Thesis (Ph. D.)--Physics, Georgia Institute of Technology, 2007. / de Heer, Walter A., Committee Chair ; Chou, Meiyin, Committee Member ; Whetten, Robert L., Committee Member ; First, Phillip N., Committee Member ; Pummer, Earl Ward, Committee Member.

This thesis describes the application of coupled-cluster theory to model systems of metallic solids and cold-atom gases. First, I give an overview of both ground- and excited-state coupled cluster theory as background for the main topics in this thesis. Next, I evaluate the accuracy of several cost-saving approaches in estimating the coupled cluster correlation energy for a model metallic system, the uniform electron gas, in the complete basis set and thermodynamic limits. After that, I present calculations of the spectral function of the uniform electron gas in these same limits, the results of which are rationalized by applying a bosonized coupled-cluster theory to an approximate, simplified Hamiltonian that couples plasmons to a structureless core hole state. Finally, I show how coupled-cluster theory captures the many-body nature of two-component Fermi gases with tunable, attractive interactions.

Thesis (PhD)--Stellenbosch University, 2014. / ENGLISH ABSTRACT: The study of α clustering is a well-established topic of research in nuclear physics. Recent experimental evidence has revealed the first 2+ excitation of the Hoyle state in 12C, which is known to have a strong α cluster structure. The idea of multi-particle α cluster structures in light nuclear matter has received much attention from theoretical investigations of late. This research has profound implications in the fields of both nuclear structure and nuclear astrophysics. The 20Ne nucleus is a good example for α clustering, since many of its states are known to have α clustering structures. Few low spin states are known at high excitation energies of this nucleus. It is predicted that this nucleus contains a 0+ 5-α cluster state, a so-called ‘Hoyle analogue state’, above its 5-α break-up threshold at Ex = 19.17 MeV. This thesis presents a study of the 20Ne nucleus with the 22Ne(p,t)20Ne reaction at laboratory angles θlab =(0◦, 7◦, 16◦, 27◦). The iThemba LABS K600 magnetic spectrometer was employed with a beam of energy Elab = 60 MeV, incident upon a 22Ne gas target held intact by Aramid foils. The aim was to search for low spin states in 20Ne at excitation energies above Ex = 15 MeV, and, possibly, to find an indication of the 5-α cluster state. Three narrow states were discovered at energies of Ex = 20.59 MeV, Ex = 21.16 MeV and Ex = 21.80 MeV. Calculations performed with the isobaric multiplet mass equation indicate that these states may be T = 2 isobaric analogue states of three known states in 20O. However, shell-model calculations indicate that these states may also have T = 0 or T = 1 isospin values. There is also evidence of a new state at Ex = 17.67 MeV and, possibly, of a collection of new states which could not be resolved at Ex = 18.84 MeV. A tentative candidate for the desired 5-α cluster state was observed, but this will require another measurement with cleaner background to be confirmed. / AFRIKAANSE OPSOMMING: Alfa bondelvorming is ’n gevestigde navorsingsonderwerp in kernfisika. Daar is onlangse bewyse vir die ontdekking van die 2+ opwekking van die Hoyle toestand in 12C, wat ’n erkende alfa bondelstruktuur het. Die idee van multi-alfa bondelstrukture in ligte kerne het onlangs baie aandag geniet in teoretiese ondersoeke. Hierdie navorsing het besondere implikasies vir kernstruktuur, sowel as vir kernastrofisika. Die 20Ne kern bied ‘n ideale voorbeeld vir alfa bondelvorming aangesien dit bekend is dat baie van die kern se toestande alfa bondelstrukture het. Min lae-spin toestande is by ho¨e opwekenergie¨e bekend in hierdie kern. Daar word voorspel dat ’n 0+ 5-alfa bondeltoestand, ’n sogenaamde ‘Hoyle analoogtoestand’, bo die drumpel vir 5-alfa verval by Ex = 19.17 MeV bestaan. Hierdie tesis beskryf ‘n studie van die 20Ne kern met die 22Ne(p,t)20Ne reaksie by laboratorium hoeke van θlab =(0◦, 7◦, 16◦, 27◦). Die K600 magnetiese spektrometer van iThemba LABS is gebruik met ‘n proton bundel, by ‘n energie van Elab = 60 MeV, wat op ‘n 22Ne gas teiken omhul met aramid foelies gerig is. Die doel was om lae-spin toestande in 20Ne by opwekenergie¨e bo Ex = 15 MeV op te spoor, en om moontlik ook ‘n aanduiding van die 5-alfa bondeltoestand te vind. Drie smal toestande is by energie¨e van Ex = 20.59 MeV, Ex = 21.16 MeV en Ex = 21.80 MeV opgespoor. Berekeninge wat met die isobariese multiplet massa vergelyking uitgevoer is, dui daarop dat hierdie toestande T = 2 isobariese analoogtoestande van drie bekende toestande in 20O kan wees, hoewel skilmodel berekeninge ook T = 0 en T = 1 kandidate aandui. Daar is ook bewyse van ’n nuwe toestand by Ex = 17.67 MeV, en moontlik van ’n versameling nuwe toestande rondom Ex = 18.84 MeV wat nie uitmekaar geken kon word nie. ‘n Tentatiewe kandidaat vir die gesogte 5-alfa bondel toestand is waargeneem, maar nog ‘n meting met ’n skoner agtergrond word vir bevestiging benodig.

Ferroelectric and ferromagnetic alloy clusters are produced and studied in molecular beams. Nb clusters doped with 1-3 impurity atoms are ferroelectric with low transition temperatures. The alloy clusters with an even number of valence electrons have larger dipole moments than those with odd number of valence electrons. The ferroelectricity is suppressed by magnetic impurities or thermal excitations, and is enhanced by Au and Al doping. The observations strongly suggest that electron-pairing interactions exist in Nb clusters, which indicates Cooper pairing in clusters. The magnetic moments of Co clusters doped with small fraction of Mn,V and Al are studied and compared with those of the bulk alloys. CoMn alloy clusters have enhanced average magnetic moments with Mn doping, which is opposite to the behavior of bulk CoMn. CoV and CoAl alloy clusters behave similarly to their bulk counterparts. We explain the experimental results using the virtual-bound-state model. Finally, the magnetic properties of BiMn clusters are studied in molecular beams. The Mn local moments are found to couple ferromagnetically or ferrimagnetically depending on the composition of the clusters.

Magnetic properties of cobalt clusters (20 N 200) were studied in molecular beams. The magnetization of cobalt clusters is studied at a broad range of temperatures, magnetic fields and clusters sizes. It is shown that the agnetization of ferromagnetic clusters in a cluster beam can be understood as an adiabatic process using the avoided crossing theory. Besides the ground state that bears magnetic moment of about 2 Bohr magneton per atom, an excited state that has 1 Bohr magneton per atom was discovered for every cobalt cluster observed. The energy separations between the two states was investigated by photo-ionization experiments. The ionization threshold shows that the energy gap between the two states is on the order of 0.1 eV for small clusters (N 100) and vanishes for larger clusters. Experiments also show that the polarizability of the excited state is lower than that of the ground state, which indicates a significant electronic tructure difference between the two states. Two states are also found for iron clusters (20 N 200) for which the magnetic moments per atom are about 3 Bohr magneton for the ground state and 1 Bohr magneton for the excited states. This explains the fractional magnetic moments as well as the local magnetic order observed above the Curie temperatures for iron group ferromagnets. Further experiments show two states for manganese clusters for which the ground state has magnetic moment of 1 Bohr magneton per atom in about the same size range. This suggests that the two states are a universal phenomenon of 3d transition metal clusters, which originate from the interaction between 3d and 4s electrons.

Thesis (Ph. D.)--University of Washington, 1998. / Vita. Includes bibliographical references (leaves [97]-102).

Thesis (MSc)--Stellenbosch University, 2014. / ENGLISH ABSTRACT: The binary cluster model is used to investigate the properties of exotic structures and decays in various nuclei. A simple method is described to choose the possible clusters forming within the nucleus, by assuming the nucleus consists of a mixture of up to four different core-cluster pairs. A phenomenological potential is then used, with optimized parameters, to describe the even-even 222-232Th nuclei within the binary cluster model, by calculating exotic decay half-lives, reduced electromagnetic transition probabilities, and energy spectra. After finding that all experimentally observed heavy ion emissions are predicted with the model and calculated structure observables are reproduced within good agreement of the experimentally measured values, the model was extended to include the even-even 230-234U, 236-240Pu and 222-224Ra nuclei. We found that almost all experimentally observed heavy ion emissions are predicted within the model with reasonable accuracy in the calculated halflives, and found good agreement in the other calculated structure observables. / AFRIKAANSE OPSOMMING: Die binêre bondel model is gebruik om die eienskappe van eksotiese strukture en verval in verskeie nukliedes te ondersoek. ’n Eenvoudige metode is gebruik om die moontlike bondels wat binne die nuklied vorm te kies, deur die aanname te maak dat die nuklied uit ’n mengsel van tot vier verskillende kern-bondel pare bestaan. ’n Fenomenologiese potensiaal is dan gebruik, met optimale parameters, om die ewe-ewe 222-232Th nukliedes met die binêre bondel model te beskryf, deur eksotiese verval halfleeftye, verminderde elektromagnetiese oorgangswaarskynlikhede, en energie spektra te bereken. Nadat daar gevind is dat alle vrygestelde swaar ione wat eksperimenteel waargeneem is deur die model voorspel word, en berekende struktuur waarneembares tot goeie ooreenstemming met die eksperimentele waardes produseer is, is die model uitgebrei om die ewe-ewe 230-234U, 236-240Pu en 222-224Ra nukliedes in te sluit. Daar is gevind dat byna alle vrygestelde swaar ione wat eksperimenteel waargeneem is deur die model voorspel word met redelike akkuraathied in die berekening van hul halfleeftye, asook dat daar goeie ooreenstemming tussen die berekende en eksperimentele waardes van ander struktuur waarneembares was.

Thesis (PhD (Physics))-University of Stellenbosch, 2009. / ENGLISH ABSTRACT: A binary cluster model is used to investigate the properties of the ground state band of 212Po, modelled as a 208Pb-alpha core-cluster system. The results obtained using a microscopic corecluster potential are compared to those obtained with a purely phenomenological potential. The two potentials were found to exhibit similar surface behaviour and thus give similar predictions for the ground state alpha decay half-life. They however generate very different energy spectra, with the results from the phenomenological potential clearly superior. We optimize the phenomenological potential parameters, and propose an additional short range interaction to improve the underbinding generally found for the J¼ = 0+ ground state. We then investigate two possible scenarios for generating the negative parity states in 212Po. We find that both are necessary in order to produce low-lying negative parity states which are able to decay via electric dipole transitions to the positive parity states of the ground state band. Finally we present a novel calculation of the properties of the low-lying positive and negative parity states of 218Rn described as a doubly closed 208Pb core plus a 10Be cluster. / AFRIKAANSE OPSOMMING: ’n Binêre bondel model word gebruik om die eienskappe van die grondtoestands energie band van 212Po, te modeleer as ’n 208Pb-alpha kern-bondel sisteem te ondersoek. Die resultate verkry vanaf ’n mikroskopiese kern-bondel potentiaal word vergelyk met die wat verkry is met ’n suiwer fenomenologiese potentiaal. Die twee potentiale is verkry om dieselfde oppervlakte toestande voor te stel en gee sodoende dieselfde voorspellings vir die grondtoestand alpha verval halfleeftyd. Alhoewel dit baie verskillende energie spektra genereer, toon die resultate van die fenomenologiese potentiaal dat dit duidelik beter is. Ons optimiseer hierdie fenomenologiese parameters en stel ’n addisionele kort ry-afstands interaksie voor om die algemene ondergebondenheid wat oor die algemeen by die J¼ = 0+ grondtoestand voorkom, te verbeter. Ons ondersoek ook hierdie twee moontlike scenarios om die negatiewe pariteitstoestande in 212Po te genereer. Ons vind dat beide scenarios noodsaaklik is om laagliggende pariteitstoestande te produseer, sodat verval deur elektriese dipool oorgange na die positiewe pariteitstoestande van die grondtoestandsband moontlik is. Laagliggende positiewe en negatiewe pariteitstoestande, van die 218Rn wat beskryf word as ’n dubbelgeslote 208Pb kern en ’n 10Be bondel.

De nombreux noyaux atomiques et hypernoyaux se modélisent comme des structures à trois corps. C'est le cas, par exemple, de noyaux à halo, comme 6He, ou de noyaux stables, comme 12C et 9Be. <p>En effet, 6He se caractérise comme un système à trois corps, formé d'un coeur (une particule alpha) et de deux neutrons de valence faiblement liés. Le noyau de 12C peut s'étudier comme un système lié formé de trois particules alphas, tandis que 9Be peut être décrit comme la liaison de deux particules alphas et d'un neutron.<p><p>Dans les exemples précédents, les particules alphas sont des amas de nucléons. Elles possèdent donc une structure interne dont il faut tenir compte en raison du principe de Pauli.<p><p>Les modèles les plus réalistes pour décrire les structures à trois corps sont les modèles "microscopiques". Ces modèles prennent en compte explicitement tous les nucléons et respectent exactement le principe d'antisymétrisation de Pauli. Cependant, l'application de ces modèles est fortement limitée en pratique, car ils exigent de trop nombreux et trop longs calculs.<p>Par conséquent, pour simplifier considérablement les calculs et permettre l'étude des structures à trois corps, des modèles moins détaillés, de type "semi-microscopiques", sont également développés. Dans ces modèles, on représente les amas de nucléons comme de simples particules ponctuelles. Dans ce cas, la modélisation consiste à construire les potentiels effectifs entre les amas, puis à les employer dans les modèles à trois corps.<p><p>Dans ce travail, nous avons développé les modèles "semi-microscopiques à trois corps". Les potentiels effectifs entre amas sont directement déduits des forces entre nucléons (selon la RGM à 2 corps). Ces potentiels sont "non-locaux", et dépendent des énergies des amas qui interagissent. Ils permettent de simuler le principe de Pauli et les échanges de nucléons entre les amas. La dépendance en l'énergie se révèle être un inconvénient dans les modèles à trois corps. Les potentiels effectifs sont par conséquent transformés en de nouveaux potentiels (non-locaux) indépendants de l'énergie, bien adaptés aux modèles à trois corps. Les modèles "semi-microscopiques" sont beaucoup plus simples et plus rapides que les modèles "microscopiques". Ils fournissent les fonctions d'onde des états liés à trois corps des noyaux légers et hypernoyaux. Cela permet d'une part de comprendre les propriétés spectroscopiques nucléaires, et d'autre part, cela ouvre la voie pour de futurs modèles de réactions nucléaires impliquant les structures à trois corps.<p><p>/<p><p>Several atomic nuclei and hypernuclei can be modelled as three-body structures: e.g. two-neutron halo nuclei, such as 6He, and other nuclei, such as 12C and 9Be.<p>Indeed 6He can be represented as a three-body system, made up of a core (an alpha particle) and two weakly bound valence neutrons. The 12C nucleus can be studied as a bound system formed by three alpha particles, while the 9Be nucleus can be described as the binding of two alpha particles and one neutron.<p><p>In these typical examples, the alpha particles are clusters of nucleons. They have an internal structure that must be taken into account because of the Pauli principle.<p><p>The most realistic models are the "microscopic models". In these models, all the nucleons are taken into account, and the Pauli antisymmetrisation principle is fully respected. However, the application of the "microscopic models" is limited in practice, because they require too many laborious calculations.<p>Therefore, in order to greatly simplify the calculations, "semi-microscopic models" are developed. In those models, the clusters of nucleons are treated as ("structureless") pointlike particles. The models then consist in determining the effective potentials between the clusters, and in using them in three-body models.<p><p>In the present work, we have developed "semi-microscopic models". The effective potentials between the clusters are directly obtained from the interactions between nucleons (according to the two-cluster RGM). These potentials are "nonlocal", and depend on the energy of the interacting clusters. The non-locality is a direct consequence of the Pauli principle and the exchanges of nucleons between the clusters. The energy-dependence of the potentials turns out to be a drawback in three-body models. Therefore, the effective potentials are transformed into energy-independent potentials, which can be used in three-body models. The "semi-microscopic models" are much simpler and faster than the "microscopic models". They provide the three-body bound-state wave functions (i.e. the spectroscopic properties and the structure) of light nuclei and hypernuclei. Such wave functions are also the basic ingredient that will be used in future reactions models. / Doctorat en Sciences de l'ingénieur / info:eu-repo/semantics/nonPublished