770 research outputs found
Localized Control of Curie Temperature in Perovskite Oxide Film by Capping-layer- induced Octahedral Distortion
With reduced dimensionality, it is often easier to modify the properties of
ultra-thin films than their bulk counterparts. Strain engineering, usually
achieved by choosing appropriate substrates, has been proven effective in
controlling the properties of perovskite oxide films. An emerging alternative
route for developing new multifunctional perovskite is by modification of the
oxygen octahedral structure. Here we report the control of structural oxygen
octahedral rotation in ultra-thin perovskite SrRuO3 films by the deposition of
a SrTiO3 capping layer, which can be lithographically patterned to achieve
local control. Using a scanning Sagnac magnetic microscope, we show increase in
the Curie temperature of SrRuO3 due to the suppression octahedral rotations
revealed by the synchrotron x-ray diffraction. This capping-layer-based
technique may open new possibilities for developing functional oxide materials.Comment: Main-text 5 pages, SI 6 pages. To appear in Physical Review Letter
Our distorted view of magnetars: application of the Resonant Cyclotron Scattering model
The X-ray spectra of the magnetar candidates are customarily fitted with an
empirical, two component model: an absorbed blackbody and a power-law. However,
the physical interpretation of these two spectral components is rarely
discussed. It has been recently proposed that the presence of a hot plasma in
the magnetosphere of highly magnetized neutron stars might distort, through
efficient resonant cyclotron scattering, the thermal emission from the neutron
star surface, resulting in the production of non-thermal spectra. Here we
discuss the Resonant Cyclotron Scattering (RCS) model, and present its XSPEC
implementation, as well as preliminary results of its application to Anomalous
X-ray Pulsars and Soft Gamma-ray Repeaters.Comment: 5 pages, 5 color figures; Astrophysics & Space Science, in press
("Isolated Neutron Stars"; London, UK
High-energy characteristics of the accretion-powered millisecond pulsar IGR J17591-2342 during its 2018 outburst
IGR J17591-2342 is a recently INTEGRAL discovered accreting millisecond X-ray
pulsar that went into outburst around July 21, 2018. To better understand the
physics acting in these systems during the outburst episode we performed
detailed temporal-, timing- and spectral analyses across the 0.3-300 keV band
using data from NICER, XMM-Newton, NuSTAR and INTEGRAL. The hard X-ray 20-60
keV outburst profile is composed of four flares. During the maximum of the last
flare we discovered a type-I thermonuclear burst in INTEGRAL JEM-X data. We
derived a distance of 7.6+/-0.7 kpc, adopting Eddington luminosity limited
photospheric radius expansion burst emission and assuming anisotropic emission.
In the timing analysis using all NICER 1-10 keV monitoring data we observed a
rather complex behaviour starting with a spin-up period, followed by a
frequency drop, a episode of constant frequency and concluding with irregular
behaviour till the end of the outburst. The 1-50 keV phase distributions of the
pulsed emission, detected up to 120 keV using INTEGRAL ISGRI data, was
decomposed in three Fourier harmonics showing that the pulsed fraction of the
fundamental increases from ~10% to ~17% going from ~1.5 to ~4 keV, while the
harder photons arrive earlier than the soft photons for energies <10 keV. The
total emission spectrum of IGR J17591-2342 across the 0.3-150 keV band could
adequately be fitted in terms of an absorbed compPS model yielding as best fit
parameters a column density of N_H=(2.09+/-0.05) x 10^{22} /cm2, a blackbody
seed photon temperature kT_bb,seed of 0.64+/- 0.02 keV, electron temperature
kT_e=38.8+/-1.2 keV and Thomson optical depth Tau_T=1.59+/-0.04. The fit
normalisation results in an emission area radius of 11.3+/-0.5 km adopting a
distance of 7.6 kpc. Finally, the results are discussed within the framework of
accretion physics- and X-ray thermonuclear burst theory.Comment: Submitted to A&A, 15 page
Gamma-ray Luminosity and Death Lines of Pulsars with Outer Gaps
We re-examine the outer gap size by taking the geometry of the dipole
magnetic field into account. Furthermore, we also consider that instead of
taking the gap size at half of the light cylinder radius to represent the
entire outer gap it is more appropriate to average the entire outer gap size
over the distance. When these two factors are considered, the derived outer gap
size f is not only the function of period P and magnetic field B of the neutron
star, but also the function of the average radial distance to the neutron star
. We use this new outer gap model to study -ray luminosity of
pulsars, and to study the death lines of -ray emission of the pulsars.
Our model can predict the -ray luminosity of individual pulsar if its
and are known. Since different pulsars have different ,
this explains why some -ray pulsars have very similar and but
have very different -ray luminosities. In determining the death line of
-ray pulsars, we have used a new criterion based on concrete physical
reason. In estimate of the fractional size of the outer gap, two possible X-ray
fields are considered: (i) X-rays are produced by the neutron star cooling and
polar cap heating, and (ii)X-rays are produced by the bombardment of the
relativistic particles from the outer gap on the stellar surface. Since it is
very difficult to measure in general, we use a Monte Carlo method to
simulate the properties of -ray pulsars in our galaxy. We find that
this new outer gap model predicts many more weak -ray pulsars. For all
simulated -ray pulsars with self-sustained outer gaps, -ray
luminosity satisfies ; where
the value of depends on the ensitivity of the -ray detector.Comment: 18 pages, 6 figures, accepted for publication in Ap
Antiferromagnetism and phase separation in electronic models for doped transition-metal oxides
We investigate the ground state properties of electronic models for doped
manganites and nickelates. An effective t - J like Hamiltonian is derived from
the case of strong Hund coupling between the conduction electrons and localized
spins by means of the projection technique. An attractive interaction for
conduction electrons and an anti-ferromagnetic coupling of the localized spin
are obtained. A large ratio of the attraction to effective electron hopping,
which is modulated by the spin background, will lead to the phase separation.
The anti-ferromagnetic phase and the phase separation appear in the case of
either high or low density of electrons. The possible relevance of the phase
separation to the charge stripe phase in the manganites and nickelates is
discussed.Comment: 12 pages, ReVTEX, 3 figures. To appear in Phys. Rev. B (RC), (01Oct.,
1998
Magnetic versus crystal field linear dichroism in NiO thin films
We have detected strong dichroism in the Ni x-ray absorption
spectra of monolayer NiO films. The dichroic signal appears to be very similar
to the magnetic linear dichroism observed for thicker antiferromagnetic NiO
films. A detailed experimental and theoretical analysis reveals, however, that
the dichroism is caused by crystal field effects in the monolayer films, which
is a non trivial effect because the high spin Ni ground state is not
split by low symmetry crystal fields. We present a practical experimental
method for identifying the independent magnetic and crystal field contributions
to the linear dichroic signal in spectra of NiO films with arbitrary
thicknesses and lattice strains. Our findings are also directly relevant for
high spin and systems such as LaFeO, FeO,
VO, LaCrO, CrO, and Mn manganate thin films
The first multi-wavelength campaign of AXP 4U 0142+61 from radio to hard X-rays
For the first time a quasi-simultaneous multi-wavelength campaign has been
performed on an Anomalous X-ray Pulsar from the radio to the hard X-ray band.
4U 0142+61 was an INTEGRAL target for 1 Ms in July 2005. During these
observations it was also observed in the X-ray band with Swift and RXTE, in the
optical and NIR with Gemini North and in the radio with the WSRT. In this paper
we present the source-energy distribution. The spectral results obtained in the
individual wave bands do not connect smoothly; apparently components of
different origin contribute to the total spectrum. Remarkable is that the
INTEGRAL hard X-ray spectrum (power-law index 0.79 +/- 0.10) is now measured up
to an energy of ~230 keV with no indication of a spectral break. Extrapolation
of the INTEGRAL power-law spectrum to lower energies passes orders of magnitude
underneath the NIR and optical fluxes, as well as the low ~30 microJy (2 sigma)
upper limit in the radio band.Comment: 6 pages, 1 figure. To be published in the proceedings of the
conference "Isolated Neutron Stars: from the Interior to the Surface" (April
24-28, 2006, London, UK), eds. S. Zane, R. Turolla and D. Pag
Parity forbidden excitations of Sr2CuO2Cl2 revealed by optical third-harmonic spectroscopy
We present the first study of nonlinear optical third harmonic generation in
the strongly correlated charge-transfer insulator Sr2CuO2Cl2. For fundamental
excitation in the near-infrared, the THG spectrum reveals a strongly resonant
response for photon energies near 0.7 eV. Polarization analysis reveals this
novel resonance to be only partially accounted for by three-photon excitation
to the optical charge-transfer exciton, and indicates that an even-parity
excitation at 2 eV, with a_1g symmetry, participates in the third harmonic
susceptibility.Comment: Requires RevTeX v4.0beta
Characterization of regional meniscal cell and chondrocyte phenotypes and chondrogenic differentiation with histological analysis in osteoarthritic donor-matched tissues
Meniscus degeneration is closely related to the progression of knee osteoarthritis (OA). However, there is currently a lack of quantitative and objective metrics to assess OA meniscal cell phenotypes. In this study we investigated the phenotypic markers and chondrogenic potency of avascular and vascular meniscal cells and chondrocytes from medial OA knee joints (n = 10). Flow cytometry results showed that a significantly greater percentage of meniscal cells were positive for CD49b, CD49c and CD166 compared to donor-matched chondrocytes after 14 days in monolayer culture. The integrins, CD49b and CD29, were expressed at a significantly higher level on avascular meniscal cells derived from tissues with a more degenerated inner border than non-degenerate menisci, suggesting that the integrin family may play an important role in meniscus OA pathology. Collagen fibres arranged in a "tree-like" formation within the meniscus appeared to have less blood vessels associated with them in the vascular region of the most degenerate menisci, which may indicate that such structures are involved in the pathological process. We have demonstrated that meniscal cells derived from the lateral meniscus in medial OA patients have chondrogenic capacity in vitro and hence could represent a potential cell source to consider for meniscus tissue engineering
Implementation of the Projector Augmented Wave LDA+U Method: Application to the Electronic Structure of NiO
The so-called local density approximation plus the multi-orbital mean-field
Hubbard model (LDA+U) has been implemented within the all-electron projector
augmented-wave method (PAW), and then used to compute the insulating
antiferromagnetic ground state of NiO and its optical properties. The
electronic and optical properties have been investigated as a function of the
Coulomb repulsion parameter U. We find that the value obtained from constrained
LDA (U=8 eV) is not the best possible choice, whereas an intermediate value
(U=5 eV) reproduces the experimental magnetic moment and optical properties
satisfactorily. At intermediate U, the nature of the band gap is a mixture of
charge transfer and Mott-Hubbard type, and becomes almost purely of the
charge-transfer type at higher values of U. This is due to the enhancement of
the oxygen 2p states near the top of the valence states with increasing U
value.Comment: 23 pages, 6 figures, submitted to Phys. Rev.
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