Hysteresis of Finite Arrays of Magnetic nano Dots

Hysteresis curves for finite arrays of $N\times N$ ferromagnetic nano dots subject to the dipole-dipole interaction are investigated for $N=2... 13$. Spin arrangements up to N=6 are presented, which indicate the onset of bulk-like behavior associated with odd (N=5) and even (N=6) systems. The effect of field misalignment on the hysteresis loops is also studied for $N=3... 6$. The area $A_N$ of the hysteresis loop is studied as a function of $N$. We find that $A_N-A_\infty$ approximately scales as $N^{-{3/2}}$ for $N$ odd and as $N^{-2}$ for $N$ even.Comment: 6 pages, 8 figures, Submitted to PR

Anisotropic Transport Properties of Ferromagnetic-Superconducting Bilayers

We study the transport properties of vortex matter in a superconducting thin film separated by a thin insulator layer from a ferromagnetic layer. We assume an alternating stripe structure for both FM and SC layers as found in [7]. We calculate the periodic pinning force in the stripe structure resulting from a highly inhomogeneous distribution of the vortices and antivortices. We show that the transport properties in FM-SC bilayer are highly anisotropic. In the absence of random pinning it displays a finite resistance for the current perpendicular to stripes and is superconducting for the current parallel to stripes. The average vortex velocity, electric field due to the vortex motion, Josephson frequency and higher harmonics of the vortex oscillatory motion are calculated.Comment: 4 pages, 2figures, Submitted to PR

Ferromagnetic Annulus And Superconducting Vortices

The screening currents induced in a superconducting film by a magnetic annulus whose magnetization is perpendicular to the superconductor are calculated. We show that close to the superconductor transition temperature $T_c$ particular values of the magnetization and radii of the annulus make the creation of superconducting vortices energy favorable. We also show that the magnetic annulus offers an alternative tool for vortex pinning in the superconductor. Statistical mechanical properties of this system are discussed.Comment: 4 pages, 2 figures, submitted to PR

Dissipation and coherent effects in narrow superconducting channels

We apply the time dependent Ginzburg-Landau equations (TDGL) to study small ac currents of frequency $\omega$ in superconducting channels narrow on the scale of London penetration depth. We show that TDGL have $t$-dependent and spatially uniform solutions that describe the order parameter with an oscillating part of the double frequency coexisting with an ac electric field. We evaluate the Ohmic losses (related neither to the flux flow nor to the phase slips) and show that the resistivity reduction on cooling through the critical temperature $T_c$ should behave as $(T_c-T)^2/\omega^2$. If the channel is cut out of an anisotropic material in a direction other than the principal axes, the transverse phase difference and the Josephson voltage between the channel sides are generated.Comment: 5 pages, 1 figures, Accepted for publication in Phys. Rev.

Superconducting Vortices and Elliptical Ferromagnetic Textures

In this article an analytical and numerical study of superconducting thin film with ferromagnetic textures of elliptical geometries in close proximity is presented. The screening currents induced in the superconductor due to the magnetic texture are calculated. Close to the superconducting transition temperature $T_c$ the spontaneous creation of superconducting vortices becomes energy favorable depending on the value of the magnetization and the geometrical quantities of the magnetic texture. The creation of vortices by elliptic dots is more energy favorable than those created by circular ones. The superconductor covered by elliptic dots array exhibits anisotropic transport properties.Comment: 4 pages, 5figure

Theory of pinning in a Superconducting Thin Film Pierced by a Ferromagnetic Columnar Defect

This is an analytical study of pinning and spontaneous vortex phase is a system consisting of a superconducting thin film pierced by a long ferromagnetic columnar defect of finite radius $R$. The magnetic fields, screening currents, energy and pinning forces for this system are calculated. The interaction between the magnetic field of vortices and the magnetization outside the plane of the film and its close proximity enhances vortex pinning significantly. Spontaneous vortex phase appears when the magnetization of the columnar defect is increased above a critical value. Transitions between phases characterized by different number of flux quanta are also studied. These results are generalized to the case when the superconductor is pierced by an array of columnar defects.Comment: 6 pages, 4 figures, Accepted for publication in Phys. Rev.

The evolving SARS-CoV-2 epidemic in Africa: Insights from rapidly expanding genomic surveillance

INTRODUCTION Investment in Africa over the past year with regard to severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) sequencing has led to a massive increase in the number of sequences, which, to date, exceeds 100,000 sequences generated to track the pandemic on the continent. These sequences have profoundly affected how public health officials in Africa have navigated the COVID-19 pandemic. RATIONALE We demonstrate how the first 100,000 SARS-CoV-2 sequences from Africa have helped monitor the epidemic on the continent, how genomic surveillance expanded over the course of the pandemic, and how we adapted our sequencing methods to deal with an evolving virus. Finally, we also examine how viral lineages have spread across the continent in a phylogeographic framework to gain insights into the underlying temporal and spatial transmission dynamics for several variants of concern (VOCs). RESULTS Our results indicate that the number of countries in Africa that can sequence the virus within their own borders is growing and that this is coupled with a shorter turnaround time from the time of sampling to sequence submission. Ongoing evolution necessitated the continual updating of primer sets, and, as a result, eight primer sets were designed in tandem with viral evolution and used to ensure effective sequencing of the virus. The pandemic unfolded through multiple waves of infection that were each driven by distinct genetic lineages, with B.1-like ancestral strains associated with the first pandemic wave of infections in 2020. Successive waves on the continent were fueled by different VOCs, with Alpha and Beta cocirculating in distinct spatial patterns during the second wave and Delta and Omicron affecting the whole continent during the third and fourth waves, respectively. Phylogeographic reconstruction points toward distinct differences in viral importation and exportation patterns associated with the Alpha, Beta, Delta, and Omicron variants and subvariants, when considering both Africa versus the rest of the world and viral dissemination within the continent. Our epidemiological and phylogenetic inferences therefore underscore the heterogeneous nature of the pandemic on the continent and highlight key insights and challenges, for instance, recognizing the limitations of low testing proportions. We also highlight the early warning capacity that genomic surveillance in Africa has had for the rest of the world with the detection of new lineages and variants, the most recent being the characterization of various Omicron subvariants. CONCLUSION Sustained investment for diagnostics and genomic surveillance in Africa is needed as the virus continues to evolve. This is important not only to help combat SARS-CoV-2 on the continent but also because it can be used as a platform to help address the many emerging and reemerging infectious disease threats in Africa. In particular, capacity building for local sequencing within countries or within the continent should be prioritized because this is generally associated with shorter turnaround times, providing the most benefit to local public health authorities tasked with pandemic response and mitigation and allowing for the fastest reaction to localized outbreaks. These investments are crucial for pandemic preparedness and response and will serve the health of the continent well into the 21st century

Agent-Specific Responses in the Cingulate Cortex During Economic Exchanges

Interactions with other responsive agents lie at the core of all social exchange. During a social exchange with a partner, one fundamental variable that must be computed correctly is who gets credit for a shared outcome; this assignment is crucial for deciding on an optimal level of cooperation that avoids simple exploitation. We carried out an iterated, two-person economic exchange and made simultaneous hemodynamic measurements from each player's brain. These joint measurements revealed agent-specific responses in the social domain (“me” and “not me”) arranged in a systematic spatial pattern along the cingulate cortex. This systematic response pattern did not depend on metrical aspects of the exchange, and it disappeared completely in the absence of a responding partner