Energy studies of the constituents of a mass-selected Gallium Liquid Metal Ion Source (LMIS) are presented. Results were obtained showing double energy loss peaks for Ga$\sbsp{2}{+},$ Ga$\sbsp{3}{+},$ and Ga$\sbsp{4}{+}$ which have not previously been reported in the literature. These loss peaks coincided with higher LMIS emission current, and some of the heavier mass peaks in the LMIS mass spectrum were observed to simultaneously diminish. The relationship of these two observations is discussed in terms of classical liquid drop fission theory. This theory predicts classical fissioning barriers and gives a range of possible parents. The fissioning times of various assumed parents for the given daughters is calculated and inferences concerning the manner of fissioning--asymmetric or symmetric--and their viability are presented. The apparatus used to perform the experiment and its implementation is discussed. / Source: Dissertation Abstracts International, Volume: 55-08, Section: B, page: 3383. / Major Professor: David M. Lind. / Thesis (Ph.D.)--The Florida State University, 1994.

Experimental results concerning the multiphonon scattering of He from the LiF(001) surface, the structure of the NiO(001) surface probed by $\sp3$He scattering, and the structure and dynamics of the CoO(001) surface are presented. Experiments measuring the multiphonon background of He scattered from the LiF surface were used to model the form factor of the interaction for the atom-surface system. This modeling yielded a surface Debye frequency of $\omega\sb{D} = 6.8\times10\sp{13}$ rad/s, the stiffness parameter in the Mott-Jackson matrix element $\beta\ =\ 9.5\ {\rm\A}\sp{-1}$, and a parallel "cutoff" factor $Q\sb{c}\ =\ 8.5\ {\rm\A}\sp{-1}$ for this system. Experiments to determine the structure of NiO using $\sp3$He as the incident probe atom were performed to search for an interaction between the antiferromagnetic ordering in the NiO surface and the nuclear spin in the $\sp3$He. These experiments showed no evidence of any magnetic scattering. It was shown that the mixing of $\sp3$He and $\sp4$He can be used to produce $\sp3$He beams with velocity distributions unobtainable from pure $\sp3$He. Experiments scattering He from the surface of a single crystal CoO sample were used to estimate the peak-to-peak corrugation of CoO for He with an incident wave vector of 6.7 A$\sp{-1}$ at 0.19 A. In addition, the Rayleigh phonon surface dispersion was measured over approximately half of the Brillouin zone. / Source: Dissertation Abstracts International, Volume: 55-09, Section: B, page: 3953. / Major Professor: James G. Skofronick. / Thesis (Ph.D.)--The Florida State University, 1994.

High resolution helium atom scattering (HAS) experiments were performed on the disordered surface of KCN(001) in the rhombohedral phase and on expitaxial grown layers of KBr on RbCl(001). The experiments on KCN used the time-of-flight (TOF) method to measure multiphonon interactions of the incident helium atoms with the crystal surface as a function of incident energy and crystal temperature. These measurements were used to model a cutoff factor ($Q\sb{c} = 4.5{\rm\A}\sp{-1}$) for parallel momentum transfer, a range parameter ($\beta$ = 4.5A$\sp{-1}$) of an exponential potential $V\propto$ exp(-$\beta z$), and a surface Debye temperature ($\Theta\sb{S}$ = 195K). / The experiments with RbCl involved the deposition of one to three monolayers of KBr onto the RbCl(001) surface, which was monitored in situ by attenuation of the atomic helium beam. Inelastic scattering experiments were performed on each atomic layer to determine the surface phonon dispersion curves as a function of coverage. This experiment was performed to validate and also, to provide additional results that will extend the shell model for ionic insulators. Measurements were made in both the ${\overline{\Gamma M}}$ and ${\overline{\Gamma X}}$ high symmetry directions for coverages of one, two, and three monolayers. The KBr/RbCl system which is nearly perfectly lattice matched is compared to the KBr/NaCl system which has lattice registry of 6 KBr to 7 NaCl. / Source: Dissertation Abstracts International, Volume: 55-09, Section: B, page: 3955. / Major Professor: J. G. Skofronick. / Thesis (Ph.D.)--The Florida State University, 1994.

The two-dimensional antiferromagnetic Heisenberg model and $t - J$ model are studied on the square lattice aimed to understand certain normal state properties of the high-temperature copper-oxide superconductors. Fundamental physical issues such as the role of quantum spin fluctuations, and hole dynamics are addressed. We constructed a variational wave function for the Heisenberg model, which includes spin-spin correlations and possesses antiferromagnetic long range order. Long and short wave length spin excitations are calculated using this wave function and compared with neutron scattering and Raman scattering experiments. Loop-expansions up to two-loop level are applied to study the spectral function of a single hole described by the $t - J$ model. Our numerical solutions extrapolated to the thermodynamical limit show that the hole spectral function consists of a sharp lowest-energy quasihole peak and a series of well defined peaks above it, which correspond to the "string" excitations. The single hole spectral weight of the t-J model is also calculated using the Green's function Monte Carlo method to further clarify the controversial question of whether or not the Fermi liquid theory breaks down in this model due to a vanishing spectral weight. / Source: Dissertation Abstracts International, Volume: 54-03, Section: B, page: 1473. / Major Professor: Efstratios Manousakis. / Thesis (Ph.D.)--The Florida State University, 1993.

The penetration of a quasistatic electric field into an epitaxial film of Yba$\sb2$Cu$\sb3$O$\sb{\rm 7-x}$ was studied around the film's transition temperature. The electric field penetration into the YBa$\sb2$Cu$\sb3$O$\sb{\rm 7-x}$ electrode was shown to vary by as much as 100A in the temperature regime 60K $<$ T $<$ 110K. The distance between the physical interface and the effective electrical interface is at least 100A at temperatures within 25K of T$\sb{\rm c}$. The direction of interfacial movement is reversed at T$\sb{\rm c}$. The data are consistent with the continuous movement, with changing temperature, of an interface between two regions, one penetrated by the electric field and one devoid of electric field. / The data indicate far stronger temperature dependance and magnitude of the delocalization forces along the c-axis of the material in both the superconducting and normal state than could be expected from any conventional theory of metals. / Applying a dc bias to the films, the effect of charge carrier concentration on T$\sb{\rm c}$ was studied. The first and second derivatives of T$\sb{\rm c}$ with respect to surface carrier concentration were determined to be, respectively, near zero and negative. / Source: Dissertation Abstracts International, Volume: 54-03, Section: B, page: 1471. / Major Professor: Louis R. Testardi. / Thesis (Ph.D.)--The Florida State University, 1993.

The two-dimensional t-J model on the square lattice is studied as a relevant model to capture the essential physics of the high-temperature copper-oxide superconductors. In order to gain understanding of the basic physics of the model, fundamental issues such as the motion of a single hole and the binding of two holes in a quantum antiferromagnet are addressed. A numerical approach is followed, combining a variation calculation with the use of the Green's function Monte Carlo method, applied in this thesis for the first time to study the t-J model in the presence of mobile holes. Important insight is obtained on the effect of a single hole on the antiferromagnetic background and on the occurrence of binding of two holes. It is found that a critical value $(t/J)\sb{c}$ of the parameter t/J of the model exists such that hole binding no longer takes place for $t/J>(t/J)\sb{c},$ with $(t/J)\sb{c}\sim3.7.$ The value that t/J should have in the real material is estimated to be about 3, in order for the model to be relevant to superconductivity. Further research developments are discussed. / Source: Dissertation Abstracts International, Volume: 53-10, Section: B, page: 5271. / Major Professor: Efstratios Manousakis. / Thesis (Ph.D.)--The Florida State University, 1992.

Strongly correlated materials are a class of materials that cannot be properly described by the Density Functional Theory (DFT), which is a single-particle approximation to the original many-body electronic Hamiltonian. These systems contain d or f orbital electrons, i.e., transition metals, actinides, and lanthanides compounds, for which the electron-electron interaction (correlation) effects are too strong to be described by the single-particle approximation of DFT. Therefore, complementary many-body methods have been developed, at the model Hamiltonians level, to describe these strong correlation effects. Dynamical Mean Field Theory (DMFT) and Rotationally Invariant Slave-Boson (RISB) approaches are two successful methods that can capture the correlation effects for a broad interaction strength. However, these many-body methods, as applied to model Hamiltonians, treat the electronic structure of realistic materials in a phenomenological fashion, which only allow to describe their properties qualitatively. Consequently, the combination of DFT and many body methods, e.g., Local Density Approximation augmented by RISB and DMFT (LDA+RISB and LDA+DMFT), have been recently proposed to combine the advantages of both methods into a quantitative tool to analyze strongly correlated systems. In this dissertation, we studied the possible improvements of these approaches, and tested their accuracy on realistic materials. This dissertation is separated into two parts. In the first part, we studied the extension of DMFT and RISB in three directions. First, we extended DMFT framework to investigate the behavior of the domain wall structure in metal-Mott insulator coexistence regime by studying the unstable solution describing the domain wall. We found that this solution, differing qualitatively from both the metallic and the insulating solutions, displays an insulating-like behavior in resistivity while carrying a weak metallic character in its electronic structure. Second, we improved DMFT to describe a Mott insulator containing spin-propagating and chargeless fermionic excitations, spinons. We found the spinon Fermi-liquid, in the Mott insulating phase, is immiscible to the electron Fermi-liquid, in the metallic phase, due to the strong scattering between spinons in a metal. Third, we proposed a new approach within the slave-boson (Gutzwiller) framework that allows to describe both the low energy quasiparticle excitation and the high energy Hubbard excitation, which cannot be captured within the original slave-boson framework. In the second part, we applied LDA+RISB to realistic materials modeling. First, we tested the accuracy of LDA+RISB on predicting the structure of transition metal compounds, CrO, MnO, FeO, CoO, CoS, and CoSe. Our results display remarkable agreements with the experimental observations. Second, we applied LDA+RISB to analyze the nature of the Am-O chemical bonding in the CsAm(CrO_4)_2 crystal. Our results indicate the Am-O bonding has strongly covalent character, and they also address the importance of the correlation effects to describe the experimentally observed electronic structure. In summary, we proposed three extensions within DMFT and RISB framework, which allow to investigate the domain wall structure in metal-Mott insulator coexistence regime, the metal-to-Mott-insulator transition with spinons excitation in the Mott-insulating phase, and the Hubbard excitation within RISB approach. Furthermore, we demonstrated that LDA+RISB is a reliable approximation to the strongly correlated materials by applying it to the transition metal compounds and the Americian chromate compounds. / A Dissertation submitted to the Department of Physics in partial fulfillment of the requirements for the degree of Doctor of Philosophy. / Summer Semester 2017. / July 12, 2017. / Density functional theory, Electronic structure simulations, Many-body methods, Mott transition, Strongly correlated system / Includes bibliographical references. / Vladimir Dobrosavljevic, Professor Directing Dissertation; Naresh S. Dalal, University Representative; Efstratios Manousakis, Committee Member; Luis Balicas, Committee Member; Jorge Piekarewicz, Committee Member.

Quantum computing gives novel way of computing using quantum mechanics, which furthers human knowledge and has exciting applications. Quantum systems with diluted spins such as rare earth ions hosted in single crystal, molecule-based magnets etc. are promising qubits candidates to form the basis of a quantum computer. High sensitivity measurement and coherent control of these spin systems are crucial for their practical usage as qubits. The micro-SQUID (direct-current micrometer-sized Superconducting QUantum Interference Device) is capable to measure magnetization of spin system with high sensitivity. For example, the micro-SQUID technique can measure magnetic moments as small as several thousand μB as shown by the study of [W. Wernsdorfer, Supercond. Sci. Technol. 22, 064013 (2009)]. Here we develop a novel on-chip setup that combines the micro-SQUID sensitivity with microwave excitation. Such setup can be used for electron spin resonance measurements or coherent control of spins utilizing the high sensitivity of micro-SQUID for signal detection. To build the setup, we studied the fabrication process of the micro-SQUID, which is made of weak-linked Josephson junctions. The SQUID as a detector is integrated on the same chip with a shorted coplanar waveguide, so that the microwave pulses can be applied through the waveguide to excite the sample for resonance measurements. The whole device is plasma etched from a thin (∼20nm) niobium film, so that the SQUID can work at in large in-plane magnetic fields of several tesla. In addition, computer simulations are done to find the best design of the waveguide such that the microwave excitation field is sufficiently strong and uniformly applied to the sample. The magnetization curve of Mn₁₂ molecule-based magnet sample is measured to prove the proper working of the micro-SQUID. Electron spin resonance measurement is done on the setup for gadolinium ions diluted in a CaWO₄ single crystal. The measurement shows clear evidence of the resonance signal from the 1st transition of the gadolinium ions' energy levels, which shows the setup is successfully built. Due to the high sensitivity of micro-SQUID and the ability to concentrate microwave energy in small areas of the chip, this setup can detect signals from a small number of spins (10⁷) in a small volume (several μm³). / A Dissertation submitted to the Department of Physics in partial fulfillment of the requirements for the degree of Doctor of Philosophy. / Summer Semester 2017. / June 28, 2017. / Resonance, SQUID, waveguide / Includes bibliographical references. / Irinel Chiorescu, Professor Directing Dissertation; Naresh Dalal, University Representative; Laura Reina, Committee Member; Pedro Schlottmann, Committee Member; Peng Xiong, Committee Member.

Source: Dissertation Abstracts International, Volume: 41-01, Section: B, page: 0250. / Thesis (Ph.D.)--The Florida State University, 1980.

A new approach to the high-temperature series expansion which is applicable to systems with complicated energy level schemes such as magnetic systems with crystal-field anisotropy of arbitrary strength has been formulated. We compare this approach with the original Green's function formulation of Wand and Lee and discuss the advantage of the present technique. We apply the approach developed here to the spin-one hard-axis Heisenberg ferromagnet and obtain the first four terms in the high-temperature series expansion for the free energy, the magnetic susceptibility, and the specific heat. The formula obtained for the free energy of the spin-one hard-axis ferromagnet also describes spin-one systems with rhombic anisotropy and reduces to the spin-one ferromagnet with an easy-axis anisotropy and to the spin-one simple Heisenberg systems by setting the appropriate matrix elements equal to zero. The method can be extended to treat systems with spin greater than one with general crystal field symmetry. The calculation of higher order terms is rendered tractable using the approach developed here. / Source: Dissertation Abstracts International, Volume: 41-02, Section: B, page: 0612. / Thesis (Ph.D.)--The Florida State University, 1980.