DSpace Coleção:
http://www.repositorio.ufc.br/handle/riufc/212
2019-03-19T16:14:50ZAcoplamento spin-fônon nos multiferróicos CuX2 (X = Br, Cl)
http://www.repositorio.ufc.br/handle/riufc/40298
Título: Acoplamento spin-fônon nos multiferróicos CuX2 (X = Br, Cl)
Autor(es): Medeiros, Fabio Eduardo de Oliveira
Abstract: With the growth of interest in muitrferroic materials in the last years, many techniques have been used in their base study in order to provide more knowledge about the operation of their ferrous characteristics, being them ferroelectricity, ferro/antiferromagnetism and ferroelasticity. In the present work the Raman scattering spectroscopy technique was applied in order to observe the behavior of the active Raman vibrational modes of the multiferroic copper chloride (CuCl2) and copper bromide (CuBr2) subjected to low temperatures and the possible existence of the spin-phonon coupling present in them. On the other hand, a study using Ramam spectroscopy and simultaneous thermal analysis of thermogravimetry (TG) and differential scanning calorimetry (DCS) was carried out by monitoring the transition of copper chloride from its dihydrated and orthorhombic phase (CuCl2⋅2H2O) for its anhydrous and monoclinic phase (CuCl2) when subjected to heating dehydration.2019-01-01T00:00:00ZÍndice de atividade magnética baseado na mecânica estatística não-extensiva de Tsallis para estrelas do tipo M
http://www.repositorio.ufc.br/handle/riufc/40063
Título: Índice de atividade magnética baseado na mecânica estatística não-extensiva de Tsallis para estrelas do tipo M
Autor(es): Rios, Luiz Daniel Alves
Abstract: Stellar activity is strongly related to magnetic fields that evolve according to their rate of rotation.
The period of the magnetic cycle and the period of rotation are correlated in such a way that
the slow rotators have larger magnetic cycles. In addition, the overall level of magnetic activity
changes over time after cycles, and local levels change according to fluctuations, ranging from
time scales from a few seconds to several hours, days, or weeks. As mentioned by Mathur
et al. (MATHUR et al., 2014), faster rotating stars show shorter activity cycles, while slower
rotating stars generally have similar or longer Sun cycle times. As the magnetic activity that
emerges from the surface of the stars is a physical mechanism produced by the stellar dynamo,
the long-term variations due to the rotational period play an important role in understanding the
level of activity on the star surface. The magnetic activity is measured by the behavior of the
photon flux obtained by the time series. For this work, we use the time series of stars observed by
the Kepler mission, in a total of 34 stars of the spectral type M. We believe that the fluctuations of
this flux at different scales do not obey the Boltzmann-Gibbs canonical distribution and therefore
the appearance of heavy tails in the distributions can give us an idea about the physical source
that operates the stellar magnetic activity at different time scales . To this end, we will investigate
the behavior of these distributions in light of the non-extensive statistical mechanics of C. Tsallis.
More specifically, we use the entropic index q of Tsallis as a measure of the tail extension of
the distribution which, in turn, is correlated to the level of stellar magnetic activity. We also
defined a new magnetic index hSqi related to generalized standard deviation sq and based on
non-extensive statistical mechanics. Thus we can find an index similar to that obtained by Mathur
but with a more appropriate physical explanation, that is, taking into account the effects of long
tail distribution. Finally, our work may open a new field of research in time series astrophysics
within the context of non-extensiveness.2019-01-01T00:00:00ZEfeitos da violação da simetria de Lorentz na polarização de ondas gravitacionais
http://www.repositorio.ufc.br/handle/riufc/40026
Título: Efeitos da violação da simetria de Lorentz na polarização de ondas gravitacionais
Autor(es): Ferreira Filho, Mapse Barroso
Abstract: This work aims to solve Einstein equation in a scenario with Lorentz symmetry violation for gravitational waves polarization. It will be commented about standard model extension, spontaneous and explicit violation of a symmetry as well as their consequences. The bumblebee model will be used for the study of Lorentz violation, where the terms that break off the symmetry are included in the lagrangian of the gravitational theory. The Euler-Lagrange equations are used to determine the modified graviton propagator, where we expand the lagrangian up to second order of the perturbed gravitational field. We see that, in this scenario, the dispersion relation of graviton is different of the usual one, where we have a term that selects a preferred direction in the spacetime. Besides that, the graviton still with two degrees of freedom, despite the existence of bumblebee field. Then, the modified wave equation for perturbation field is solved and we compare the polarization states of the gravitational wave solution modified with the usual case. We show that for a bumblebee field being timelike or in the same direction of wave momentum we have no changes in the polarization tensor. But for a bumblebee field being in another diretion we have a modified polarization tensor for the graviton. Besides that, we compute some bounds for the parameter of the bumblebee field by comparing the group velocity here computed with a group velocity of a massive graviton.2019-01-01T00:00:00ZMonocamada de fosforeno na presença de campo magnético dependente do tempo
http://www.repositorio.ufc.br/handle/riufc/40006
Título: Monocamada de fosforeno na presença de campo magnético dependente do tempo
Autor(es): Nascimento, João Pedro Gomes do
Abstract: In this work we study electrons and holes in monolayer phosphorene under a low-intensity time-dependent magnetic field 𝑩(𝑡). By considering 𝑩(𝑡)=(0,0,𝐵(𝑡)) and choosing the symmetric gauge, 𝑨(𝒓,𝑡)=−(1/2)𝒓×𝑩(𝑡), we use the Lewis and Riesenfeld method to obtain the wave functions for electrons and holes. From those solutions, we calculate the uncertainties, the Fisher information, the quantum-mechanical energy expectation value and the transition probabilities. We apply the results to the fields 𝑩(𝑡)=𝐵0𝒌 and 𝑩(𝑡)=(𝐵02+𝐵12𝑐𝑜𝑠2(𝜇𝑡))𝟏/𝟐𝒌. The uncertainties for the ground state show squeezing phenomenon depending on the intensity of the oscillatory magnetic field. In this system, we also verify that the Fisher information provides more accurate uncertainty measures than the standard deviations. In the constant magnetic field intensity case, the energy varies linearly with the quantum numbers 𝑛 and 𝑚 and 𝐵0. As expected, no transitions takes place because the states describing the particle are stationary. In the oscillatory case, we observe that the energy oscillates in time, increasing linearly with the Landau levels n and m and nonlinearly with the magnetic field. The (𝑘,𝑙)→(𝑛,𝑚) transitions take place only for 𝑙=𝑚. We also investigate the (0,0)→(𝑛,0) and (1,𝑙)→(2,𝑙) probability transitions.2019-01-01T00:00:00Z