9,550 research outputs found
Linear spin wave theory for single-Q incommensurate magnetic structures
Linear spin wave theory provides the leading term in the calculation of the
excitation spectra of long-range ordered magnetic systems as a function of
. This term is acquired using the Holstein-Primakoff approximation
of the spin operator and valid for small fluctuations of the ordered
moment. We propose an algorithm that allows magnetic ground states with general
moment directions and single-Q incommensurate ordering wave vector using a
local coordinate transformation for every spin and a rotating coordinate
transformation for the incommensurability. Finally we show, how our model can
determine the spin wave spectrum of the magnetic C-site langasites with
incommensurate order.Comment: 12 pages, 3 figures, cite this paper if you use SpinW
(http://www.psi.ch/spinw
The duration distribution of Swift Gamma-Ray Bursts
Decades ago two classes of gamma-ray bursts were identified and delineated as
having durations shorter and longer than about 2 s. Subsequently indications
also supported the existence of a third class. Using maximum likelihood
estimation we analyze the duration distribution of 888 Swift BAT bursts
observed before October 2015. Fitting three log-normal functions to the
duration distribution of the bursts provides a better fit than two log-normal
distributions, with 99.9999% significance. Similarly to earlier results, we
found that a fourth component is not needed. The relative frequencies of the
distribution of the groups are 8% for short, 35% for intermediate and 57% for
long bursts which correspond to our previous results. We analyse the redshift
distribution for the 269 GRBs of the 888 GRBs with known redshift. We find no
evidence for the previously suggested difference between the long and
intermediate GRBs' redshift distribution. The observed redshift distribution of
the 20 short GRBs differs with high significance from the distributions of the
other groups.Comment: accepte
Thermally induced coherence in a Mott insulator of bosonic atoms
Conventional wisdom is that increasing temperature causes quantum coherence
to decrease. Using finite temperature perturbation theory and exact
calculations for the strongly correlated bosonic Mott insulating state we show
a practical counter-example that can be explored in optical lattice
experiments: the short-range coherence of the Mott insulating phase can
increase substantially with increasing temperature. We demonstrate that this
phenomenon originates from thermally produced defects that can tunnel with
ease. Since the near zero temperature coherence properties have been measured
with high precision we expect these results to be verifiable in current
experiments.Comment: 5 pages, 3 figure
Implications of spatial distributions of snow mass and melt rate for snow-cover depletion: observations in a subarctic mountain catchment
Hybrid A/D converter for 200 deg C operation
A 12 bit A/D converter was designed and developed which will operate at 200 C with .05 linearity, 1/accuracy, 350 WSec conversion time, and only 455 mW power consumption. This product also necessitated the development of a unique three metal system in which aluminum wire bonding is done utilizing aluminum bonding pads, gold wire bonding to all gold areas, and employment of a nickel interface between gold and aluminum connections. This system totally eliminates the formation of a intermetallics at the bonding interface which can lead to bond failure. This product represents an advancement in electronics as it proved the operation of integrated circuits at high temperature, as well as providing information about both the electrical and mechanical reliability of hybrid circuits at 200 C
On the Azimuthal Stability of Shock Waves around Black Holes
Analytical studies and numerical simulations of time dependent axially
symmetric flows onto black holes have shown that it is possible to produce
stationary shock waves with a stable position both for ideal inviscid and for
moderately viscous accretion disks.
We perform several two dimensional numerical simulations of accretion flows
in the equatorial plane to study shock stability against non-axisymmetric
azimuthal perturbations. We find a peculiar new result. A very small
perturbation seems to produce an instability as it crosses the shock, but after
some small oscillations, the shock wave suddenly transforms into an asymmetric
closed pattern, and it stabilizes with a finite radial extent, despite the
inflow and outflow boundary conditions are perfectly symmetric. The main
characteristics of the final flow are: 1) The deformed shock rotates steadily
without any damping. It is a permanent feature and the thermal energy content
and the emitted energy vary periodically with time. 2) This behavior is also
stable against further perturbations. 3) The average shock is still very strong
and well defined, and its average radial distance is somewhat larger than that
of the original axially symmetric circular shock. 4) Shocks obtained with
larger angular momentum exhibit more frequencies and beating phenomena. 5) The
oscillations occur in a wide range of parameters, so this new effect may have
relevant observational consequences, like (quasi) periodic oscillations, for
the accretion of matter onto black holes. Typical time scales for the periods
are 0.01 and 1000 seconds for black holes with 10 and 1 million solar mass,
respectively.Comment: 15 pages, 7 figures, accepted by the Astrophysical Journa
The Twist of the Draped Interstellar Magnetic Field Ahead of the Heliopause: A Magnetic Reconnection Driven Rotational Discontinuity
Based on the difference between the orientation of the interstellar
and the solar magnetic fields, there was an expectation that the magnetic field
direction would rotate dramatically across the heliopause (HP). However, the
Voyager 1 spacecraft measured very little rotation across the HP. Previously we
showed that the twists as it approaches the HP and acquires a strong
T component (East-West). Here we establish that reconnection in the eastern
flank of the heliosphere is responsible for the twist. On the eastern flank the
solar magnetic field has twisted into the positive N direction and reconnects
with the Southward pointing component of the . Reconnection drives a
rotational discontinuity (RD) that twists the into the -T direction
and propagates upstream in the interstellar medium towards the nose. The
consequence is that the N component of is reduced in a finite width
band upstream of the HP. Voyager 1 currently measures angles
() close to solar values. We present MHD simulations
to support this scenario, suppressing reconnection in the nose region while
allowing it in the flanks, consistent with recent ideas about reconnection
suppression from diamagnetic drifts. The jump in plasma (the plasma to
magnetic pressure) across the nose of HP is much greater than in the flanks
because the heliosheath is greater there than in the flanks.
Large-scale reconnection is therefore suppressed in the nose but not at the
flanks. Simulation data suggest that will return to its pristine
value past the HP.Comment: 19 pages, 5 figures, submitte
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