Física - PPGF
https://repositorio.ufscar.br/handle/ufscar/8207
2024-03-28T12:58:44ZTeoria de controle ótimo aplicada a sistemas quânticos abertos
https://repositorio.ufscar.br/handle/ufscar/19271
Teoria de controle ótimo aplicada a sistemas quânticos abertos
Fernandes, Marllos Eustáquio Fonseca
This thesis, grounded in the field of quantum computing, explores diverse and crucial topics for the advancement of this area. Firstly, we introduce an innovative "double-bang" solution using optimal control theory for state preparation in the context of quantum teleportation. Combined with the adiabatic door teleportation protocol, this approach provides an experimentally viable way for the implementation of universal quantum computing. Subsequently, we shift our focus to adiabatic quantum computing, revealing the robustness of the adiabatic entanglement protocol to noise and proposing enhancements to the teleportation protocol through the inclusion of local control fields. Collectively, these studies contribute to the progress of quantum computing, offering practical insights for future implementations in Noisy Intermediate-Scale Quantum (NISQ) devices. Finally, we apply the Krotov method to find optimized controls in open quantum systems, highlighting the effectiveness of non-unitary optimization in state preparation, although unitary optimization often proves equivalent in the implementation of quantum gates.
2023-11-10T00:00:00ZPropriedades magnéticas e magnetoelétricas de compósitos multiferroicos de Pb[Zr0,53Ti0,47]O3/NixCo1-xFe2O4
https://repositorio.ufscar.br/handle/ufscar/19247
Propriedades magnéticas e magnetoelétricas de compósitos multiferroicos de Pb[Zr0,53Ti0,47]O3/NixCo1-xFe2O4
Dalla Costa, Leonardo José
New technologies are arising from the research and development of magnetism, especially those related to processing, information storage and sensors, are becoming increasingly important. in this regard, magnetoelectric multiferroic materials have been extensively tested for technological applications. Composite multiferroic materials are a combination of two materials with different ferroic orders, such as ferroelectricity and ferrimagnetism. In magnetoelectric compounds, magnetoelectricity is observed due to the mechanics between a piezoelectric material (ferroelectric phase), with a magnetostrictive material, such as ferrites. Ferrites can be made of different elements, like cobalt (CFO) and nickel (NFO), these two examples present opposite behavior in their coercive fields and decreasing values of remanent magnetization and saturation magnetization. Which results in different magnetostrictive responses. In this Ph.D. thesis we propose to study compounds of Pb[ZrₓTi₁₋ₓ]O₃ (PZT) with NixCo1-xFe2O4 ferrites (N/CFO) with 0≤x≤1, in the molar proportion of 80% PZT and 20% N /CFO with the aim of varying the magnetoelectric behavior of these compounds by varying the influence of modified Nickel or cobalt. The results obtained demonstrated that when synthesizing N/CFO ferrites it was possible to vary the effective anisotropy and, as a consequence, the magnetostrictive behavior of these ferrites. It should also be noted that the saturation magnetization of the compounds depends on the electrical polarization, being reduced by up to 15%, but without changing the coercivity, a drop that occurs due to the presence of a high electric field inside the composite can lead to a change in the valence of the ions of Faith. Magnetoelectric measurements as a function of the magnetic field frequency showed a change in the resonance frequency as a function of the Ni concentration of the ferrite. The results obtained indicate that the replacement of Co2+ by Ni2+ in ferrites structure have significantly alters the magnetoelectric properties of the investigated compounds.
2023-10-27T00:00:00ZModelos de branas em (5+1) dimensões e o espectro dos leptóns carregados
https://repositorio.ufscar.br/handle/ufscar/19038
Modelos de branas em (5+1) dimensões e o espectro dos leptóns carregados
Gauy, Henrique
This manuscript is concerned with a mechanism for explaining the mass spectrum of
the charged leptons within (5 + 1)-dimensional braneworlds. As a preliminary objective,
new two co-dimensional thick braneworlds ought to be proposed and investigated. By
considering a twofold-warp factor constructed from two intersecting warp factors and
scalar fields that generates the extra dimension defect, an alternative bulk configuration
is examined. With the brane localization thus driven by two crossing scalar fields, the
possible solvable models obtained from such a two co-dimensional setup are systematically
discussed. The obtained solutions are classified into two subsets, string and intersecting-like.
The intersecting-like models are sorted in six different models organized into two subsets
for which some of their physical properties are evaluated. For models I and II , in the
first subset, Einstein equation solutions are rigidly defined, up to some arbitrary constant.
For models III , IV , V and V I , in the second subset, an additional degree of freedom not
constrained by Einstein equations is admitted. For all intersecting-like models, gravity and
standard matter fields are shown to be localized in the vicinity of the brane. Finally, by
studying the zero modes of leptons, which are localized by the inclusion of a non-trivial
gauge field, over asymmetric versions of the classified braneworlds, one is able to model
the masses of the electron, muon and tauon based on two parameters: the gauge field
strength and the ratio between the sizes of the co-dimensions.
2023-10-27T00:00:00ZMagnetic fields as a tool to control superconducting devices
https://repositorio.ufscar.br/handle/ufscar/18978
Magnetic fields as a tool to control superconducting devices
Chaves, Davi Araujo Dalbuquerque
The quantum nature and dissipation-free flow of electric currents in superconducting materials have taken a central role as technological developments push forward the necessity of new methods for processing and dealing with the amount of information generated by modern consumption patterns. To enable such superconducting technologies, it is necessary to understand the underlying Physics dictating the behavior of superconductors, unveiling or allowing to control phenomena useful for applications. Due to its intrinsic relationship with the superconducting state, applied magnetic fields emerge as natural candidates for manipulating the properties of superconducting materials.
These ideas motivated the research conducted in the context of this thesis, which is structured as a collection of studies in which different superconducting systems are subjected to magnetic fields, aiming to investigate and control their behavior. In the first set of results presented, the temperature of a plain Nb thin film is reduced under different magnetic field cooling conditions. Then, flux penetration patterns are studied by magneto-optical imaging (MOI). The results demonstrate how the applied field spatial distribution and direction influence the ability of a superconducting device to transport electrical currents without dissipation, without the need for any complex nanofabrication steps. Such influence is due to the emergence of different trapped flux configurations in the superconductor, either facilitating or hampering further magnetic flux penetration, effectively reducing or increasing the effective maximum current the film can carry in the superconducting state.
It is true, however, that for most applications in superconducting technology, nanofabrication is required, in some cases demanding the creation of regions of suppressed superconductivity called weak-links. A different study investigates normal and superconducting state properties of Nb films patterned with a single weak-link fabricated by focused ion beam (FIB) milling. The investigation quantifies the suppression of superconducting properties and the modification of the normal flow of electrons, finding that these are linked by the degree of impurities introduced by the nanofabrication. One interesting effect is the emergence of a local peak in the magnetic field-dependent magnetization of the patterned samples. In a separate work, we employ MOI to investigate these specimens under applied fields near such local peak. This study reveals that the patterned films undergo a behavior transformation from a weak-link to a strong-link, enabling more current to flow between the unpatterned Nb regions. Quantifying the MOI data allows us to understand the flux dynamics responsible for the peak effect.
In a different study, the properties of a dc superconducting quantum interference device (SQUID) presenting two parallel weak-links comprised of asymmetric constrictions of a superconducting amorphous MoGe film were investigated. It is possible to influence the behavior of such devices by modifying their geometry. The study demonstrates how understanding the relationship between the device and applied magnetic fields and currents allows preparing the SQUID in multiple energy states, readable at the same field value, thus allowing its use as a multilevel memory element. Finally, we aim to consolidate MOI as a reliable tool to quantitatively study the behavior of superconducting films under ac magnetic fields. For that, we investigate the independence of the thermomagnetic history on the ac magnetic susceptibility response of an amorphous MoSi film. This study relies on the possibility of emulating ac effects by cycling an applied dc field. The results are successfully compared with standard SQUID-based magnetometry while taking advantage of the local spatial resolution of MOI to reveal the quantitative behavior of individual flux avalanche events and the presence of zones of flux annihilation at interfaces between positive and negative flux regions.
2023-10-16T00:00:00Z