67,873 research outputs found
Reply on `comment on our paper `Single two-level ion in an anharmonic-oscillator trap: Time evolution of the Q function and population inversion ''
We show here that the model Hamiltonian used in our paper for ion vibrating
in a q-analog harmonic oscillator trap and interacting with a classical
single-mode light field is indeed obtained by replacing the usual bosonic
creation and annihilation operators of the harmonic trap model by their
q-deformed counterparts. The approximations made in our paper amount to using
for the ion-laser interaction in a q-analog harmonic oscillator trap, the
operator F_{q}=exp{-(|\epsilon|^2}/2)}exp{i\epsilon A^{\dagger}}exp{i\epsilon
A}, which is analogous to the corresponding operator for ion in a harmonic
oscillator trap that is . In our article we do not claim to have diagonalized the
operator, , for which the basis states
|g,m> and |e,m> are not analytic vectors.Comment: Revtex, 4pages. To be Published in Physical Review A59, NO.4(April
99
Composition dependent magnetic properties of iron oxide - polyaniline nanoclusters
Gamma - Iron Oxide prepared by sol -gel process was used to produce
nanocomposites with polyaniline of varying aniline concentrations. TEM shows
the presence of chain like structure for lower polyaniline concentration. The
room temperature hysteresis curves show finite coercivity of 160 Oe for all the
composites while the saturation magnetization was found to decrease with
increasing polymer content. ZFC - FC magnetisation measurements indicate high
blocking temperatures. It is believed that this indicates a strongly
interacting system, which is also shown by our TEM results. Monte Carlo
simulations performed on a random anisotropy model with dipolar and exchange
inteactions match well with experimental results.Comment: 9 (nine) pages, 6 figures (jpeg and eps
GALAH Survey: Chemically Tagging the Thick Disk
The GALAH survey targets one million stars in the southern hemisphere down to
a limiting magnitude of V = 14 at the Anglo- Australian Telescope. The project
aims to measure up to 30 elemental abundances and radial velocities (~1 km/s
accuracy) for each star at a resolution of R = 28000. These elements fall into
8 independent groups (e.g. alpha, Fe peak, r-process). For all stars, Gaia will
provide distances to 1% and transverse velocities to 1 km/s or better, giving
us a 14D set of parameters for each star, i.e. 6D phase space and 8D abundance
space. There are many scientic applications but here we focus on the prospect
of chemically tagging the thick disk and making a direct measurement of how
stellar migration evolves with cosmic time.Comment: Barcelona conference (Dec 1-5, 2014): The Milky Way Unravelled by
Gaia, eds. Soubiran, Figueras, Walton; 8 page
Low temperature hopping magnetotransport in paramagnetic single crystals of cobalt doped ZnO
Long needle-shaped single crystals of Zn1-xCoxO were grown at low
temperatures using a molten salt solvent technique, up to x=0.10. The
conduction process at low temperatures is determined to be by Mott variable
range hopping. Both pristine and cobalt doped crystals clearly exhibit a
crossover from negative to positive magnetoresistance as the temperature is
decreased. The positive magnetoresistance of the Zn1-xCoxO single crystals
increases with increased Co concentration and reaches up to 20% at low
temperatures (2.5 K) and high fields (>1 T). SQUID magnetometry confirms that
the Zn1-xCoxO crystals are predominantly paramagnetic in nature and the
magnetic response is independent of Co concentration. The results indicate that
cobalt doping of single crystalline ZnO introduces localized electronic states
and isolated Co2+ ions into the host matrix, but that the magnetotransport and
magnetic properties are decoupled.Comment: 7 pages, 9 figures, submitted to Physical Review
A critical layer model for turbulent pipe flow
A model-based description of the scaling and radial location of turbulent
fluctuations in turbulent pipe flow is presented and used to illuminate the
scaling behaviour of the very large scale motions. The model is derived by
treating the nonlinearity in the perturbation equation (involving the Reynolds
stress) as an unknown forcing, yielding a linear relationship between the
velocity field response and this nonlinearity. We do not assume small
perturbations. We examine propagating modes, permitting comparison of our
results to experimental data, and identify the steady component of the velocity
field that varies only in the wall-normal direction as the turbulent mean
profile. The "optimal" forcing shape, that gives the largest velocity response,
is assumed to lead to modes that will be dominant and hence observed in
turbulent pipe flow.
An investigation of the most amplified velocity response at a given
wavenumber-frequency combination reveals critical layer-like behaviour
reminiscent of the neutrally stable solutions of the Orr-Sommerfeld equation in
linearly unstable flow. Two distinct regions in the flow where the influence of
viscosity becomes important can be identified, namely a wall layer that scales
with and a critical layer, where the propagation velocity is equal
to the local mean velocity, that scales with in pipe flow. This
framework appears to be consistent with several scaling results in wall
turbulence and reveals a mechanism by which the effects of viscosity can extend
well beyond the immediate vicinity of the wall.Comment: Submitted to the Journal of Fluid Mechanics and currently under
revie
On the correspondence between Koopman mode decomposition, resolvent mode decomposition, and invariant solutions of the Navier-Stokes equations
The relationship between Koopman mode decomposition, resolvent mode
decomposition and exact invariant solutions of the Navier-Stokes equations is
clarified. The correspondence rests upon the invariance of the system operators
under symmetry operations such as spatial translation. The usual interpretation
of the Koopman operator is generalised to permit combinations of such
operations, in addition to translation in time. This invariance is related to
the spectrum of a spatio-temporal Koopman operator, which has a travelling wave
interpretation. The relationship leads to a generalisation of dynamic mode
decomposition, in which symmetry operations are applied to restrict the dynamic
modes to span a subspace subject to those symmetries. The resolvent is
interpreted as the mapping between the Koopman modes of the Reynolds stress
divergence and the velocity field. It is shown that the singular vectors of the
resolvent (the resolvent modes) are the optimal basis in which to express the
velocity field Koopman modes where the latter are not a priori known
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