Two phase transitions in (d(x2-y2)+is)-wave superconductors

We study numerically the temperature dependencies of specific heat, susceptibility, penetration depth, and thermal conductivity of a coupled (d(x2-y2) + is)-wave Bardeen-Cooper-Schrieffer (BCS) superconductor in the presence of a weak s-wave component (1) on square lattice and (2) on a lattice with orthorhombic distortion. As the temperature is lowered past the critical temperature T-c, a less ordered superconducting phase is created in d(x2-y2) wave, which changes to a more ordered phase in (d(x2-y2) + is) wave at T-c1. This manifests in two second-order phase transitions. The two phase transitions are identified by two jumps in specific heat at T-c and T-c1. The temperature dependencies of the superconducting observables exhibit a change from power-law to exponential behavior as temperature is lowered below T-c1 and confirm the new phase transition. (C) 1999 Elsevier B.V. B.V. All rights reserved.

Critical temperature for first-order phase transitions in confined systems

We consider the Euclidean D-dimensional -lambda vertical bar phi vertical bar(4)+eta vertical bar rho vertical bar(6) (lambda,eta > 0) model with d (d <= D) compactified dimensions. Introducing temperature by means of the Ginzburg-Landau prescription in the mass term of the Hamiltonian, this model can be interpreted as describing a first-order phase transition for a system in a region of the D-dimensional space, limited by d pairs of parallel planes, orthogonal to the coordinates axis x(1), x(2),..., x(d). The planes in each pair are separated by distances L-1, L-2, ... , L-d. We obtain an expression for the transition temperature as a function of the size of the system, T-c({L-i}), i = 1, 2, ..., d. For D = 3 we particularize this formula, taking L-1 = L-2 = ... = L-d = L for the physically interesting cases d = 1 (a film), d = 2 (an infinitely long wire having a square cross-section), and for d = 3 (a cube). For completeness, the corresponding formulas for second-order transitions are also presented. Comparison with experimental data for superconducting films and wires shows qualitative agreement with our theoretical expressions.

Incorporating memory effects in phase separation processes

We consider the modification of the Cahn-Hilliard equation when a time delay process through a memory function is taken into account. We then study the process of spinodal decomposition in fast phase transitions associated with a conserved order parameter. Finite-time memory effects are seen to affect the dynamics of phase transition at short times and have the effect of delaying, in a significant way, the process of rapid growth of the order parameter that follows a quench into the spinodal region. These effects are important in several systems characterized by fast processes, like non-equilibrium dynamics in the early universe and in relativistic heavy-ion collisions. (C) 2006 Elsevier B.V. All rights reserved.

Numerical approximation of the Ginzburg-Landau equation with memory effects in the dynamics of phase transitions

Coordenação de Aperfeiçoamento de Pessoal de Nível Superior (CAPES)

Non-local effects at the onset of the chiral transition

Inspired by analytic results obtained for a systematic expansion of the memory kernel in dissipative quantum mechanics, we propose a phenomenological procedure to incorporate non-markovian corrections to the Langevin dynamics of an order parameter in field theory systematically. In this note, we restrict our analysis to the onset of the evolution. As an example, we consider the process of phase conversion in the chiral transition.

Anomalous diffusion of pions at RHIC

After pointing out the difference between normal and anomalous diffusion, we consider a hadron resonance cascade (HRC) model simulation for particle emission at RHIC and point out that rescattering in an expanding hadron resonance gas leads to a heavy tail in the source distribution. The results are compared to recent PHENIX measurements of the tail of the particle emitting source in Au+Au collisions at RHIC. In this context, we show how can one distinguish experimentally the anomalous diffusion of hadrons from a second order QCD phase transition.

Magnetic field induced lattice effects in a quasi-two-dimensional organic conductor close to the Mott metal-insulator transition

Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP)

Study of potassium additive on the phase formation and ferroelectric properties of 0.9PMN-0.1PT ceramics

A recent and innovative method to include Ti into the columbite precursor has permitted to synthesize 0.9PMN-0.1PT powders with high homogeneity. The present work describes this methodology, named modified columbite method, showing that the reaction between MN(T)and PbO at 800 degrees C for 2 h results in perovskite single-phase. The crystal structure alterations in the columbite and perovskite phases obtained by this methodology and the effects of potassium doping were investigated by the Rietveld method. Changes in the powder morphology, density and weight loss during the sintering process were also studied. Conclusively, potassium does not affect significantly the perovskite amount, but reduces the particle and grain sizes. This dopant also changes the relaxor behavior of 0.9PMN-0.1 PT ceramic, reducing the dielectric loss and enhancing the diffuseness of the phase transition. (C) 2005 Published by Elsevier Ltd and Techna Gronp S.r.l.

RECENT HEAVY-ION EXPERIMENTS AND STATISTICAL-MODEL OF HADRONS

The combined CERN and Brookhaven heavy ion (H.I.) data supports a scenario of hadron gas which is in chemical and thermal equilibrium at a temperature T of about 140 MeV. Using the Brown-Stachel-Welke model (which gives 150 MeV) we show that in this scenario, the hot nucleons have mass 3 pi T and the pi and rho mesons have masses close to pi T and 2 pi T, respectively. A simple model with pions and quarks supports the co-existence of two phases in these heavy ion experiments, suggesting a second order phase transition. The masses of the pion, rho and the nucleon are intriguingly close to the lattice screening masses.

Temperature and concentration dependence of heat capacity of model aqueous solutions

Heat capacities of binary aqueous solutions of different concentrations of sucrose, glucose, fructose, citric acid, malic acid, and inorganic salts were measured with a differential scanning calorimeter in the temperature range from 5degreesC to 65degreesC. Heat capacity increased with increasing water content and increasing temperature. At low concentrations, heat capacity approached that of pure water, with a less pronounced effect of temperature, and similar abnormal behavior of pure water with a minimum around 30degreesC-40degreesC. Literature data, when available agreed relatively well with experimental values. A correction factor, based on the assumption of chemical equilibrium between liquid and gas phase in the Differential Scanning Calorimeter, was proposed to correct for the water evaporation due to temperature rise. Experimental data were fitted to predictive models. Excess molar heat capacity was calculated using the Redlich-Kister equation to represent the deviation from the additive ideal model.

Thermogravimetric and UV-vis spectroscopic studies of chromium redox reactions in rutile pigments

In this study undoped and Cr, Sb or Mo doped TiO(2) were synthesized by polymeric precursor method and characterized by X-ray diffraction, UV-VIS spectroscopy, infrared spectroscopy and thermogravimetry (TG). The TG curves showed a continuous mass loss assigned to the hydroxyl elimination and Cr(6+) reduction. Doped TiO(2) samples showed a higher mass loss assigned to water and gas elimination at lower temperatures. In these doped materials a decrease in the anatase-rutile phase transition temperature was observed. After calcination at 1,000 A degrees C, rutile was obtained as a single phase material without the presence of Cr(6+).

QCD phase transitions from relativistic hadron models

The models of translationally invariant infinite nuclear matter in the relativistic mean field models are very interesting and simple, since the nucleon can connect only to a constant vector and scalar meson field. Can one connect these to the complicated phase transitions of QCD? For an affirmative answer to this question, one must consider models where the coupling contstants to the scalar and vector fields depend on density in a nonlinear way, since as such the models are not explicitly chirally invariant. Once this is ensured, indeed one can derive a quark condensate indirectly from the energy density of nuclear matter which goes to zero at large density and temperature. The change to zero condensate indicates a smooth phase transition. © Springer-Verlag 1996.

Insulator-to-metal transition driven by clustering of solitons in polyacetylene

It's believed that the simple Su-Schrieffer-Heeger Hamiltonian can not predict the insulator to metal transition of transpolyacetylene (t-PA). The soliton lattice configuration at a doping level y=6% still has a semiconductor gap. Disordered distributions of solitons close the gap, but the electronic states around the Fermi energy are localized. However, within the same framework, it is possible to show that a cluster of solitons can produce dramatic changes in the electronic structure, allowing an insulator-to-metal transition.

Model for stars of interacting bosons and fermions

In this paper we introduce a current-current type interaction term in the Lagrangian density of gravity coupled to complex scalar fields, in the presence of a degenerated Fermi gas. For low transferred momenta, such a term, which might account for the interaction among boson and fermion constituents of compact stellar objects, is subsequently reduced to a quadratic one in the scalar sector. This procedure enforces the use of a complex radial field counterpart in the equations of motion. The real and the imaginary components of the scalar field exhibit different behavior as the interaction increases. The results also suggest that the Bose-Fermi system undergoes a phase transition for a suitable choice of the coupling constant.

A joint theoretical and kinetic investigation on the fragmentation of (N-halo)-2-amino cycloalkanecarboxylates

A combined theoretical and experimental study to elucidate the molecular mechanism for the Grob fragmentation of different (N-halo)-2-amino cyclocarboxylates with the nitrogen atom in exocyclic position: (N-Cl)-2-amino cyclopropanecarboxylate (1), (N-Cl)-2-amino cyclobutanecarboxylate (2), (N-Cl)-2-amino cyclopentanecarboxylate (3) and (N-Cl)-2-amino cyclohexanecarboxylate (4), and the corresponding acyclic compounds, (N-Cl)-2-amino isobutyric acid (A), (N-Cl)-2-amino butyric acid (B), has been carried out. The kinetics of decomposition for these compounds and related bromine derivatives were experimentally determined by conventional and stopped-flow UV spectrophotometry. The reaction products have been analyzed by GC and spectrophotometry. Theoretical analysis is based in the localization of stationary points (reactants and transition structures) on the potential energy surface. Calculations were carried out at B3LYP/6-31+G* and MP2/6-31+G* computing methods in the gas phase, while solvent effects have been included by means the self-consistent reaction field theory, PCM continuum model, at MP2/6-31+G* and MP4/6-31+G*//MP2/6-31+G* calculation levels. Based on both experimental and theoretical results, the different Grob fragmentation processes show a global synchronicity index close to 0.9, corresponding to a nearly concerted process. At the TSs, the N-Cl bond breaking is more advanced than the C-C cleavage process. An antiperiplanar configuration of these bonds is reached at the TSs, and this geometrical arrangement is the key factor governing the decomposition. In the case of 1 and 2 the ring strain prevents this spatial disposition, leading to a larger value of the activation barrier. Natural population analysis shows that the polarization of the N-Cl and C-C bonds along the bond-breaking process can be considered the driving force for the decomposition and that a negative charge flows from the carboxylate group to the chlorine atom to assist the reaction pathway. A comparison of theoretical and experimental results shows the relevance of calculation level and the inclusion of solvent effects for determining accurate unimolecular rate coefficients for the decomposition process. © 2002 Published by Elsevier Science B.V.