J. E. Moreira

Brazilian elections: voting for a scaling democracy

The proportional elections held in Brazil in 1998 and 2002 display identical statistical signatures. In particular, the distribution of votes among candidates includes a power-law regime. We suggest that the rationale behind this robust scaling invariance is a multiplicative process in which the voters choice for a candidate is governed by a product of probabilities.

Phase transitions in LiKSO4

Abstract Raman spectroscopy and dielectric constant measurements were used to study the phase transitions in LiKSO 4 below room temperature. Three transitions with large hysteresis were observed. Transition temperatures were 20K, 160K and 240K when cooling and 30K, 192K and 290K when heating the sample. Some considerations are made about the crystal structure of LiKSO 4 in its low temperature phases.

The ferromagnetic Ising model on a Voronoi–Delaunay lattice

We investigate the two-dimensional ferromagnetic Ising model in the Voronoi–Delaunay tesselation. In this study, we assume that the coupling factor J varies with the distance r between the first neighbors as J(r)∝e−αr, with α⩾0. The disordered system is simulated applying the single-cluster Monte Carlo update algorithm and the reweighting technique. We calculate the critical point exponents γ/ν,β/ν and ν for this model and find that this random system belongs to the same universality class as the pure two-dimensional ferromagnetic Ising model.

Self-organized criticality in the El Niño Southern oscillation

The Fourier analysis has been applied to observational sea surface temperature (SST) data from the Southern Pacific. The spectral response of the daily temperature fluctuations indicates that the so-called El Nino Southern Oscillation (ENSO) belongs to the class of dynamical phenomena which are in a self-organized critical state. This has implications on the predictability of the significant events in the ocean-atmosphere interaction process. A toy model is used to point out the similarities of this system with other large scale phenomena, as earthquakes and volcanic eruptions, which are considered to be consistent with the hypothesis of self-organized criticality. The qualitative agreement between observational data and results of numerical simulations demonstrates the validity extent of the theoretical approach.

Raman study of the tetragonal → monoclinic phase transition in KDP

Abstract Crystals of KH2PO4 (KDP) undergo a phase transition from a tetragonal to a monoclinic phase when heated from room temperature to a temperature above 453K. This work presents a clear evidence of this phase transition through Raman scattering observations. The monoclinic phase is metastable and persists even when the material is brought back to room temperature under controlled conditions.

Pre-resonant Raman effect of CrO42− in a metasilicate glass

Abstract Pre-resonant Raman effect of chromate ion, CrO42−, was observed in a metasilicate glass with molar composition 2Na2O · 1CaO · 3SiO2 containing 1.0 wt% of Cr2O3. Raman spectra were measured by the conventional 90° scattering geometry and by the microprobe Raman spectroscopic techniques. The presence of chromate ions in the glass is favoured by the glass composition and oxidizing conditions during the glass melting, and they are responsible for optical absorption bands at 370 and 250 nm. Raman spectrum of the undoped glass presents bands at 625, 860 and 980 cm−1, and the presence of chromate ions gives rise to additional bands at 365, 850 and a shoulder at 890 cm−1. An enhancement of the 850 cm−1 Raman band is observed with decreasing laser exciting wavelength. The exciting frequency dependence of the intensity of this band is discussed in terms of theoretical models given in the literature.

Evidence for a diffuse transition in lead chloride

Abstract The Raman spectra and the electrical conductivity of lead chloride single crystals at various temperatures, up to the melting point, have been investigated. From the results of both techniques we have found evidence of the beginnings of a diffuse transition to a state of high conductivity at temperatures close to the melting point of PbCl 2 .

Phase transitions in KDP induced by uniaxial pressure

Raman spectra of KH2PO4 (KDP) under uniaxial pressure were studied at various temperatures around the transition point from the paraelectric to the ferroelectric phases. For pressures along the [110] direction in the paraelectric phase, and along the [100] direction in the ferroelectric phase, many qualitative modifications were observed in the Raman spectra, most of them in geometries corresponding to unidimensional representations. These modifications can be interpreted as phase transitions due to changes in the symmetry sites of the ions, induced by the applied uniaxial stress.

Order-disorder phase transition in BaCl2•2H2O crystals

Crystals of BaCl2.2H2O were studied by Raman spectroscopy and dielectric constant measurements, from 12K to room temperature. Both techniques gave evidence of an order-disorder phase transition occurring around 200K. The conclusion drawn is that half of the water molecules in the unit cell change their site symmetry during the transition.

Low-frequency relaxation processes in LiNbO3: Fe single crystals

Abstract The study of the complex dielectric function of LiNbO3:Fe single crystals over a wide range of Fe concentrations revealed the presence of well-defined low-frequency relaxations associated to the Fe ion. These resonances are very sensitive to heat treatment and doping. Such low-frequency relaxation processes in LiNbO3:Fe could be important for a better understanding of this material due to its large application in nonlinear optics and especially in holographic storage techniques.