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 $100$ nm in a single ultrathin layer of the Heusler alloy Co$_2$FeAl (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

Active fault segmentation in Northern Tunisia

Active shortening structures in Northern Tunisia have developed by tectonic inversion since the Pliocene, after Late Miocene extensional collapse of the whole region. Restored Plio-Quaternary deformation observed on reflection seismic lines indicates deformation rates around 0.6–0.8 mm/yr in the studied segments and larger amounts of shortening to the West of Northern Tunisia (16%) than to the East (7%), which suggests tectonic inversion started earlier to the West and later propagated eastwards, reaching Northeastern Tunisia in the Late Pliocene. This shortening is registered on striated pebbles in Quaternary alluvial terraces and fault-slip data giving two populations of strain ellipsoids with N–S and WNW-ESE maximum shortening. Morphometric analysis in combination with field fault segmentation mapping show that topographic uplift and drainage rejuvenation occurs in relation to 20–30 km long ENE-WSW reverse fault segments and related antiforms that are offset and linked by E-W to WNW-ESE dextral and NE-SW-oriented sinistral faults. The largest fully linked fault system is the Alia-Thibar fault. This 130 km long fault zone shows an helicoidal geometry with five different fault segments, including reverse, dextral, sinistral and oblique faults. Due to the young age of tectonic inversion, after late Miocene extensional collapse of the region, the present relief of Northern Tunisia is characteristic of a young thrust and fold belt, with dominating axial valleys along synforms and an incipient transverse drainage development propagating from West to East.This study was supported by research projects CGL2015-67130- C2-1-R, Erasmus Mundus External Cooperation Window and by Scientific Cooperation Agreement 0534 between the Office National des Mines (ONM), the Tunis el Manar University and the Group for Relief and Active Processes Analysis (ARPA) from the University of Granada. Proyecto del Ministerio de Ciencia e innovación PID2019-107138RB-I00 and P18-RT- 3632 of the Junta de Andalucia. We are grateful to the Tunisian Company of Petroleum Activities (ETAP) for sharing their Reflection Seismic Data. Midland Valley Exploration Ltd is gratefully acknowledged for supporting ASI (Academic Licence Initiative) and providing Academic licenses of Move™ to the Department of Geodynamics of the Granada University. Revisions by Jacques Deverchere and an anonymous reviewer greatly helped improve the manuscript

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 a static disorder, based on the 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

Room-Temperature Skyrmions at Zero Field in Exchange-Biased Ultrathin Films

We demonstrate that magnetic skyrmions with a mean diameter around 60 nm can be stabilized at room temperature and zero external magnetic field in an exchange-biased Pt/Co/Ni80Fe20/Ir20Mn80 multilayer stack. This is achieved through an advanced optimization of the multilayer-stack composition in order to balance the different magnetic energies controlling the skyrmion size and stability. Magnetic imaging is performed both with magnetic force microscopy and scanning nitrogen-vacancy magnetometry, the latter providing unambiguous measurements at zero external magnetic field. In such samples, we show that exchange bias provides an immunity of the skyrmion spin texture to moderate external-magnetic-field perturbations, in the tens-of-millitesla range, which is an important feature for applications such as memory devices. These results establish exchange-biased multilayer stacks as a promising platform toward the effective realization of memory and logic devices based on magnetic skyrmions