3,012 research outputs found
Atomistic perspective of long lifetimes of small skyrmions at room temperature
The current development to employ magnetic skyrmions in novel spintronic
device designs has led to a demand for room temperature-stable skyrmions of
ever smaller size. We present extensive studies on skyrmion stability in
atomistic magnetic systems in two- and three-dimensional geometries. We show
that for materials described by the same micromagnetic parameters, the
variation of the atomistic exchange between different neighbors, the stacking
order, and the number of layers of the atomic lattice can significantly
influence the rate of the thermally activated decay of a skyrmion. These
factors alone are important considerations, but it is shown that their
combination can open up novel avenues of materials design in the search for
sub-10nm skyrmions, as their lifetime can be extended by several orders of
magnitude
Collision Detection and Administration Methods for Many Particles with Different Sizes
This paper deals with the calculation of the motion and the adminis-tration of the contacts for systems with many colliding bodies of round shape and possibly large size-differences. Both two dimensional (2D) and three dimensional (3D) cases are investigated, while the efficiency of the employed algorithms is compared. For the integration of the equations of motion, standard methods are used, but to reduce the effort for collision detection, more sophisticated administration algorithms for the neighbor-hood search are prosented. Especially for large systems with many parti-cles and a wide, polydisperse size distribution, this is a challenge. Three methods, the Verlet-Neighbor List (VL), the Linked Cell (LC) method, and the Linked Linear List (LLL), are discussed and compared for 2D and 3D. Only LLL performs well for strongly different particle sizes
Antiskyrmions stabilized at interfaces by anisotropic Dzyaloshinskii-Moriya interaction
Chiral magnets are an emerging class of topological matter harbouring
localized and topologically protected vortex-like magnetic textures called
skyrmions, which are currently under intense scrutiny as a new entity for
information storage and processing. Here, on the level of micromagnetics we
rigorously show that chiral magnets cannot only host skyrmions but also
antiskyrmions as least-energy configurations over all non-trivial homotopy
classes. We derive practical criteria for their occurrence and coexistence with
skyrmions that can be fulfilled by (110)-oriented interfaces in dependence on
the electronic structure. Relating the electronic structure to an atomistic
spin-lattice model by means of density-functional calculations and minimizing
the energy on a mesoscopic scale applying spin-relaxation methods, we propose a
double layer of Fe grown on a W(110) substrate as a practical example. We
conjecture that ultrathin magnetic films grown on semiconductor or heavy metal
substrates with symmetry are prototype classes of materials hosting
magnetic antiskyrmions.Comment: 20 pages (11 pages + 9 pages supplementary material
On the commutability of homogenization and linearization in finite elasticity
We study non-convex elastic energy functionals associated to (spatially)
periodic, frame indifferent energy densities with a single non-degenerate
energy well at SO(n). Under the assumption that the energy density admits a
quadratic Taylor expansion at identity, we prove that the Gamma-limits
associated to homogenization and linearization commute. Moreover, we show that
the homogenized energy density, which is determined by a multi-cell
homogenization formula, has a quadratic Taylor expansion with a quadratic term
that is given by the homogenization of the quadratic term associated to the
linearization of the initial energy density
Elimination of spiral waves in cardiac tissue by multiple electrical shocks
We study numerically the elimination of a spiral wave in cardiac tissue by application of multiple shocks of
external current. To account for the effect of shocks we apply a recently developed theory for the interaction
of the external current with cardiac tissue. We compare two possible feedback algorithms for timing of the
shocks: a "local" feedback algorithm (using an external electrode placed directly on the tissue) and a
"global" feedback algorithm (using the electrocardiogram). Our main results are: application of the
external current causes a parametric resonant drift similar to that reported in previous model computations; the
ratio of the threshold of elimination of the spiral wave by multiple shocks to the threshold of conventional
single shock defibrillation in our model for cardiac tissue is about 0.5, while earlier, less realistic models
predicted the value about 0.2; we show that an important factor for successful defibrillation is the location of
the feedback electrode and the best results are achieved if the feedback electrode or the ECG lead is located at
the boundary (or edge) of the cardiac tissue; the "local" and the "global" feedback algorithms show similar
efficiency
Towards many colors in FISH on 3D-preserved interphase nuclei
The article reviews the existing methods of multicolor FISH on nuclear targets, first of all, interphase chromosomes. FISH proper and image acquisition are considered as two related components of a single process. We discuss (1) M-FISH (combinatorial labeling + deconvolution + widefield microscopy); (2) multicolor labeling + SIM (structured illumination microscopy); (3) the standard approach to multicolor FISH + CLSM (confocal laser scanning microscopy; one fluorochrome - one color channel); (4) combinatorial labeling + CLSM; (5) non-combinatorial labeling + CLSM + linear unmixing. Two related issues, deconvolution of images acquired with CLSM and correction of data for chromatic Z-shift, are also discussed. All methods are illustrated with practical examples. Finally, several rules of thumb helping to choose an optimal labeling + microscopy combination for the planned experiment are suggested. Copyright (c) 2006 S. Karger AG, Basel
Universal spectral form factor for chaotic dynamics
We consider the semiclassical limit of the spectral form factor of
fully chaotic dynamics. Starting from the Gutzwiller type double sum over
classical periodic orbits we set out to recover the universal behavior
predicted by random-matrix theory, both for dynamics with and without time
reversal invariance. For times smaller than half the Heisenberg time
, we extend the previously known -expansion to
include the cubic term. Beyond confirming random-matrix behavior of individual
spectra, the virtue of that extension is that the ``diagrammatic rules'' come
in sight which determine the families of orbit pairs responsible for all orders
of the -expansion.Comment: 4 pages, 1 figur
Periodic-orbit theory of universal level correlations in quantum chaos
Using Gutzwiller's semiclassical periodic-orbit theory we demonstrate
universal behaviour of the two-point correlator of the density of levels for
quantum systems whose classical limit is fully chaotic. We go beyond previous
work in establishing the full correlator such that its Fourier transform, the
spectral form factor, is determined for all times, below and above the
Heisenberg time. We cover dynamics with and without time reversal invariance
(from the orthogonal and unitary symmetry classes). A key step in our reasoning
is to sum the periodic-orbit expansion in terms of a matrix integral, like the
one known from the sigma model of random-matrix theory.Comment: 44 pages, 11 figures, changed title; final version published in New
J. Phys. + additional appendices B-F not included in the journal versio
Negative optical inertia for enhancing the sensitivity of future gravitational-wave detectors
We consider enhancing the sensitivity of future gravitational-wave detectors
by using double optical spring. When the power, detuning and bandwidth of the
two carriers are chosen appropriately, the effect of the double optical spring
can be described as a "negative inertia", which cancels the positive inertia of
the test masses and thus increases their response to gravitational waves. This
allows us to surpass the free-mass Standard Quantum Limit (SQL) over a broad
frequency band, through signal amplification, rather than noise cancelation,
which has been the case for all broadband SQL-beating schemes so far considered
for gravitational-wave detectors. The merit of such signal amplification
schemes lies in the fact that they are less susceptible to optical losses than
noise cancelation schemes. We show that it is feasible to demonstrate such an
effect with the {\it Gingin High Optical Power Test Facility}, and it can
eventually be implemented in future advanced GW detectors.Comment: 7 pages, 3 figure
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