388 research outputs found
Orbital-Order Driven Ferroelectricity and Dipolar Relaxation Dynamics in Multiferroic GaMoS
We present the results of broadband dielectric spectroscopy of GaMoS,
a lacunar spinel system that recently was shown to exhibit non-canonical,
orbitally-driven ferroelectricity. Our study reveals complex relaxation
dynamics of this multiferroic material, both above and below its Jahn-Teller
transition at T K. Above T, two types of
coupled dipolar-orbital dynamics seem to compete: relaxations within
cluster-like regions with short-range polar order like in relaxor
ferroelectrics and critical fluctuations of only weakly interacting dipoles,
the latter resembling the typical dynamics of order-disorder type
ferroelectrics. Below the Jahn-Teller transition, the onset of orbital order
drives the system into long-range ferroelectric order and dipolar dynamics
within the ferroelectric domains is observed. The coupled dipolar and orbital
relaxation behavior of GaMoS above the Jahn-Teller transition markedly
differs from that of the skyrmion host GaVS, which seems to be linked
to differences in the structural distortions of the two systems on the
unit-cell level.Comment: 6 pages, 3 figures + Supplemental Material (2 pages, 2 figures
Strong-field general relativity and quasi-periodic oscillations in x-ray binaries
Quasi-periodic oscillations (QPOs) at frequencies near 1000 Hz were recently
discovered in several x-ray binaries containing neutron stars. Two sources show
no correlation between QPO frequency and source count rate (Berger et al. 1996,
Zhang et al. 1996). We suggest that the QPO frequency is determined by the
Keplerian orbital frequency near the marginally stable orbit predicted by
general relativity in strong gravitational fields (Muchotrzeb-Czerny 1986,
Paczynski 1987, Kluzniak et al. 1990). The QPO frequencies observed from 4U
1636-536 imply that the mass of the neutron star is 2.02 +/- 0.12 solar masses.
Interpretation of the 4.1 keV absorption line observed from 4U 1636-536 (Waki
et al. 1984) as due to Fe XXV ions then implies a neutron star radius of 9.6
+/-0.6 km.Comment: 4 pages, uses aas2pp4.sty, submitted to ApJ
Disc-Jet coupling in the LMXB 4U1636-53 from INTEGRAL
We report on the spectral analysis results of the neutron star, atoll type,
low mass X-ray Binary 4U1636-53 observed by INTEGRAL and BeppoSAX satellites.
Spectral behavior in three different epochs corresponding to three different
spectral states has been deeply investigated. Two data set spectra show a
continuum well described by one or two soft blackbody plus a Comptonized
components with changes in the Comptonizing electrons and black body
temperature and the accretion rates, which are typical of the spectral
transitions from high to low state. In one occasion INTEGRAL spectrum shows,
for first time in this source, a hard tail dominating the emission above 30
keV. The total spectrum is fitted as the sum of a Comptonized component similar
to soft state and a power-law component (Gamma=2.76), indicating the presence
of a non thermal electron distribution of velocities. In this case, a
comparison with hard tails detected in soft states from neutron stars systems
and some black hole binaries suggests that a similar mechanism could originate
these components in both cases.Comment: 6 pages, 4 figures, 2 tables. accepted Ap
Equilibrium configurations for quark-diquark stars and the problem of Her X-1 mass
We report new calculations of the physical properties of a quark-diquark
plasma. A vacuum contribution is taken into account and is responsible for the
appearance of a stable state at zero pressure and at a baryon density of about
2.2 times the nuclear matter density in this model. The resulting equation of
state was used to integrate numerically the Tolman-Oppenheimer-Volkoff
equations. The mass-radius relationship has been derived from a series of
equilibrium configurations constituted by a mixture of quarks and diquarks.
These stellar models, which are representative of a whole class, may be helpful
to understand the possible compactness of the X-ray source Her X-1 and related
objects.Comment: 15 pp., PlainTex file + 3 figures available upon request at
[email protected]. Submitted to Int. Jour. Mod. Phys.
Stroboscopic Laser Diagnostics for Detection of Ordering in One-Dimensional Ion beam
A novel diagnostic method for detecting ordering in one-dimensional ion beams
is presented. The ions are excited by a pulsed laser at two different positions
along the beam and fluorescence is observed by a group of four
photomultipliers. Correlation in fluorescence signals is firm indication that
the ion beam has an ordered structure.Comment: 7 pages, REVTEX, fig3 uuencoded, figs 1-2 available upon request from
[email protected], to appear in Phys. Rev.
Long-Term Pancreas Allograft Survival in Simultaneous Pancreas-Kidney Transplantation by Era: UNOS registry analysis
Non-Detection of Gravitationally Redshifted Absorption Lines in the X-ray Burst Spectra of GS 1826-24
During a 200 ks observation with the XMM-Newton Reflection Grating
Spectrometer, we detected 16 type-I X-ray bursts from GS 1826-24. We combined
the burst spectra in an attempt to measure the gravitational redshifts from the
surface of the neutron star. We divided the composite GS 1826-24 burst spectrum
into three groups based on the blackbody temperature during the bursts. The
spectra do not show any obvious discrete absorption lines. We compare our
observations with those of EXO 0748-676.Comment: 4 pages, 4 figures; accepted for publication in ApJ
Atomic Spectral Features During Thermonuclear Flashes on Neutron Stars
The gravitational redshift measured by Cottam, Paerels and Mendez for the
neutron star (NS) in the low-mass X-ray binary EXO 0748-676 depends on the
identification of an absorption line during a type I burst as the H
line from hydrogenic Fe. We show that Fe is present above the photosphere as
long as during the burst. In
this limit, the total Fe column is for incident material of solar abundances and only depends on the
nuclear physics of the proton spallation. The Fe destruction creates many heavy
elements with which may imprint photo-ionization edges on the NS spectra
during a radius expansion event or in a burst cooling tail. Detecting these
features in concert with those from Fe would confirm a redshift measurement. We
also begin to address the radiative transfer problem, and find that a
concentrated Fe layer with and column (depending on the line depth) above the hotter
continuum photosphere is required to create the H line of the observed
strength. This estimate must be refined by considerations of non-LTE effects as
well as resonant line transport. Until these are carried out, we cannot say
whether the Fe column from accretion and spallation is in conflict with the
observations. We also show that hydrogenic Fe might remain in the photosphere
due to radiative levitation from the high burst flux.Comment: Substantially revised version, to appear in Ap J Letter
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