11,655 research outputs found
Husimi-Wigner representation of chaotic eigenstates
Just as a coherent state may be considered as a quantum point, its
restriction to a factor space of the full Hilbert space can be interpreted as a
quantum plane. The overlap of such a factor coherent state with a full pure
state is akin to a quantum section. It defines a reduced pure state in the
cofactor Hilbert space. The collection of all the Wigner functions
corresponding to a full set of parallel quantum sections defines the
Husimi-Wigner reresentation. It occupies an intermediate ground between drastic
suppression of nonclassical features, characteristic of Husimi functions, and
the daunting complexity of higher dimensional Wigner functions. After analysing
these features for simpler states, we exploit this new representation as a
probe of numerically computed eigenstates of chaotic Hamiltonians. The
individual two-dimensional Wigner functions resemble those of semiclassically
quantized states, but the regular ring pattern is broken by dislocations.Comment: 21 pages, 7 figures (6 color figures), submitted to Proc. R. Soc.
Thermal entanglement witness for materials with variable local spin lengths
We show that the thermal entanglement in a spin system using only magnetic
susceptibility measurements is restricted to the insulator materials. We
develop a generalization of the thermal entanglement witness that allows us to
get information about the system entanglement with variable local spin lengths
that can be used experimentally in conductor or insulator materials. As an
application, we study thermal entanglement for the half-filled Hubbard model
for linear, square and cubic clusters. We note that it is the itinerancy of
electrons that favors the entanglement. Our results suggest a weak dependence
between entanglement and external spin freedom degrees.Comment: 4 pages, 3 figure
Stochastic Electron Acceleration During the NIR and X-ray Flares in Sagittarius A*
Recent near-IR (NIR) and X-ray observations of Sagittarius A*'s spectrum have
yielded several strong constraints on the transient energization mechanism,
justifying a re-examination of the stochastic acceleration model proposed
previously for these events. We here demonstrate that the new results are fully
consistent with the acceleration of electrons via the transit-time damping
process. But more importantly, these new NIR and X-ray flares now can constrain
the source size, the gas density, the magnetic field, and the wave energy
density in the turbulent plasma. Future simultaneous multi-wavelength
observations with good spectral information will, in addition, allow us to
study their temporal evolution, which will eventually lead to an accurate
determination of the behavior of the plasma just minutes prior to its
absorption by the black hole.Comment: 13 pages, 4 figures, Submitted to ApJ
Fabrication of free-standing ordered fluorescent polymer nanofibres by electrospinning
The authors are grateful to the Engineering and Physical Sciences Research Council for financial support.We demonstrate a static fabrication approach to make free-standing ordered arrays of fluorescent nanofibres through control of the transverse electrospinning field. The alignment and the density of the nanofibre arrays are optimised by careful design of both the source and collector electrode geometries which can control the transverse electric field over the full path of the jet. In doing so, we fabricate suspended fluorescent nanofibres with an aspect ratio of 10(4), and with a substantially increased density and order parameter (by a factor of similar to 10 compared to random deposition). Electrostatic modelling suggests that the field distribution of the component is the main contribution to the ordering between the plates. This method offers increased efficiency for the creation of ordered fibres collected over a small area and the characterisation of their photoluminescent properties.Publisher PDFPeer reviewe
Infall models of Class 0 protostars
We have carried out radiative transfer calculations of infalling, dusty
envelopes surrounding embedded protostars to understand the observed properties
of the recently identified ``Class 0'' sources. To match the far-infrared peaks
in the spectral energy distributions of objects such as the prototype Class 0
source VLA 1623, pure collapse models require mass infall rates
\sim10^{-4}\msunyr. The radial intensity distributions predicted by
such infall models are inconsistent with observations of VLA 1623 at sub-mm
wavelengths, in agreement with the results of Andre et al. (1993) who found a
density profile of rather than the expected gradient. To resolve this conflict, while still invoking
infall to produce the outflow source at the center of VLA 1623, we suggest that
the observed sub-mm intensity distribution is the sum of two components: an
inner infall zone, plus an outer, more nearly constant-density region. This
explanation of the observations requires that roughly half the total mass
observed within 2000 AU radius of the source lies in a region external to the
infall zone. The column densities for this external region are comparable to
those found in the larger Oph A cloud within which VLA 1623 is embedded. The
extreme environments of Class 0 sources lead us to suggest an alternative or
additional interpretation of these objects: rather than simply concluding with
Andre et al. that Class 0 objects only represent the earliest phases of
protostellar collapse, and ultimately evolve into older ``Class I'' protostars,
we suggest that many Class 0 sources could be the protostars of very dense
regions. (Shortened)Comment: 22 pages, including 3 PostScript figures, accepted for publication in
The Astrophysical Journa
Molecular Tracers of Embedded Star Formation in Ophiuchus
In this paper we analyze nine SCUBA cores in Ophiuchus using the
second-lowest rotational transitions of four molecular species (12CO, 13CO,
C18O, and C17O) to search for clues to the evolutionary state and
star-formation activity within each core. Specifically, we look for evidence of
outflows, infall, and CO depletion. The line wings in the CO spectra are used
to detect outflows, spectral asymmetries in 13CO are used to determine infall
characteristics, and a comparison of the dust emission (from SCUBA
observations) and gas emission (from C18O) is used to determine the fractional
CO freeze-out.
Through comparison with Spitzer observations of protostellar sources in
Ophiuchus, we discuss the usefulness of CO and its isotopologues as the sole
indicators of the evolutionary state of each core. This study is an important
pilot project for the JCMT Legacy Survey of the Gould Belt (GBS) and the
Galactic Plane (JPS), which intend to complement the SCUBA-2 dust continuum
observations with HARP observations of 12CO, 13CO, C18O, and C17O J = 3 - 2 in
order to determine whether or not the cold dust clumps detected by SCUBA-2 are
protostellar or starless objects.
Our classification of the evolutionary state of the cores (based on molecular
line maps and SCUBA observations) is in agreement with the Spitzer designation
for six or seven of the nine SCUBA cores. However, several important caveats
exist in the interpretation of these results, many of which large mapping
surveys like the GBS may be able to overcome to provide a clearer picture of
activity in crowded fields.Comment: 43 pages including 19 postscript figures. Accepted for publication in
the PAS
Studying Parallel Evolutionary Algorithms: The cellular Programming Case
Parallel evolutionary algorithms, studied to some extent over the past few years, have proven empirically worthwhileâthough there seems to be lacking a better understanding of their workings. In this paper we concentrate on cellular (fine-grained) models, presenting a number of statistical measures, both at the genotypic and phenotypic levels. We demonstrate the application and utility of these measures on a specific example, that of the cellular programming evolutionary algorithm, when used to evolve solutions to a hard problem in the cellular-automata domain, known as synchronization
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