Browsing by Subject "DOMAIN-WALLS"

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  • Estevez, Virginia; Laurson, Lasse (2017)
    We study vortex domain wall dynamics in wide permalloy strips driven by applied magnetic fields and spin-polarized electric currents. As recently reported [V. Estevez and L. Laurson, Phys. Rev. B 93, 064403 (2016)], for sufficiently wide strips and above a threshold field, periodic dynamics of the vortex core are localized in the vicinity of one of the strip edges, and the velocity drop typically observed for narrow strips is replaced by a high-velocity plateau. Here, we analyze this behavior in more detail by means of micromagnetic simulations. We show that the high-velocity plateau originates from a repeated double switching of the magnetic vortex core, underlying the periodic vortex core dynamics in the vicinity of the strip edge, i.e., the "attraction-repulsion" effect. We also discuss the corresponding dynamics driven by spin-polarized currents, as well as the effect of including quenched random structural disorder to the system.
  • Hindmarsh, Mark; Lizarraga, Joanes; Lopez-Eiguren, Asier; Urrestilla, Jon (2020)
    In the QCD axion dark matter scenario with postinflationary Peccei-Quinn symmetry breaking, the number density of axions, and hence the dark matter density, depends on the length of string per unit volume at cosmic time t, by convention written zeta/t(2). The expectation has been that the dimensionless parameter zeta tends to a constant zeta(0), a feature of a string network known as scaling. It has recently been claimed that in larger numerical simulations zeta shows a logarithmic increase with time, while theoretical modeling suggests an inverse logarithmic correction. Either case would result in a large enhancement of the string density at the QCD transition, and a substantial revision to the axion mass required for the axion to constitute all of the dark matter. With a set of new simulations of global strings, we compare the standard scaling (constant-zeta) model to the logarithmic growth and inverse-logarithmic correction models. In the standard scaling model, by fitting to linear growth in the mean string separation xi = t/root zeta, we find zeta(0) = 1.19 +/- 0.20. We conclude that the apparent corrections to zeta are artifacts of the initial conditions, rather than a property of the scaling network. The residuals from the constant-zeta (linear xi) fit also show no evidence for logarithmic growth, restoring confidence that numerical simulations can be simply extrapolated from the Peccei-Quinn symmetry-breaking scale to the QCD scale. Reanalysis of previous work on the axion number density suggests that recent estimates of the axion dark matter mass in the postinflationary symmetry-breaking scenario we study should be increased by about 50%.
  • Volovik, G. E.; Zhang, K. (2020)
    The composite cosmological objects-Kibble-Lazarides-Shafi (KLS) walls bounded by strings and cosmic strings terminated by Nambu monopoles-could be produced during the phase transitions in the early universe. Recent experiments in superfluid He-3 reproduced the formation of the KLS domain walls, which opened the new arena for the detailed study of those objects in a human controlled system with different characteristic lengths. These composite defects are formed by two successive symmetry breaking phase transitions. In the first transition the strings are formed, then in the second transition the string becomes the termination line of the KLS wall. In the same manner, in the first transition monopoles are formed, and then in the second transition these monopoles become the termination points of strings. Here we show that in the vicinity of the second transition the composite defects can be described by relative homotopy groups. This is because there are two well-separated length scales involved, which give rise to two different classes of the degenerate vacuum states, R-1 and R-2, and the composite objects correspond to the nontrivial elements of the group pi(n) (R-1, R-2). We discuss this on example of the so-called polar distorted B phase, which is formed in the two-step phase transition in liquid He-3 distorted by aerogel. In this system the string monopoles terminate spin vortices with an even winding number, while KLS string walls terminate on half-quantum vortices. In the presence of magnetic field, vortex skyrmions are formed, and the string monopole transforms to the nexus. We also discuss the integer-valued topological invariants of those objects. Our consideration can be applied to the composite defects in other condensed matter and cosmological systems.