4,695 research outputs found
Tracing large-scale structures in circumstellar disks with ALMA
Planets are supposed to form in circumstellar disks. The gravitational
potential of a planet perturbs the disk and leads to characteristic structures,
i.e. spiral waves and gaps, in the disk's density profile. We perform a
large-scale parameter study of the observability of these planet-induced
structures in circumstellar disks with ALMA. On the basis of HD and MHD
simulations, we calculated the disk temperature structure and (sub)mm images of
these systems. These were used to derive simulated ALMA images. Because
appropriate objects are frequent in Taurus, we focused on a distance of 140pc
and a declination of 20{\deg}. The explored range of star-disk-planet
configurations consists of 6 HD simulations (including magnetic fields and
different planet masses), 9 disk sizes, 15 total disk masses, 6 different
central stars, and two different grain size distributions. On almost all scales
and in particular down to a scale of a few AU, ALMA is able to trace disk
structures induced by planet-disk interaction or by the influence of magnetic
fields on the wavelength range between 0.4 and 2.0mm. In most cases, the
optimum angular resolution is limited by the sensitivity. However, within the
range of typical masses of protoplanetary disks (0.1-0.001Msun) the disk mass
has a minor impact on the observability. It is possible to resolve disks down
to 2.67e-6Msun and trace gaps induced by a planet with M_p/M_s = 0.001 in disks
with 2.67e-4Msun with a signal-to-noise ratio greater than three. The central
star has a major impact on the observability of gaps, as well as the considered
maximum grainsize of the dust in the disk. In general, it is more likely to
trace planet-induced gaps in our MHD models, because gaps are wider in the
presence of magnetic fields. We also find that zonal flows resulting from MRI
create gap-like structures in the disk's re-emission radiation, which are
observable with ALMA.Comment: 17 pages, 21 figure
Existence of vertical spin stiffness in Landau-Lifshitz-Gilbert equation in ferromagnetic semiconductors
We calculate the magnetization torque due to the spin polarization of the
itinerant electrons by deriving the kinetic spin Bloch equations based on the
- model. We find that the first-order gradient of the magnetization
inhomogeneity gives rise to the current-induced torques, which are consistent
to the previous works. At the second-order gradient, we find an effective
magnetic field perpendicular to the spin stiffness filed. This field is
proportional to the nonadiabatic parameter . We show that this vertical
spin stiffness term can significantly modify the domain-wall structure in
ferromagnetic semiconductors and hence should be included in the
Landau-Lifshitz-Gilbert equation in studying the magnetization dynamics.Comment: 7 pages, 4 figure
Magnetization reversal and anomalous coercive field temperature dependence in MnAs epilayers grown on GaAs(100) and GaAs(111)B
The magnetic properties of MnAs epilayers have been investigated for two
different substrate orientations: GaAs(100) and GaAs(111). We have analyzed the
magnetization reversal under magnetic field at low temperatures, determining
the anisotropy of the films. The results, based on the shape of the
magnetization loops, suggest a domain movement mechanism for both types of
samples. The temperature dependence of the coercivity of the films has been
also examined, displaying a generic anomalous reentrant behavior at T200 K.
This feature is independent of the substrate orientation and films thickness
and may be associated to the appearance of new pinning centers due to the
nucleation of the -phase at high temperatures.Comment: 9 pages, 7 figure
Domain walls in (Ga,Mn)As diluted magnetic semiconductor
We report experimental and theoretical studies of magnetic domain walls in an
in-plane magnetized (Ga,Mn)As dilute moment ferromagnetic semiconductor. Our
high-resolution electron holography technique provides direct images of domain
wall magnetization profiles. The experiments are interpreted based on
microscopic calculations of the micromagnetic parameters and
Landau-Lifshitz-Gilbert simulations. We find that the competition of uniaxial
and biaxial magnetocrystalline anisotropies in the film is directly reflected
in orientation dependent wall widths, ranging from approximately 40 nm to 120
nm. The domain walls are of the N\'eel type and evolve from near-
walls at low-temperatures to large angle [10]-oriented walls and small
angle [110]-oriented walls at higher temperatures.Comment: 5 pages, 4 figure
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
A smooth cascade of wrinkles at the edge of a floating elastic film
The mechanism by which a patterned state accommodates the breaking of
translational symmetry by a phase boundary or a sample wall has been addressed
in the context of Landau branching in type-I superconductors, refinement of
magnetic domains, and compressed elastic sheets. We explore this issue by
studying an ultrathin polymer sheet floating on the surface of a fluid,
decorated with a pattern of parallel wrinkles. At the edge of the sheet, this
corrugated profile meets the fluid meniscus. Rather than branching of wrinkles
into generations of ever-smaller sharp folds, we discover a smooth cascade in
which the coarse pattern in the bulk is matched to fine structure at the edge
by the continuous introduction of discrete, higher wavenumber Fourier modes.
The observed multiscale morphology is controlled by a dimensionless parameter
that quantifies the relative strength of the edge forces and the rigidity of
the bulk pattern.Comment: 4 pages, 4 figure
Determination of the micromagnetic parameters in (Ga,Mn)As using domain theory
The magnetic domain structure and magnetic properties of a ferromagnetic
(Ga,Mn)As epilayer with perpendicular magnetic easy-axis are investigated. We
show that, despite strong hysteresis, domain theory at thermodynamical
equilibrium can be used to determine the micromagnetic parameters. Combining
magneto-optical Kerr microscopy, magnetometry and ferromagnetic resonance
measurements, we obtain the characteristic parameter for magnetic domains
, the domain wall width and specific energy, and the spin stiffness
constant as a function of temperature. The nucleation barrier for magnetization
reversal and the Walker breakdown velocity for field-driven domain wall
propagation are also estimated
Dust sedimentation and self-sustained Kelvin-Helmholtz turbulence in protoplanetary disk mid-planes. I. Radially symmetric simulations
We perform numerical simulations of the Kelvin-Helmholtz instability in the
mid-plane of a protoplanetary disk. A two-dimensional corotating slice in the
azimuthal--vertical plane of the disk is considered where we include the
Coriolis force and the radial advection of the Keplerian rotation flow. Dust
grains, treated as individual particles, move under the influence of friction
with the gas, while the gas is treated as a compressible fluid. The friction
force from the dust grains on the gas leads to a vertical shear in the gas
rotation velocity. As the particles settle around the mid-plane due to gravity,
the shear increases, and eventually the flow becomes unstable to the
Kelvin-Helmholtz instability. The Kelvin-Helmholtz turbulence saturates when
the vertical settling of the dust is balanced by the turbulent diffusion away
from the mid-plane. The azimuthally averaged state of the self-sustained
Kelvin-Helmholtz turbulence is found to have a constant Richardson number in
the region around the mid-plane where the dust-to-gas ratio is significant.
Nevertheless the dust density has a strong non-axisymmetric component. We
identify a powerful clumping mechanism, caused by the dependence of the
rotation velocity of the dust grains on the dust-to-gas ratio, as the source of
the non-axisymmetry. Our simulations confirm recent findings that the critical
Richardson number for Kelvin-Helmholtz instability is around unity or larger,
rather than the classical value of 1/4Comment: Accepted for publication in ApJ. Some minor changes due to referee
report, most notably that the clumping mechanism has been identified as the
streaming instability of Youdin & Goodman (2005). Movies of the simulations
are still available at http://www.mpia.de/homes/johansen/research_en.ph
Nearly Optimal Private Convolution
We study computing the convolution of a private input with a public input
, while satisfying the guarantees of -differential
privacy. Convolution is a fundamental operation, intimately related to Fourier
Transforms. In our setting, the private input may represent a time series of
sensitive events or a histogram of a database of confidential personal
information. Convolution then captures important primitives including linear
filtering, which is an essential tool in time series analysis, and aggregation
queries on projections of the data.
We give a nearly optimal algorithm for computing convolutions while
satisfying -differential privacy. Surprisingly, we follow
the simple strategy of adding independent Laplacian noise to each Fourier
coefficient and bounding the privacy loss using the composition theorem of
Dwork, Rothblum, and Vadhan. We derive a closed form expression for the optimal
noise to add to each Fourier coefficient using convex programming duality. Our
algorithm is very efficient -- it is essentially no more computationally
expensive than a Fast Fourier Transform.
To prove near optimality, we use the recent discrepancy lowerbounds of
Muthukrishnan and Nikolov and derive a spectral lower bound using a
characterization of discrepancy in terms of determinants
- …