4,273 research outputs found
Stationary point approach to the phase transition of the classical XY chain with power-law interactions
The stationary points of the Hamiltonian H of the classical XY chain with
power-law pair interactions (i.e., decaying like r^{-{\alpha}} with the
distance) are analyzed. For a class of "spinwave-type" stationary points, the
asymptotic behavior of the Hessian determinant of H is computed analytically in
the limit of large system size. The computation is based on the Toeplitz
property of the Hessian and makes use of a Szeg\"o-type theorem. The results
serve to illustrate a recently discovered relation between phase transitions
and the properties of stationary points of classical many-body Hamiltonian
functions. In agreement with this relation, the exact phase transition energy
of the model can be read off from the behavior of the Hessian determinant for
exponents {\alpha} between zero and one. For {\alpha} between one and two, the
phase transition is not manifest in the behavior of the determinant, and it
might be necessary to consider larger classes of stationary points.Comment: 9 pages, 6 figure
Nonlinear dielectric response at the excess wing of glass-forming liquids
We present nonlinear dielectric measurements of glass-forming glycerol and
propylene carbonate applying electrical fields up to 671 kV/cm. The
measurements extend to sufficiently high frequencies to allow for the
investigation of the nonlinear behavior in the regime of the so-far mysterious
excess wing, showing up in the loss spectra of many glass formers as a second
power law at high frequencies. Surprisingly, we find a complete lack of
nonlinear behavior in the excess wing, in marked contrast to the
alpha-relaxation where, in agreement with previous reports, a strong increase
of dielectric constant and loss is found.Comment: 8 pages (including 3 pages Supplementary Information), 4 + 1 figures.
Revised according to suggestions of referee
Physical Structure of the Planetary Nebula NGC 3242 from the Hot Bubble to the Nebular Envelope
One key feature of the interacting stellar winds model of the formation of
planetary nebulae (PNe) is the presence of shock-heated stellar wind confined
in the central cavities of PNe. This so-called hot bubble should be detectable
in X-rays. Here we present XMM-Newton observations of NGC 3242, a
multiple-shell PN whose shell morphology is consistent with the interacting
stellar winds model. Diffuse X-ray emission is detected within its inner shell
with a plasma temperature ~2.35\times10^6 K and an intrinsic X-ray luminosity
~2\times10^30 ergs s^(-1) at the adopted distance of 0.55 kpc. The observed
X-ray temperature and luminosity are in agreement with "ad-hoc" predictions of
models including heat conduction. However, the chemical abundances of the
X-ray-emitting plasma seem to imply little evaporation of cold material into
the hot bubble, whereas the thermal pressure of the hot gas is unlikely to
drive the nebular expansion as it is lower than that of the inner shell rim.
These inconsistencies are compounded by the apparent large filling factor of
the hot gas within the central cavity of NGC 3242. Subject headings: planetary
nebulae: individual (NGC 3242)Comment: Accepted the publication in Astronomical Journa
Electronic Properties of Charged Centers in Si0â‚‚-like Glasses
Contains report on one research project.Joint Services Electronics Program (Contract DAAG29-78-C-0020
Songs of sand and grit : a collection of narrative literary journalism
Includes bibliographical references (leaves 87-92).The introduction explicates the nature of narrative literary journalism and situates the four stories within this edifice. This body of work experiments with the classical form of narrative literary journalism. The Soul Searcher tells the story of the ethnomusicologist John Turest-Swartz who wants to promote the little known rural musician Louisa Steenkamp. It traces the development of an indigenous South African band over a period of one year. Thumeka in the Dunes is a career portrait in the context of urban nature conservation. The protagonist Thumeka Mdlazi is both part of the obstacle - the community threatening the nature reserve - and its solution, as a protector of the dunes. A Place to Live under the Rainbow deals with the repeated attacks against Somali refugees in South Africa portraying victims and perpetrators alike. On a meta-level, the story also reflects on the topic of xenophobia in a more discursive style. A Runner's Mind is a first-person creative non-fiction narrative. The core of the story is the motivation for ultra-marathon runner Randall Turner to keep running. This account is framed by my own personal reflections on running and non-running. All stories are anchored in a South African social setting and reflect people's struggles and small achievements in overcoming seemingly hopeless situations
Pinpointing the Position of the Post-AGB Star at the Core of RAFGL 2688 using Polarimetric Imaging with NICMOS
We have used infrared polarimetric imaging with NICMOS to determine precisely
the position of the star that illuminates (and presumably generated) the
bipolar, pre-planetary reflection nebula RAFGL 2688 (the Egg Nebula). The
polarimetric data pinpoint the illuminating star, which is not detected
directly at wavelengths less than or equal to 2 microns, at a position well
within the dark lane that bisects the nebula, 0.55" (about 550 AU) southwest of
the infrared peak which was previously detected at the southern tip of the
northern polar lobe. The inferred position of the central star corresponds to
the geometric center of the tips of the four principle lobes of near-infrared
H2 emission; identifying the central star at this position also reveals the
strong point symmetric structure of the nebula, as seen both in the intensity
and polarization structure of the polar lobes. The polarimetric and imaging
data indicate that the infrared peak directly detected in the NICMOS images is
a self-luminous source and, therefore, is most likely a distant binary
companion to the illuminating star. Although present theory predicts that
bipolar structure in pre-planetary and planetary nebulae is a consequence of
binary star evolution, the separation between the components of the RAFGL 2688
binary system, as deduced from these observations, is much too large for the
presence of the infrared companion to have influenced the structure of the
RAFGL 2688 nebula.Comment: 15 pages, 6 figures, to appear in The Astrophysical Journa
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