles, seems very promising for the usage in ELFSE, as they provide a
significant higher hydrodynamic drag than the other label types.
The results are compared to theoretical predictions, and we investigate how
the efficiency of the ELFSE method can be improved by using smartly designed
drag-tags.Comment: 32 pages, 11 figures, submitted to Macromolecule
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