47,441 research outputs found
Transverse Quark Distribution in Mesons - QCD Sum Rule Approach -
QCD sum rules are used to compute the first few moments of the mesonic quark
momentum. Transverse, longitudinal and mixed transverse-longitudinal components
are examined. The transverse size of the pion is shown to be dictated by the
gluon condensate, even though the mass and the longitudinal distribution are
dominated by the quark condensate. The implications of our results for color
transparency physics and finite temperature QCD are discussed.Comment: 8 pages, Latex, Univ. of Washington preprint DOE/ER/40427-24-N9
Density Functional Theory and Molecular Dynamics Studies on Energetics and Kinetics for Electro-Active Polymers: PVDF and P(VDF-TrFE)
We use first principles methods to study static and dynamical mechanical
properties of the ferroelectric polymer Poly(vinylidene fluoride) (PVDF) and
its copolymer with trifluoro ethylene (TrFE). We use density functional theory
[within the generalized gradient approximation (DFT-GGA)] to calculate
structures and energetics for various crystalline phases for PVDF and
P(VDF-TrFE). We find that the lowest energy phase for PVDF is a non-polar
crystal with a combination of trans (T) and gauche (G) bonds; in the case of
the copolymer the role of the extra (bulkier) F atoms is to stabilize T bonds.
This leads to the higher crystallinity and piezoelectricity observed
experimentally. Using the MSXX first principles-based force field (FF) with
molecular dynamics (MD), we find that the energy barrier necessary to nucleate
a kink (gauche pairs separated by trans bonds) in an all-T crystal is much
lower (14.9 kcal/mol) in P(VDF-TrFE) copolymer than in PVDF (24.8 kcal/mol).
This correlates with the observation that the polar phase of the copolymer
exhibits a solid-solid a transition to a non-polar phase under heating while
PVDF directly melts. We also studied the mobility of an interface between a
polar and non-polar phases under uniaxial stress; we find a lower threshold
stress and a higher mobility in the copolymer as compared with PVDF. Finally,
considering plastic deformation under applied shear, we find that the chains
for P(VDF-TrFE) have a very low resistance to sliding, particularly along the
chain direction. The atomistic characterization of these "unit mechanisms"
provides essential input to mesoscopic or macroscopic models of electro-active
polymers.Comment: 15 pages 9 figures Electro-active polyme
Remote spectral imaging with simultaneous extraction of 3D topography for historical wall paintings
PRISMS (Portable Remote Imaging System for Multispectral Scanning) is designed for in situ, simultaneous high resolution spectral and 3D topographic imaging of wall paintings and other large surfaces. In particular, it can image at transverse resolutions of tens of microns remotely from distances of tens of metres, making high resolution imaging possible from a fixed position on the ground for areas at heights that is difficult to access. The spectral imaging system is fully automated giving 3D topographic mapping at millimetre accuracy as a by-product of the image focusing process. PRISMS is the first imaging device capable of both 3D mapping and spectral imaging simultaneously without additional distance measuring devices. Examples from applications of PRISMS to wall paintings at a UNESCO site in the Gobi desert are presented to demonstrate the potential of the instrument for large scale 3D spectral imaging, revealing faded writing and material identification
Non-volatile resistive switching in dielectric superconductor YBCO
We report on the reversible, nonvolatile and polarity dependent resistive
switching between superconductor and insulator states at the interfaces of a
Au/YBaCuO (YBCO)/Au system. We show that the
superconducting state of YBCO in regions near the electrodes can be reversibly
removed and restored. The possible origin of the switching effect may be the
migration of oxygen or metallic ions along the grain boundaries that control
the intergrain superconducting coupling. Four-wire bulk resistance measurements
reveal that the migration is not restricted to interfaces and produce
significant bulk effects.Comment: 4 pages, 4 figures, corresponding author: C. Acha ([email protected]
Negative Refraction of Excitations in the Bose-Hubbard Model
Ultracold atoms in optical lattices provide a unique opportunity to study
Bose- Hubbard physics. In this work we show that by considering a spatially
varying onsite interaction it is possible to manipulate the motion of
excitations above the Mott phase in a Bose-Hubbard system. Specifically, we
show that it is possible to "engineer" regimes where excitations will
negatively refract, facilitating the construction of a flat lens.Comment: 6 pages, 4 figure
Dirac-Schr\"odinger equation for quark-antiquark bound states and derivation of its interaction kerne
The four-dimensional Dirac-Schr\"odinger equation satisfied by
quark-antiquark bound states is derived from Quantum Chromodynamics. Different
from the Bethe-Salpeter equation, the equation derived is a kind of first-order
differential equations of Schr\"odinger-type in the position space. Especially,
the interaction kernel in the equation is given by two different closed
expressions. One expression which contains only a few types of Green's
functions is derived with the aid of the equations of motion satisfied by some
kinds of Green's functions. Another expression which is represented in terms of
the quark, antiquark and gluon propagators and some kinds of proper vertices is
derived by means of the technique of irreducible decomposition of Green's
functions. The kernel derived not only can easily be calculated by the
perturbation method, but also provides a suitable basis for nonperturbative
investigations. Furthermore, it is shown that the four-dimensinal
Dirac-Schr\"odinger equation and its kernel can directly be reduced to rigorous
three-dimensional forms in the equal-time Lorentz frame and the
Dirac-Schr\"odinger equation can be reduced to an equivalent
Pauli-Schr\"odinger equation which is represented in the Pauli spinor space. To
show the applicability of the closed expressions derived and to demonstrate the
equivalence between the two different expressions of the kernel, the t-channel
and s-channel one gluon exchange kernels are chosen as an example to show how
they are derived from the closed expressions. In addition, the connection of
the Dirac-Schr\"odinger equation with the Bethe-Salpeter equation is discussed
Spin-Orbit Coupling and Ion Displacements in Multiferroic TbMnO3
The electronic and magnetic properties of TbMnO3 leading to its ferroelectric
(FE) polarization were investigated on the basis of relativistic density
functional theory (DFT) calculations. In agreement with experiment, we show
that the spin-spiral plane of TbMnO3 can be either the bc- or ab-plane, but not
the ac-plane. As for the mechanism of FE polarization, our work reveals that
the "pure electronic" model by Katsura, Nagaosa and Balatsky (KNB) is
inadequate in predicting the absolute direction of FE polarization. For the
ab-plane spin-spiral state of TbMnO3, the direction of FE polarization
predicted by the KNB model is opposite to that predicted by DFT calculations.
In determining the magnitude and the absolute direction of FE polarization in
spin-spiral states, it is found crucial to consider the displacements of the
ions from their ecntrosymmetric positions
Strange meson-nucleon states in the quark potential model
The quark potential model and resonating group method are used to investigate
the bound states and/or resonances. The model potential consists of
the t-channel and s-channel one-gluon exchange potentials and the confining
potential with incorporating the QCD renormalization correction and the
spin-orbital suppression effect in it. It was shown in our previous work that
by considering the color octet contribution, use of this model to investigate
the low energy elastic scattering leads to the results which are in pretty
good agreement with the experimental data. In this paper, the same model and
method are employed to calculate the masses of the bound systems.
For this purpose, the resonating group equation is transformed into a standard
Schr\"odinger equation in which a nonlocal effective interaction
potential is included. Solving the Schr\"odinger equation by the variational
method, we are able to reproduce the masses of some currently concerned
states and get a view that these states possibly exist as
molecular states. For the system, the same calculation gives no support to
the existence of the resonance which was announced
recently.Comment: 15 pages, 4 figure
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