677 research outputs found
Giant Electroresistance in Ferroelectric Tunnel Junctions
The interplay between the electron transport in metal/ferroelectric/metal
junctions with ultrathin ferroelectric barriers and the polarization state of a
barrier is investigated. Using a model which takes into account screening of
polarization charges in metallic electrodes and direct quantum tunneling across
a ferroelectric barrier we calculate the change in the tunneling conductance
associated with the polarization switching. We find the conductance change of a
few orders of magnitude for metallic electrodes with significantly different
screening lengths. This giant electroresistance effect is the consequence of a
different potential profile seen by transport electrons for the two opposite
polarization orientations.Comment: 4 page
A combined theoretical and experimental study of the low temperature properties of BaZrO3
Low temperature properties of BaZrO3 are revealed by combining experimental
techniques (X-ray diffraction, neutron scattering and dielectric measurements)
with theoretical first-principles-based methods (total energy and linear
response calculations within density functional theory, and effective
Hamiltonian approaches incorporating/neglecting zero-point phonon vibrations).
Unlike most of the perovskite systems, BaZrO3 does not undergo any
(long-range-order) structural phase transition and thus remains cubic and
paraelectric down to 2 K, even when neglecting zero-point phonon vibrations. On
the other hand, these latter pure quantum effects lead to a negligible thermal
dependency of the cubic lattice parameter below ~ 40 K. They also affect the
dielectricity of BaZrO3 by inducing an overall saturation of the real part of
the dielectric response, for temperatures below ~ 40 K. Two fine structures in
the real part, as well as in the imaginary part, of dielectric response are
further observed around 50-65 K and 15 K, respectively. Microscopic origins
(e.g., unavoidable defects and oxygen octahedra rotation occurring at a local
scale) of such anomalies are suggested. Finally, possible reasons for the facts
that some of these dielectric anomalies have not been previously reported in
the better studied KTaO3 and SrTiO3 incipient ferroelectrics are also
discussed.Comment: 8 pages, 5 figures, submitted to Physical Review
Magnetodielectric effect and optic soft mode behaviour in quantum paraelectric EuTiO3 ceramics
Infrared reflectivity and time-domain terahertz transmission spectra of
EuTiO3 ceramics revealed a polar optic phonon at 6 - 300K, whose softening is
fully responsible for the recently observed quantum paraelectric behaviour.
Even if our EuTiO3 ceramics show lower permittivity than the single crystal due
to a reduced density and/or small amount of secondary pyrochlore Eu2Ti2O7
phase, we confirmed the magnetic field dependence of the permittivity, also
slightly smaller than in single crystal. Attempt to reveal the soft phonon
dependence at 1.8K on the magnetic field up to 13T remained below the accuracy
of our infrared reflectivity experiment
Morphodynamics of a wave dominated embayed beach on an irregular rocky coastline
Praias dominadas por ondas ao longo de litorais rochosos recortados são consideradas de grande potencial turÃstico, devido a beleza de seus recursos naturais. A compreensão das interações entre os processos oceanográficos, geomorfológicos e a herança geológica dominante neste tipo de praias ainda requer maiores investigações que permitam qualificar nossa compreensão e ajudar a preservar o equilÃbrio dinâmico natural destes ambientes. Através da abordagem morfodinâmica clássica, descrevemos pela primeira vez as variações morfológicas da praia de Boiçucanga, localizada em uma baÃa entre promontórios rochosos no municÃpio de São Sebastião, SP; região conhecida por belas praias arenosas incrustadas nos vales da Serra do Mar. Foram realizados levantamentos morfológicos ao longo de 15 meses, entre abril de 2008 e setembro de 2009, buscando avaliar as relações entre a morfologia e as caracterÃsticas hidrodinâmicas e sedimentares da praia de Boiçucanga. Os dados de campo permitiram a aplicação de modelos da literatura clássica de morfodinâmica praial, como: classificação de estágios morfológicos; número de bancos; altura relativa da maré; grau de exposição da praia; e o grau de embaiamento da praia. A combinação dos resultados dos levantamentos de campo, em especial dos perfis morfológicos da praia e das caracterÃscas dos sedimentos na zona de espraiamento, bem como a aplicação dos modelos morfodinâmicos clássicos, permitiram uma melhor compreensão do papel da herança geológica - posição e alcance dos promontórios mar adentro - sobre as assinaturas morfodinâmicas encontradas ao longo do arco praial de Boiçucanga; sendo uma assinatura mais energética com tendências refletivas, favorecendo o desenvolvimento de feições praiais rÃtmicas (e.g. cúspides praiais) e correntes de retorno controladas pela topografia, e outra menos energética com tendências dissipativas.Wave-dominated embayed beaches on irregular rocky coastlines are highly appreciated for their landscapes and tourism potential. Yet our understanding about the interaction of the oceanographic processes with the geological inheritance in this type of beaches still needs further investigation to better understand their natural balance. In this study, we apply the classical approach of morphodynamics to describe for the first time the variations in the Boiçucanga beach morphology, an embayed beach located in São Sebastião City on an irregular rocky coastline that is well known for its beautiful beaches embedded in the valleys of Serra do Mar. Field surveys were performed over 15 months, between April 2008 and September 2009, seeking to evaluate the interactions among the morphology, sediments and hydrodynamic characteristics at Boiçucanga. All data generated during the field surveys have supported the application of the following morphodynamic models: Classification of morphological beach stages [O]; Number of nearshore sandbars [B*]; Relative tide range [RTR]; Beach exposure on embayed coasts [Ro/a and ß]; and Embayment beach scaling parameter [d] to determine the geomorphological behavior of the beach on monthly and annual time scales. From these models, Boiçucanga is classified as a deep embayed beach exposed to highenergy waves, with a predominant reflective profile, which favors the development of beach cusps and topographically controlled rip currents. The combined results of the field observations and applied models allowed us to better understand the role of geological heritage in the two morphodynamic signatures found at the same beach arch. For future studies, we will focus on numerical models to increase the understanding of the hydrodynamic processes that govern the sediment transport in Boiçucanga
Characterization of high-temperature PbTe p-n junctions prepared by thermal diffusion and by ion-implantation
We describe here the characteristics of two types of high-quality PbTe
p-n-junctions, prepared in this work: (1) by thermal diffusion of In4Te3 gas
(TDJ), and (2) by ion implantation (implanted junction, IJ) of In (In-IJ) and
Zn (Zn-IJ). The results, as presented here, demonstrate the high quality of
these PbTe diodes. Capacitance-voltage and current-voltage characteristics have
been measured. The measurements were carried out over a temperature range from
~ 10 K to ~ 180 K. The latter was the highest temperature, where the diode
still demonstrated rectifying properties. This maximum operating temperature is
higher than any of the earlier reported results.
The saturation current density, J0, in both diode types, was ~ 10^-5 A/cm2 at
80 K, while at 180 K J0 ~ 10^-1 A/cm2 in TDJ and ~ 1 A/cm2 in both
ion-implanted junctions. At 80 K the reverse current started to increase
markedly at a bias of ~ 400 mV for TDJ, and at ~550 mV for IJ. The ideality
factor n was about 1.5-2 for both diode types at 80 K. The analysis of the C-V
plots shows that the junctions in both diode types are linearly graded. The
analysis of the C-V plots allows also determining the height of the junction
barrier, the concentrations and the concentration gradient of the impurities,
and the temperature dependence of the static dielectric constant. The
zero-bias-resistance x area products (R0Ae) at 80 K are: 850 OHMcm2 for TDJ,
250 OHMcm2 for In-IJ, and ~ 80 OHMcm2 for Zn-IJ, while at 180 K R0Ae ~ 0.38
OHMcm2 for TDJ, and ~ 0.1 OHMcm2 for IJ. The estimated detectivity is: D* ~
10^10 cmHz^(1/2)/W up to T=140 K, determined mainly by background radiation,
while at T=180 K, D* decreases to 108-107 cmHz^(1/2)/W, and is determined by
the Johnson noise
Variações morfológicas espaço-temporais entre as praias de Cibratel e Itanhaém-Suarão, Estado de São Paulo
The main morphological difference between Cibratel and Itanhaém ÂSuarão beaches is due to the presence of the Itanhaém river mouth in the second one. Eight monthly beach profiles were done during fourteen months. Simultaneously sediments were sampled in the shoreface and incident waves characteristics were visually estimated. Sediment sampling and bathymetry were done in the adjacent submerged area. The results show that, despite of the predominance of the action from the Tropical South Atlantic Anticyclone (TSAA) over the wave regime, the sedimentation process dynamics from the beaches was mainly controlled by the incidence of frontal systems, leading to a more effective northeastward longshore transporto Aerial photographs and beach sediment characteristics confirm such tendency. At Cibratel beach, the southwestward transport caused by TSAA was more evident than in Itanhaém - Suarão beach. Modifications in the beach dynamics, caused by the presence ofthe Itanhaém river mouth occurred only in restricted areas of its vicinities.As praias de Cibratel e Itanhaém-Suarão tem como fator diferencial a presença da desembocadura do Rio Itanhaém na Praia Itanhaém-Suarão. Os trabalhos realizados consistiram na realização mensal de 8 perfis praiais durante um perÃodo de 14 meses. Concomitantemente, houve coletas de sedimentos da face praial e observações visuais dos trens de ondas incidentes. Atividades complementares como fotointerpretação; caracterizações climáticas, batimétricas e dos sedimentos de superfÃcie de fundo da zona submersa até 8 metros de profundidade também foram realizadas. Os resultados obtidos demonstraram que, apesar do predomÃnio da ação do Anticiclone Tropical do Atlântico Sul (ATAS), a dinâmica sedimentar de ambas as praias esteve controlada principalmente pela incidência de sistemas frontais que resultou em um transporte mais efetivo rumo NE. O estudo das fotografias aéreas e a análise dos sedimentos praiais confirmaram uma maior efetividade da deriva litorânea neste rumo. Na Praia de Cibratel, a ação da ATAS foi mais notória que na de Itanhaém-Suarão, causando em ambas a ação da deriva longitudinal rumo SW. A desembocadura fluvial do Rio Itanhaém exerceu influência somente em trechos praiais mais próximos a ela, alterando os mecanismos de transporte litorâneo
Dynamics of relaxor ferroelectrics
We study a dynamic model of relaxor ferroelectrics based on the spherical
random-bond---random-field model and the Langevin equations of motion. The
solution to these equations is obtained in the long-time limit where the system
reaches an equilibrium state in the presence of random local electric fields.
The complex dynamic linear and third-order nonlinear susceptibilities
and , respectively, are calculated as
functions of frequency and temperature. In analogy with the static case, the
dynamic model predicts a narrow frequency dependent peak in ,
which mimics a transition into a glass-like state.Comment: 15 pages, Revtex plus 5 eps figure
Influence of uniaxial tensile stress on the mechanical and piezoelectric properties of short-period ferroelectric superlattice
Tetragonal ferroelectric/ferroelectric BaTiO3/PbTiO3 superlattice under
uniaxial tensile stress along the c axis is investigated from first principles.
We show that the calculated ideal tensile strength is 6.85 GPa and that the
superlattice under the loading of uniaxial tensile stress becomes soft along
the nonpolar axes. We also find that the appropriately applied uniaxial tensile
stress can significantly enhance the piezoelectricity for the superlattice,
with piezoelectric coefficient d33 increasing from the ground state value by a
factor of about 8, reaching 678.42 pC/N. The underlying mechanism for the
enhancement of piezoelectricity is discussed
Caracterização do Transporte Sedimentar Litorâneo junto à foz do Rio Itanhaém sob ação do Anticiclone Tropical Continental e dos Ciclones Extra-Tropicais
The study of the shore and beach environment is one of the most investigated issues in the last decades on the Brazilian coast. This kind of research involves scientists from several areas due to the large amount of variables interfering into this kind of environment. Meteorological studies, for instance are essential to the comprehension of beach evolution. The foreshore is the less known compartment among the existent beach compartment in such terms of complexity and difficulty that involves this kind of study. This work had as objective the monitoring of the foreshore under the influence of the Continental Tropical Anticyclone (ATC). The monitoring was made in the region of Itanhaém, central coast of São Paulo state. Near the outlet of Itanhaém River suspension material were collected and profiles of currentmetry and thermosalinometry were made. The meteorological specifications were obtained from properly literature. The observation of waves was done visually. The results showed that the ATC effects were punctual, causing the increase of the local rainfall without change of the incident wave trains characteristics. The plume of suspended material was inducted by the river discharge due to the rains, causing the increasing of the transport capacity from Itanhaém River
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