3,580 research outputs found
Capturing of a Magnetic Skyrmion with a Hole
Magnetic whirls in chiral magnets, so-called skyrmions, can be manipulated by
ultrasmall current densities. Here we study both analytically and numerically
the interactions of a single skyrmion in two dimensions with a small hole in
the magnetic layer. Results from micromagnetic simulations are in good
agreement with effective equations of motion obtained from a generalization of
the Thiele approach. Skyrmion-defect interactions are described by an effective
potential with both repulsive and attractive components. For small current
densities a previously pinned skyrmion stays pinned whereas an unpinned
skyrmion moves around the impurities and never gets captured. For higher
current densities, j_c1 < j < j_c2, however, single holes are able to capture
moving skyrmions. The maximal cross section is proportional to the skyrmion
radius and to Sqrt(alpha), where alpha is the Gilbert damping. For j > j_c2 all
skyrmions are depinned. Small changes of the magnetic field strongly change the
pinning properties, one can even reach a regime without pinning, j_c2=0. We
also show that a small density of holes can effectively accelerate the motion
of the skyrmion and introduce a Hall effect for the skyrmion.Comment: 11 page
Edge instabilities and skyrmion creation in magnetic layers
We study both analytically and numerically the edge of two-dimensional
ferromagnets with Dzyaloshinskii-Moriya (DM) interactions, considering both
chiral magnets and magnets with interface-induced DM interactions. We show that
in the field-polarized ferromagnetic phase magnon states exist which are bound
to the edge, and we calculate their spectra within a continuum field theory.
Upon lowering an external magnetic field, these bound magnons condense at a
finite momentum and the edge becomes locally unstable. Micromagnetic
simulations demonstrate that this edge instability triggers the creation of a
helical phase which penetrates the field-polarized state within the bulk. A
subsequent increase of the magnetic field allows to create skyrmions close to
the edge in a controlled manner.Comment: 10 pages, 8 figures; (v2) minor corrections, published versio
Hydrodynamic long-time tails after a quantum quench
After a quantum quench, a sudden change of parameters, generic many particle
quantum systems are expected to equilibrate. A few collisions of quasiparticles
are usually sufficient to establish approximately local equilibrium. Reaching
global equilibrium is, however, much more difficult as conserved quantities
have to be transported for long distances to build up a pattern of fluctuations
characteristic for equilibrium. Here we investigate the quantum quench of the
one-dimensional bosonic Hubbard model from infinite to finite interaction
strength U using semiclassical methods for weak, and exact diagonalization for
strong quenches. Equilibrium is approached only slowly, as t^{-1/2} with
subleading corrections proportional to t^{-3/4}, consistent with predictions
from hydrodynamics. We show that these long-time tails determine the relaxation
of a wide range of physical observables.Comment: 5 pages + 4 pages Supplementary Materia
Magnetic skyrmions and skyrmion clusters in the helical phase of CuOSeO
Skyrmions are nanometric spin whirls that can be stabilized in magnets
lacking inversion symmetry. The properties of isolated skyrmions embedded in a
ferromagnetic background have been intensively studied. We show that single
skyrmions and clusters of skyrmions can also form in the helical phase and
investigate theoretically their energetics and dynamics. The helical background
provides natural one-dimensional channels along which a skyrmion can move
rapidly. In contrast to skyrmions in ferromagnets, the skymion-skyrmion
interaction has a strong attractive component and thus skyrmions tend to form
clusters with characteristic shapes. These clusters are directly observed in
transmission electron microscopy measurements in thin films of CuOSeO.
Topological quantization, high mobility and the confinement of skyrmions in
channels provided by the helical background may be useful for future
spintronics devices.Comment: 5 pages, 3 figures, 4 pages supplemen
Control of Energy Transfer Between Pyrene‐ and Perylene‐Nucleosides by the Sequence of DNA‐Templated Supramolecular Assemblies
DNA was used as supramolecular scaffold to order chromophores and control their optical properties. Ethynylpyrene as energy donor was attached to 2′‐desoxy‐2‐aminoadenosine that binds selectively to thymidines (T) in the template. Ethynylperylene as acceptor was attached to 2′‐desoxyuridine that is complementary to 2′‐desoxyadenosine (A). This donor‐acceptor pair was assembled along single‐stranded DNA templates of different A−T sequences to investigate the sequence control of the energy transfer between the chromophores. The fluorescence intensities increase in the mixed assemblies along the DNA templates from A10T10 over (AATT)5 to (AT)10, although these templates provide equal numbers of potential binding sites for the two different nucleoside chromophore conjugates and exhibit similar absorbances. This shows that the sequence selective assembly of the two building blocks along DNA templates is programmable and alters the fluorescence readout. Such sequence‐controlled supramolecular chemistry represents the key element for future functional π‐systems in materials for light harvesting of solar energy
Test of the isotropy of the speed of light using a continuously rotating optical resonator
We report on a test of Lorentz invariance performed by comparing the
resonance frequencies of one stationary optical resonator and one continuously
rotating on a precision air bearing turntable. Special attention is paid to the
control of rotation induced systematic effects. Within the photon sector of the
Standard Model Extension, we obtain improved limits on combinations of 8
parameters at a level of a few parts in . For the previously least
well known parameter we find . Within the Robertson-Mansouri-Sexl test theory, our measurement
restricts the isotropy violation parameter to
, corresponding to an eightfold improvement with
respect to previous non-rotating measurements.Comment: 4 pages, 3 figures, to appear in Phys. Rev. Let
Indigo in the nanochannels of zeolite L : towards a new type of colorant
A host-guest based colorant was synthesized by intercalating indigo molecules into the nanochannels of zeolite L (ZL). Reductive washing thereby ensured the efficient and selective removal of non-intercalated indigo molecules. The UV-vis diffuse reflectance spectrum of the product after intercalation and reductive washing (designated as indigo-ZL) was found to resemble the solution spectrum of indigo, leading to the conclusion that the formation of indigo aggregates is prevented due to the steric constraints imposed by the microporous structure of ZL. The application of indigo-ZL on cotton was tested by roll coating. The light absorption properties of the resulting textile prints showed no significant alteration when compared to the pure indigo-ZL powder. The UV-vis diffuse reflectance spectra of mixtures consisting of indigo-ZL and a further ZL-based colorant were successfully predicted by the weighted addition of the respective primary spectra
Discrimination between S²⁻ and SO²⁻ Sites in Mixed Crystal Compounds Containing Co and Co/Mo Clusters by ⁵⁹Co-NMR Spectroscopy, and the Relevance for HDS Catalysis
In aqueous solutions of the mixed crystal compounds with formal composition K8[Co4-xMoxS4-y(SO)y(CN)12·nH2O (x = 0–2; y = 0–2) i) there are distinct CoS, Co(SO), and Co(SO)2 sites and ii) no hetero-metal clusters are present. These results, revealed by 59Co–NMR, could principally not be disclosed by single crystal X-ray structure analysis. The NMR data also suggest a stable dinuclear cobalt-sulphur cluster in solution. The relevance for HDS catalysis is discussed
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