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Jacob Linder - One of the best experts on this subject based on the ideXlab platform.
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universal absence of walker breakdown and linear Current Velocity relation via spin orbit torques in coupled and single domain wall motion
Physical Review B, 2017Co-Authors: Vetle Risinggard, Jacob LinderAbstract:We consider theoretically domain wall motion driven by spin-orbit and spin Hall torques. We find that it is possible to achieve universal absence of Walker breakdown for all spin-orbit torques using experimentally relevant spin-orbit coupling strengths. For spin-orbit torques other than the pure Rashba spin-orbit torque, this gives a linear Current-Velocity relation instead of a saturation of the Velocity at high Current densities. The effect is very robust and is found in both soft and hard magnetic materials, as well as in the presence of the Dzyaloshinskii-Moriya interaction and in coupled domain walls in synthetic antiferromagnets, where it leads to very high domain wall velocities. Moreover, recent experiments have demonstrated that the switching of a synthetic antiferromagnet does not obey the usual spin Hall angle dependence, but that domain expansion and contraction can be selectively controlled toggling only the applied in-plane magnetic field magnitude and not its sign. We show that the combination of spin Hall torques and interlayer exchange coupling produces the necessary relative velocities for this switching to occur.
Chaudry Masood Khalique - One of the best experts on this subject based on the ideXlab platform.
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solitary waves with the madelung fluid description a generalized derivative nonlinear schrodinger equation
Communications in Nonlinear Science and Numerical Simulation, 2016Co-Authors: Chaudry Masood KhaliqueAbstract:Abstract Within the framework of the Madelung fluid description, we will derive bright and dark (including gray- and black-soliton ) envelope solutions for a generalized derivative nonlinear Schrodinger model i ∂ Ψ ∂ t = ∂ 2 Ψ ∂ x 2 + i a ∂ ∂ x ( | Ψ | 2 Ψ ) + b | Ψ | 2 Ψ , by virtue of the corresponding solitary wave solutions for the stationary Gardner equations. Note that we only consider the motion with stationary-profile Current Velocity case and exclude the motion with constant Current Velocity case for a ≠ 0; on the other hand, our results are derived under suitable assumptions for the Current Velocity associated with corresponding boundary conditions of the fluid density, and under corresponding parametric constraints.
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envelope bright and dark soliton solutions for the gerdjikov ivanov model
Nonlinear Dynamics, 2015Co-Authors: Xing Lu, Jianping Yu, Chaudry Masood KhaliqueAbstract:Within the context of the Madelung fluid description, investigation has been carried out on the connection between the envelope soliton-like solutions of a wide family of nonlinear Schrodinger equations and the soliton-like solutions of a wide family of Korteweg–de Vries or Korteweg–de Vries-type equations. Under suitable hypothesis for the Current Velocity, the Gerdjikov–Ivanov envelope solitons are derived and discussed in this paper. For a motion with the stationary profile Current Velocity, the fluid density satisfies a generalized stationary Gardner equation, which possesses bright- and dark-type (including gray and black) solitary waves due to associated parametric constraints, and finally envelope solitons are found correspondingly for the Gerdjikov–Ivanov model. Moreover, this approach may be useful for studying other nonlinear Schrodinger-type equations.
Vetle Risinggard - One of the best experts on this subject based on the ideXlab platform.
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universal absence of walker breakdown and linear Current Velocity relation via spin orbit torques in coupled and single domain wall motion
Physical Review B, 2017Co-Authors: Vetle Risinggard, Jacob LinderAbstract:We consider theoretically domain wall motion driven by spin-orbit and spin Hall torques. We find that it is possible to achieve universal absence of Walker breakdown for all spin-orbit torques using experimentally relevant spin-orbit coupling strengths. For spin-orbit torques other than the pure Rashba spin-orbit torque, this gives a linear Current-Velocity relation instead of a saturation of the Velocity at high Current densities. The effect is very robust and is found in both soft and hard magnetic materials, as well as in the presence of the Dzyaloshinskii-Moriya interaction and in coupled domain walls in synthetic antiferromagnets, where it leads to very high domain wall velocities. Moreover, recent experiments have demonstrated that the switching of a synthetic antiferromagnet does not obey the usual spin Hall angle dependence, but that domain expansion and contraction can be selectively controlled toggling only the applied in-plane magnetic field magnitude and not its sign. We show that the combination of spin Hall torques and interlayer exchange coupling produces the necessary relative velocities for this switching to occur.
Nicolas F Martin - One of the best experts on this subject based on the ideXlab platform.
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Current Velocity data on dwarf galaxy ngc 1052 df2 do not constrain it to lack dark matter
The Astrophysical Journal, 2018Co-Authors: Nicolas F Martin, Michelle L M Collins, Nicolas Longeard, Erik J TollerudAbstract:It was recently proposed that the globular cluster system of the very low surface-brightness galaxy NGC1052-DF2 is dynamically very cold, leading to the conclusion that this dwarf galaxy has little or no dark matter. Here, we show that a robust statistical measure of the Velocity dispersion of the tracer globular clusters implies a mundane Velocity dispersion and a poorly constrained mass-to-light ratio. Models that include the possibility that some of the tracers are field contaminants do not yield a more constraining inference. We derive only a weak constraint on the mass-to-light ratio of the system within the half-light radius (M/LV < 6.7 at the 90-percent confidence level) or within the radius of the furthest tracer (M/LV < 8.1 at the 90-percent confidence level). This limit may imply a mass-to-light ratio on the low end for a dwarf galaxy but many Local Group dwarf galaxies fall well within this contraint. With this study, we emphasize the need to reliably account for measurement uncertainties and to stay as close as possible to the data when determining dynamical masses from very small data sets of tracers.
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Current Velocity data on dwarf galaxy ngc1052 df2 do not constrain it to lack dark matter
arXiv: Astrophysics of Galaxies, 2018Co-Authors: Nicolas F Martin, Michelle L M Collins, Nicolas Longeard, Erik J TollerudAbstract:It was recently proposed that the globular cluster system of the very low surface-brightness galaxy NGC1052-DF2 is dynamically very cold, leading to the conclusion that this dwarf galaxy has little or no dark matter. Here, we show that a robust statistical measure of the Velocity dispersion of the tracer globular clusters implies a mundane Velocity dispersion and a poorly constrained mass-to-light ratio. Models that include the possibility that some of the tracers are field contaminants do not yield a more constraining inference. We derive only a weak constraint on the mass-to-light ratio of the system within the half-light radius (M/L_V<6.7 at the 90% confidence level) or within the radius of the furthest tracer (M/L_V<8.1 at the 90% confidence level). This limit may imply a mass-to-light ratio on the low end for a dwarf galaxy but many Local Group dwarf galaxies fall well within this contraint. With this study, we emphasize the need to reliably account for measurement uncertainties and to stay as close as possible to the data when determining dynamical masses from very small data sets of tracers.
F Ziemer - One of the best experts on this subject based on the ideXlab platform.
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the near surface Current Velocity determined from image sequences of the sea surface
IEEE Transactions on Geoscience and Remote Sensing, 2001Co-Authors: C M Senet, J Seemann, F ZiemerAbstract:A method to measure the ocean's near-surface Current Velocity vector based on the analysis of remote sea-surface image sequences was developed. The spatial and temporal records were transformed to the wavenumber-frequency domain, resulting in a three-dimensional (3-D) image power spectrum. In the spectrum, the signal energy of the waves is localized on a shell defined by the dispersion relation of surface waves. The sum of the sensor's Velocity and the near-surface Current profile deforms the dispersion shell due to the Doppler-frequency shift. An iterative least-squares fitting technique and an error-estimation model was implemented. To improve the method's accuracy, spectral wave energy found in higher harmonics of the dispersion shell and aliasing effects are taken into account. The most important nonlinear mechanism leading to higher harmonics is explained as resulting from wave shadowing due to the low grazing angles typical for ground- or ship-based radars. The improved method is examined analytically and is tested with Monte Carlo simulations. The variation of the shape of the measured or simulated 3-D image spectra, especially the peak wavenumber, the directional spread, and the main travel direction, controls the behavior and accuracy of the technique. A comparison of velocities acquired by nautical radar and independent Doppler log Current measurements is presented. The technique's accuracy, its limits, and its adaptability are discussed. Additional improvements are proposed.
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an iterative technique to determine the near surface Current Velocity from time series of sea surface images
OCEANS Conference, 1997Co-Authors: C M Senet, J Seemann, F ZiemerAbstract:Time series of spatial wave images recorded from a nautical radar are transformed with a fast Fourier transformation to the wavenumber frequency domain resulting in a three dimensional image power spectrum. The spectral energy is localized on a shell defined by the surface wave dispersion relation. The sum of the sensor's Velocity and the near surface Current profile deforms the dispersion shell due to the Doppler frequency shift. An iterative least square fitting technique and an error estimation model has been implemented recently. Higher harmonics and aliasing effects are taken into account to increase the method's accuracy. The most important nonlinear mechanism leading to higher harmonics can be explained by wave shadowing due to the low grazing angles typical for ground or ship based radars. Temporal undersampling leads to aliasing. Both effects result in additional signals which are used to increase the technique's accuracy. In this paper the technique is examined analytically and with Monte Carlo simulation runs. The variation of shape of the measured or simulated three dimensional spectra-especially the peak wavenumber, the directional spread, and the main travel direction-controls the behaviour and accuracy of the technique. Comparisons with independent Doppler log Current measurements are presented. With the results the technique's accuracy, its limits, and its consistency with conventional instrumentation are shown. Additional improvements are presented which lead to higher accuracy and the technique's applicability to high ship speeds.
