CaCu_3Ti_4O_12/CaTiO_3 Composite Dielectrics: A Ba/Pb-free Ceramics with High Dielectric Constants

We have measured dielectric properties of Ca$_{1+x}$Cu$_{3-x}$Ti$_4$O$_{12}$ ($x$ = 0, 0.1, 0.5, 1, 1.5, 2, 2.9 and 3), and have found that Ca$_2$Cu$_2$Ti$_4$O$_{12}$ (a composite of CaCu$_3$Ti$_4$O$_{12}$ and CaTiO$_3$) exhibits a high dielectric constant of 1800 with a low dissipation factor of 0.02 below 100 kHz from 220 to 300 K. These are comparable to (or even better than) those of the Pb/Ba-based ceramics, which could be attributed to a barrier layer of CaTiO$_3$ on the surface of the CaCu$_3$Ti$_4$O$_{12}$ grains. The composite dielectric ceramics reported here are environmentally benign as they do not contain Ba/Pb.Comment: 4 pages, 4 figures, Appl. Phys. Lett. (scheduled on July 25, 2005

Incidence of Trypanosoma cruzi infection in two Guatemalan communities.

The prevalence of human infection by Trypanosoma cruzi was assessed using an enzyme-linked immunosorbent assay (ELISA) in a serological survey in 1998 of 2 rural communities (SMH and PS) in Guatemala. In SMH (Department of Zacapa), where Rhodnius prolixus was the principal vector, the seroprevalence amongst 373 people tested was 38.8%. In PS (Department of Santa Rosa), where the main vector was Triatoma dimidiata, 8.9% of the 428 people tested were seropositive. The overall prevalence of seropositivity was higher in females than in males in both SMH (40% vs 36%) and PS (11.9% vs 4.9%), although this difference was significant only in PS. Historical seroconversion rates, estimated retrospectively by fitting a transmission model to the age-prevalence curves, were 3.8% per year in SMH and 0.5% per year in PS. There was some indication of a recent reduction in incidence in both villages. In PS, but not in SMH, both the observed prevalence and the estimated incidence rates were significantly higher in females than in males

Dynamic Programming and Learning Models for Management of a Nonnative Species

Nonnative invasive species result in sizeable economic damages and expensive control costs. Because dynamic optimization models break down if controls depend in complex ways on past controls, non-uniform or scale-dependent spatial attributes, etc., decision support systems that allow learning may be preferred. We compare three models of an invasive weed in California’s grazing lands: (1) a stochastic dynamic programming model, (2) a reinforcement-based, experience-weighted attraction (EWA) learning model, and (3) an EWA model that also includes stochastic forage growth and penalties for repeated application of environmentally harmful control techniques. Results indicate that EWA learning models may be appropriate for invasive species management.Invasive weed species, optimal control, adaptive management

Surface reconstruction and ferroelectricity in PbTiO3_3 thin films

Surface and ferroelectric properties of PbTiO$_3$ thin films are investigated using an interatomic potential approach with parameters computed from first-principles calculations. We show that a model developed for the bulk describes properly the surface properties of PbTiO$_3$. In particular, the antiferrodistortive surface reconstruction, recently observed from X-ray scattering, is correctly reproduced as a result of the change in the balance of long-range Coulombic and short-range interactions at the surface. The effects of the surface reconstruction on the ferroelectric properties of ultrathin films are investigated. Under the imposed open-circuit electrical boundary conditions, the model gives a critical thickness for ferroelectricity of 4 unit cells. The surface layer, which forms the antiferrodistortive reconstruction, participates in the ferroelectricity. A decrease in the tetragonality of the films leads to the stabilization of a phase with non-vanishing in-plane polarization. A peculiar effect of the surface reconstruction on the in-plane polarization profile is found.Comment: 6 pages, 5 figure

First principles investigation of ferroelectricity in epitaxially strained Pb2_2TiO4_4

The structure and polarization of the as-yet hypothetical Ruddlesden-Popper compound Pb$_2$TiO$_4$ are investigated within density-functional theory. Zone enter phonons of the high-symmetry K$_2$NiF$_4$-type reference structure, space group $I4/mmm$, were calculated. At the theoretical ground-state lattice constants, there is one unstable infrared-active phonon. This phonon freezes in to give the $I2mm$ ferroelectric state. As a function of epitaxial strain, two additional ferroelectric phases are found, with space groups $I4mm$ and $F2mm$ at compressive and tensile strains, respectively.Comment: 4 pages, 4 figure

Effect of ferroelectric layers on the magnetocapacitance properties of superlattices-based oxide multiferroics

A series of superlattices composed of ferromagnetic La$_{0.7}$Ca$_{0.3}$MnO$_3$ (LCMO) and ferroelectric/paraelectric Ba$_{1-x}$Sr$_x$TiO$_3$ (0$\leq$x$\leq$1) were deposited on SrTiO$_3$ substrates using the pulsed laser deposition. Films of epitaxial nature comprised of spherical mounds having uniform size are obtained. Magnetotransport properties of the films reveal a ferromagnetic Curie temperature in the range of 145-158 K and negative magnetoresistance as high as 30%, depending on the type of ferroelectric layers employed for their growth (\QTR{it}{i.e.} '\QTR{it}{x'} value). Ferroelectricity at temperatures ranging from 55 K to 105 K is also observed, depending on the barium content. More importantly, the multiferroic nature of the film is determined by the appearance of negative magnetocapacitance, which was found to be maximum around the ferroelectric transition temperature (3% per \QTR{it}{tesla}). These results are understood based on the role of the ferroelectric/paraelectric layers and strains in inducing the multiferroism.Comment: Accepted to Applied Physics Letter

Structural, electronic and magnetic properties of SrRuO3_3 under epitaxial strain

Using density functional theory within the local spin density approximation, structural, electronic and magnetic properties of SRO are investigated. We examine the magnitude of the orthorhombic distortion in the ground state and also the effects of applying epitaxial constraints, whereby the influence of large (in the range of $\pm 4$) in-plane strain resulting from coherent epitaxy, for both [001] and [110] oriented films, have been isolated and investigated. The overall pattern of the structural relaxations reveal coherent distortions of the oxygen octahedra network, which determine stability of the magnetic moment on the Ru ion. The structural and magnetic parameters exhibit substantial changes allowing us to discuss the role of symmetry and possibilities of magneto-structural tuning of \SRO-based thin film structures.Comment: 11 page

Phase stability and structural temperature dependence in sodium niobate: A high resolution powder neutron diffraction study

We report investigation of structural phase transitions in technologically important material sodium niobate as a function of temperature on heating over 300-1075 K. Our high resolution powder neutron diffraction data show variety of structural phase transitions ranging from non-polar antiferrodistortive to ferroelectric and antiferroelectric in nature. Discontinuous jump in lattice parameters is found only at 633 K that indicates that the transition of orthorhombic antiferroelectric P (space group Pbcm) to R (space group Pbnm) phase is first order in nature, while other successive phase transitions are of second order. New superlattice reflections appear at 680 K (R phase) and 770 K (S phase) that could be indexed using an intermediate long-period modulated orthorhombic structure whose lattice parameter along direction is 3 and 6 times that of the CaTiO3-like Pbnm structure respectively. The correlation of superlattice reflections with the phonon instability is discussed. The critical exponent ({\beta}) for the second order tetragonal to cubic phase transition at 950 K, corresponds to a value {\beta}$\approx 1/3$, as obtained from the temperature variation of order parameters (tilt angle and intensity of superlattice reflections). It is argued that this exponent is due to a second order phase transition close to a tricritical point. Based on our detailed temperature dependent neutron diffraction studies, the phase diagram of sodium niobate is presented that resolves existing ambiguities in the literature.Comment: 21 Pages, 8 Figure

Farm Revenue Assurance or Income Insurance?

Exact date of working paper unknown.Farm revenue insurance has public appeal as a potential means to reduce annual variation in farm income while also reducing the cost of commodity programs. For some time, economists have been attracted to the idea of farm revenue insurance as a policy instrument addressing farm economic instability. Public enthusiasm for the idea intensified after endorsement by the Iowa 1995 Farm Bill Study Team. The broad appeal of the proposal was apparent: it had the support of 11 organizations ranging in philosophy from the Iowa Farm Bureau to the National Farmers Organization. This paper lists advantages and disadvantages of revenue insurance. Preliminary empirical analysis suggests how that program may affect farm income associated with various crops and regions. We conclude with a proposal to make revenue insurance workable as an income insurance program administered through the federal income tax