Submitted by Reginaldo Soares de Freitas (reginaldo.freitas@ufv.br) on 2015-11-10T15:55:30Z No. of bitstreams: 1 texto completo.pdf: 2161122 bytes, checksum: e5f431c07c8bf11e4baf6184921d753d (MD5) / Made available in DSpace on 2015-11-10T15:55:30Z (GMT). No. of bitstreams: 1 texto completo.pdf: 2161122 bytes, checksum: e5f431c07c8bf11e4baf6184921d753d (MD5) Previous issue date: 2014-03-18 / Coordenação de Aperfeiçoamento de Pessoal de Nível Superior / Nesta tese s ̃o estudados efeitos termodinˆmicos e de defeitos estruturais nos materiais a a conhecidos como gelos de spin artificiais, por meio de simula ̧ ̃es computacionais em co redes quadradas. No caso da termodinˆmica, damos ˆnfase aos efeitos da temperatura a e sobre as excita ̧oes elementares: os monopolos magn ́ticos de Nambu e suas cordas. c ̃ e Usando t ́cnicas de Monte Carlo foram obtidas quˆntidades termodinˆmicas como calor e a a espec ́ ıfico, densidade de cargas e a separa ̧ ̃o m ́dia entre pares de carga como fun ̧ao ca e c ̃ da temperatura. Nossos c ́lculos mostram que o calor espec ́ a ıfico e a separa ̧ ̃o m ́dia ca e entre pares de cargas exibem, respectivamente, um pico acentuado e um m ́ximo global a e c ̃ na mesma temperatura Tc ≈ 7, 2 D/kB , em que D ́ a constante de intera ̧ao dipolar e kB a constante de Boltzmann. Analisando a dependˆncia dessas quantidades com o e tamanho do sistema verificamos um crescimento logar ́ ıtmico das amplitudes m ́ximas a ca e do calor espec ́ ıfico, Cmax , e da separa ̧ ̃o m ́dia entre pares, dmax , com o aumento do tamanho do sistema. Dessa forma, nossos resultados indicam que monopolos comple- tamente isolados seriam encontrados apenas no limite termodinˆmico. Entretanto, os a resultados tamb ́m sugerem que para temperaturas T ≥ Tc , o sistema exibe uma fase e a a com monopolos separados, apesar de dmax n ̃o ser maior que poucos parˆmetros de rede para sistemas reais. Na segunda parte desta tese, n ́s estudamos o caso em que a rede o ́ constru ́ com um defeito estrutural como, por exemplo, uma ilha deformada. Isto e ıda ́ feito considerando um arranjo quadrado de nanoilhas contendo apenas uma unica e ́ ilha diferente das demais. Esta diferen ̧a ́ incorporada no momento magn ́tico (spin) c e e da ilha defeituosa, sendo muitos casos estudados, inclusive o caso especial no qual a ilha est ́ ausente (vacˆncia). N ́s mostramos que sobre os v ́rtices adjacentes da na- a a o e noilha “malformada” existe um par de cargas magn ́ticas opostas. Tamb ́m obtivemos e e o potencial intera ̧ ̃o efetivo entre as excita ̧oes elementares (um par de monopolos de ca c ̃ Nambu e suas cordas) com a ilha defeituosa, sendo um problema envolvendo quatro cargas magn ́ticas (dois pares de polos opostos) e uma corda. / In this thesis, we study the thermodynamics and structural defects effects in artificial spin ice systems, by mean of computer simulations in square lattices. In the case of ther- modynamics, we give emphasis to the effects of temperature on elementary excitations: the Nambu magnetic monopoles and their strings. By using Monte Carlo techniques, we obtain the specific heat, the density of charges and the average separation of a pair of charges as a function of temperature. Our calculations show that the specific heat and average separation of a pair of charges exhibit a sharp peak and global maximum, respectively, at the same temperature Tc = 7, 2 D/kB , where D is the strength of the dipolar interaction and kB is the Boltzmann constant. Analysing the dependence of these quantities with the size of the system, we verify a logarithmic growth of the ma- ximum amplitude of the specific heat, Cmax and the avarege separation, dmax , with the increasing of system’s size. Then, our results suggest that completely isolated charges could be found only at thermodynamic limit. However, for temperatures T ≥ Tc the system exibits a phase with separated monopoles despite of the maximun separation, dmax , not exceed a few lattice spacings for viable artificial materials. In the second part of this thesis, we study the case where the lattice is building with a structural defect such as a defective island. This is done by considering a square array of islands containing only one island different from all others. This difference is incorporated in the magnetic moment (spin) of the “imperfect” island and several cases are studied, including the special situation in which this distinct spin is zero (vacancy). We show that the two extreme points of a defective island behave like two opposite magnetic charges. Then, the effective interaction between the elementary excitations (a pair of Nambu monopoles and their string) with the defective island is a problem involving four magnetic charges (two pairs of opposite poles) and a string.

Thesis (MScEng (Electrical and Electronic Engineering))--University of Stellenbosch, 2006. / This thesis focuses on the coaxial-microstrip T-resonator measurement technique to determine the aperture plane capacitance and radiation resistance of electrically short monopoles above a finite ground plane at Radio Frequencies. The determination of these impedance parameters is of interest as they are needed in the Thévenin equivalent circuit of the monopole to establish the relationship between the monopole, the electromagnetic field within which it is immersed, and its surrounding medium properties. Electromagnetically short monopoles are used in various applications such as in permittivity and biomedical measurement techniques and near-field metrology. The coaxial-microstrip device allows the measurement of transmission coefficient nulls when connected to the network analyser. By measuring the frequency shift in transmission null points and analysing the results, the capacitance of short monopoles of various lengths can be obtained. The radiation resistance of the monopole can also be determined by comparing the depth of transmission coefficient nulls points at various resonant frequencies. Monopoles ranging from flush-mounted to various sizes of protruding centre conductors, both bare and insulated, are investigated in the frequencies ranging from 45 MHz to 3 GHz. The experimental results compare well with FEKO numerical predictions at the ranges where this technique is applicable. The technique is however not applicable near and beyond the resonant frequency of the monopole where it is considered to be electrically long. Measurements to obtain monopole impedance parameters were conducted by connecting the device to the HP 8510C Network Analyser. To enhance accuracy, the Network Analyser was calibrated using a high quality calibration kit with precisely defined standards. The technique used was found to accurately yield capacitances in the range of 0.0211 to 1 pF and radiation resistances in the range of 0.9245 to 12.1 Ohms. These results were obtained for monopoles of length 0.0062 < h/l < 0.24. Results from literature, viz. W.R Scott, R.W.P. King and L.J. Cooper, show radiation resistance in the ranges: 2.5 to 35 Ohms (0.079 < h/l < 0.22), 1.26 to 74.08 Ohms (0.026 < h/l < 0.28) and 5.59 to 74.98 Ohms (0.1 < 0.28) for electrically short monopoles. The results of this work are compared with those in the literature and show variations of less than 6%. A technique capable of accurately measuring capacitance and radiation resistance of the monopole at various lengths of protruding centre conductor has been developed. The capacitance for a flush probe compares very well (agreement is less than 6%) when compared with those obtained by the previous researchers. The technique is capable of yielding accurate results radio frequencies in the challenging low radiation resistance range around 1 Ohm.

No description available.

Cette thèse traite des propriétés des glaces de spins artificielles de type kagomé. Il s'agit de réseaux de nano-aimants magnétiques que nous avons fabriqués par dépôt d'un film mince, lithographie électronique et gravure ionique. Les éléments de la maille appelés brins possèdent des tailles typiques que 500 nm de long, 100 nm de large et 10nm d'épaisseur. L'intérêt de ces brins mésoscopiques repose sur la possibilité d'en déterminer la configuration magnétique par imagerie. Les caractérisations réalisées après désaimantation nous ont permis de mettre en évidence l'impact des différents types de désaimantation et de faire apparaître pour la première fois un polycristal de charges. Pour approfondir notre compréhension de ce système, nous avons aussi réalisé une étude numérique. Nous avons montré que les brins ne se comportent pas comme des spins d'Ising. En effet, la configuration d'aimantation d'un brin peut s'apparenter à une configuration homogène avec un domaine de fermeture aux deux extrémités. Nous avons étudié l'impact de ces configurations de bout de brins dans le renversement de l'aimantation des réseaux. Nous montrons également expérimentalement que le renversement peut être très anisotrope. L'origine de cette anisotropie a été étudiée. Enfin, nous montrons numériquement que lorsqu'une configuration interdite est stabilisée, les domaines de fermeture ferment le flux laissant apparaître deux chiralités qui possèdent des champs de disparition différents lorsque le champ est appliqué hors axe de la nanostructure. D'un point de vue expérimental, nous avons tenté de mettre en évidence l'existence de cette chiralité des monopoles / The subject of this thesis is the study of artificial kagome spin ices which are frustrated networks of nanomagnets. These arrays are made using thin film deposition, electron beam lithography and ion beam etching. The typical sizes of each nanomagnet are a length of 500nm, a width of 100nm and a thickness of 10nm with a separation between nanomagnets of 50nm. The interest of these frustrated networks relies on the possibility to measure the magnetic configurations by imagery and extract the macrospin configurations. In this work we have especially compared different demagnetization procedures (field and thermal) that allowed us to highlight their impact on the configurations and we have shown for the first time an emergent polycristal of charges. To have a better understanding of our system, we have also conducted a numerical study using finite difference methods. We have shown that nanomagnets do not behave like Ising spin. Indeed, the magnetic configuration is shown to be homogeneous with domains at extremities. In the array, the domains close the flux at a vertex and the effects of those domains on the magnetization reversal of our networks have been studied. We have also shown experimentally that the reversal can be anisotropic. The origin of this anisotropy has been studied. Finally, we have numerically shown that, when a forbidden configuration is stabilized, the closure of the flux at the vertex leads to chiralities of the forbidden state. These chiralities possess different annihilation fields when the fields are applied out of the nanostructure axis. From an experimental point of view, we tried to show the existence of this monopole chirality using adapted field histories

Switched and adaptive parasitic monopole array radiating structures are investigated. Antenna design is orientated toward increasing practicability for implementation in terrestrial wireless communication systems. A number of antennas are designed with the aid of optimization and commercial simulation software. Simulation procedure was verified with the experimental manufacture and measurement of the arrays. The antennas presented in this thesis comprise an active monopole surrounded by a ring of parasitic monopoles. Parasitic radiators are constructed with static loading to enable simple experimental realization. Beam positions of an electrically steered equivalent antenna are thus simulated. Antenna symmetry ensures the beam can be reproduced throughout the azimuth. Complex antenna geometries require antenna design through optimization. A genetic algorithm is employed with HFSS and NEC for electromagnetic analysis. The robust optimization method couples with simulation software flexibility to provide an effective design tool for arbitrary structures. The genetic algorithm is employed strictly for design and not complete structural optimization. Dual band, five and six element switched parasitic antennas are presented. Lumped elemental loading along the radiators provide resonance and directed radiation at two GSM frequencies. Load value, radiator dimension and spacing are incorporated as design parameters. Experimentally built, 10dB return loss bandwidths of 17.2% and 9.6% and front to back ratios of 12.6dB and 8.4dB at 900MHz and 1900MHz respectively are measured. To reduce the ground requirements of monopole arrays, a skirted ground structure for switched parasitic antennas is analyzed. A six element switched parasitic monopole array with conductive ground skirt exhibits a front to back ratio of 10.7dB and main lobe gain of 6.4dBi at 1.575GHz. Radiation is not elevated despite lateral ground terminating at the parasitic elements. Skirt height is observed to linearly control radiation elevation, depressing the principal lobe through 40 degrees from 23 degrees above the horizontal. The Electronically Steerable Passive Array Radiator or ESPAR antenna is an adaptive parasitic monopole array. An ESPAR radiating structure incorporating a conductive ground skirt is designed for operation at 2.4GHz. Utility is confirmed with a frequency sensitivity analysis showing consistent electrical characteristics over an 8.1% bandwidth. The antenna design is improved with optimization to reduce average principal lobe elevation from 25 degrees to 9.7 degrees.

In this thesis, we study the subject of geometrical phases in detail by considering its various forms. We focus primarily on the relation between quantum geometrical phases and magnetic monopoles, and study how one can make use of the concepts of geometrical phases to define magnetic monopoles.

Computer simulations of metal-organic frameworks are conducted to both investigate the mechanism of hydrogen sorption and to elucidate a detailed, molecular-level understanding of the physical interactions that can lead to successful material design strategies. To this end, important intermolecular interactions are identified and individually parameterized to yield a highly accurate representation of the potential energy landscape. Polarization, one such interaction found to play a significant role in H 2 sorption, is included explicitly for the first time in simulations of metal-organic frameworks. Permanent electrostatics are usually accounted for by means of an approximate fit to model compounds. The application of this method to simulations involving metal-organic frameworks introduces several substantial problems that are characterized in this work. To circumvent this, a method is developed and tested in which atomic point partial charges are computed more directly, fit to the fully periodic electrostatic potential. In this manner, long-range electrostatics are explicitly accounted for via Ewald summation. Grand canonical Monte Carlo simulations are conducted employing the force field parameterization developed here. Several of the major findings of this work are: Polarization is found to play a critical role in determining the overall structure of H 2 sorbed in metal-organic frameworks, although not always the determining factor in uptake. The parameterization of atomic point charges by means of a fit to the periodic electrostatic potential is a robust, efficient method and consistently results in a reliable description of Coulombic interactions without introducing ambiguity associated with other procedures. After careful development of both hydrogen and framework potential energy functions, quantitatively accurate results have been obtained. Such predictive accuracy will aid greatly in the rational, iterative design cycle between experimental and theoretical groups that are attempting to design metal-organic frameworks for a variety of purposes, including H 2 sorption and CO2 sequestration.

The physical world is, for the most part, modelled using second order ordinary differential equations. The time-dependent simple harmonic oscillator and the Ermakov-Pinney equation (which together form an Ermakov system) are two examples that jointly and separately describe many physical situations. We study Ermakov systems from the point of view of the algebraic properties of differential equations. The idea of generalised Ermakov systems is introduced and their relationship to the Lie algebra sl(2, R) is explained. We show that the 'compact' form of generalized Ermakov systems has an infinite dimensional Lie algebra. Such algebras are usually associated only with first order equations in the context of ordinary differential equations. Apart from the Ermakov invariant which shares the infinite-dimensional algebra of the 'compact' equation, the other three integrals force the dimension of the algebra to be reduced to the three of sl(2, R). Subsequently we establish a new class of Ermakov systems by considering equations invariant under sl(2, R) (in two dimensions) and sl(2, R) EB so(3) (in three dimensions). The former class contains the generalized Ermakov system as a special case in which the force is velocity-independent. The latter case is a generalization of the classical equation of motion of the magnetic monopole which is well known to possess the conserved Poincare vector. We demonstrate that in fact there are three such vectors for all equations of this type. / Thesis (M.Sc.)-University of Natal, 1993.

In this thesis we study the connection between continuum quantum field theory and corresponding lattice field theory, specifically for two cases: Landau gauge fixing and ’t Hooft-Polyakov monopoles. To study non-perturbative phenomena such as the confinement mechanism of quarks and gluons and dynamical chiral symmetry breaking in Quantum Chromodynamics (QCD), there are two major approaches: the Dyson-Schwinger equations (DSEs) approach, which is based on the covariant continuum formulation, and lattice gauge theory. The strength and beauty of lattice gauge theory is due to the fact that gauge invariance is manifest and fixing a gauge is not required. In the covariant continuum formulation of gauge theories, on the other hand, one has to deal with the redundant degrees of freedom due to gauge invariance and has to fix gauge (most popularly, Landau gauge). There, the gauge-fixing machinery is based on the so-called Faddeev-Popov procedure or more generally, the Becchi-Rouet-Stora-Tyutin (BRST) symmetry. Beyond perturbation theory this is aggravated by the existence of so-called Gribov copies, however, that satisfy the same gauge-fixing condition, but are related by gauge transformations, and are thus physically equivalent. When attempting to fix Landau gauge on the lattice to make a connection with its continuum counterpart, this ambiguity manifests itself in the Neuberger 0/0 problem that asserts that the expectation value of any physical observable will always be of the indefinite form 0/0. We explain the topological nature of this problem and how the complete cancellation of Gribov copies can be avoided in a modified lattice Landau gauge based on a new definition of gauge fields on the lattice as stereographically projected link variables. For compact U(1), where the Gribov copy problem is related to the classification the local minima of XY spin glass models, we explicitly show that there still remain Gribov copies but their number is exponentially reduced in lower dimensional models. We then formulate the corresponding Faddeev-Popov procedure on the lattice, for these models. Moreover, we explicitly demonstrate that the proposed modification circumvents the Neuberger 0/0 problem for lattices of arbitrary dimensions for compact U(1). Applied to the maximal Abelian subgroup this will avoid the perfect cancellation amongst the remaining Gribov copies for SU(N), and so the corresponding BRST formulation is also then possible for generic SU(N), in particular, for the Standard Model groups. For higher dimensional lattices, the gauge fixing conditions for both the standard and the modified lattice Landau gauges are systems of multivariate nonlinear equations, solving which in general is a highly non-trivial task. However, we show that these systems can be interpreted as systems of polynomial equations. They can then be solved exactly by computational Algebraic Geometry, the Groebner basis technique in particular, and numerically by the Polynomial Homotopy Continuation method. ’t Hooft-Polyakov monopoles play an important role in high energy physics due to their presence in grand unified theories and their usefulness in studying non-perturbative properties of quantum field theories through electric-magnetic dualities. In the second part of the thesis, we study adjoint Higgs models, which exhibit ’t Hooft-Polyakov monopoles, and have been extensively analyzed using semi-classical analysis in the continuum. However, to study them in a fully nonperturbative fashion, it is essential to put the theory on the lattice. Here, we investigate twisted C-periodic boundary conditions in SU(N) gauge field theory with an adjoint Higgs field and show that for even N with a suitable twist one can impose a non-zero magnetic charge relative to each of N − 1 residual U(1)’s in the broken phase, thereby creating ’t Hooft-Polyakov magnetic monopoles. This makes it possible then to use lattice Monte-Carlo simulations to study the properties of these monopoles in the full quantum theory and compare them with the existing results in the continuum. / Thesis (Ph.D.) -- University of Adelaide, School of Chemistry and Physics, 2010

Ανασκόπηση τηε θεωρίας Dirac σχετικά με τα μαγνητικά μονόπολα και αποτελέσματα της. Ανασκόπηση εννοιών από την τοπολογία και την διαφορική γεωμετρία, με την βοήθεια των οποίων, περιγράφουμε την θεωρία των t'Hooft-Polyakov, και συγκεκριμένα την λύση που βρήκαν που αντιστοιχεί σε μαγνητικό μονόπολο στην μη αβελιανή θεωρία με ομάδα βαθμίδας την SU(2). Τέλος, εξετάζονται τα γενικά χαρακτηριστικά των μονοπολικών λύσεων με τοπολογικά επιχειρήματα και γενικεύονται τα αποτελέσματα προηγούμενων ενοτήτων. / Dirac theory of magnetic monopoles. Aspects of topology and differential geometry. The t'Hooft - Polyakov solution in non abelian gauge theory with gauge group SU(2). General properties of monopole solutions from a topological point of view.