11,657 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
Control of ELT false alarms
The statistics of emergency locator transmitter (ELT) alarms are presented. The primary sources of data include ELT Incident Logs, Service Difficulty Reports, and Frequency Interference Reports. The number of reported and unreported alarms is discussed, as are seasonal variations, duration of ELT transmissions, and cost of silencing. Origin, causes, and possible strategies for reducing the impact of alarms on the aviation community are considered
Entanglement loss in molecular quantum-dot qubits due to interaction with the environment
We study quantum entanglement loss due to environmental interaction in a
condensed matter system with a complex geometry relevant to recent proposals
for computing with single electrons at the nanoscale. We consider a system
consisting of two qubits, each realized by an electron in a double quantum dot,
which are initially in an entangled Bell state. The qubits are widely separated
and each interacts with its own environment. The environment for each is
modeled by surrounding double quantum dots placed at random positions with
random orientations. We calculate the unitary evolution of the joint system and
environment. The global state remains pure throughout. We examine the time
dependence of the expectation value of the bipartite Clauser-Horne-Shimony-Holt
(CHSH) and Brukner-Paunkovi\'c-Rudolph-Vedral (BPRV) Bell operators and explore
the emergence of correlations consistent with local realism. Though the details
of this transition depend on the specific environmental geometry, we show how
the results can be mapped on to a universal behavior with appropriate scaling.
We determine the relevant disentanglement times based on realistic physical
parameters for molecular double-dots.Comment: 14 pages, 3 figure
Applying MOG to lensing: Einstein rings, Abell 520 and the Bullet Cluster
We investigate gravitational lensing in the context of the MOG modified
theory of gravity. Using a formulation of the theory with no adjustable or
fitted parameters, we present the MOG equations of motion for slow,
nonrelativistic test particles and for ultrarelativistic test particles, such
as rays of light. We demonstrate how the MOG prediction for the bending of
light can be applied to astronomical observations. Our investigation first
focuses on a small set of strong lensing observations where the properties of
the lensing objects are found to be consistent with the predictions of the
theory. We also present an analysis of the colliding clusters 1E0657-558 (known
also as the Bullet Cluster) and Abell 520; in both cases, the predictions of
the MOG theory are in good agreement with observation.Comment: 14 pages, 2 figures; final proof before publicatio
Quantum ergodicity for restrictions to hypersurfaces
Quantum ergodicity theorem states that for quantum systems with ergodic
classical flows, eigenstates are, in average, uniformly distributed on energy
surfaces. We show that if N is a hypersurface in the position space satisfying
a simple dynamical condition, the restrictions of eigenstates to N are also
quantum ergodic.Comment: 22 pages, 1 figure; revised according to referee's comments. To
appear in Nonlinearit
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
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