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São Cristóvão, Brazil

Passos A.L.,Federal University of Sergipe | De Albuquerque D.F.,Federal University of Sergipe | Filho J.B.S.,Instituto Federal Of Sergipe
Physica A: Statistical Mechanics and its Applications | Year: 2016

This work presents a representation of the Kagome and pyrochlore lattices using Monte Carlo simulation as well as some results of the critical properties. These lattices are composed corner sharing triangles and tetrahedrons respectively. The simulation was performed employing the Cluster Wolf Algorithm for the spin updates through the standard ferromagnetic Ising Model. The determination of the critical temperature and exponents was based on the Histogram Technique and the Finite-Size Scaling Theory. © 2016 Elsevier B.V. All rights reserved.

Freitas A.S.,Instituto Federal Of Sergipe
Solid State Communications | Year: 2016

In this paper I present a Quantum Monte Carlo (QMC) study of the magnetic properties of an antiferromagnetic spin ladder (C5H12N)2CuBr4. This compound is the prototype of the Heisenberg model for a two leg spin ladder in the presence of an external magnetic field. The susceptibility phase diagram has a rounded peak in the vicinity of T=7.4 K, obeys Troyer's law for low temperatures, and Curie's law for high temperatures. I also study the susceptibility diagram in low temperatures and I found the spin gap Δ=9.26 K, in good concordance with the experimental value, 9.5 K. In high field, I present a diagram of magnetization as a function of temperature. In the vicinity of a critical field, Hci, the magnetization scales with T1/2 and this result was found also in the QMC simulation. In all the results, there is a very good concordance with the experimental data. I also show in this paper that the spin gap is null and the susceptibility is proportional to T for low temperatures when relatively high values of the ladders' coupling is taken in account. © 2016 Elsevier Ltd.

Ribeiro A.N.,Federal University of Sergipe | Ribeiro A.N.,Instituto Federal Of Sergipe | MacEdo C.A.,Federal University of Sergipe
Physical Review A - Atomic, Molecular, and Optical Physics | Year: 2012

The Hubbard model describes interacting itinerant fermion systems and is the simplest model capable of describing the essential physics of strongly correlated electron systems. In this model, both spin correlations and charge correlations are important for the entanglement, thus the magnetic susceptibility is not an appropriate thermal entanglement witness. The specific heat reflects spin correlations and charge correlations. We obtain a relation that permits the specific heat to be used directly as a thermal entanglement witness. We calculate, away from half filling, the entanglement critical temperature T E, below which entanglement is detected, for small linear clusters and the hypercubic lattice in the limit of infinite dimensions by using the exact numerical diagonalization method and the dynamical mean-field theory, respectively. We find, in both cases, that T E increases with increasing strength of the on-site Coulomb repulsion U. © 2012 American Physical Society.

Dos S. Rezende M.V.,Instituto Federal Of Sergipe | Montes P.J.R.,Instituto Federal Of Sergipe Ifs | Soares F.M.D.S.,Federal University of Sergipe | Santos C.D.,Federal University of Sergipe | Valerio M.E.G.,Federal University of Sergipe
Journal of Synchrotron Radiation | Year: 2014

Xerogels of strontium chlorate and aluminium chlorate doped with europium (un-co-doped) and co-doped with rare earth ions (Ln = Gd, Dy, Er and Y) were prepared using the proteic sol-gel route. Synchrotron radiation was used to investigate the effect of different co-dopants on the Eu3+ → Eu2+ reduction process during the synthesis of the samples. Samples were excited at the Eu L III-edge and the XANES regions were analyzed. The results suggest that some of the Eu ions can be stabilized in the divalent state and that it is difficult to completely reduce Eu3+ to Eu 2+ during thermal treatment. The mechanisms of the Eu reduction processes are explained by a proposed model based on the incorporation of charge-compensation defects.© 2014 International Union of Crystallography.

Freitas A.S.,Instituto Federal Of Sergipe | De Albuquerque D.F.,Federal University of Sergipe
Solid State Communications | Year: 2016

We describe the phase diagram of binary and ternary disordered alloys using the mixed-bond Ising model, via effective field theory (EFT). For example, we describe the Fe-Al alloy as a mixed-bond system instead of as diluted alloy. In our approach, we obtain the percolation threshold for some lattices and describe the lines of ferro-paramagnetic transition of Fe-Al, Fe-Mn, Fe-Mn-Al and Fe-Ni-Mn alloys and we obtain good agreement with the experimental data. © 2015 Elsevier Ltd

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