2,354 research outputs found
The impact of celestial pole offset modelling on VLBI UT1 Intensive results
Very Long Baseline Interferometry (VLBI) Intensive sessions are scheduled to
provide operational Universal Time (UT1) determinations with low latency. UT1
estimates obtained from these observations heavily depend on the model of the
celestial pole motion used during data processing. However, even the most
accurate precession-nutation model, IAU 2000/2006, is not accurate enough to
realize the full potential of VLBI observations. To achieve the highest
possible accuracy in UT1 estimates, a celestial pole offset (CPO), which is the
difference between the actual and modelled precession-nutation angles, should
be applied. Three CPO models are currently available for users. In this paper,
these models have been tested and the differences between UT1 estimates
obtained with those models are investigated. It has been shown that neglecting
CPO modelling during VLBI UT1 Intensive processing causes systematic errors in
UT1 series of up to 20 microarcseconds. It has been also found that using
different CPO models causes the differences in UT1 estimates reaching 10
microarcseconds. Obtained results are applicable to the satellite data
processing as well.Comment: 8 pp., accepted for publication in Journal of Geodes
Analysis of the Accuracy of Prediction of the Celestial Pole Motion
VLBI observations carried out by global networks provide the most accurate
values of the precession-nutation angles determining the position of the
celestial pole; as a rule, these results become available two to four weeks
after the observations. Therefore, numerous applications, such as satellite
navigation systems, operational determination of Universal Time, and space
navigation, use predictions of the coordinates of the celestial pole. In
connection with this, the accuracy of predictions of the precession- nutation
angles based on observational data obtained over the last three years is
analyzed for the first time, using three empiric nutation models---namely,
those developed at the US Naval Observatory, the Paris Observatory, and the
Pulkovo Observatory. This analysis shows that the last model has the best of
accuracy in predicting the coordinates of the celestial pole. The rms error for
a one-month prediction proposed by this model is below 100 microarcsecond.Comment: 13 p
The influence of Galactic aberration on precession parameters determined from VLBI observations
The influence of proper motions of sources due to Galactic aberration on
precession models based on VLBI data is determined. Comparisons of the linear
trends in the coordinates of the celestial pole obtained with and without
taking into account Galactic aberration indicate that this effect can reach 20
as per century, which is important for modern precession models. It is
also shown that correcting for Galactic aberration influences the derived
parameters of low-frequency nutation terms. It is therefore necessary to
correct for Galactic aberration in the reduction of modern astrometric
observations
Cross-relaxation and phonon bottleneck effects on magnetization dynamics in LiYF4:Ho3+
Frequency and dc magnetic field dependences of dynamic susceptibility in
diluted paramagnets LiYF:Ho have been measured at liquid helium
temperatures in the ac and dc magnetic fields parallel to the symmetry axis of
a tetragonal crystal lattice. Experimental data are analyzed in the framework
of microscopic theory of relaxation rates in the manifold of 24
electron-nuclear sublevels of the lowest non-Kramers doublet and the first
excited singlet in the Ho ground multiplet split by the crystal
field of S symmetry. The one-phonon transition probabilities were computed
using electron-phonon coupling constants calculated in the framework of
exchange charge model and were checked by optical piezospectroscopic
measurements. The specific features observed in field dependences of the in-
and out-of-phase susceptibilities (humps and dips, respectively) at the
crossings (anti-crossings) of the electron-nuclear sublevels are well
reproduced by simulations when the phonon bottleneck effect and the cross-spin
relaxation are taken into account
Parity doubling in particle physics
Parity doubling in excited hadrons is reviewed. Parity degeneracy in hadrons
was first experimentally observed 40 years ago. Recently new experimental data
on light mesons caused much excitement and renewed interest to the phenomenon,
which still remains to be enigmatic. The present retrospective review is an
attempt to trace the history of parity doubling phenomenon, thus providing a
kind of introduction to the subject. We begin with early approaches of 1960s
(Regge theory and dynamical symmetries) and end up with the latest trends
(manifestations of broader degeneracies and AdS/QCD). We show the evolution of
various ideas about parity doubling. The experimental evidence for this
phenomenon is scrutinized in the non-strange sector. Some experiments of 1960s
devoted to the search for missing non-strange bosons are re-examined and it is
argued that results of these experiments are encouraging from the modern
perspective.Comment: Version to appear in Int. J. Mod. Phys. A, 63 pages, 9 figure
Supersymmetry and a Time-Dependent Landau System
A general technique is outlined for investigating supersymmetry properties of
a charged spin-\half quantum particle in time-varying electromagnetic fields.
The case of a time-varying uniform magnetic induction is examined and shown to
provide a physical realization of a supersymmetric quantum-mechanical system.
Group-theoretic methods are used to factorize the relevant Schr\"odinger
equations and obtain eigensolutions. The supercoherent states for this system
are constructed.Comment: 47 pages, submitted to Phys. Rev. A, LaTeX, IUHET 243 and
LA-UR-93-20
Decoherence window and electron-nuclear cross-relaxation in the molecular magnet V 15
Rabi oscillations in the V_15 Single Molecule Magnet (SMM) embedded in the
surfactant DODA have been studied at different microwave powers. An intense
damping peak is observed when the Rabi frequency Omega_R falls in the vicinity
of the Larmor frequency of protons w_N, while the damping time t_R of
oscillations reaches values 10 times shorter than the phase coherence time t_2
measured at the same temperature. The experiments are interpreted by the N-spin
model showing that t_R is directly associated with the decoherence via
electronic/nuclear spin cross-relaxation in the rotating reference frame. It is
shown that this decoherence is accompanied with energy dissipation in the range
of the Rabi frequencies w_N - sigma_e < Omega_R < w_N, where sigma_e is the
mean super-hyperfine field (in frequency units) induced by protons at SMMs.
Weaker damping without dissipation takes place outside this dissipation window.
Simple local field estimations suggest that this rapid cross-relaxation in
resonant microwave field observed for the first time in SMMV_15 should take
place in other SMMs like Fe_8 and Mn_12 containing protons, too
One-Parameter Squeezed Gaussian States of Time-Dependent Harmonic Oscillator and Selection Rule for Vacuum States
By using the invariant method we find one-parameter squeezed Gaussian states
for both time-independent and time-dependent oscillators. The squeezing
parameter is expressed in terms of energy expectation value for
time-independent case and represents the degree of mixing positive and negative
frequency solutions for time-dependent case. A {\it minimum uncertainty
proposal} is advanced to select uniquely vacuum states at each moment of time.
We show that the Gaussian states with minimum uncertainty coincide with the
true vacuum state for time-independent oscillator and the Bunch-Davies vacuum
for a massive scalar field in a de Sitter spacetime.Comment: 13 Pages, ReVTeX, no figure
Coherent states of non-relativistic electron in magnetic-solenoid field
We construct coherent states of a nonrelativistic electron in the
magnetic-solenoid field, which is a superposition of the Aharonov-Bohm field
and a collinear uniform magnetic field. In the problem under consideration
there are two kind of coherent states, the first kind corresponds to classical
trajectories which embrace the solenoid and the second one to trajectories
which do not. Mean coordinates in the constructed coherent states are moving
along classical trajectories, the coherent states maintain their form under the
time evolution, and represent a complete set of functions, which can be useful
in semi classical calculations. In the absence of the Aharonov-Bohm filed these
states are reduced to the well-known in the case of uniform magnetic field
Malkin-Man'ko coherent states.Comment: 11 pages, version accepted for publication in J. Phys. A, 3 figures
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Symplectic structure of the moduli space of flat connections on a Riemann surface
We consider canonical symplectic structure on the moduli space of flat
{\g}-connections on a Riemann surface of genus with marked points.
For {\g} being a semisimple Lie algebra we obtain an explicit efficient
formula for this symplectic form and prove that it may be represented as a sum
of copies of Kirillov symplectic form on the orbit of dressing
transformations in the Poisson-Lie group and copies of the
symplectic structure on the Heisenberg double of the Poisson-Lie group (the
pair () corresponds to the Lie algebra {\g}).Comment: 20 page
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