941 research outputs found
Bulk and Surface Magnetization of Co atoms in Rutile Ti_[1-x]Co_xO_[2-delta] Thin Films Revealed by X-Ray Magnetic Circular Dichroism
We have studied magnetism in Ti_[1-x]Co_xO_[2-\delta] thin films with various
x and \delta by soft x-ray magnetic circular dichroism (XMCD) measurements at
the Co L_[2,3] absorption edges. The estimated ferromagnetic moment by XMCD was
0.15-0.24 \mu\beta/Co in the surface, while in the bulk it was 0.82-2.25
\mu\beta/Co, which is in the same range as the saturation magnetization of
1.0-1.5 \mu\beta/Co. Theseresults suggest that the intrinsic origin of the
erromagnetism. The smaller moment of Co atom at surface is an indication of a
magnetically dead layer of a few nm thick at the surface of the thin films.Comment: This Paper is accepted in J. of Phys: Conds. Matte
Luminosity Correlations for Gamma-Ray Bursts and Implications for Their Prompt and Afterglow Emission Mechanisms
We present the relation between the (z- and k-corrected) spectral lags, tau, for the standard Swift energy bands 50 - 100 keV and 100 - 200 keV and the peak isotropic luminosity, L(sub iso) (a relation reported first by Norris et al.), for a subset of 12 long Swift GRBs taken from a recent study of this relation by Ukwatta et al. The chosen GRBs are also a subset of the Dainotti et al. sample, a set of Swift GRBs of known redshift, employed in establishing a relation between the (GRB frame) luminosity, L(sub x), of the shallow (or constant) flux portion of the typical XRT GRB-afterglow light curve and the (GRB frame) time of transition to the normal decay rate, T(sub brk). We also present the L(sub x) - T(sub brk) relation using only the bursts common in the two samples. The two relations exhibit a significant degree of correlation (rho = -0.65 for the L(sub iso) - tao and rho = -0.88 for the L(sub x) -T(sub brk) relation) and have surprisingly similar best-fit power law indices (-1.19 +/- 0.17 for L(sub iso) - tau and -1.10 +/- 0.03 for L(sub x) - T(sub brk)). Even more surprisingly, we noted that although tau and T(sub brk) represent different GRB time variables, it appears that the first relation (L(sub iso) - tao) extrapolates into the second one for timescales tau similar to T(sub brk) This fact suggests that these two relations have a common origin, which we conjecture to be kinematic. This relation adds to the recently discovered relations between properties of the prompt and afterglow GRB phases, indicating a much more intimate relation between these two phases than hitherto considered
Temperature dependent optical properties of -GaO thin films
We determined the complex dielectric functions of -GaO
using optical transmittance and reflectance spectroscopies at temperatures from
10 K to room temperature. The measured dielectric-function spectra reveal
distinct structures at a bandgap energy. We fitted a model dielectric function
based on the electronic energy-band structure to these experimental data. We
analyzed the temperature dependence of the band-gap with a model based on
phonon dispersion effects. One could explain it in terms of phonon-related
parameters such as the optical phonon temperature. We compare phonon-related
properties of -GaO with those of a large variety of
element and binary semiconductors.Comment: 10 pages, 6 png figures, using arxiv.st
Mass Outflows from Dissipative Shocks in Hot Accretion Flows
We consider stationary, axisymmetric hydrodynamic accretion flows in Kerr
geometry. As a plausible means of efficiently separating a small population of
nonthermal particles from the bulk accretion flows, we investigate the
formation of standing dissipative shocks, i.e. shocks at which fraction of the
energy, angular momentum and mass fluxes do not participate in the shock
transition of the flow that accretes onto the compact object but are lost into
collimated (jets) or uncollimated (winds) outflows. The mass loss fraction (at
a shock front) is found to vary over a wide range (0 - 95%) depending on flow's
angular momentum and energy. On the other hand, the associated energy loss
fraction appears to be relatively low (<1%) for a flow onto a non-rotating
black hole case, whereas the fraction could be an order of magnitude higher
(<10%) for a flow onto a rapidly-rotating black hole. By estimating the escape
velocity of the outflowing particles with a mass-accretion rate relevant for
typical active galactic nuclei, we find that nearly 10% of the accreting mass
could escape to form an outflow in a disk around a non-rotating black hole,
while as much as 50% of the matter may contribute to outflows in a disk around
a rapidly-rotating black hole. In the context of disk-jet paradigm, our model
suggests that shock-driven outflows from accretion can occur in regions not too
far from a central engine. Our results imply that a shock front under some
conditions could serve as a plausible site where (nonthermal) seed particles of
the outflows (jets/winds) are efficiently decoupled from bulk accretion.Comment: 25 pages, 10 black&white figures, Accepted to Ap
Photoreflectance spectroscopy of BiOCl epitaxial thin films
We have observed a new optical transition in the photoreflectance spectra of
indirect-gap BiOCl thin films, which were grown on SrTiO substrates. The
position of this transition is close in energy to its bulk critical point
energy. Moreover, these are significantly lower than a higher-lying direct-type
critical point from an energetic point of view. The spectral line shape
analysis for our observed signal suggests the presence of an excitonic effect
of this compound. We determined its dependence of the optical anomaly on
temperature ranging from 80 K to room temperature. We adopted the Varshni model
for this analysis. At last, we compared phononic properties of BiOCl with those
of an element and binary semiconductors.Comment: 9 pages, 4 figures, submitted to Jpn. J. Appl. Phy
Giant Planar Hall Effect in Epitaxial (Ga,Mn)As Devices
Large Hall resistance jumps are observed in microdevices patterned from
epitaxial (Ga,Mn)As layers when subjected to a swept, in-plane magnetic field.
This giant planar Hall effect is four orders of magnitude greater than
previously observed in metallic ferromagnets. This enables extremely sensitive
measurements of the angle-dependent magnetic properties of (Ga,Mn)As. The
magnetic anisotropy fields deduced from these measurements are compared with
theoretical predictions.Comment: 3 figure
Poynting-Robertson effect on black-hole-driven winds
Layers of ionized plasma, in the form of winds ejected from the accretion
disk of Supermassive Black Holes (SMBHs) are frequently observed in Active
Galactic Nuclei (AGNs). Winds with a velocity often exceeding are called
Ultra-Fast-Outflows (UFOs) and thanks to their high power they can play a key
role in the co-evolution between the SMBH and the host galaxy. In order to
construct a proper model of the properties of these winds, it is necessary to
consider special relativistic corrections due to their very high velocities. We
present a derivation of the Poynting-Robertson effect (P-R effect) and apply it
to the description of the dynamics of UFOs. The P-R effect is a special
relativistic correction which breaks the isotropy of the radiation emitted by a
moving particle funneling the radiation in the direction of motion. As a result
of the conservation of the four-momentum, the emitting particles are subjected
to a drag force and decelerate. We provide a derivation of the drag force
caused by the P-R effect starting from general Lorentz transformations and
assuming isotropic emission in the gas reference frame. Then, we derive the
equations to easily implement this drag force in future simulations. Finally,
we apply them in a toy model in which the gas particles move radially under the
influence of the gravitation force, the radiation pressure and the drag due to
the P-R effect. P-R effect plays an important role in determining the velocity
profile of the wind. For a wind launched from (where stands
for the Schwarzschild radius), the asymptotic velocity reached by the wind is
between % and % smaller than the one it would possess if we neglect the
effect. This shows that the P-R effect should be taken into account when
studying the dynamics of high-velocity, photoionized outflows in general.Comment: Accepted for publication on Astronomy & Astrophysics. 7 pages, 4
figure
A Global Picture of AGN Winds
We present a unified structure for accretion powered sources across their entire luminosity range from accreting galactic black holes to the most luminous quasars, with emphasis on AGN and their phenomenology. Central to this end is the notion of MHD winds launched from the accretion disks that power these objects. This work similar in spirit to that of Elvis of more that a decade ago, provides, on one hand, only the broadest characteristics of these objects, but on the other, also scaling laws that allow one to make contact with objects of different luminosity. The conclusion of this work is that AGN phenomenology can be accounted for in terms of dot(m), the wind mass flux in units of the Eddington value, the observer's inclination angle theta and alpha_OX the logarithmic slope between UV and X-ray flares. However given the well known correlation between alpha(sub ox) and UV Luminosity, we conclude that the AGN structure depends on only two parameters. The small number of model parameters hence suggests that an understanding of the global AGN properties maybe within reach
Magnetic oxide semiconductors
Magnetic oxide semiconductors, oxide semiconductors doped with transition
metal elements, are one of the candidates for a high Curie temperature
ferromagnetic semiconductor that is important to realize semiconductor
spintronics at room temperature. We review in this paper recent progress of
researches on various magnetic oxide semiconductors. The magnetization,
magneto-optical effect, and magneto-transport such as anomalous Hall effect are
examined from viewpoint of feasibility to evaluate the ferromagnetism. The
ferromagnetism of Co-doped TiO2 and transition metal-doped ZnO is discussed.Comment: 26 pages, 5 tables, 6 figure
Ultrafast optical control of magnetization in EuO thin films
All-optical pump-probe detection of magnetization precession has been
performed for ferromagnetic EuO thin films at 10 K. We demonstrate that the
circularly-polarized light can be used to control the magnetization precession
on an ultrafast time scale. This takes place within the 100 fs duration of a
single laser pulse, through combined contribution from two nonthermal
photomagnetic effects, i.e., enhancement of the magnetization and an inverse
Faraday effect. From the magnetic field dependences of the frequency and the
Gilbert damping parameter, the intrinsic Gilbert damping coefficient is
evaluated to be {\alpha} \approx 3\times10^-3.Comment: 5 pages, 3 figures, accepted for publication in Phys. Rev.
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