555,658 research outputs found
Multiple Quantum NMR Dynamics in Dipolar Ordered Spin Systems
We investigate analytically and numerically the Multiple Quantum (MQ) NMR
dynamics in systems of nuclear spins 1/2 coupled by the dipole-dipole
interactions in the case of the dipolar ordered initial state. We suggest two
different methods of MQ NMR. One of them is based on the measurement of the
dipolar temperature in the quasi-equilibrium state which establishes after the
time of order T2 after the MQ NMR experiment. The other method uses an
additional resonance 45^0 -pulse after the preparation period of the standard
MQ NMR experiment in solids. Many-spin clusters and correlations are created
faster in such experiments than in the usual MQ NMR experiments and can be used
for the investigation of many-spin dynamics of nuclear spins in solids.Comment: 11 pages, 3 figures. accepted for publication in Physical Review
Variability of the soft X-ray excess in IRAS 13224-3809
We study the soft excess variability of the narrow line Seyfert 1 galaxy IRAS
13224-3809. We considered all five archival XMM-Newton observations, and we
applied the 'flux-flux plot' (FFP) method. We found that the flux-flux plots
were highly affected by the choice of the light curves' time bin size, most
probably because of the fast and large amplitude variations, and the intrinsic
non-linear flux--flux relations in this source. Therefore, we recommend that
the smallest bin-size should be used in such cases. Hence, We constructed FFPs
in 11 energy bands below 1.7 keV, and we considered the 1.7-3 keV band, as
being representative of the primary emission. The FFPs are reasonably well
fitted by a 'power-law plus a constant' model. We detected significant positive
constants in three out of five observations. The best-fit slopes are flatter
than unity at energies below keV, where the soft excess is
strongest. This suggests the presence of intrinsic spectral variability. A
power-law-like primary component, which is variable in flux and spectral slope
(as ) and a soft-excess component, which varies
with the primary continuum (as ),
can broadly explain the FFPs. In fact, this can create positive `constants',
even when a stable spectral component does not exist. Nevertheless, the
possibility of a stable, soft--band constant component cannot be ruled out, but
its contribution to the observed 0.2-1 keV band flux should be less than %. The model constants in the FFPs were consistent with zero in one
observation, and negative at energies below 1 keV in another. It is hard to
explain these results in the context of any spectral variability scenario, but
they may signify the presence of a variable, warm absorber in the source.Comment: Accepted for publication in A&A, 10 pages, 7 figure
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
Fine Tuning Free Paradigm of Two Measures Theory: K-Essence, Absence of Initial Singularity of the Curvature and Inflation with Graceful Exit to Zero Cosmological Constant State
The dilaton-gravity sector of the Two Measures Field Theory (TMT)is explored
in detail in the context of cosmology. The model possesses scale invariance
which is spontaneously broken due to the intrinsic features of the TMT
dynamics. The effective model represents an explicit example of the effective
k-essence resulting from first principles without any exotic term in the
fundamental action. Depending of the choice of regions in the parameter space,
TMT exhibits different possible outputs for cosmological dynamics: a) Absence
of initial singularity of the curvature while its time derivative is singular.
This is a sort of "sudden" singularities studied by Barrow on purely kinematic
grounds. b) Power law inflation in the subsequent stage of evolution. Depending
on the region in the parameter space (but without fine tuning) the inflation
ends with a graceful exit either into the state with zero cosmological constant
(CC) or into the state driven by both a small CC and the field phi with a
quintessence-like potential. c) Possibility of resolution of the old CC
problem. From the point of view of TMT, it becomes clear why the old CC problem
cannot be solved (without fine tuning) in conventional field theories. d) TMT
enables two ways for achieving small CC without fine tuning of dimensionfull
parameters: either by a seesaw type mechanism or due to a correspondence
principle between TMT and conventional field theories (i.e theories with only
the measure of integration sqrt{-g} in the action. e) There is a wide range of
the parameters such that in the late time universe: the equation-of-state
w=p/\rho <-1; w asymptotically (as t\to\infty) approaches -1 from below; \rho
approaches a constant, the smallness of which does not require fine tuning of
dimensionfull parameters.Comment: 37 pages, 20 figures. Minor misprints corrected, reference added. The
final version published in Phys. Rev.
Relationship between probabilities of the state transfers and entanglements in spin systems with simple geometrical configurations
In this paper we derive analytical relations between probabilities of the
excited state transfers and entanglements calculated by both the Wootters and
positive partial transpose (PPT) criteria for the arbitrary spin system with
single excited spin in the external magnetic field and Hamiltonian commuting
with . We apply these relations to study the arbitrary state transfers and
entanglements in the simple systems of nuclear spins having two- and
three-dimensional geometrical configurations with Hamiltonian. It is
shown that High-Probability State Transfers (HPSTs) are possible among all four
nodes placed in the corners of the rectangle with the proper ratio of sides as
well as among all eight nodes placed in the corners of the parallelepiped with
the proper ratio of sides. Entanglements responsible for these HPSTs have been
identified.Comment: 27pages, 10 figure
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