Quantum fidelity approach to the ground state properties of the 1D ANNNI model in a transverse field

In this work we analyze the ground-state properties of the $s=1/2$ one-dimensional ANNNI model in a transverse field using the quantum fidelity approach. We numerically determined the fidelity susceptibility as a function of the transverse field $B_x$ and the strength of the next-nearest-neighbor interaction $J_2$, for systems of up to 24 spins. We also examine the ground-state vector with respect to the spatial ordering of the spins. The ground-state phase diagram shows ferromagnetic, paramagnetic, floating, $\Braket{2,2}$ phases, and we predict an infinite number of modulated phases in the thermodynamic limit ($L \rightarrow \infty$). The transition lines separating the modulated phases seem to be of second-order, whereas the line between the floating and the $\Braket{2,2}$ phases is possibly of first-order.Comment: 10 pages, 20 figure

Griffiths phases in the strongly disordered Kondo necklace model

The effect of strong disorder on the one-dimensional Kondo necklace model is studied using a perturbative real-space renormalization group approach which becomes asymptotically exact in the low energy limit. The phase diagram of the model presents a random quantum critical point separating two phases; the {\em random singlet phase} of a quantum disordered XY chain and the random Kondo phase. We also consider an anisotropic version of the model and show that it maps on the strongly disordered transverse Ising model. The present results provide a rigorous microscopic basis for non-Fermi liquid behavior in disordered heavy fermions due to Griffiths phases.Comment: 4 pages, 4 figure

Quantum first order phase transitions

The scaling theory of critical phenomena has been successfully extended for classical first order transitions even though the correlation length does not diverge in these transitions. In this paper we apply the scaling ideas to quantum first order transitions. The usefulness of this approach is illustrated treating the problems of a superconductor coupled to a gauge field and of a biquadratic Heisenberg chain, at zero temperature. In both cases there is a latent energy associated with their discontinuous quantum transitions. We discuss the effects of disorder and give a general criterion for it's relevance in these transitions.Comment: 6 pages, 2 figures, misprints corrected and a reference added. Version published in PHYSICA

The agrarian question in São Paulo, Brazil: the debate over the end of the colonato and the appearance of the boia-fria in the agriculture

El artículo presenta el debate acerca del colonato, trabajador de la cultura del café en Oeste Paulista, proponiendo una interpretación de ello como parte del proceso de autonomización del capital y de aplicación de la colonización sistemática en Brasil, necesitando, pues, de la cesión mediada de tierras al trabajador. Posteriormente, se enfoca algunas teorizaciones clásicas sobre el proceso de limitación de la existencia del colonato en las haciendas cafeteras paulistas. Por último, se centra el análisis en la recuperación de un debate académico hecho entre Verena Stolcke y José Graziano da Silva, acerca de la modernización de la agricultura y el fin del colonato. Con ello, se procura explicitar las divergencias entre las interpretaciones del proceso y se analiza esas perspectivas como realces de aspectos propios de un objeto contradictorio.The paper presents the debate over the colonato, a regular and widespread form of worker in the West São Paulo coffee crops, grasping an interpretation of the process of capital autonomization and of the application of the systematic colonization within Brazil. Therefore, a colonization which required the mediated cession of the means of production to the worker. Afterwards, it deals with certain classic theories on the process of limitations of the colonato in the coffee farms. Then, it focus the analysis in an academic controversy between Verena Stolcke and José Graziano da Silva, upon the agriculture modernization and the end of the colonato. Thus, it aims at explain the differences of interpretation of the same process as the highlights of a contradictory object.O artigo discute o colonato, modo de trabalho da cafeicultura paulista do Oeste Paulista, propondo interpretá-lo como parte do processo de autonomização do capital e de aplicação da colonização sistemática no Brasil, necessitando, pois, da cessão mediada de terras para o trabalhador. Também aborda algumas teorizações clássicas sobre o processo de limitação da existência do colonato nas fazendas cafeeiras paulistas e, finalmente, recupera um debate acadêmico travado entre Verena Stolcke e José Graziano da Silva sobre a modernização da agricultura e o fim do colonato. Com isso, procura explicitar as divergências nas interpretações do mesmo processo e analisa essas perspectivas como realces de aspectos pró- prios de um objeto contraditório

Phase transitions in the two-dimensional super-antiferromagnetic Ising model with next-nearest-neighbor interactions

We use Monte Carlo and Transfer Matrix methods in combination with extrapolation schemes to determine the phase diagram of the 2D super-antiferromagnetic (SAF) Ising model with next-nearest-neighbor (nnn) interactions in a magnetic field. The interactions between nearest-neighbor (nn) spins are ferromagnetic along x, and antiferromagnetic along y. We find that for sufficiently low temperatures and fields, there exists a region limited by a critical line of 2nd-order transitions separating a SAF phase from a magnetically induced paramagnetic phase. We did not find any region with either first-order transition or with re-entrant behavior. The nnn couplings produce either an expansion or a contraction of the SAF phase. Expansion occurs when the interactions are antiferromagnetic, and contraction when they are ferromagnetic. There is a critical ratio R_c = 1/2 between nnn- and nn-couplings, beyond which the SAF phase no longer exists.Comment: 12 pages, 10 figure

Breakdown of the perturbative renormalization group for S >= 1 random antiferromagnetic spin chains

We investigate the application of a perturbative renormalization group (RG) method to random antiferromagnetic Heisenberg chains with arbitrary spin size. At zero temperature we observe that initial arbitrary probability distributions develop a singularity at J=0, for all values of spin S. When the RG method is extended to finite temperatures, without any additional assumptions, we find anomalous results for S >= 1. These results lead us to conclude that the perturbative scheme is not adequate to study random chains with S >= 1. Therefore a random singlet phase in its more restrictive definition is only assured for spin-1/2 chains.Comment: 5 pages, 3 figures. To appear in Physical Review

"MESMO PROIBIDO OLHAI POR NÓS": LIBERDADE DE EXPRESSÃO, CENSURA E RELIGIÃO NO CARNAVAL CARIOCA

Carnaval e religião ligam-se de forma umbilical e apresentam uma longa trajetória comum; que vai desde a origem religiosa da manifestação carnavalesca na Antiguidade até a fixação oficial da festa no calendário cristão-romano no período anterior à Quaresma. Contudo, apesar da manifestação comum entre a esfera religiosa e a esfera cultural/artística da festa, em muitos aspectos esse imbricamento não impede que surjam tensões entre Religião e Carnaval que, não raro, buscam solução no Direito. Nesse sentido, o objetivo desse ensaio é analisar o aspecto jurídico da relação Religião e o desfile de Carnaval na Marques de Sapucaí no Rio de Janeiro. A partir do estudo do caso jurídico entre a Arquidiocese do Rio de Janeiro e a Escola de Samba Beija-flor de Nilópolis, provocado pelo enredo “Ratos e Urubus: larguem minha fantasia” de 1989 da Escola de Samba Beija-Flor de Nilópolis, buscar-se-á compreender os aspectos sociojurídicos evocados na disputa jurídica, o princípio da liberdade de expressão, os símbolos religiosos e a censura