2,482 research outputs found
Towards the physical vacuum of cosmic inflation
There have been long debates about the initial condition of inflationary
perturbations. In this work we explicitly show the decay of excited states
during inflation via interactions. For this purpose, we note that the folded
shape non-Gaussianity can be interpreted as the decay of the non-Bunch-Davies
initial condition. The one loop diagrams with non-Bunch-Davies propagators are
calculated to uncover the decay of such excited states. The observed smallness
of non-Gaussianity keeps the window open for probing inflationary initial
conditions and trans-Planckian physics
Antifactors of regular bipartite graphs
Let be a bipartite graph, where and are color classes and
is the set of edges of . Lov\'asz and Plummer \cite{LoPl86} asked
whether one can decide in polynomial time that a given bipartite graph admits a 1-anti-factor, that is subset of such that for
all and for all . Cornu\'ejols \cite{CHP}
answered this question in the affirmative. Yu and Liu \cite{YL09} asked
whether, for a given integer , every -regular bipartite graph
contains a 1-anti-factor. This paper answers this question in the affirmative
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Laser Surface Texturing, Crystallization and Scribing of Thin Films in Solar Cell Applications
Thin films have been considered for use in terrestrial solar cell applications because of their significantly reduced cost compared with bulk crystalline silicon. However, their overall efficiency and stability are less than that of their bulk crystalline counterpart. The work presented in this thesis seeks to investigate these issues via a series of experimental and numerical analysis of the influences of laser processing on microstructure, optical and electrical properties of two absorber materials, a-Si:H (hydrogenated amorphous silicon) and CdTe (cadmium telluride). a-Si:H thin film solar cells suffer from disadvantages of low efficiencies and light induced degradation. A one-step laser processing is investigated for introducing light-trapping structure and crystallization on a-Si:H thin films, which can potentially simultaneously alleviate the two weaknesses of a-Si:H. The nanoscale conical and pillar-shaped spikes formed on the surface of a-Si:H films by irradiation of both femtosecond (fs) infrared and nanosecond (ns) excimer lasers enhanced light absorption, while the formation of a mixture of hydrogenated nanocrystalline silicon (nc-Si:H) and a-Si:H after crystallization suggests that the overall material stability can potentially improve. It is shown that growth is a more dominant spike formation mechanism in excimer laser processing, rather than ablation which is dominant during fs laser texturing. Experimental and analytical approaches are also developed revealing the effect of hydrogen on texturing behavior and crystallization during excimer laser irradiation, and a step-by-step crystallization process is proposed to prevent the hydrogen from diffusing out in order to reduce the defect density. In addition, a comparison of absorptance spectra for various surface morphologies and crystallinity is developed and the absorptance across the solar spectrum shows that the combination of surface texturing and crystallization induced by laser processing is very promising for a-Si:H thin film solar cell applications. CdTe thin-film solar cells are the basis of a significant technology with major commercial impact on terrestrial photovoltaic production, since CdTe leads to substantial cost reduction. Laser scribing is a key process used to increase thin-film solar panel efficiency through the formation of serial interconnections to reduce photocurrent and resistance losses. Currently, scribing is performed using glass-side laser processes which have led to increased scribe quality. Defects formed during scribing such as micro cracks, film delamination, thermal effect and tapered sidewall geometries, however, still keep solar panels from reaching their theoretical efficiencies. In this study, a ns Nd:YAG laser operating at the fundamental (1064nm) or frequency-doubled (532nm) wavelengths is employed for pattern 1 (P1) and 2 (P2) scribing on CdTe thin-film solar cells. The experimental investigation shows that film removal mechanisms for different materials are due to laser-induced ablation, thermal stress and micro-explosion processes. The formation mechanisms and mitigation techniques of the defects during micro-explosion process are studied. A fully-coupled thermal and mechanical finite element model is developed to analyze the laser-induced spatio-temporal temperature and thermal stress distribution responsible for SnO2:F film removal, and a plasma expansion model is also investigated to simulate the film removal of CdTe/Cds multilayer due to the micro-explosion process. The characterization of removal qualities will enable the process optimization and design required to enhance solar module efficiency
Numerical simulation of subcontinent lithosphere dynamics: craton stability, evolution and formation
Through geodynamical modelling, two hypotheses about the craton stability and evolution were
revisited and an important process of cratonization is investigated. Unlike most previous, related
numerical studies, non-Newtonian rheology with composition dependence was used in these
studies, and the rheological parameters are thus directly comparable with laboratory experiment
of mantle. The first hypothesis, that the cratonic lithosphere is “isopycnic”, is found to be not
strictly necessary for craton stability and longevity. The high viscosity of the cratonic litho-
sphere due to compositional effects on the mantle rheology is found to be essential to maintain a
thickness difference between cratonic and non-cratonic lithosphere for over billions of years and
it allows a modest negative buoyancy of the cratonic root, depending on the strengthening factor
due to the compositional effects. The second hypothesis to be tested is that mantle plume im-
pingements cause rapid, significant removal of subcontinental lithosphere. The results presented
in this thesis show that the erosion caused by a plume impact on a continent that is strong
enough to have survived billions of years of Earth’s history is rather limited. A special weaken-
ing mechanism of such highly viscous and buoyant roots is required to reactivate this cratonic
lithosphere and thus cause significant thinning within 10s of Myrs. The fluid/melt-rock interac-
tion during mantle metasomatism is probably the most likely mechanism to modify and weaken
depleted cratonic lithosphere. Therefore, metasomatic weakening is essential for the significant
thinning of subcontinental lithosphere observed, e.g.at North China Craton and Namibia, south-
ern African, no matter whether caused by a plume impact or another tectonic event.
Using the reasonable compositional effects on the buoyancy and rheology of mantle rocks
from the above studies, numerical experiments are performed to study the formation of thick
cratonic lithosphere from a layered, depleted mantle material. In this scenario, substantial tec-
tonic shortening and thickening of previously depleted material seems to be an essential ingre-
dient to initiate the cratonization process. Afterwards, gravitational self-thickening will cause
further thickening. Compositional buoyancy resists Rayleigh-Taylor instability collapse and
stabilizes the thick cratonic root, while the secular cooling also has a stabilizing effect on the
cratonic root by reducing the thermal buoyancy contrast between lithosphere and asthenosphere
and increasing mantle viscosity. The presented numerical results are consistent with the vertical
movement of cratonic peridotite as suggested on petrological grounds
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