In this work, we study the phase diagram of Fe1−qAlq alloys via site diluted spin-2 ferromagnetic Ising model by employing effective field theory. In contrast with previous studies, here, we incorporate in such model the high-spin S = 2 Fe2+ ion. We consider the same kind of the exchange interaction that of the iron-nickel alloys and we obtain an excellent theoretical description of the experimental data of the T–q phase diagram over the entire range of Al concentration q. In particular, the present calculation provides an excellent agreement with the experimental results for both the anomalous region of Al concentration q (i.e., q<0.2) and for the value of q at which the critical temperature Tc(q) vanishes (i.e., q=qc).
In this work, we proposed a variant of the SIR model, taking as based on models used to describe the epidemic outbreak in South Korea and Portugal, to study the COVID-19epidemic curve in Brazil. The model presented here describes with reasonable agreement the number of COVID-19 cases registered in Brazil between February 26 and May 18, 2020 based on the hypothesis that there a large number no notified cases (11 to 1) and variation in contagion rate according to social isolation measures and greater or lesser exposure to the virus (highest rate in beginning from epidemic). To this end, we introduced an exposure factor, called β 1 /β 2 , which allows us to describe the influence of factors such as social isolation on dispersal from disease. The results also corroborate a phenomenon observed in countries that registered a high growth in cases in short period of time, to example of Italy, Spain and USA: if isolation measures are imposed late, the total number of cases explodes when the epidemic is approaching from peak, which implies a higher exposure rate in the first days of case registration.From the data collected, we made the linear adjustment of the infected curve according to the time between the 41st and 74th days since the official notification of the first case and we obtained a high infection rate in the period, close to 4.0. The result indicates that the relaxation of social exclusion measures contributed to the high increase in cases in the period. This result reinforces the adoption of the model that differentiates unexposed susceptible from those that are most exposed to contagion. The model also predicts that the peak epidemic outbreak in Brazil, based on the number of cases, will occur around in late May and early June.
We study the phase diagram in the H-T plane of the potassium jarosite compound KFe(3)(OH)(6)(SO(4))(2) for the antiferromagnetic XY model with Dzyaloshinskii-Moriya (DM) interaction using the mean-field theory for different value of DM. In our approach, we obtain the tricritical point in the H-T plane and the adjustment has a strong correlation with experimental data.
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