Thesis (Ph. D.)--Florida State University, 2003. / Advisor: Dr. Peng Xiong, Florida State University, College of Arts and Sciences, Dept. of Physics. Title and description from dissertation home page (viewed Apr. 9, 2004). Includes bibliographical references.

NMR is a powerful analytical spectroscopic tool used to perform detailed studies of structure and dynamics of molecules in solution. However, despite NMR's excellent spectral sensitivity, most NMR methods suffer from low detection sensitivity. This low detection sensitivity results largely from extremely small (Boltzmann) nuclear spin polarization at thermal equilibrium--in even the strongest of magnets. This dissertation focuses on selected research areas that maybe used to combat the limitations presented by NMR and measure weak spectral responses with atomic-scale precision. In particular, these methods involve the use of laser-polarized xenon, liquid crystals, and polarization transfer (cross-polarization) techniques to enhance NMR sensitivity and/or measure weak interactions. The potential use of these tools to study host-guest interactions is of particular interest. In certain systems the sensitivity problem of conventional NMR/MRI can be overcome by applying optical pumping (OP) methods to enhance nuclear spin polarization. For instance, OP of noble gases (such as xenon) is employed to dramatically increase their nuclear spin polarization by transferring angular momentum of laser light to electronic and then nuclear spins. Next, cryptophane complexes are ideal choices for fundamental studies of prototypical host-guest interactions. Of general interest when studying host-guest interactions is how (1) physical confinement at the nanoscale and (2) interactions between guest and host may affect the properties, dynamics, interactions, and/or reactivity of a trapped molecule and the host/guest complex as a whole. As a more specific example, we are interested in probing host-guest dynamic coupling, which refers to the relative motion of the guest within the host, determined by the relative sizes and geometries--as well as the interactions involved. With the development of new NMR methods and techniques, we hope to gain insight into mechanisms that underlie complex formation by probing the structures, dynamics and energetic contributions involved in ligand binding, where molecular contributions such as: orientational and motional freedom of the guest; and structure, dynamics, and ordering of the host can influence the behavior of inclusion complexes.

abstract: A theoretical study of a three-dimensional (3D) N/S interface with arbitrary spin polarization and interface geometry is presented. The 3D model gives the same intrinsic spin polarization and superconducting gap dependence as the 1D model. This demonstrates that the 1D model can be use to t 3D data. Using this model, a Heusler alloy is investigated. Andreev reflection measurements show that the spin polarization is 80% in samples sputtered on unheated MgO(100) substrates and annealed at high temperatures. However, the spin polarization is considerably smaller in samples deposited on heated substrates. Ferromagnetic FexSi􀀀x alloys have been proposed as potential spin injectors into silicon with a substantial spin polarization. Andreev Reflection Spectroscopy (ARS) is utilized to determine the spin polarization of both amorphous and crystalline Fe65Si35 alloys. The amorphous phase has a significantly higher spin polarization than that of the crystalline phase. In this thesis, (1111) Fe SmO0:82F0:18FeAs and Pb superconductors are used to measure the spin polarization of a highly spin-polarized material, La0:67Sr0:33MnO3. Both materials yield the same intrinsic spin polarization, therefore, Fe-superconductors can be used in ARS. Based on the behavior of the differential conductance for highly spin polarized LSMO and small polarization of Au, it can be concluded that the Fe-Sc is not a triplet superconductor. Zero bias anomaly (ZBA), in point contact Andreev reflection (PCAR), has been utilized as a characteristic feature to reveal many novel physics. Complexities at a normal metal/superconducting interface often cause nonessential ZBA-like features, which may be mistaken as ZBA. In this work, it is shown that an extrinsic ZBA, which is due to the contact resistance, cannot be suppressed by a highly spin-polarized current while a nonessential ZBA cannot be affected the contact resistance. Finally, Cu/Cu multilayer GMR structures were fabricated and the GMR% measured at 300 K and 4.5 K gave responses of 63% and 115% respectively. Not only do the GMR structures have a large enhancement of resistance, but by applying an external magnetic eld it is shown that, unlike most materials, the spin polarization can be tuned to values of 0.386 to 0.415 from H = 0 kOe to H = 15 kOe. / Dissertation/Thesis / Doctoral Dissertation Physics 2015

Made available in DSpace on 2015-05-14T12:14:08Z (GMT). No. of bitstreams: 1 parte1.pdf: 2147467 bytes, checksum: 06d61abcd760c3d3a8cc6b1d61af88fa (MD5) Previous issue date: 2008-08-21 / Coordenação de Aperfeiçoamento de Pessoal de Nível Superior - CAPES / In this work, we performed an analysis of the structural and electronic stability of nanostructures of Boron Nitride (BN), such as layers, tubes and cones, when doped with Carbon, through first-principles calculations as implemented in code SIESTA. We found that substitutional doping of Carbon for either a single Boron or a single Nitrogen atom produces significant changes in the conductive properties of each material. Such replacement process transforms the formerly insulating material, in a n-type conductor, donnor of electrons, for Boron replaced by Carbon, and the p-type, acceptor of electrons, for the Nitrogen substitution. Furthermore, we also performed calculations with spin-polarization and found occurrence of spontaneous magnetization of 1μB for all doped structures, except for tube (6,0), which presented a magnetic moment of 0;2μB. This magnetization is attributed to the unpaired electron located in the pz orbital of carbon. It was also noted that the doped layers become more energetically stable as the number of atoms increases. To the tubes, increased stability occurs with the increase in diameter combined with the consequent increase in the number of atoms. At the cones, stability energy is reduced with the increase in the angle of disclination. These effects are the result of a combination of percentage concentration of the defect and the greater or lesser degree of hybridization. / Neste trabalho, fizemos uma análise da estabilidade estrutural e eletrônica de nanoestruturas de Nitreto de Boro (BN), tais como planos, tubos e cones, quando dopadas com carbono, através do uso de cálculos de primeiros princípios como implementado no código SIESTA. Encontramos que a dopagem substitucional de carbono por boro ou nitrogênio provoca mudanças significativas nas propriedades condutoras de cada material. Tais substituições transformam o material que antes era isolante, em condutor do tipo n, doador de elétrons, para o boro substituído pelo carbono, e do tipo p, receptor de elétrons, para a substituição do nitrogênio. Além disso, realizamos cálculos com polarização de spin e verificamos a ocorrência de um momento magnético de 1μB para todas as estruturas dopadas, com exceção do tubo (6;0) que apresentou um momento magnético de 0;2μB. Essa magnetização é atribuída ao elétron desemparelhado localizado no orbital pz do carbono. Foi verificado também que os planos dopados se tornam mais estáveis energeticamente à medida que o número de átomos aumenta. Para os tubos, o aumento da estabilidade ocorre com o aumento do diâmetro combinado com o consequente aumento do número de átomos. Já nos cones, a estabilidade energética é diminuída com o aumento do ângulo de disclinação. Esses efeitos são fruto de uma combinação entre porcentagem de concentração do defeito e do maior ou menor grau de hibridização.

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