José Antonio Eiras

Bio: José Antonio Eiras is an academic researcher from Federal University of São Carlos. The author has contributed to research in topics: Dielectric & Ferroelectricity. The author has an hindex of 34, co-authored 369 publications receiving 4491 citations.

Phenomenological description of the diffuse phase transition in ferroelectrics

Abstract: Features of the diffuse phase transition in lead magnesium niobate and strontium barium niobate, typical relaxor ferroelectric materials, were studied as a function of temperature and frequency. A new empirical equation for a phenomenological description of the temperature dependence of the dielectric permittivity (e') peak is proposed. In fact, the proposed equation provides an excellent fitting of the experimental curves at temperatures into and above the dielectric dispersion region, enabling us to calculate some characteristic parameters of the phase transitions in ferroelectric materials.

Effects of the postannealing atmosphere on the dielectric properties of (Ba, Sr)TiO3 capacitors: Evidence of an interfacial space charge layer

Abstract: The dielectric properties of ~Ba, Sr!TiO 3 films were found to be remarkably sensitive to the postannealing treatment atmosphere. This study demonstrates that postannealing in an oxygen atmosphere increases the dielectric relaxation phenomenon and that postannealing in a nitrogen atmosphere produces a slight dielectric relaxation. Such dependence of the dielectric relaxation was related both to oxygen vacancies and to the presence of negatively charged oxygen, trapped at the grain boundary and/or at the electrode/dielectric film interface. © 2000 American Institute of Physics. @S0003-6951~00!00817-2#

90° domain wall relaxation and frequency dependence of the coercive field in the ferroelectric switching process

Abstract: The mechanisms involved in the polarization switching process in soft and hard Pb(Zr53,Ti47)O3 (PZT) bulk ceramics were investigated through the dependency of the hysteresis loop on the frequency. In order to determine the influence of the defects on the domain switching dynamics, the samples were characterized in the virgin state and after a fatigue or a depinning process. The frequency dependence of the polarization revealed a strong relaxation of the 90° domain walls at ∼100 Hz. The results also revealed a strong influence of the kind of defect and their distribution in the ferroelectric matrix on the domain switching dynamics, which were reflected in the frequency dependence of the coercive field and the percentage of the backswitching. Initially, it was observed that the frequency dependence of the coercive field for the soft and the hard PZT in the virgin state had just one rate of change per decade in the entire frequency range investigated, which is the standard behavior found in the literature. However, after the fatigue or the depinning process, two rates of changes were noticed. Consequently, evidence of an upper-frequency limit for the coercive field changes was found. The percentage of the backswitching and its behavior for the soft PZT was almost independent of the fatigue state in the entire frequency range investigated. Nevertheless, for the hard PZT, an opposite behavior was verified. The reorientation of the domains was modeled as occurring in a viscous medium where several forces, such as viscous and restoring forces, act on them.

Raman scattering study of the PbZr 1 − x Ti x O 3 system: Rhombohedral-monoclinic-tetragonal phase transitions

Abstract: In this paper we present a Raman-scattering study of the phase transitions in the ${\mathrm{PbZr}}_{1\ensuremath{-}x}{\mathrm{Ti}}_{x}{\mathrm{O}}_{3}$ systems around the morphotropic phase boundary over a wide temperature range. The boundary between rhombohedral and monoclinic phases was found to be a quasivertical line between $x=0.46$ and $x=0.47.$ We also studied the monoclinic-tetragonal phase boundary and our spectroscopic results agree very well with those reported by using x-ray diffraction.

Fast parallel algorithms for short-range molecular dynamics

Abstract: Three parallel algorithms for classical molecular dynamics are presented. The first assigns each processor a fixed subset of atoms; the second assigns each a fixed subset of inter-atomic forces to compute; the third assigns each a fixed spatial region. The algorithms are suitable for molecular dynamics models which can be difficult to parallelize efficiently—those with short-range forces where the neighbors of each atom change rapidly. They can be implemented on any distributed-memory parallel machine which allows for message-passing of data between independently executing processors. The algorithms are tested on a standard Lennard-Jones benchmark problem for system sizes ranging from 500 to 100,000,000 atoms on several parallel supercomputers--the nCUBE 2, Intel iPSC/860 and Paragon, and Cray T3D. Comparing the results to the fastest reported vectorized Cray Y-MP and C90 algorithm shows that the current generation of parallel machines is competitive with conventional vector supercomputers even for small problems. For large problems, the spatial algorithm achieves parallel efficiencies of 90% and a 1840-node Intel Paragon performs up to 165 faster than a single Cray C9O processor. Trade-offs between the three algorithms and guidelines for adapting them to more complex molecular dynamics simulations are also discussed.