Regular black holes in f(G)f(G) gravity

In this work, we study the possibility of generalizing solutions of regular black holes with an electric charge, constructed in general relativity, for the $f(G)$ theory, where $G$ is the Gauss-Bonnet invariant. This type of solution arises due to the coupling between gravitational theory and nonlinear electrodynamics. We construct the formalism in terms of a mass function and it results in different gravitational and electromagnetic theories for which mass function. The electric field of these solutions are always regular and the strong energy condition is violated in some region inside the event horizon. For some solutions, we get an analytical form for the $f(G)$ function. Imposing the limit of some constant going to zero in the $f(G)$ function we recovered the linear case, making the general relativity a particular case.Comment: 22 pages, 25 figures.Version published in EPJ

The quantum H3H_3 integrable system

The quantum $H_3$ integrable system is a 3D system with rational potential related to the non-crystallographic root system $H_3$. It is shown that the gauge-rotated $H_3$ Hamiltonian as well as one of the integrals, when written in terms of the invariants of the Coxeter group $H_3$, is in algebraic form: it has polynomial coefficients in front of derivatives. The Hamiltonian has infinitely-many finite-dimensional invariant subspaces in polynomials, they form the infinite flag with the characteristic vector \vec \al\ =\ (1,2,3). One among possible integrals is found (of the second order) as well as its algebraic form. A hidden algebra of the $H_3$ Hamiltonian is determined. It is an infinite-dimensional, finitely-generated algebra of differential operators possessing finite-dimensional representations characterized by a generalized Gauss decomposition property. A quasi-exactly-solvable integrable generalization of the model is obtained. A discrete integrable model on the uniform lattice in a space of $H_3$-invariants "polynomially"-isospectral to the quantum $H_3$ model is defined.Comment: 32 pages, 3 figure

Superfluid to normal phase transition in strongly correlated bosons in two and three dimensions

Using quantum Monte Carlo simulations, we investigate the finite-temperature phase diagram of hard-core bosons (XY model) in two- and three-dimensional lattices. To determine the phase boundaries, we perform a finite-size-scaling analysis of the condensate fraction and/or the superfluid stiffness. We then discuss how these phase diagrams can be measured in experiments with trapped ultracold gases, where the systems are inhomogeneous. For that, we introduce a method based on the measurement of the zero-momentum occupation, which is adequate for experiments dealing with both homogeneous and trapped systems, and compare it with previously proposed approaches.Comment: 13 pages, 11 figures. http://link.aps.org/doi/10.1103/PhysRevA.86.04362

Fronteiras de espaço e tempo em Vida e Morte de M.J. Gonzaga de Sá (1919) de Lima Barreto

Vida e Morte de M.J. Gonzaga de Sá (1919) foi o último romance publicado em vida por Lima Barreto. A ação da narrativa se passa na capital da República, no início do século XX, período das reformas urbanísticas, do apagamento das marcas do passado colonial brasileiro e do surgimento da cidade letrada. O presente trabalho pretende discutir a visão histórica de Lima Barreto nesse romance que destoa do corro progressista do início do século. Nesse sentido, o presente e o passado são fundidos no olhar lírico do protagonista Gonzaga de Sá e na escrita crítica do personagem-narrador Augusto Machado.Organização, execução e patrocínio: UNILA e Itaipu-Paraguay - Parceria: NELOOL/UFSC & Universidad de VIG

Ferromagnetism of the Hubbard Model at Strong Coupling in the Hartree-Fock Approximation

As a contribution to the study of Hartree-Fock theory we prove rigorously that the Hartree-Fock approximation to the ground state of the d-dimensional Hubbard model leads to saturated ferromagnetism when the particle density (more precisely, the chemical potential mu) is small and the coupling constant U is large, but finite. This ferromagnetism contradicts the known fact that there is no magnetization at low density, for any U, and thus shows that HF theory is wrong in this case. As in the usual Hartree-Fock theory we restrict attention to Slater determinants that are eigenvectors of the z-component of the total spin, {S}_z = sum_x n_{x,\uparrow} - n_{x,\downarrow}, and we find that the choice 2{S}_z = N = particle number gives the lowest energy at fixed 0 < mu < 4d.Comment: v2: Published version. 30 pages latex. Changes in title, abstract, introductio