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

Comparação entre tomografia computadorizada e ressonância magnética nos esclarecimento etiológico de epilepsias parciais.

Trabalho de Conclusão de Curso - Universidade Federal de Santa Catarina, Centro de Ciências da Saúde, Departamento de Clínica Médica, Curso de Medicina, Florianópolis, 200

Absence of Chaos in Bohmian Dynamics

The Bohm motion for a particle moving on the line in a quantum state that is a superposition of n+1 energy eigenstates is quasiperiodic with n frequencies.Comment: 1 pag

Multicritical Points And Reentrant Phenomenon In The BEG Model

The Blume - Emery - Griffiths model is investigated by use of the cluster variation method in the pair approximation. We determine the regions of the phase space where reentrant phenomenon takes place. Two regions are found, depending on the sign of the reduced quadrupole - quadrupole coupling strength $\xi$. For negative $\xi$ we find Para-Ferro-Para and Ferro-Para-Ferro-Para transition sequences; for positive $\xi$, a Para$_-$-Ferro-Para$_+$ sequence. Order parameters, correlation functions and specific heat are given in some typical cases. By-products of this work are the equations for the critical and tricritical lines.Comment: 14 pages, figures available upon reques

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

Fluctuating asymmetry and environmental stress : understanding the role of trait history

While fluctuating asymmetry (FA; small, random deviations from perfect symmetry in bilaterally symmetrical traits) is widely regarded as a proxy for environmental and genetic stress effects, empirical associations between FA and stress are often weak or heterogeneous among traits. A conceptually important source of heterogeneity in relationships with FA is variation in the selection history of the trait(s) under study, i.e. traits that experienced a (recent) history of directional change are predicted to be developmentally less stable, potentially through the loss of canalizing modifiers. Here we applied X-ray photography on museum specimens and live captures to test to what extent the magnitude of FA and FA-stress relationships covary with directional shifts in traits related to the flight apparatus of four East-African rainforest birds that underwent recent shifts in habitat quality and landscape connectivity. Both the magnitude and direction of phenotypic change varied among species, with some traits increasing in size while others decreased or maintained their original size. In three of the four species, traits that underwent larger directional changes were less strongly buffered against random perturbations during their development, and traits that increased in size over time developed more asymmetrically than those that decreased. As we believe that spurious relationships due to biased comparisons of historic (museum specimens) and current (field captures) samples can be ruled out, these results support the largely untested hypothesis that directional shifts may increase the sensitivity of developing traits to random perturbations of environmental or genetic origin

Master Operators Govern Multifractality in Percolation

Using renormalization group methods we study multifractality in percolation at the instance of noisy random resistor networks. We introduce the concept of master operators. The multifractal moments of the current distribution (which are proportional to the noise cumulants $C_R^{(l)} (x, x^\prime)$ of the resistance between two sites x and $x^\prime$ located on the same cluster) are related to such master operators. The scaling behavior of the multifractal moments is governed exclusively by the master operators, even though a myriad of servant operators is involved in the renormalization procedure. We calculate the family of multifractal exponents ${\psi_l}$ for the scaling behavior of the noise cumulants, $C_R^{(l)} (x, x^\prime) \sim | x - x^\prime |^{\psi_l /\nu}$, where $\nu$ is the correlation length exponent for percolation, to two-loop order.Comment: 6 page