4,599 research outputs found
The multiple quantum NMR dynamics in systems of equivalent spins with the dipolar ordered initial state
The multiple quantum (MQ) NMR dynamics in the system of equivalent spins with
the dipolar ordered initial state is considered. The high symmetry of the MQ
Hamiltonian is used in order to develop the analytical and numerical methods
for an investigation of the MQ NMR dynamics in the systems consisting of
hundreds of spins from "the first principles". We obtain the dependence of the
intensities of the MQ NMR coherences on their orders (profiles of the MQ NMR
coherences) for the systems of spins. It is shown that these
profiles may be well approximated by the exponential distribution functions. We
also compare the MQ NMR dynamics in the systems of equivalent spins having two
different initial states, namely the dipolar ordered state and the thermal
equilibrium state in the strong external magnetic field.Comment: 11 pages 4 figure
Matrix elements of the electromagnetic operator between kaon and pion states
We compute the matrix elements of the electromagnetic (EM) operator between
kaon and pion states, using lattice QCD with maximally twisted-mass fermions
and two flavors of dynamical quarks (Nf = 2). The EM operator is renormalized
non-perturbatively in the RI'/MOM scheme and our simulations cover pion masses
as light as 260 MeV and three values of the lattice spacing, ranging from ~
0.07 up to ~ 0.1 fm. At the physical point our preliminary result for the K ->
pion tensor form factor at zero-momentum transfer is fT[K\pi](0) =
0.42(2_stat), which differs significantly from the old quenched result
fT[K\pi](0) = 0.78(6) obtained by the SPQcdR Collaboration with pion masses
above 500 MeV. We investigate the source of this difference and conclude that
it is mainly related to the chiral extrapolation of the quenched data. For the
case of the tensor charge of the pion we obtain the preliminary value
fT[\pi\pi](0) = 0.200(14_stat), which can be compared with the result
fT[\pi\pi](0) = 0.216(34) obtained at Nf = 2 by the QCDSF Collaboration using
higher pion masses.Comment: 7 pages, 5 figures, contribution to the XXVIII International
Symposium on Lattice Field Theory, Lattice2010, June 14-19, 2010,
Villasimius, Ital
Exact results on spin dynamics and multiple quantum dynamics in alternating spin-1/2 chains with XY-Hamiltonian at high temperatures
We extend the picture of a transfer of nuclear spin-1/2 polarization along a
homogeneous one-dimensional chain with the XY-Hamiltonian to the inhomogeneous
chain with alternating nearest neighbour couplings and alternating Larmor
frequencies. To this end, we calculate exactly the spectrum of the spin-1/2
XY-Hamiltonian of the alternating chain with an odd number of sites. The exact
spectrum of the XY-Hamiltonian is also applied to study the multiple quantum
(MQ) NMR dynamics of the alternating spin-1/2 chain. MQ NMR spectra are shown
to have the MQ coherences of zero and second orders just as in the case
of a homogeneous chain. The intensities of the MQ coherences are calculated.Comment: 10 pages, 4 figure
Dating COINS: Kinematic Ages for Compact Symmetric Objects
We present multi-epoch VLBA observations of Compact Symmetric Objects (CSOs)
from the COINS sample (CSOs Observed In the Northern Sky). These observations
allow us to make estimates of, or place limits on, the kinematic ages for those
sources with well-identified hot spots. This study significantly increases the
number of CSOs with well-determined ages or limits. The age distribution is
found to be sharply peaked under 500 years, suggesting that many CSOs die
young, or are episodic in nature, and very few survive to evolve into FR II
sources like Cygnus A. Jet components are found to have higher velocities than
hot spots which is consistent with their movement down cleared channels. We
also report on the first detections of significant polarization in two CSOs,
J0000+4054 (2.1%) and J1826+1831 (8.8%). In both cases the polarized emission
is found in jet components on the stronger side of the center of activity.Comment: 34 pages including 7 figures, Accepted to ApJ on Dec 7, 200
Spin-resolved electron-impact ionization of lithium
Electron-impact ionization of lithium is studied using the convergent
close-coupling (CCC) method at 25.4 and 54.4 eV. Particular attention is paid
to the spin-dependence of the ionization cross sections. Convergence is found
to be more rapid for the spin asymmetries, which are in good agreement with
experiment, than for the underlying cross sections. Comparison with the recent
measured and DS3C-calculated data of Streun et al (1999) is most intriguing.
Excellent agreement is found with the measured and calculated spin asymmetries,
yet the discrepancy between the CCC and DS3C cross sections is very large
Kinematic Age Estimates for 4 Compact Symmetric Objects from the Pearson-Readhead Survey
Based on multi-epoch observations at 15 and 43 GHz with the Very Long
Baseline Array (VLBA) we detect significant angular expansions between the two
hot spots of 4 Compact Symmetric Objects (CSOs). From these relative motions we
derive kinematic ages of between 300 and 1200 years for the radio emission.
These ages lend support to the idea that CSOs are produced in a recent phase of
activity. These observations also allow us to study the evolution of the hot
spots dynamically in individual sources. In all 4 sources the hot spots are
separating along the source axis, but in 1031+567 the tip of one hot spot
appears to be moving almost orthogonally to the source axis. Jet components,
seen in 3 of the 4 sources observed, are found to be moving relativistically
outward from the central engines towards the more slowly moving hot spots.Comment: in press at ApJ for v. 541 Oct. 1, 2000, 23 page LaTeX document
includes 6 postscript figure
Compact Symmetric Objects as Radio Flux Density Calibrators
We present results from the first intensive monitoring campaign of a sample
of Compact Symmetric Objects (CSOs). We observed seven CSOs at 8.5 GHz over a
period of eight months, with an average spacing between observations of 2.7
days. Our results show that, as predicted, the flux densities of the CSOs are
extremely stable; the mean RMS variability of the sample was 0.7% in flux
density. The low variability of the CSOs makes them excellent flux density
calibrators at this frequency. We recommend that at least four CSOs be included
in any VLA monitoring campaign which requires precise epoch-to-epoch
calibration, such as those to measure gravitational lens time delays. The CSO
data enable the correction of small systematic errors in the primary flux
calibration.Comment: To appear in the Astronomical Journal. 19 pages, 5 figure
Remote sensing of ice crystal asymmetry parameter using multi-directional polarization measurements – Part 1: Methodology and evaluation with simulated measurements
We present a new remote sensing technique to infer the average asymmetry parameter of ice crystals near cloud top from multi-directional polarization measurements. The method is based on previous findings that (a) complex aggregates of hexagonal crystals generally have scattering phase matrices resembling those of their components; and (b) scattering phase matrices systematically vary with aspect ratios of crystals and their degree of microscale surface roughness. Ice cloud asymmetry parameters are inferred from multi-directional polarized reflectance measurements by searching for the closest fit in a look-up table of simulated polarized reflectances computed for cloud layers that contain individual, randomly oriented hexagonal columns and plates with varying aspect ratios and roughness values. The asymmetry parameter of the hexagonal particle that leads to the best fit with the measurements is considered the retrieved value. For clouds with optical thickness less than 5, the cloud optical thickness must be retrieved simultaneously with the asymmetry parameter, while for optically thicker clouds the asymmetry parameter retrieval is independent of cloud optical thickness. Evaluation of the technique using simulated measurements based on the optical properties of a number of complex particles and their mixtures shows that the ice crystal asymmetry parameters are generally retrieved to within 5%, or about 0.04 in absolute terms. The retrieval scheme is largely independent of calibration errors, range and sampling density of scattering angles and random noise in the measurements. The approach can be applied to measurements of past, current and future airborne and satellite instruments that measure multi-directional polarized reflectances of ice-topped clouds
Chandra Discovery of a 300 kpc X-ray Jet in the GPS Quasar PKS1127-145
We have discovered an X-ray jet with Chandra imaging of the z=1.187
radio-loud quasar PKS1127-145. In this paper we present the Chandra X-ray data,
follow-up VLA observations, and optical imaging using the HST WFPC2. The X-ray
jet contains 273+/-5 net counts in 27ksec and extends ~30 arcsec, from the
quasar core, corresponding to a minimum projected linear size of ~330/h_50 kpc.
The evaluation of the X-ray emission processes is complicated by the observed
offsets between X-ray and radio brightness peaks. We discuss the problems posed
by these observations to jet models. In addition, PKS1127-145 is a Giga-Hertz
Peaked Spectrum radio source, a member of the class of radio sources suspected
to be young or ``frustrated'' versions of FRI radio galaxies. However the
discovery of an X-ray and radio jet extending well outside the host galaxy of
PKS1127-145 suggests that activity in this and other GPS sources may be
long-lived and complex.Comment: 22 pages, 11 ps figures, 1 figure in a JPG file, 3 tables. AASTEX.
Accepted by The Astrophysical Journa
Evolution of spin entanglement and an entanglement witness in multiple-quantum NMR experiments
We investigate the evolution of entanglement in multiple-quantum (MQ) NMR
experiments in crystals with pairs of close nuclear spins-1/2. The initial
thermodynamic equilibrium state of the system in a strong external magnetic
field evolves under the non-secular part of the dipolar Hamiltonian. As a
result, MQ coherences of the zeroth and plus/minus second orders appear. A
simple condition for the emergence of entanglement is obtained. We show that
the measure of the spin pair entanglement, concurrence, coincides qualitatively
with the intensity of MQ coherences of the plus/minus second order and hence
the entanglement can be studied with MQ NMR methods. We introduce an
Entanglement Witness using MQ NMR coherences of the plus/minus second order.Comment: 5 pages, 2 figure
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