Numerical simulation techniques play an important role due to their flexibility in dealing with a broad range of complex geometries and material responses. This flexibility requires substantial computational time and memory. Most numerical methods use structured grid for graphical discretization, although this approach is straightforward it is not ideal for smoothly curved boundaries. In this thesis the two-dimensional Transmission Line Modelling (TLM) method based on unstructured meshes is adopted. TLM is an established numerical simulation technique that has been employed in a variety of applications area. Using unstructured meshes to discretize the problem domain permits smooth boundary presentation which provides significant enhancement in the flexibility and accuracy of the TLM simulations. An algorithm is developed to implement Unstructured Transmission Line Modelling (UTLM) which is carefully designed for simplicity and scalability of model size. Several examples are employed to test the accuracy and efficiency of the UTLM simulations. Delaunay meshes, as a type of unstructured meshes, provide good quality triangles but have the disadvantage of providing close to zero transmission line length which has impact on the maximum permissible time step for stable operation. In this thesis, a simple perturbation method for relaxing the minimum link length and clustering triangles in pairs is presented, which permits substantial increase in time step and hence computational runtime to be made without compromising the simulation stability or accuracy. Also, a new model for relaxing the short link lines that fall on the boundaries is presented. UTLM method is based on temporal and spatial sampling of electromagnetic fields which results in dispersion. In this thesis, dispersion characteristics of the unstructured TLM mesh are investigated and compared against structured TLM results for different mesh sizes and shapes. Unlike the structured TLM mesh, the unstructured mesh gives rise to spatial mode coupling. Intermodal coupling behaviour is investigated in a statistical manner upon the change of the mesh local characteristics.

The thermally induced flux motion and the elemental pinning force, fp, for Nb thin films (1000-5000A) were measured for applied magnetic fields ranging from 0.3 to 7.5G, and temperatures from 4.22K to 5.72K. / The magnitude of f$\sb{\rm p}$(H,d,T) ranged from 10$\sp{-12}$ to 10$\sp{-11}$ N/m which is approximately 5 orders of magnitude smaller than Lorentz force depinning measurements made on Nb for the high field regime$\sp{16}$ (flux line lattice), as well as the low field regime$\sp{15}$ (isolated essentially non-interacting fluxoids). Some of these results are similar to the works of Huebener, et al, who first found a large discrepancy between the transport current method (J x B) and the thermal method, S$\nabla$T, when calculating the pinning force on a flux line lattice structure. A model was proposed to explain the discrepancy in terms of an electron scattering effect at or near the grain boundaries and extending into the grains which produces a channeling effect whereby the transport electrons flow away from the trapped fluxoids such that their local Lorentz force per unit length is less than the value calculated from J$\sb{\rm ave\Phi o}$. / The $\Delta\Phi$ vs $\Delta$T data for small $\Delta$T values, implied a spectrum of pinning force values where the fluxoids that were depinned and had substantially weaker pinning forces than the majority of the remaining trapped flux lines. Using statistical arguments, the qualitative features of the $\Delta\Phi$(H,d) data and f$\sb{\rm p}$(H,d) is explained. The data exhibited a magnetic field threshold, below which there is no flux motion for the temperature range studied. The value of the minimum required applied field necessary for flux motion increases with increasing thickness. / Source: Dissertation Abstracts International, Volume: 49-12, Section: B, page: 5358. / Major Professor: William G. Moulton. / Thesis (Ph.D.)--The Florida State University, 1988.

Thesis advisor: Michael Graf / The R5T4 compounds (R = rare earth, T = Ge or Si) are interesting because the magnetic properties are very sensitively dependent on slight changes in the crystalline structure. Yb5Ge4 is one such compound, with (presumed) antiferromagnetic order occurring at TN = 1.7 K. We are interested in the effects of substituting Li in place of one Yb atom. Previous measurements of the magnetic properties of polycrystalline Yb4LiGe4 using NMR, specific heat, and resistance measurements at temperatures down to 0.5 K and in magnetic fields up to 4 Tesla were made to compare results with the parent compound. The resistance measurements showed a maximum at 1.1 K, which may indicate the onset of magnetic order. Thus we performed μSR measurements on Yb4LiGe4 and Yb5Ge4, and analysis of the data confirmed magnetic ordering (possibly antiferromagnetic) at 1.1 K. The μSR measurements also revealed a dependence on the magnetic history of the sample. Currently we are studying the pressure dependence of the (presumed) Néel Temperature in order to explore whether increased pressure can drive the TN to 0 K, and results will be discussed. / Thesis (BS) — Boston College, 2010. / Submitted to: Boston College. College of Arts and Sciences. / Discipline: Physics Honors Program. / Discipline: Physics.

Thesis advisor: Ziqiang Wang / The discovery of high-Tc superconductivity in cuprates, quantum Hall effect greatly challenge the single-electron understanding of condensed matter physics. In contrast to phonon-mediated BCS mechanism, the unconventional high-Tc superconductivity is widely believed to come from strongly electronic correlation. Strong electron-electron repulsion leads to the interplay among spin, charge, orbital and lattice degrees of freedom, resulting in high-temperature superconductivity, charge or spin density wave, Mott insulator, orbital order, nematicity etc. On the other hand, quantum Hall effect brings us the realization of the mathematical concept of topology in condensed matter. Topology has been widely explored in the topological insulator, topological superconductors, symmetry protected topological order etc. In this dissertation, we study theoretically the physics of electronic correlation and topology in various systems, including superconductivity in single layer CuO₂, electronic nematicity in FeSe, chiral spin density wave in honeycomb lattice and antiferromagnetic Chern insulator in 2D non-centrosymmetric systems. / Thesis (PhD) — Boston College, 2018. / Submitted to: Boston College. Graduate School of Arts and Sciences. / Discipline: Physics.

An Electromagnetic Pulse (EMP) resulting from, for example, the detonation of a nuclear weapon is characterised by a wave of electromagnetic activity able to couple with power lines and electro-sensitive equipment with the potential of rendering an establishment or on a greater scale, a whole city impotent. Protection against such occurrences is of paramount importance. It is now accepted that an important consideration when devising protective schemes against such phenomena is an accurate understanding of the effects on propagating waveforms such as those coupled to wires, when electrical breakdown of the dielectric material surrounding such wires occurs, otherwise known as electrical discharge. Such issues can occur around the affected wires if the electric fields generated exceed the dielectric strength of the surrounding medium, typically air or soil. Under these circumstances, the signature of the coupled waveform is known to change in characteristic ways. The form and degree of distortion needs to be understood if the harmful effects are to be prevented by protection systems put in place. The purpose of this thesis is to first describe the mechanisms that lead to the development of the Nuclear–Electromagnetic Pulse (NEMP) and the mechanisms of the discharge that can result once such pulses have coupled to a wire. Next, some of the previous corona-modelling approaches are discussed. Many of the modelling approaches have been applied to 1-D transmission-line simulations. When 3-D simulations have been performed, the Finite-Difference (Time Domain) or FD-TD approach seems to be the preferred method. At the time of writing, no 3-D Transmission Line simulations of discharge phenomena around wires were available. Hence, here, the 3-D Transmission Line Modelling Method (TLM) is described with a view to modelling such behaviour. In particular, the Embedded-Wire-Node (EWN) is used to model the discharge development around the wire. This is a fine-wire technique used to reduce computational fatigue. The node can be adapted to accept changes related to electrical discharge allowing for a real-time, self-consistent recreation of such effects. The 3-D TLM approach proves to be a decent candidate to the modelling of such behaviour. Both advantages and disadvantages of this method are discussed.

This thesis is concerned with the synthesis of linear-optimal heave control of electromagnetic suspension systems. The methods of computer-aided analysis and simulation were employed in this research. The intrinsic properties of electromagnetic suspension system were investigated to facilitate the synthesis and to provide guidelines for the design of electromagnets or linear motors suitable for use in suspension systems. The technique of complementary filtering, which resolves the conflicting requirements of high stiffness to load and high compliance for ride-comfort, was further developed. This led to the ability of directly comparing the merits of systems with different configurations and determining the optimal natural frequency. Together with a novel way of removing steady state gap error during the traversing of transition curves, the filtering technique was applied to the system using fixed-gain and sliding-mode variable-structure control methods. The latter method is well known for its abi1ity in maintaining closed-loop characteristics in the presence of disturbance. However, further development was required before it was applied. The resultant suspension system gave a 20% improvement in ride-quality over the best published result which was carried out under similar test conditions. Even so, theoretical analysis showed that a four-fold improvement may be possible.

Combining stainless steel with polyester fibres adds an attractive conductive behaviour to the yarn. Once knitted in such a manner, fabrics develop sensing properties that make them suitable for wearable applications as a consequence of the variation of their conductivity when subject to structural deformations. The use of such textiles as accurate strain/motion sensors can be considered the new frontier in wearable sensor applications, in opposition to their use as simple strain/motion detectors as mainly done until now. This thesis aims to characterise the electrical properties of knitted conductive textiles made of 20% stainless steel and 80% polyester fibres in the context of their application as fabric strain sensors. However, the use of conductive textiles as sensors is challenging and requires the combined study of their electrical and mechanical behaviours. In the first part of this thesis, the fabric resistance variation is analysed with a purposely built measurement system during a considerable number of stretch-recovery cycles performed by an Instron tensile machine. From the resulting electrical characterisation, it is possible to select the best knit pattern candidates for the second part of the study, which regards their integration into a supporting garment and further investigation of their electrical and mechanical properties in view of their application as wearable motion sensors. Two acquisition systems capture both free and constrained motions of a participant's knee and the electrical resistance variation due to combined deformation of samples and supporting leggings. In both parts of this thesis, a detailed analysis in terms of linearity, stability, sensitivity and hysteresis confirms that the overall sensing performance progressively decreases during extended use as a consequence of the short-term irreversible fabric deformation and does not significantly recover after short rest periods. For the development and use of high performance fabric-based strain sensors, it is therefore evident the importance of the long-term maintaining of a reversible stretch-recovery behaviour not only of the sensing fabrics but also of the supporting garments they are integrated in.

The rational design of ligand architectures that enables the development of well-defined metallic constructs has long been recognized as an avenue toward controlling and/or tuning the properties of the corresponding material. To date, one of the most widely studied ligand systems is 2,2’;6’,2”-terpyridine, whose tridentate chelating environment allows for coordination to a variety of metal ions with a wide range of geometries. The development of a terpy-like framework which incorporates a boratriazine moiety into the central ring has been recently reported by the Brusso group, namely 2,2-difluoro-4,6-bis(2-pyridyl)-1,3-dihydro-1,3,5,2-triazaborinine (Py2F2BTA) and 2,2-difluoro-4,6-bis(2-4,6-pyrimidinyl)-1,3-dihydro-1,3,5,2-triazaborinine (Pm2F2BTA). These compounds constitute unfused analogues to the extensively studied boron dipyrromethene (bodipy) family of luminophores, which are known for their high thermal resistance, chemical robustness, low photodegradation, and interesting photophysical signatures. While numerous bodipy derivatives have been studied, metal coordination with these compounds remains an intricate property to explore. Thus, the work presented herein represents the first investigations into the coordination chemistry of these novel ligand frameworks. Chapter one serves as an introduction to coordination chemistry, exploring important concepts as they relate to the field; the origin of our boratriazine based ligands is also explored within this chapter. Chapter two aims to describe trends in coordination and reactivity of these ligands with iron and cobalt metal sources of different oxidation states. The third and fourth chapter focuses on the targeted use of azide and thiocyanate ligands, respectively, to promote interesting magnetic interactions in related complexes. In the fifth chapter, luminescent metal complexes synthesized through the use of d10 metals are described. Conclusions and future outlooks are then presented in the final section, chapter six.

The third part of this thesis is on one hole problem of asymmetric T and T -- Jz model, which we found can be turned into a single body problem and is solvable. We get the spectrum of the model and calculate as a result the mass of the hole. This model may be related to the two species fermion coexisting system which may be realized in the optical lattice. / This thesis project is mainly inspired by the newly discovered superconductor NaxCoO2 · yH 2O. It has a layered structure with an almost separated single band out of the inner band electrons. It becomes the theoretical focus because it is the first layered strongly correlated system that shows superconductivity besides the cuprate superconductors. It may help people to understand the superconductivity in the cuprate system. In this thesis we mainly focus our attention on the strongly correlated models on triangular lattice. These models are expected to provide a theoretical background for the Nax CoO2 · yH2O system. We investigate the magnetic and superconductivity properties of these models on the triangular lattice. By the mean field calculation of different magnetic orders we got a phase diagram of the triangular lattice Hubbard model for different magnetic orders in the Hubbard model in the triangular lattice. To further investigate the superconductivity pairing symmetry of the triangular lattice, we use the standard variational Monte Carlo method to find which kind of pairing symmetry is stabilized in the triangular lattice T -- J -- V model. Our finding is that the extended- f wave pairing is most stable in the lattice. / To further understand the properties of the related model in the electron interacting system in the triangular lattice, we also solve the few particle problem in the triangular lattice. For two particles with the T -- U -- J -- V model in the triangular lattice, we got the exact solutions. For four particles of the Hubbard model in the triangular lattice we got the asymptotic solutions. We discuss the existing of the bound states in these solutions and the pairing symmetry of these bound states. For system with two-hole and one down-spin away from the all up spin background we also get a solution utilizing a large U expansion. We discussed the relation of this solution with the Nagaoka states. / Fan, Rui = 三角格子上磁性與超導的研究 / 樊睿. / "September 2007." / Adviser: Lin Hai Qing. / Source: Dissertation Abstracts International, Volume: 70-03, Section: B, page: 1719. / Thesis (Ph.D.)--Chinese University of Hong Kong, 2007. / Includes bibliographical references (p. 83-87). / Electronic reproduction. Hong Kong : Chinese University of Hong Kong, [2012] System requirements: Adobe Acrobat Reader. Available via World Wide Web. / Electronic reproduction. [Ann Arbor, MI] : ProQuest Information and Learning, [200-] System requirements: Adobe Acrobat Reader. Available via World Wide Web. / Abstracts in English and Chinese. / School code: 1307. / Fan, Rui = San jiao ge zi shang ci xing yu chao dao de yan jiu / Fan Rui.

This thesis investigates the electron-phonon interactions in fabricated Semiconductor Devices i.e. Schottky Diodes, Quantum Well (QW) embedded p-i-n photodiodes, two dimensional electron gas (2DEG) and two-dimensional hole gas (2DHG) structures all on (311) GaAs substrates. Fast electrical measurements were performed on Schottky diodes fabricated on the (311B) GaAs surface. Optical excitation generates a compressive strain pulse in an Al film which is at the opposite side of the GaAs substrate. Two coherent acoustic phonon modes were generated in the (311B) GaAs substrate: quasi-longitudinal phonons (QLA) and quasi-transverse phonons (QTA). These two modes were detected successfully using the Schottky diode, the dependence of QLA and QTA signals on different applied bias, pump powers and temperatures were investigated. The dependence of signals on bias reveals that the temporal position of the signals for QLA and QTA moves slightly earlier times with increasing reverse bias because the width of the depletion layer increases with increasing negative bias. The first peak arrival time for QLA phonons in the measured signal moves to earlier times with increasing pump power. This was caused by the nonlinear acoustic properties of the GaAs substrate. Conversely, QTA phonons arrival time was not dependent on pump power which indicates the elastic nonlinear effects are not crucial for QTA signals. Investigation of the temperature dependence reveals that the arrival time changes to later times with increasing temperature because of the decreasing sound velocity with increasing temperature. The polarity of the experimental signals was compared to the simulated signals of electron-phonon coupling interactions concludes the predominant role of piezoelectric phonon-electric transformation mechanism over deformation potential. Detailed theoretical calculations show the dominance of the piezoelectric mechanism up to frequencies of 70GHz and 250GHz for longitudinal and transverse phonons respectively. Low dimensional structures i.e. Quantum well embedded p-i-n photodiode detector (QW-p-i-n), two-dimensional electron gas (2DEG) and two-dimensional hole gas (2DHG), fabricated on the (311) GaAs surface, were also investigated as detectors for longitudinal and transverse acoustic signals. The first experimental evidence of high frequency tail in transverse signal is observed using GaAs based QW-p-i-n detector. The longitudinal signal confirms the existence of the GHz oscillations, with the most significant occurring 170 GHz, while in transverse acoustic signal we observed a high frequency tail from 300 ps to 800 ps. Fast Fourier transforms (FFT) analysis of this tail shows the existence of high frequency phonons up to 200 GHz. Photocurrent pump-probe measurements was also performed with three different Al film thicknesses (20nm, 30nm and 100nm). Experiments performed with 20nm film thickness clearly shows a weak signal at 250 GHz in the amplitude spectrum which is not observed for 30 nm and 100 nm aluminium films. This observation indicates that the thinner aluminium films can generate higher frequency phonons. The longitudinal and transverse signals were also detected by the 2DHG; for QLA predominantly the peak is negative while for QTA it is positive in contrast to the 2DEG experiments due to the reason that in 2DEG experiments the origin of the signal is due to the decrease in carrier density but for 2DHG the signal is due to the increase in carrier density. This thesis studies the interaction of subterahertz acoustic phonons with charge carriers with the aim of developing high speed acoustoelectric detectors for subterahertz compressional and shear acoustic waves, and it has been shown that the Schottky diodes are successful in this application.