Scattering of vortex pairs in 2D easy-plane ferromagnets

Vortex-antivortex pairs in 2D easy-plane ferromagnets have characteristics of solitons in two dimensions. We investigate numerically and analytically the dynamics of such vortex pairs. In particular we simulate numerically the head-on collision of two pairs with different velocities for a wide range of the total linear momentum of the system. If the momentum difference of the two pairs is small, the vortices exchange partners, scatter at an angle depending on this difference, and form two new identical pairs. If it is large, the pairs pass through each other without losing their identity. We also study head-tail collisions. Two identical pairs moving in the same direction are bound into a moving quadrupole in which the two vortices as well as the two antivortices rotate around each other. We study the scattering processes also analytically in the frame of a collective variable theory, where the equations of motion for a system of four vortices constitute an integrable system. The features of the different collision scenarios are fully reproduced by the theory. We finally compare some aspects of the present soliton scattering with the corresponding situation in one dimension.Comment: 13 pages (RevTeX), 8 figure

On-column 2p bound state with topological charge \pm1 excited by an atomic-size vortex beam in an aberration-corrected scanning transmission electron microscope

Atomic-size vortex beams have great potential in probing materials' magnetic moment at atomic scales. However, the limited depth of field of vortex beams constrains the probing depth in which the helical phase front is preserved. On the other hand, electron channeling in crystals can counteract beam divergence and extend the vortex beam without disrupting its topological charge. Specifically, in this paper, we report atomic vortex beams with topological charge \pm1 can be coupled to the 2p columnar bound states and propagate for more 50 nm without being dispersed and losing its helical phase front. We gave numerical solutions to the 2p columnar orbitals and tabulated the characteristic size of the 2p states of two typical elements, Co and Dy, for various incident beam energies and various atomic densities. The tabulated numbers allow estimates of the optimal convergence angle for maximal coupling to 2p columnar orbital. We also have developed analytic formulae for beam energy, convergence-angle, and hologram dependent scaling for various characteristic sizes. These length scales are useful for the design of pitch-fork apertures and operations of microscopes in the vortex-beam imaging mode.Comment: 30 pages, 7 figures, Microscopy and Microanalysis, in pres

Two Bipolar Outflows and Magnetic Fields in a Multiple Protostar System, L1448 IRS 3

We performed spectral line observations of CO J=2-1, 13CO J=1-0, and C18O J=1-0 and polarimetric observations in the 1.3 mm continuum and CO J=2-1 toward a multiple protostar system, L1448 IRS 3, in the Perseus molecular complex at a distance of ~250 pc, using the BIMA array. In the 1.3 mm continuum, two sources (IRS 3A and 3B) were clearly detected with estimated envelope masses of 0.21 and 1.15 solar masses, and one source (IRS 3C) was marginally detected with an upper mass limit of 0.03 solar masses. In CO J=2-1, we revealed two outflows originating from IRS 3A and 3B. The masses, mean number densities, momentums, and kinetic energies of outflow lobes were estimated. Based on those estimates and outflow features, we concluded that the two outflows are interacting and that the IRS 3A outflow is nearly perpendicular to the line of sight. In addition, we estimated the velocity, inclination, and opening of the IRS 3B outflow using Bayesian statistics. When the opening angle is ~20 arcdeg, we constrain the velocity to ~45 km/s and the inclination angle to ~57 arcdeg. Linear polarization was detected in both the 1.3 mm continuum and CO J=2-1. The linear polarization in the continuum shows a magnetic field at the central source (IRS 3B) perpendicular to the outflow direction, and the linear polarization in the CO J=2-1 was detected in the outflow regions, parallel or perpendicular to the outflow direction. Moreover, we comprehensively discuss whether the binary system of IRS 3A and 3B is gravitationally bound, based on the velocity differences detected in 13CO J=1-0 and C18O J=1-0 observations and on the outflow features. The specific angular momentum of the system was estimated as ~3e20 cm^2/s, comparable to the values obtained from previous studies on binaries and molecular clouds in Taurus.Comment: ApJ accepted, 20 pages, 2 tables, 10 figure

Evolution of heavy quark distribution function in quark-gluon plasma: using the Iterative Laplace Transform Method

The "Iterative Laplace Transform Method" is used to solve the Fokker-Planck equation for finding the time evolution of the heavy quarks distribution functions such as charm and bottom in quark gluon plasma. These solutions will lead us to calculation of nuclear suppression factor RAA. The results have good agreement with available experiment data from the PHENIX collaboration.Comment: 13 pages, 4 figure