DSpace Coleção:
http://www.repositorio.ufc.br/handle/riufc/218
2019-10-23T20:20:45ZProperties of quasi particles on two dimensional materials and related structures
http://www.repositorio.ufc.br/handle/riufc/47010
Título: Properties of quasi particles on two dimensional materials and related structures
Autor(es): Castro, Luan Vieira de Castro
Abstract: In this thesis, we use analytical techniques to study properties of two dimensional systems for which the carriers can be described as quasi particles. In the first part, we consider confinement of quasi electrons on AA-stacked bilayer graphene quantum dots. In order to describe this confinment, an appropriate boundary condition is derived. Then, we use this condition for the calculation of the energy levels of the quantum dot with and without an external applied perpendicular magnetic field. Finally, we compare our analytical result with numerical tight-binding calculations. Our results show that due to the nature of the boundary conditions there is intrisic symmetry in the system related with the Dirac K and K 0 states. Also, we see that the dependence of the energy levels on the radius of the system is inversely proportinal to the radius, which is a direct consequence of the Dirac nature of electrons in AA-stacked bilayers. Motivated by recent experimental observations, in the second part we investigate properties of acoustic plasmons. Plasmons are quasi particles describing a collective modes of electrons in the presence of an external electric field. Here, we consider a 2D electron liquid for which plasmon modes can be achieved in the presence of a top gate. Using a semiclassical approach we calculate the plasmon dispersion with many-body corrections beyond Random Phase Approximation. Another important result is the calculation of the damping of acoustic plasmon modes identifying the crossover between the collisionless and hydrodynamic regime. Finally, we calculate the coupling to a near-field probe. This allow the identification of how efficiently observe these modes experimentaly. In the last part, we extend the results obtained in the second part by adding an external perpendicular magnetic field to the structure. The presence of the field generates hybridization of the plasmon modes, known as Bernstein modes. Finally, we show thate electron-electron interactions have fundamental role in the observation of these hybrid modes.2019-01-01T00:00:00ZEstudo de transições de fase por Espectroscopia Raman em monocristais de L-asparagina monohidratada
http://www.repositorio.ufc.br/handle/riufc/44168
Título: Estudo de transições de fase por Espectroscopia Raman em monocristais de L-asparagina monohidratada
Autor(es): Moreno, Antônio Jeferson de Deus
Abstract: Polarized Raman spectra of cristais of 1-asparagine monohidrated over the range 3500-40 cm-1 were recorded since 10K untill the room temperature. The data obtained have been used to propose a general assignment of the vibrational fundamentals at room temperature on the base of temperature dependence of some modes and correlations with frequencies assigned in other aminoacids, principally the anidrous 1-asparagine which look like our compound. The coupling of the four zwitterions in the unit cell lead to 237 optical modes [60A + 59B1 + 59B2 + 59B3]. These data confirm that 1-asparagine monohydrated crystallize in an orthorhombic form with space group symmetry given by P212121 (D2). A great number of solid substances can be modeled as a dynamical system consisting of harmonic oscillators. However, some materials, as l-asparagine monohydrated and other hydrogen-bounded crystals, behave as anharmonic system. This leads to an important mechanism of broadening and shifting of the Raman lines. Temperature dependence polarized Raman studies show that although there are difference among Raman spectra below and above 160K, the selection rules are consistent with orthorhombic crystal structure in the temperature range 10-300K. There are changes in the numbers of external modes of B2 representation and in the internal modes of A representation. We also observed little changes in the slope of the temperature dependence of the Raman parameters. It was shown that the 1-asparagine monohidrated crystal axhibits various phase transitons with hidrostatic pressure. The first pressure-induced phase transition occur below 0.2 GPa and is accompanied by abrupt changes in the external and internal modes. The change of external modes of this region look like the observed changes at 160K in the temperature-dependence Raman spectra. The second phase transition occurs between 0,6 and 0,9 GPa and the third pressure induced phase transition is observed between 0,9 and 1,3 GPa. From the Raman data we observe the appearance of three hidrostatic pressureinduced phase transitions. Wowever, we expect tha further phase transitions can be observed for pressures above 2 GPa.1996-01-01T00:00:00ZPropriedades eletrônicas e de transporte em nanoestruturas de metais de transição dicalcogenados
http://www.repositorio.ufc.br/handle/riufc/43592
Título: Propriedades eletrônicas e de transporte em nanoestruturas de metais de transição dicalcogenados
Autor(es): Costa, Ana Luiza Mariano Torres
Abstract: In this thesis we study the electronic and transport properties of heterojunctions formed by di erent layers of distinct types of TMDCs, which are three-dimensional materials composed of triatomic layers of type MX2, where M represents an atom of the transition metal family and X represents a atom of the calcogens family. We use the Density Functional Theory (DFT) functional theory for the study of the electronic properties of the systems considering MoS2, MoSe2, WS2 and WSe2 whose band structure is semiconductor and NbS2, NbSe2, CoS2 and CoSe2 which are metal TMDCs. Having these characteristics as a base, vertical heterojunctions formed by a metallic layer stacked under a semiconductive layer were hypothesized. This double con guration is what we know as bilayer. Heterojunctions formed by metallic layers were also studied. In the model used in this thesis for electronic structure was considered the contribution of spin to the structure
of bands. The results show that spin has considerable relevance pointing to this type of structure as a potential candidate for applications in the emerging branch of spintronics in the future. In addition, from the original bilayers were constructed models for the study of the electronic transport in the transverse direction to the plane of the layers. For this, the formalism of Landauer-B uttiker was used for ballistic transport. The results for the quantum conductance showed that it is strongly dependent on the spin components, also showing a diode behavior, not only in the metal-semiconductor junction but also in the metal-metal junction. Finally, it has also been investigated how the electronic properties of a MoS2 trilayer are modi ed when it is subjected to an axial stress.2019-01-01T00:00:00ZUma abordagem generalizada para a definição e caracterização de bacias hidromorfológicas
http://www.repositorio.ufc.br/handle/riufc/41730
Título: Uma abordagem generalizada para a definição e caracterização de bacias hidromorfológicas
Autor(es): Oliveira, Rubens Soares de
Abstract: Drainage basins play a fundamental role in Hydrology and Geomorphology. However, the lack of a proper definition for these regions still represents an open problem. Here, we propose a new hierarchical algorithm to define all sub-basins of a given (basin) landscape. We introduce a model to delineate multiple drainage
basins through an extension of the Invasion Percolation-Based Algorithm (IPBA). In order to prove the potential of our approach, we apply it to real and artificial datasets. We observe that the perimeter and area distributions of basins and antibasins display long tails extending over several orders of magnitude and following approximately power-law behaviors. Moreover, the exponents of these power laws depend on spatial correlations and are invariant under the landscape orientation, not only for terrestrial, but lunar and martian landscapes. The terrestrial and martian results are statistically identical, which suggests that a hypothetical martian river would present similarity to the terrestrial rivers. Finally, we propose a theoretical value for the Hack's exponent based on the fractal dimension of watersheds, γ = D/2. We measure γ = 0.52 ± 0.01 for Earth, which is close to our estimation of γ ≈ 0.55. Our study suggests that Hack's law can have its origin purely in the maximum and minimum lines of the landscapes. We too apply our method to the Amazon
basin and find that the obtained sub-basins are consistent with those reported in the literature. Finally, we also observe that the perimeter and area distributions of subbasins exhibit long tails following approximately power-law behaviors and that their exponents decrease with the hierarchy of the sub-basins.2019-01-01T00:00:00Z