480 research outputs found
Ordering Policy Rules with an Unconditional Welfare Measure
The unconditional expectation of social welfare is often used to assess alternative macroeconomic policy rules in applied quantitative research. This paper provides a detailed analysis of such policies. It sets out the unconditionally optimal (UO) policy problem and derives a linear-quadratic (LQ) version of that problem that approximates the exact non-linear problem. The properties of UO policies are analyzed through a series of examples and contrasted with the timeless perspective (TP), exposited in Benigno and Woodford (2012). Some substantive implications for optimal monetary policy are explored
Piezoelectric nonlinearity and frequency dispersion of the direct piezoelectric response of BiFeO3 ceramics
We report on the frequency and stress dependence of the direct piezoelectric
d33 coefficient in BiFeO3 ceramics. The measurements reveal considerable
piezoelectric nonlinearity, i.e., dependence of d33 on the amplitude of the
dynamic stress. The nonlinear response suggests a large irreversible
contribution of non-180{\deg} domain walls to the piezoelectric response of the
ferrite, which, at present measurement conditions, reached a maximum of 38% of
the total measured d33. In agreement with this interpretation, both types of
non-180{\deg} domain walls, characteristic for the rhombohedral BiFeO3, i.e.,
71{\deg} and 109{\deg}, were identified in the poled ceramics using
transmission electron microscopy (TEM). In support to the link between
nonlinearity and non-180{\deg} domain wall contribution, we found a correlation
between nonlinearity and processes leading to deppining of domain walls from
defects, such as quenching from above the Curie temperature and
high-temperature sintering. In addition, the nonlinear piezoelectric response
of BiFeO3 showed a frequency dependence that is qualitatively different from
that measured in other nonlinear ferroelectric ceramics, such as "soft"
(donor-doped) Pb(Zr,Ti)O3 (PZT); possible origins of this dispersion are
discussed. Finally, we show that, once released from pinning centers, the
domain walls can contribute extensively to the electromechanical response of
BiFeO3; in fact, the extrinsic domain-wall contribution is relatively as large
as in Pb-based ferroelectric ceramics with morphotropic phase boundary (MPB)
composition, such as PZT. This finding might be important in the search of new
lead-free MPB compositions based on BiFeO3 as it suggests that such
compositions might also exhibit large extrinsic domain-wall contribution to the
piezoelectric response.Comment: 38 pages, 11 figure
Landau thermodynamic potential for BaTiO_3
In the paper, the description of the dielectric and ferroelectric properties
of BaTiO_3 single crystals using Landau thermodynamic potential is addressed.
Our results suggest that when using the sixth-power free energy expansion of
the thermodynamic potential, remarkably different values of the fourth-power
coefficient, \beta (the coefficient of P^4_i terms), are required to adequately
reproduce the nonlinear dielectric behavior of the paraelectric phase and the
electric field induced ferroelectric phase, respectively. In contrast, the
eighth-power expansion with a common set of coefficients enables a good
description for both phases at the same time. These features, together with the
data available in literature, strongly attest to the necessity of the
eighth-power terms in Landau thermodynamic potential of BaTiO_3. In addition,
the fourth-power coefficients, \beta and \xi (the coefficient of P^2_i P^2_j
terms), were evaluated from the nonlinear dielectric responses along [001],
[011], and [111] orientations in the paraelectric phase. Appreciable
temperature dependence was evidenced for both coefficients above T_C. Further
analysis on the linear dielectric response of the single domain crystal in the
tetragonal phase demonstrated that temperature dependent anharmonic
coefficients are also necessary for an adequate description of the dielectric
behavior in the ferroelectric phase. As a consequence, an eighth-power
thermodynamic potential, with some of the anharmonic coefficients being
temperature dependent, was proposed and compared with the existing potentials.
In general, the potential proposed in this work exhibits a higher quality in
reproducing the dielectric and ferroelectric properties of this prototypic
ferroelectric substance.Comment: 7 figures, 5 table
Using Touch Technology to Foster Storytelling in the Preschool Classroom
This practitioner research explores ways children engage in literacy learning through storytelling with the use of touch technology in a VPK (Voluntary Pre-Kindergarten) classroom. With access to diverse touch technology devices but no experience using these technologies, a VPK teacher explored strategies to use the resources to enhance literacy learning in the classroom with the support of a professional learning community (PLC). The PLC consisted of a masterâs student, university faculty, school director, and a technology liaison. The implementation of this study took place over three weeks, and every week children created a different story. Collected data include photographs, student voice recordings, anecdotal notes, and a reflective journal. The three weeks of implementation data showed how touch technology provided a new modality of learning representation for young children in my classroom. The findings suggest that multiliteracies complemented traditional literacy, storytelling enhanced childrenâs communication, and touch technology functionality went beyond literacy skills
Anharmonicity of BaTiO_3 single crystals
By analyzing the dielectric non-linearity with the Landau thermodynamic
expansion, we find a simple and direct way to assess the importance of the
eighth order term. Following this approach, it is demonstrated that the eighth
order term is essential for the adequate description of the para/ferroelectric
phase transition of BaTiO_3. The temperature dependence of the quartic
coefficient \beta is accordingly reconsidered and is strongly evidenced by the
change of its sign above 165 C. All these findings attest to the strong
polarization anharmonicity of this material, which is unexpected for classical
displacive ferroelectrics.Comment: 4 figures, to be published in Phys. Rev.
Tailoring the interfacial interactions in ferroelectric fluorinated polymer/ceramic nanocomposites
In this work composites of PVDF-TrFE containing 60 vol% untreated and surface modified BaTiO3 were produced by solvent casting with two procedures. Their morphology and structure were characterized by scanning electron microscope, X-ray diffraction and differential scanning calorimetry. The effect of the processing conditions and of the surface modification of BaTiO3 on the viscoelastic, dielectric and piezoelectric properties was investigated. The surface modification of BaTiO3 allowed obtaining composite films with low porosity and good filler dispersion, and hence higher storage modulus and lower loss tangent, in a wider processing window. Furthermore it reduced the dielectric losses at low frequency and modified the decay kinetic of the d33 piezoelectric coefficient with respect to composites made with untreated particles
Theory and Phenomenology of Vector Mesons in Medium
Electromagnetic probes promise to be direct messengers of (spectral
properties of) hot and dense matter formed in heavy-ion collisions, even at
soft momentum transfers essential for characterizing possible phase
transitions. We examine how far we have progressed toward this goal by
highlighting recent developments, and trying to establish connections between
lattice QCD, effective hadronic models and phenomenology of dilepton
production.Comment: 8 pages latex incl. 12 ps/eps files; invited plenary talk at Quark
Matter 2006 conference, Shanghai (China), Nov. 14-20, 200
Piezoresponse Force Spectroscopy of Ferroelectric Materials
Piezoresponse Force Spectroscopy (PFS) has emerged as a powerful technique
for probing highly localized switching behavior and the role of microstructure
and defects on switching. The application of a dc bias to a scanning probe
microscope tip in contact with a ferroelectric surface results in the
nucleation and growth of a ferroelectric domain below the tip, resulting in
changes in local electromechanical response. Resulting hysteresis loops
contains information on local ferroelectric switching behavior. The signal in
PFS is the convolution of the volume of the nascent domain and the probing
volume of the tip. Here, we analyze the signal formation mechanism in PFS by
deriving the main parameters of domain nucleation in a semi-infinite material
and establishing the relationships between domain parameters and PFM signal
using a linear Greens function theory. The effect of surface screening and
finite Debye length on the switching behavior is established. In particular, we
predict that the critical nucleus size in PFM is controlled by the surface
screening mechanism and in the absence of screening, tip-induced switching is
impossible. Future prospects of PFS to study domain nucleation in the vicinity
of defects, local switching centers in ferroelectrics, and unusual polarization
states in low-dimensional ferroelectrics are discussed.Comment: 74 pages, 18 figures, 3 appendices, sent to Phys. Rev.
QCD in the nuclear medium and effects due to Cherenkov gluons
The equations of in-medium gluodynamics are proposed. Their classical lowest
order solution is explicitly shown for a color charge moving with constant
speed. For nuclear permittivity larger than 1 it describes emission of
Cherenkov gluons resembling results of classical electrodynamics. The values of
the real and imaginary parts of the nuclear permittivity are obtained from the
fits to experimental data on the double-humped structure around the away-side
jet obtained at RHIC. The dispersion of the nuclear permittivity is predicted
by comparing the RHIC, SPS and cosmic ray data. This is important for LHC
experiments. Cherenkov gluons may be responsible for the asymmetry of dilepton
mass spectra near rho-meson, observed in the SPS experiment with excess in the
low-mass wing of the resonance. This feature is predicted to be common for all
resonances. The "color rainbow" quantum effect might appear according to higher
order terms of in-medium QCD if the nuclear permittivity depends on color.Comment: 29 p., 4 figs; for "Phys. Atom. Nucl." volume dedicated to 80th
birthday of L.B. Okun; minor corrections on pp. 11 and 13 in v
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