University Federal Of Uberlgndia

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University Federal Of Uberlgndia

Brazil
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Xavier J.C.,University Federal Of Uberlgndia | Xavier J.C.,University of Sao Paulo | Alcaraz F.C.,University of Sao Paulo
Physical Review B - Condensed Matter and Materials Physics | Year: 2011

Using the density matrix renormalization group, we investigate the Rényi entropy of the anisotropic spin-s Heisenberg chains in a z-magnetic field. We considered the half-odd-integer spin-s chains, with s=1/2, 3/2, and 5/2, and periodic and open boundary conditions. In the case of the spin-1/2 chain we were able to obtain accurate estimates of the new parity exponents pα(p) and pα(o) that gives the power-law decay of the oscillations of the α-Rényi entropy for periodic and open boundary conditions, respectively. We confirm the relations of these exponents with the Luttinger parameter K, as proposed by Calabrese. Moreover, the predicted periodicity of the oscillating term was also observed for some nonzero values of the magnetization m. We show that for s 1/2 the amplitudes of the oscillations are quite small and get accurate estimates of pα(p) and pα(o) become a challenge. Although our estimates of the new universal exponents pα(p) and pα(o) for the spin-3/2 chain are not so accurate, they are consistent with the theoretical predictions. © 2011 American Physical Society.


Xavier J.C.,University Federal Of Uberlgndia | Xavier J.C.,University of Sao Paulo | Alcaraz F.C.,University of Sao Paulo
Physical Review B - Condensed Matter and Materials Physics | Year: 2011

Finite-size scaling analysis turns out to be a powerful tool to calculate the phase diagram as well as the critical properties of two-dimensional classical statistical mechanics models and quantum Hamiltonians in one dimension. The most used method to locate quantum critical points is the so-called crossing method, where the estimates are obtained by comparing the mass gaps of two distinct lattice sizes. The success of this method is due to its simplicity and the ability to provide accurate results even considering relatively small lattice sizes. In this paper, we introduce an estimator that locates quantum critical points by exploring the known distinct behavior of the entanglement entropy in critical and noncritical systems. As a benchmark test, we use this new estimator to locate the critical point of the quantum Ising chain and the critical line of the spin-1 Blume-Capel quantum chain. The tricritical point of this last model is also obtained. Comparison with the standard crossing method is also presented. The method we propose is simple to implement in practice, particularly in density matrix renormalization group calculations, and provides us, like the crossing method, amazingly accurate results for quite small lattice sizes. Our applications show that the proposed method has several advantages, as compared with the standard crossing method, and we believe it will become popular in future numerical studies. © 2011 American Physical Society.


Xavier J.C.,University Federal Of Uberlgndia | Xavier J.C.,University of Sao Paulo | Alcaraz F.C.,University of Sao Paulo
Physical Review B - Condensed Matter and Materials Physics | Year: 2012

Using the density matrix renormalization group, we calculated the finite-size corrections of the entanglement α-Rényi entropy of a single interval for several critical quantum chains. We considered models with U(1) symmetry such as the spin-1/2 XXZ and spin-1 Fateev-Zamolodchikov models, as well as models with discrete symmetries such as the Ising, the Blume-Capel, and the three-state Potts models. These corrections contain physically relevant information. Their amplitudes, which depend on the value of α, are related to the dimensions of operators in the conformal field theory governing the long-distance correlations of the critical quantum chains. The obtained results together with earlier exact and numerical ones allow us to formulate some general conjectures about the operator responsible for the leading finite-size correction of the α-Rényi entropies. We conjecture that the exponent of the leading finite-size correction of the α-Rényi entropies is p α=2X ε/α for α>1 and p 1=ν, where X ε denotes the dimensions of the energy operator of the model and ν=2 for all the models. © 2012 American Physical Society.


Vernek E.,University Federal Of Uberlgndia | Orellana P.A.,Católica del Norte University | Ulloa S.E.,Ohio University
Physical Review B - Condensed Matter and Materials Physics | Year: 2010

The interplay between the coupling of an interacting quantum dot to a conduction band and its connection to localized levels has been studied in a triple quantum-dot arrangement. The electronic Dicke effect, resulting from quasiresonant states of two side-coupled noninteracting quantum dots, is found to produce important effects on the Kondo resonance of the interacting dot. We study in detail the Kondo regime of the system by applying a numerical renormalization-group analysis to a finite- U multi-impurity Anderson Hamiltonian model. We find an extreme narrowing of the Kondo resonance, as the single-particle levels of the side dots are tuned toward the Fermi level and "squeeze" the Kondo resonance, accompanied by a strong drop in the Kondo temperature, due to the presence of a supertunneling state. Further, we show that the Kondo temperature vanishes in the limit of the Dicke effect of the structure. By analyzing the magnetic moment and entropy of the three-dot cluster versus temperature, we identify a different local singlet that competes with the Kondo state, resulting in the eventual suppression of the Kondo temperature and strongly affecting the spin correlations of the structure. We further show that system asymmetries in couplings, level structure or due to Coulomb interactions, result in interesting changes in the spectral function near the Fermi level. These strongly affect the Kondo temperature and the linear conductance of the system. © 2010 The American Physical Society.


Borges H.S.,University Federal Of Uberlgndia | Sanz L.,University Federal Of Uberlgndia | Villas-Boas J.M.,University Federal Of Uberlgndia | Diniz Neto O.O.,University Federal Of Uberlgndia | Alcalde A.M.,University Federal Of Uberlgndia
Physical Review B - Condensed Matter and Materials Physics | Year: 2012

Electromagnetic induced transparency is an optical phenomenon that allows transmission of a laser beam through a dense medium by using a control laser beam. Here, we propose the use of a quantum molecule where the control laser beam is replaced by the electron tunneling between quantum dots, which can be controlled by an external electric field, opening the possibility to induce transparency and slow light with electric gates. Our results show that a transparency window appears if the tunneling strength T e and the decay rate of direct exciton Γ 1 obey the condition T e/Γ 1≤0.5. © 2012 American Physical Society.


Brito W.H.,University Federal Of Uberlgndia | Kagimura R.,University Federal Of Uberlgndia | Miwa R.H.,University Federal Of Uberlgndia
Physical Review B - Condensed Matter and Materials Physics | Year: 2012

The energetics and electronic properties of substitutional B (B C) and N (N C) doping, and BN codoping in graphene with distinct grain boundary defects were investigated by ab initio simulations. Our results indicate that a single B or N impurity atoms and an isolated BN pair prefer to incorporate into the grain boundary region. In particular, we find that the formation of N C along the grain boundary sites is an exothermic process. It suggests that hexagonal-BN (h-BN) or h-BN and carbon (h-BNC) domains may be patterned by these defective regions. The electronic properties of those doped grain boundary systems have been examined through scanning tunneling microscopy (STM) simulations and electronic band-structure calculations. We find a quite different STM picture for the B C- and N C-doped grain boundaries when compared with the same impurities on the perfect graphene sheet. © 2012 American Physical Society.


Eloy D.,University Federal Of Uberlgndia | Xavier J.C.,University Federal Of Uberlgndia
Physical Review B - Condensed Matter and Materials Physics | Year: 2012

In this work, we investigate an exactly solvable two-leg spin ladder with three-spin interactions. We obtain analytically the finite-size corrections of the low-lying energies and determine the central charge as well as the scaling dimensions. The model considered in this work is in the same universality class of critical behavior of the XX chain with central charge c=1. By using the correlation matrix method, we also study the finite-size corrections of the Rényi entropy of the ground state and of the excited states. Our results are in agreement with the predictions of the conformal field theory. © 2012 American Physical Society.


Xavier J.C.,University Federal Of Uberlgndia
Physical Review B - Condensed Matter and Materials Physics | Year: 2010

Using the density-matrix renormalization group, we investigate the critical behavior of the anisotropic Heisenberg chains with spins up to s=9/2. We show that through the relations arising from the conformal invariance and the DMRG technique it is possible to obtain accurate finite-size estimates of the conformal anomaly c, the sound velocity vs, the anomalous dimension xbulk, and the surface exponent xs of the anisotropic spin- s Heisenberg chains with relatively good accuracy without fitting parameters. Our results indicate that the entanglement entropy S (L, lA, s) of the spin- s Heisenberg chains satisfies the relation S (L, lA, s) -S (L, lA, s-1) =1/ (2s+1) for s>3/2 in the thermodynamic limit. © 2010 The American Physical Society.


Borges H.S.,University Federal Of Uberlgndia | Sanz L.,University Federal Of Uberlgndia | Villas-Boas J.M.,University Federal Of Uberlgndia | Alcalde A.M.,University Federal Of Uberlgndia
Physical Review B - Condensed Matter and Materials Physics | Year: 2010

Semiconductor quantum dots coherently driven by pulsed laser are fundamental physical systems which allow studying the dynamical properties of confined quantum states. These systems are attractive candidates for a solid-state qubit, which open the possibility for several investigations in quantum-information processing. In this work we study the effects of a specific decoherence process, the spontaneous emission of excitonic states, in a quantum dot molecule. We model our system considering a three-level Hamiltonian and solve the corresponding master equation in the Lindblad form. Our results show that the spontaneous emission associated with the direct exciton helps to build up a robust indirect exciton state. This robustness against decoherence allows potential applications in quantum memories and quantum gate architectures. We further investigate several regimes of physical parameters, showing that this process is easily controlled by tuning of external fields. © 2010 The American Physical Society.


Brito W.H.,University Federal Of Uberlgndia | Miwa R.H.,University Federal Of Uberlgndia
Physical Review B - Condensed Matter and Materials Physics | Year: 2010

We have performed a theoretical ab initio investigation, within the density-functional theory, of Au adatoms on graphene nanoribbons (Au/GNRs). We have considered armchair GNR (A-GNR) and zigzag GNR (Z-GNR). For both systems, we find an energetic preference for Au adatoms lying along the edge sites. Our calculated adsorption energies, 1.01 eV and 2.18 eV for Au adatoms on the A-GNR and Z-GNR, respectively, indicate the formation of Au-C covalent bonds. Au atoms can adsorb more easily along the zigzag edges than along the armchair edges. In addition, we examined the diffusion barriers of Au adatoms on those ribbon systems. We have considered a number of Au diffusion paths perpendicular as well as parallel to the ribbon growth directions. Our calculated energy barriers indicate that the segregation of Au adatoms from the inner sites toward the edge sites is a quite likely process. In this case, we find a net energy barrier of ∼80 meV. In contrast, the Au adatoms will face higher energy barriers for diffusion paths along the edge sites of the ribbons. The electronic-structure calculations indicate that the semiconducting character of the GNRs has been kept for low concentration of Au adatoms. Meanwhile, by increasing the concentration of Au adatoms along the edge sites, the Au/GNR systems become metallic. © 2010 The American Physical Society.

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