8,715 research outputs found
Low-temperature phases in Pb(Zr0.52Ti0.48)O3: A neutron powder diffraction study
A neutron powder diffraction study has been carried out on Pb(Zr0.52Ti0.48)O3
in order to resolve an ongoing controversy about the nature of the
low-temperature structure of this strongly-piezoelectric and
technologically-important material. The results of a detailed and systematic
Rietveld analysis at 20 K are consistent with the coexistence of two monoclinic
phases having space groups Cm and Ic respectively, in the approximate ratio
4:1, and thus support the findings of a recent electron diffraction study by
Noheda et al. [Phys. Rev. B 66, 060103 (2002)]. The results are compared to
those of two recent conflicting neutron powder diffraction studies of materials
of the same nominal composition by Hatch et al. [Phys. Rev. B 65, 212101
(2002)] and Frantti et al. [Phys. Rev. B 66, 064108 (2002)].Comment: RevTex4, 16 pages, 6 color figure
Origin Of the enhanced flexoelectricity of relaxor ferroelectrics
We have measured the bending-induced polarization of Pb(Mg1/3Nb2/3)O3-PbTiO3
single crystals with compositions at the relaxor-ferroelectric phase boundary.
The crystals display very large flexoelectricity, with flexocoupling
coefficients an order of magnitude bigger than the theoretical upper limit set
by the theories of Kogan and Tagantsev. This enhancement persists in the
paraphrase up to a temperature T* that coincides with the start of elastic
softening in the crystals. Analysis of the temperature dependence and
cross-correlation between flexoelectric, dielectric and elastic properties
indicates that the large bendinginduced polarization of relaxor ferroelectrics
is not caused by intrinsically giant flexoelectricity, but by the reorientation
of polar nanotwins that become ferroelastically active below T*
Novel Quark Fragmentation Functions and the Nucleon's Transversity Distribution
We define twist-two and twist-three quark fragmentation functions in Quantum
Chromodynamics (QCD) and study their physical implications. Using this
formalism we show how the nucleon's transversity distribution can be measured
in single pion inclusive electroproduction.Comment: 10 pages, uses PHYZZX macro package, 2 PostScript figures (added
using FIGURES). MIT-CTP-215
Existence of Multiple Vortices in Supersymmetric Gauge Field Theory
Two sharp existence and uniqueness theorems are presented for solutions of
multiple vortices arising in a six-dimensional brane-world supersymmetric gauge
field theory under the general gauge symmetry group and
with Higgs scalar fields in the fundamental representation of .
Specifically, when the space of extra dimension is compact so that vortices are
hosted in a 2-torus of volume |\Om|, the existence of a unique multiple
vortex solution representing respectively prescribed vortices
arising in the species of the Higgs fields is established under the
explicitly stated necessary and sufficient condition \[ n_i<\frac{g^2v^2}{8\pi
N}|\Om|+\frac{1}{N}(1-\frac{1}{N}[\frac{g}{e}]^2)n,\quad i=1,...,N,] where
and are the U(1) electromagnetic and SU(N) chromatic coupling constants,
measures the energy scale of broken symmetry, and is
the total vortex number; when the space of extra dimension is the full plane,
the existence and uniqueness of an arbitrarily prescribed -vortex solution
of finite energy is always ensured. These vortices are governed by a system of
nonlinear elliptic equations, which may be reformulated to allow a variational
structure. Proofs of existence are then developed using the methods of calculus
of variations.Comment: 23 page
Symmetry of high-piezoelectric Pb-based complex perovskites at the morphotropic phase boundary I. Neutron diffraction study on Pb(Zn1/3Nb2/3)O3 -9%PbTiO3
The symmetry was examined using neutron diffraction method on
Pb(Zn1/3Nb2/3)O3 -9%PbTiO3 (PZN/9PT) which has a composition at the
morphotropic phase boundary (MPB) between Pb(Zn1/3Nb2/3)O3 and PbTiO3. The
results were compared with those of other specimens with same composition but
with different prehistory. The equilibrium state of all examined specimens is
not the mixture of rhombohedral and tetragonal phases of the end members but
exists in a new polarization rotation line Mc# (orthorhombic-monoclinic line).
Among examined specimens, one exhibited tetragonal symmetry at room temperature
but recovered monoclinic phase after a cooling and heating cycle
Low-temperature phase transformations of PZT in the morphotropic phase-boundary region
We present anelastic and dielectric spectroscopy measurements of
PbZr(1-x)Ti(x)O(3) with 0.455 < x < 0.53, which provide new information on the
low temperature phase transitions. The tetragonal-to-monoclinic transformation
is first-order for x < 0.48 and causes a softening of the polycrystal Young's
modulus whose amplitude may exceed the one at the cubic-to-tetragonal
transformation; this is explainable in terms of linear coupling between shear
strain components and tilting angle of polarization in the monoclinic phase.
The transition involving rotations of the octahedra below 200 K is visible both
in the dielectric and anelastic losses, and it extends within the tetragonal
phase, as predicted by recent first-principle calculations.Comment: 4 pages, 4 figure
On Distribution Functions for Partons in Nuclei
We suggest that a previously conjectured relation between Structure Functions
(SF) for nuclei and nucleons also links distribution functions (df) for partons
in a nucleus and in nucleons. The above suggestion ensures in principle
identical results for SF , whether computed with hadronic or partonic
degrees of freedom. In practice there are differences, due to different
input. We show that the thus defined nuclear parton distribution functions
(pdf) respect standard sumrules. In addition we numerically compare some
moments of nuclear SF, and find agreement between results, using hadronic and
partonic descriptions. We present computations of EMC ratios for both, and
compare those with hadronic predictions and data. In spite of substantial
differences in the participating SF, the two representations produce
approximately the same EMC ratios. The apparent correlation between the above
deviations is ascribed to a sumrule for . We conclude with a discussion
of alternative approaches to nuclear pdf.Comment: 14 pages, 4 figure
Quark Masses: An Environmental Impact Statement
We investigate worlds that lie on a slice through the parameter space of the
Standard Model over which quark masses vary. We allow as many as three quarks
to participate in nuclei, while fixing the mass of the electron and the average
mass of the lightest baryon flavor multiplet. We classify as "congenial" worlds
that satisfy the environmental constraint that the quark masses allow for
stable nuclei with charges one, six, and eight, making organic chemistry
possible. Whether a congenial world actually produces observers depends on a
multitude of historical contingencies, beginning with primordial
nucleosynthesis, which we do not explore. Such constraints may be independently
superimposed on our results. Environmental constraints such as the ones we
study may be combined with information about the a priori distribution of quark
masses over the landscape of possible universes to determine whether the
measured values of the quark masses are determined environmentally, but our
analysis is independent of such an anthropic approach.
We estimate baryon masses as functions of quark masses and nuclear masses as
functions of baryon masses. We check for the stability of nuclei against
fission, strong particle emission, and weak nucleon emission. For two light
quarks with charges 2/3 and -1/3, we find a band of congeniality roughly 29 MeV
wide in their mass difference. We also find another, less robust region of
congeniality with one light, charge -1/3 quark, and two heavier, approximately
degenerate charge -1/3 and 2/3 quarks. No other assignment of light quark
charges yields congenial worlds with two baryons participating in nuclei. We
identify and discuss the region in quark-mass space where nuclei would be made
from three or more baryon species.Comment: 40 pages, 16 figures (in color), 4 tables. See paper for a more
detailed abstract. v4: Cleaning up minor typo
Nitrogen oxides in the boundary layer and free troposphere at the Mt. Bachelor Observatory
Nitrogen oxide (NO<sub>x</sub>=NO+NO<sub>2</sub>) observations were made at the Mt. Bachelor Observatory in central Oregon, USA (MBO; 2.73 km above sea level) during one autumn and three springtime (15 April–20 May) periods. This is the first study to discuss interannual variability in NO<sub>x</sub> for this region. NO<sub>x</sub> concentrations (mean&plusmn;1&sigma;) for spring 2007, 2008 and 2009 were 119&plusmn;65, 117&plusmn;65, and 91&plusmn;54 pptv, respectively. The difference in mean mixing ratios between 2007 and 2008 is not statistically significant, whereas the difference between these years and 2009 is significant (<i>p</i><0.01). We attribute the decline in NO<sub>x</sub> from 2007–2008 to 2009 to changes in free tropospheric synoptic conditions over the Northeast Pacific and trans-Pacific transport pathways during spring 2009. In 2009, there were: (1) higher geopotential heights over the Gulf of Alaska, (2) warmer temperatures over the Aleutian Islands/Gulf of Alaska and (3) much weaker winds throughout the North Pacific. During the autumn 2008 campaign, NO<sub>x</sub> concentrations (mean&plusmn;1&sigma;) were 175&plusmn;548 pptv. The highly non-normal distribution of data (skewness coefficient of 19.1 vs. 2.5, 2.8 and 2.4 in spring 2007, 2008 and 2009, respectively) resulted from periods of very high NO<sub>x</sub> levels. Using MODIS Rapid Response (Aqua and Terra) results, we show that during autumn our site can be heavily influenced by wildfires in western North America. This is in contrast to springtime, when the smaller positive (i.e., right) tail of the NO<sub>x</sub> distribution is driven largely by Asian long-range transport (ALRT) events. <br><br> We developed a novel means of segregating boundary layer (BL)-influenced vs. free tropospheric (FT) air. During spring 2008 we collected "chairlift soundings" of temperature, relative humidity and pressure in an effort to better understand the diurnal pattern of a BL influence at our summit station. Results from this experiment revealed that, on average, a BL influence begins around 10:00 PDT (UTC – 07:00 h) in spring. Using this information to isolate FT air, we characterize probable pollution sources and synoptic conditions for the top 20 FT NO<sub>x</sub> events over three spring campaigns. Half (<i>n</i>=10) of these 20 events were determined to be "Imported" events characterized by anomalously: (1) high geopotential heights off the west coast of North America, (2) warm temperatures stretching from the Aleutian Islands to Baja California, and (3) strong southwesterly winds in the Asian outflow region. Five events exhibited an influence from the North American continent. These events are characterized by very strong cyclonic behavior off the northwestern USA coast
- …