373 research outputs found
Supporting the Everyday Work of Scientists: Automating Scientific Workflows
This paper describes an action research project that we undertook with National Research Council Canada (NRC) scientists. Based on discussions about their \ud
difficulties in using software to collect data and manage processes, we identified three requirements for increasing research productivity: ease of use for end- \ud
users; managing scientific workflows; and facilitating software interoperability. Based on these requirements, we developed a software framework, Sweet, to \ud
assist in the automation of scientific workflows. \ud
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Throughout the iterative development process, and through a series of structured interviews, we evaluated how the framework was used in practice, and identified \ud
increases in productivity and effectiveness and their causes. While the framework provides resources for writing application wrappers, it was easier to code the applications’ functionality directly into the framework using OSS components. Ease of use for the end-user and flexible and fully parameterized workflow representations were key elements of the framework’s success. \u
Sputtered Tungsten Oxide as Hole Contact for Silicon Heterojunction Solar Cells
Reactively sputtered tungsten oxide WOx was investigated as hole contact on n type crystalline silicon. Varying the oxygen gas flow during sputtering enables variation of the WOx conductivity from 0.01 to 1000 amp; 937; cm, while the band bending at the interface and the implied fill factor FF change by 70 meV and 1.5 . SputteredWOx shows higher resistivity and higher absorption in the visible range compared with indium tin oxide ITO . Therefore, stacks of WOx and ITO are used in solar cells. It was found that at least 20 nm thick WOx is needed to prevent detrimental effects of the ITO work function on the band bending at the junction, the implied FF, and the real FF of solar cells. WOx hole contacts of different thicknesses and conductivity were applied in solar cells and it was found that the highest FF is achieved using about 20 nm thick interlayers of WOx with the highest possible conductivity. It was found that sputtering enables a drastic improvement of WOx silicon solar cells compared with thermal evaporation, due to the precise control of the WOx conductivity. Unfortunately, the resistivity of the sputteredWOx is still limiting the FF of these device
Electron-phonon interaction in quantum-dot/quantum-well semiconductor heterostructures
Polar optical phonons are studied in the framework of the dielectric
continuum approach for a prototypical quantum-dot/quantum-well (QD/QW)
heterostructure, including the derivation of the electron-phonon interaction
Hamiltonian and a discussion of the effects of this interaction on the
electronic energy levels. The particular example of the CdS/HgS QD/QW is
addressed and the system is modelled according to the spherical geometry,
considering a core sphere of material "1" surrounded by a spherically
concentric layer of material "2", while the whole structure is embedded in a
host matrix assumed as an infinite dielectric medium. The strength of the
electron-LO phonon coupling is discussed in details and the polaronic
corrections to both ground state and excited state electron energy levels are
calculated. Interesting results concerning the dependence of polaronic
corrections with the QD/QW structure size are analyzed.Comment: 8 pages, 5 figure
Faraday Rotation Spectroscopy of Quantum-Dot Quantum Wells
Time-resolved Faraday rotation studies of CdS/CdSe/CdS quantum-dot quantum
wells have recently shown that the Faraday rotation angle exhibits several
well-defined resonances as a function of probe energy close to the absorption
edge. Here, we calculate the Faraday rotation angle from the eigenstates of the
quantum-dot quantum well obtained with k.p theory. We show that the large
number of narrow resonances with comparable spectral weight observed in
experiment is not reproduced by the level scheme of a quantum-dot quantum well
with perfect spherical symmetry. A simple model for broken spherical symmetry
yields results in better qualitative agreement with experiment.Comment: 9 pages, 4 figure
Multiband theory of quantum-dot quantum wells: Dark excitons, bright excitons, and charge separation in heteronanostructures
Electron, hole, and exciton states of multishell CdS/HgS/CdS quantum-dot
quantum well nanocrystals are determined by use of a multiband theory that
includes valence-band mixing, modeled with a 6-band Luttinger-Kohn Hamiltonian,
and nonparabolicity of the conduction band. The multiband theory correctly
describes the recently observed dark-exciton ground state and the lowest,
optically active, bright-exciton states. Charge separation in pair states is
identified. Previous single-band theories could not describe these states or
account for charge separation.Comment: 10 pages of ReVTex, 6 ps figures, submitted to Phys. Rev.
Spin dynamics and level structure of quantum-dot quantum wells
We have characterized CdS/CdSe/CdS quantum-dot quantum wells using
time-resolved Faraday rotation (TRFR). The spin dynamics show that the electron
g-factor varies as a function of quantum well width and the transverse spin
lifetime of several nano-seconds is robust up to room temperature. As a
function of probe energy, the amplitude of the TRFR signal shows pronounced
resonances, which allow one to identify individual exciton transitions. While
the TRFR data are inconsistent with the conduction and valence band level
scheme of spherical quantum-dot quantum wells, a model in which broken
spherical symmetry is taken into account captures the essential features.Comment: 5 pages, 3 figure
Aluminium metallisation for interdigitated back contact silicon heterojunction solar cells
Back contact silicon heterojunction solar cells with an efficiency of 22 were manufactured, featuring a simple aluminium metallisation directly on the doped amorphous silicon films. Both the open circuit voltage and the fill factor heavily depend on the parameters of the annealing step after aluminium layer deposition. Using numerical device simulations and in accordance with the literature, we demonstrate that the changes in solar cell parameters with annealing can be explained by the formation of an aluminium silicide layer at temperatures as low as 150 C, improving the contact resistance and thus enhancing the fill factor. Further annealing at higher temperatures initialises the crystallisation of the amorphous silicon layers, yielding even lower contact resistances, but also introduces more defects, diminishing the open circuit voltag
Electron and hole states in quantum-dot quantum wells within a spherical 8-band model
In order to study heterostructures composed both of materials with strongly
different parameters and of materials with narrow band gaps, we have developed
an approach, which combines the spherical 8-band effective-mass Hamiltonian and
the Burt's envelope function representation. Using this method, electron and
hole states are calculated in CdS/HgS/CdS/H_2O and CdTe/HgTe/CdTe/H_2O
quantum-dot quantum-well heterostructures. Radial components of the wave
functions of the lowest S and P electron and hole states in typical quantum-dot
quantum wells (QDQWs) are presented as a function of radius. The 6-band-hole
components of the radial wave functions of an electron in the 8-band model have
amplitudes comparable with the amplitude of the corresponding 2-band-electron
component. This is a consequence of the coupling between the conduction and
valence bands, which gives a strong nonparabolicity of the conduction band. At
the same time, the 2-band-electron component of the radial wave functions of a
hole in the 8-band model is small compared with the amplitudes of the
corresponding 6-band-hole components. It is shown that in the CdS/HgS/CdS/H_2O
QDQW holes in the lowest states are strongly localized in the well region
(HgS). On the contrary, electrons in this QDQW and both electron and holes in
the CdTe/HgTe/CdTe/H_2O QDQW are distributed through the entire dot. The
importance of the developed theory for QDQWs is proven by the fact that in
contrast to our rigorous 8-band model, there appear spurious states within the
commonly used symmetrized 8-band model.Comment: 15 pages, 5 figures, E-mail addresses: [email protected],
[email protected]
A nonextensive approach to Bose-Einstein condensation of trapped interacting boson gas
In the Bose-Einstein condensation of interacting atoms or molecules such as
87Rb, 23Na and 7Li, the theoretical understanding of the transition temperature
is not always obvious due to the interactions or zero point energy which cannot
be exactly taken into account. The S-wave collision model fails sometimes to
account for the condensation temperatures. In this work, we look at the problem
within the nonextensive statistics which is considered as a possible theory
describing interacting systems. The generalized energy Uq and the particle
number Nq of boson gas are given in terms of the nonextensive parameter q. q>1
(q<1) implies repulsive (attractive) interaction with respect to the perfect
gas. The generalized condensation temperature Tcq is derived versus Tc given by
the perfect gas theory. Thanks to the observed condensation temperatures, we
find q ~ 0.1 for 87Rb atomic gas, q ~ 0.95 for 7Li and q ~ 0.62 for 23Na. It is
concluded that the effective interactions are essentially attractive for the
three considered atoms, which is consistent with the observed temperatures
higher than those predicted by the conventional theory
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