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Structural stability of solids from first principles theoryMagyari-Köpe, Blanka January 2002 (has links)
No description available.
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Computational Material Design : Diluted Magnetic Semiconductors for SpintronicsHuang, Lunmei January 2007 (has links)
The present thesis deals with the application of ab-initio electronic structure calculations based on density functional theory for material design. The correlation between magnetic properties and electronic structures has been investigated in detail for diluted magnetic semiconductors (DMS), which have promising application for spintronics devices. The point defects, acting as electron donor or electron acceptor, have been studied for their key role in mediating the long ranged ferromagnetic interaction between transition metal (TM) ions. The electron holes induced by electron acceptor are completely spin polarized in semiconductor and exhibit a very significant efficiency to the ferromagnetic coupling between TM ions. While the electron donor shows a negative effect to the ferromagnetism in the system. The common trend of magnetic interaction and electron charge transfer between TM ions and electron acceptors or electron donators have been outlined. The Coulomb correlation U of d electrons, which could change the energy levels of TM d band respective to the host semiconductor band, also shows a significant influence on the magnetic behavior in DMS. The crystallography phase transition under high pressure has also been studied for the iron doped with light element, carbon. Our calculated results show that interstitial carbon defect has little effect on the iron's bcc to hcp phase transition under high pressure. The other carbon iron phases, like Fe3C, has also been studied in a wide pressure range. We also present a first-principles description on the temperature dependence of elastic constant for solids. The total temperature effects are approximated as a sum of two separated parts, the thermal expansion contribution, which gives the normal linearly decreasing effect on the elastic constant with increasing temperature, and the electronic band contribution, which could lead anomalous behavior for thermal elastic constants.
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Threshold Phenomena in Random Constraint Satisfaction ProblemsConnamacher, Harold 30 July 2008 (has links)
Despite much work over the previous decade, the Satisfiability Threshold
Conjecture remains open. Random k-SAT, for constant k >= 3,
is just one family of a large number
of constraint satisfaction problems that are conjectured to have exact
satisfiability thresholds, but for which the existence and location of these
thresholds has yet to be proven.
Of those problems for which we are able to prove
an exact satisfiability threshold, each seems to be fundamentally different
than random 3-SAT.
This thesis defines a new family of
constraint satisfaction problems with constant size
constraints and domains and which
contains problems that are NP-complete and a.s.\ have exponential
resolution complexity. All four of these properties hold for k-SAT, k >= 3,
and the
exact satisfiability threshold is not known for any constraint
satisfaction problem
that has all of these properties. For each problem in the
family defined in this
thesis, we determine
a value c such that c is an exact satisfiability threshold if a certain
multi-variable function has a unique maximum at a given point
in a bounded domain. We
also give numerical evidence that this latter condition holds.
In addition to studying the satisfiability threshold, this thesis
finds exact
thresholds for the efficient behavior of DPLL using the unit clause heuristic
and a variation of the generalized unit clause heuristic,
and this thesis proves an analog
of a conjecture on the satisfiability of (2+p)-SAT.
Besides having similar properties as k-SAT, this new family of
constraint satisfaction problems
is interesting to study in its own right because it generalizes the
XOR-SAT problem and it has close ties
to quasigroups.
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244 |
Threshold Phenomena in Random Constraint Satisfaction ProblemsConnamacher, Harold 30 July 2008 (has links)
Despite much work over the previous decade, the Satisfiability Threshold
Conjecture remains open. Random k-SAT, for constant k >= 3,
is just one family of a large number
of constraint satisfaction problems that are conjectured to have exact
satisfiability thresholds, but for which the existence and location of these
thresholds has yet to be proven.
Of those problems for which we are able to prove
an exact satisfiability threshold, each seems to be fundamentally different
than random 3-SAT.
This thesis defines a new family of
constraint satisfaction problems with constant size
constraints and domains and which
contains problems that are NP-complete and a.s.\ have exponential
resolution complexity. All four of these properties hold for k-SAT, k >= 3,
and the
exact satisfiability threshold is not known for any constraint
satisfaction problem
that has all of these properties. For each problem in the
family defined in this
thesis, we determine
a value c such that c is an exact satisfiability threshold if a certain
multi-variable function has a unique maximum at a given point
in a bounded domain. We
also give numerical evidence that this latter condition holds.
In addition to studying the satisfiability threshold, this thesis
finds exact
thresholds for the efficient behavior of DPLL using the unit clause heuristic
and a variation of the generalized unit clause heuristic,
and this thesis proves an analog
of a conjecture on the satisfiability of (2+p)-SAT.
Besides having similar properties as k-SAT, this new family of
constraint satisfaction problems
is interesting to study in its own right because it generalizes the
XOR-SAT problem and it has close ties
to quasigroups.
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245 |
Optical Modeling of Amorphous and Metal Induced Crystallized Silicon with an Effective Medium ApproximationMuller, Theophillus Frederic George January 2009 (has links)
<p>Hydrogenated amorphous silicon (a-Si:H) is second only to crystalline silicon in volume manufacturing of solar cells due to its attractive characteristics for solar panel manufacturing. These are lower manufacturing costs, and the fact that it can be deposited on any surface, and in any shape even on flexible substrates. The metal induced crystallization of hydrogenated amorphous silicon has been the subject of intense scrutiny in recent years. By combining the technology of hydrogenated amorphous silicon thin films with the superior characteristics of c-Si material, it is hoped that more efficient solar cells can be produced. In this thesis we report on the metal-mediated-thermally induced changes of the structural and optical properties of hydrogenated amorphous silicon deposited by hot-wire CVD, where aluminium and nickel were used to induce crystallization. The metal-coated amorphous silicon was subjected to a thermal annealing regime of between 150 and 520° / C. The structural measurements, obtained by Raman spectroscopy, show partial crystallization occurring at 350 ° / C. At the higher annealing temperatures of 450° / C and 520° / C complete crystallization occurs. Reflection and transmission measurements in the UV-visible range were then used to extract the optical properties. By adopting the effective medium approximation a single optical model could be constructed that couldsuccessfully model material that was in different structural phases, irrespective of metal contamination. Changes in the absorption of the material in various stages of transition were confirmed with a directly measured absorption technique, and the modelled absorption closely followed the same trends This study forms part of the larger overall solar cell research project, of which the primary aim is to eventually develop a silicon solar panel that optimises the characteristics for best performance.</p>
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Studies on the effects of low-field Landau quantization in a two-dimensional electron systemZhang, Yan-wei 21 July 2005 (has links)
In this paper, we mainly discuss the transport properties of the two-dimensional gas of a high-mobility GaAs/AlGaAs semiconductor heterostructure in high magnetic fields and low temperatures. We analyzed the measured longitudinal resistivity and Hall resistivity at the five different temperatures. We observed that the classical Hall effect is valid when the magnetic field is less than 0.25 Tesla; and the quantum Hall plateaux appeared obviously when the magnetic field is larger than 1.6 Tesla. We proceeded to analyze the longitudinal resistivity oscillation occurred in the magnetic fields between 0.477 Tesla and 1.483 Tesla. According to the Lifshitz-Kosevich (LK) formula, we can get the two-dimensional electron concentration, effective mass, and quantum scattering time from the quantum magnetoresistivity oscillation measurement. Our results suggested that the applicable range of the LK formula could be broader than the generally-assumed one. In quantum Hall effect regime at high magnetic field, we can calculate the h/e2 value from the quantum Hall plateaux value.
In classical Hall effect regime, the three-dimensional electron concentration and classical mobility (classical scattering time) can be obtained. However, we find out that the zero-field Hall resistivity experimental value is not equal to zero, and this is not conformed to the standard theory. We tried to use the magnetic field shift and Hall resistivity shift to solve the problem, and compared both advantages of them.
Finally, we observed the plateau-plateau phase transitions of the two-dimensional electron system
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Self-organized Flocking With A Mobile Robot SwarmTurgut, Ali Emre 01 May 2008 (has links) (PDF)
In this thesis, we study self-organized flocking using a swarm of mobile robots. We first present a mobile robot platform having two novel sensing systems developed specifically for
swarm robotic studies. We describe its infrared-based short-range sensing system, capable of measuring the range to obstacles and detecting kin robots. In particular, we describe
a novel sensing system called the virtual heading sensor (VHS), which combines a digital compass and a wireless communication module to form a scalable method for sensing the relative headings of neighboring robots. We propose a behavior based on heading alignment and proximal control and show that it is capable of generating self-organized ocking in a group of seven robots. Then, we propose a number of metrics to evaluate the quality of flocking and use them to evaluate four main variants of this behavior. We characterize and model the sensing abilities of the robots and develop a physics-based simulator that is verified against the physical robots for flocking in open environments. After showing in simulation that we can achieve flocking in a group of up to 1000 robots in an open environment, we perform experiments to determine the performance of flocking under different controller parameters and characteristics of VHS using the predefined metrics. In the experiments, we vary the three main characteristics of VHS, namely: (1) The amount and nature of noise in heading measurement, (2) The number
of neighboring robots that can be " / heard" / , and (3) the range of wireless communication. Ourresults show that range of communication is the main factor that determines the scale of
flocking, and that the behavior is highly robust against the other two characteristics.
We extend an existing particle-based model to determine the phase transition characteristics of flocking under different VHS characteristics. An analytical treatment of the model
is also presented and verified against the results obtained from experiments in a physics-based simulator.
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Structural stability of solids from first principles theoryMagyari-Köpe, Blanka January 2002 (has links)
No description available.
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249 |
Structures and Self-Organization at Liquid Crystal Interfaces: Surface Ordering and AnchoringFeng, Xunda 15 April 2013 (has links)
No description available.
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250 |
Optical Modeling of Amorphous and Metal Induced Crystallized Silicon with an Effective Medium ApproximationMuller, Theophillus Frederic George January 2009 (has links)
<p>Hydrogenated amorphous silicon (a-Si:H) is second only to crystalline silicon in volume manufacturing of solar cells due to its attractive characteristics for solar panel manufacturing. These are lower manufacturing costs, and the fact that it can be deposited on any surface, and in any shape even on flexible substrates. The metal induced crystallization of hydrogenated amorphous silicon has been the subject of intense scrutiny in recent years. By combining the technology of hydrogenated amorphous silicon thin films with the superior characteristics of c-Si material, it is hoped that more efficient solar cells can be produced. In this thesis we report on the metal-mediated-thermally induced changes of the structural and optical properties of hydrogenated amorphous silicon deposited by hot-wire CVD, where aluminium and nickel were used to induce crystallization. The metal-coated amorphous silicon was subjected to a thermal annealing regime of between 150 and 520° / C. The structural measurements, obtained by Raman spectroscopy, show partial crystallization occurring at 350 ° / C. At the higher annealing temperatures of 450° / C and 520° / C complete crystallization occurs. Reflection and transmission measurements in the UV-visible range were then used to extract the optical properties. By adopting the effective medium approximation a single optical model could be constructed that couldsuccessfully model material that was in different structural phases, irrespective of metal contamination. Changes in the absorption of the material in various stages of transition were confirmed with a directly measured absorption technique, and the modelled absorption closely followed the same trends This study forms part of the larger overall solar cell research project, of which the primary aim is to eventually develop a silicon solar panel that optimises the characteristics for best performance.</p>
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