1,919 research outputs found
A VLBA survey of the core shift effect in AGN jets I. Evidence for dominating synchrotron opacity
The effect of a frequency dependent shift of the VLBI core position (known as
the "core shift") was predicted more than three decades ago and has since been
observed in a few sources, but often within a narrow frequency range. This
effect has important astrophysical and astrometric applications. To achieve a
broader understanding of the core shift effect and the physics behind it, we
conducted a dedicated survey with NRAO's Very Long Baseline Array (VLBA). We
used the VLBA to image 20 pre-selected sources simultaneously at nine
frequencies in the 1.4-15.4 GHz range. The core position at each frequency was
measured by referencing it to a bright, optically thin feature in the jet. A
significant core shift has been successfully measured in each of the twenty
sources observed. The median value of the core shift is found to be 1.21 mas if
measured between 1.4 and 15.4 GHz, and 0.24 mas between 5.0 and 15.4 GHz. The
core position, r, as a function of frequency, n, is found to be consistent with
an r n^-1 law. This behavior is predicted by the Blandford & Koenigl model of a
purely synchrotron self-absorbed conical jet in equipartition. No systematic
deviation from unity of the power law index in the r(n) relation has been
convincingly detected. We conclude that neither free-free absorption nor
gradients in pressure and/or density in the jet itself and in the ambient
medium surrounding the jet play a significant role in the sources observed
within the 1.4-15.4 GHz frequency range. These results support the
interpretation of the parsec-scale core as a continuous Blandford-Koenigl type
jet with smooth gradients of physical properties along it.Comment: 31 pages, 6 figures, 5 tables; accepted to Astronomy & Astrophysic
First-principles calculation of spin-orbit torque in a Co/Pt bilayer
The angular dependence of spin-orbit torque in a disordered Co/Pt bilayer is
calculated using a first-principles non-equilibrium Green's function formalism
with an explicit supercell averaging over Anderson disorder. In addition to the
usual dampinglike and fieldlike terms, the odd torque contains a sizeable
planar Hall-like term whose contribution to
current-induced damping is consistent with experimental observations. The
dampinglike and planar Hall-like torquances depend weakly on disorder strength,
while the fieldlike torquance declines with increasing disorder. The torques
that contribute to damping are almost entirely due to spin-orbit coupling on
the Pt atoms, but the fieldlike torque does not require it.Comment: 11 pages, 5 figure
Theory of spin loss at metallic interfaces
Interfacial spin-flip scattering plays an important role in magnetoelectronic
devices. Spin loss at metallic interfaces is usually quantified by matching the
magnetoresistance data for multilayers to the Valet-Fert model, while treating
each interface as a fictitious bulk layer whose thickness is times the
spin-diffusion length. By employing the properly generalized circuit theory and
the scattering matrix approaches, we derive the relation of the parameter
to the spin-flip transmission and reflection probabilities at an
individual interface. It is found that is proportional to the square
root of the probability of spin-flip scattering. We calculate the spin-flip
transmission probability for flat and rough Cu/Pd interfaces using the
Landauer-B\"uttiker method based on the first-principles electronic structure
and find in reasonable agreement with experiment.Comment: 5 pages + supplementary material, 3 figures, version accepted in
Phys. Rev. Let
Production of Dirhams at the Mint of Damascus (Dimashq) in the First Four Centuries of Islam and the Question of Near Eastern Metallic Zones
All examination of 159 hoards with 1822 dirhams minted at Damascus during the first four centuries of Islam sheds much light on a number of important questions regarding the economic history of the Near East during the
Umayyad, 'Abbasid, and Ikhshidid eras. Using the hoard-count method of
estimating mint outputs, it has been determined that the Damascus mint was
significantly active only during the Umayyad period, a time when the city was
the capital of the caliphate. Having no local silver mines and lacking revenue
transfers from the provinces after 127 H, Damascus was, at best, a marginal
mint for the production of dirhams. However, because Syria and the eastern
Mediterranean region in general fell into a gold-copper metallic zone during
the period in question, it is surprising that Damascus was at all an important
mint when the city was the capital of the Islamic world
Magnetoelectric domain wall dynamics and its implications for magnetoelectric memory
Domain wall dynamics in a magnetoelectric antiferromagnet is analyzed, and
its implications for magnetoelectric memory applications are discussed.
CrO is used in the estimates of the materials parameters. It is found
that the domain wall mobility has a maximum as a function of the electric field
due to the gyrotropic coupling induced by it. In CrO the maximal
mobility of 0.1 m/(sOe) is reached at V/nm. Fields of
this order may be too weak to overcome the intrinsic depinning field, which is
estimated for B-doped CrO. These major drawbacks for device
implementation can be overcome by applying a small in-plane shear strain, which
blocks the domain wall precession. Domain wall mobility of about 0.7
m/(sOe) can then be achieved at V/nm. A split-gate scheme is
proposed for the domain-wall controlled bit element; its extension to
multiple-gate linear arrays can offer advantages in memory density,
programmability, and logic functionality.Comment: 5 pages, 2 figures, revised and corrected version, accepted in
Applied Physics Letter
Symmetry, incommensurate magnetism and ferroelectricity: the case of the rare-earth manganites RMnO3
The complete irreducible co-representations of the paramagnetic space group
provide a simple and direct path to explore the symmetry restrictions of
magnetically driven ferroelectricity. The method consists of a straightforward
generalization of the method commonly used in the case of displacive modulated
systems and allows us to determine, in a simple manner, the full magnetic
symmetry of a given phase originated from a given magnetic order parameter. The
potential ferroic and magneto-electric properties of that phase can then be
established and the exact Landau free energy expansions can be derived from
general symmetry considerations. In this work, this method is applied to the
case of the orthorhombic rare-earth manganites RMnO3. This example will allow
us to stress some specific points, such as the differences between commensurate
or incommensurate magnetic phases regarding the ferroic and magnetoelectric
properties, the possible stabilization of ferroelectricity by a single
irreducible order parameter or the possible onset of a polarization oriented
parallel to the magnetic modulation. The specific example of TbMnO3 will be
considered in more detail, in order to characterize the role played by the
magneto-electric effect in the mechanism for the polarization rotation induced
by an external magnetic field.Comment: Conference: Aperiodic`0
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