71 research outputs found
Helical states of nonlocally interacting molecules and their linear stability: geometric approach
The equations for strands of rigid charge configurations interacting
nonlocally are formulated on the special Euclidean group, SE(3), which
naturally generates helical conformations. Helical stationary shapes are found
by minimizing the energy for rigid charge configurations positioned along an
infinitely long molecule with charges that are off-axis. The classical energy
landscape for such a molecule is complex with a large number of energy minima,
even when limited to helical shapes. The question of linear stability and
selection of stationary shapes is studied using an SE(3) method that naturally
accounts for the helical geometry. We investigate the linear stability of a
general helical polymer that possesses torque-inducing non-local
self-interactions and find the exact dispersion relation for the stability of
the helical shapes with an arbitrary interaction potential. We explicitly
determine the linearization operators and compute the numerical stability for
the particular example of a linear polymer comprising a flexible rod with a
repeated configuration of two equal and opposite off-axis charges, thereby
showing that even in this simple case the non-local terms can induce
instability that leads to the rod assuming helical shapes.Comment: 34 pages, 9 figure
Skyrmion morphology in ultrathin magnetic films
Nitrogen-vacancy magnetic microscopy is employed in quenching mode as a
non-invasive, high resolution tool to investigate the morphology of isolated
skyrmions in ultrathin magnetic films. The skyrmion size and shape are found to
be strongly affected by local pinning effects and magnetic field history.
Micromagnetic simulations including static disorder, based on a physical model
of grain-to-grain thickness variations, reproduce all experimental observations
and reveal the key role of disorder and magnetic history in the stabilization
of skyrmions in ultrathin magnetic films. This work opens the way to an
in-depth understanding of skyrmion dynamics in real, disordered media.Comment: 9 pages, 8 figures, including supplementary information
Current-induced nucleation and dynamics of skyrmions in a Co-based Heusler alloy
We demonstrate room-temperature stabilization of dipolar magnetic skyrmions
with diameters in the range of nm in a single ultrathin layer of the
Heusler alloy CoFeAl (CFA) under moderate magnetic fields. Current-induced
skyrmion dynamics in microwires is studied with a scanning Nitrogen-Vacancy
magnetometer operating in the photoluminescence quenching mode. We first
demonstrate skyrmion nucleation by spin-orbit torque and show that its
efficiency can be significantly improved using tilted magnetic fields, an
effect which is not specific to Heusler alloys and could be advantageous for
future skyrmion-based devices. We then show that current-induced skyrmion
motion remains limited by strong pinning effects, even though CFA is a magnetic
material with a low magnetic damping parameter.Comment: 5 pages, 4 figure
Tops and Writhing DNA
The torsional elasticity of semiflexible polymers like DNA is of biological
significance. A mathematical treatment of this problem was begun by Fuller
using the relation between link, twist and writhe, but progress has been
hindered by the non-local nature of the writhe. This stands in the way of an
analytic statistical mechanical treatment, which takes into account thermal
fluctuations, in computing the partition function. In this paper we use the
well known analogy with the dynamics of tops to show that when subjected to
stretch and twist, the polymer configurations which dominate the partition
function admit a local writhe formulation in the spirit of Fuller and thus
provide an underlying justification for the use of Fuller's "local writhe
expression" which leads to considerable mathematical simplification in solving
theoretical models of DNA and elucidating their predictions. Our result
facilitates comparison of the theoretical models with single molecule
micromanipulation experiments and computer simulations.Comment: 17 pages two figure
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