Relative equilibria of vortex sheets

Numerical evidence is presented for the existence of relative equilibrium configurations of vortex sheets (one-dimensional singular distributions of vorticity) in a two-dimensional ideal fluid. Point vortices are used to approximate the vortex sheets.     

Transitions from vortex lines to sheets: interplay of topology and dynamics in an anisotropic superfluid

In isotropic macroscopic quantum systems vortex lines can be formed while in anisotropic systems also vortex sheets are possible. Based on measurements of superfluid 3He-A, we present the principles which select between these two competing forms of quantized vorticity: sheets displace lines if the frequency of the external drive exceeds a critical limit. The resulting topologically stable state consists of multiple vortex sheets and has much faster dynamics than the state with vortex lines.     

Experimental dynamics of a vortex within a vortex

We report the experimental dynamics of a new two-dimensional (2D) fluid phenomenon that occurs when an intense, pointlike vortex is placed within a diffuse, circular vortex. Our observations, made using strongly magnetized electron columns to model the 2D fluid, support the analysis performed by Jin and Dubin.     

Breakdown and fusion of vortex loops and dynamics of vortex tangle

The role of the processes of reconnection of vortex filaments in the structure and dynamics of vortex tangle in turbulent HeII has been studied. The evolution of a network of vortex loops has been investigated on the basis of the “kinetic” equation for the distribution function of the number of loops in the space of their lengths. The vortex tangle properties are discussed on the basis of the exact solution obtained for this equation.     

Anomalous vortex dynamics in

We report on measurements of current–voltage (I–V) characteristics for YNi₂B₂C single crystals with weak pinning in various fields at 7.6 K. We find nonmonotonic, N-shaped I–V curves in a certain field region deep in the vortex solid phase. This behavior is anomalous, since there exists an intermediate I region where flow voltage V shows a decrease with increasing I (a driving force). While the exact nature remains unknown, this phenomenon suggests vortex motion (driving I) induced pinning.     

Micro-spot-welding of copper sheets with an IR vortex beam

Copper welding with an infrared(IR) Gaussian laser beam usually shows obvious instability, spatters, and worse surface morphology due to the Gaussian distribution, temperature-dependent IR absorption, and high thermal conductivity in copper. In this paper, the IR quasi-continuous-wave Gaussian beam was converted into a vortex ring beam with a phase-plate and then applied to the micro-spot-welding of copper sheets. The welding with the vortex beam demonstrated a significantly improved welding per...     

Simulation of two-dimensional vortex dynamics

The two-dimensional XY model is simulated with a time-dependent Ginzburg-Landau type dynamics. The data are, in the limit of small driving force, well described by the Minnhagen phenomenology for vortex dynamics of a two-dimensional superfluid. This phenomenology is different and distinguishable from the conventional AHNS phenomenology. The Minnhagen phenomenology has been observed in recent experiments on Josephson-junction arrays and high-T_c BSCCO films. The present simulations suggest that this reflects an intrinsic property of the vortex dynamics for a two-dimensional superfluid.     

Superconductive CaC6 prepared from flexible graphite sheets

The host graphite material of choice for most CaC₆ studies is highly oriented pyrolytic graphite (HOPG), which can affect the resulting properties of prepared graphite intercalation compounds (GICs). In order to gain a better understanding of the superconductivity of CaC₆, we examined CaC₆ prepared from Grafoil and PGS. To confirm their superconductivity, the dependences of the magnetic moment on temperature under a constant magnetic field were measured over 2–20 K. The resulting CaC₆ transitioned to a superconducting state at around 11 K, independent of purity and homogeneity, while slight differences that depended on the host materials were observed. The transition to a superconducting state at 11.5 K occurred provided that a small amount of CaC₆ was present. The co-existence of LiC₆ and other GICs, and the residual Li atoms in the CaC₆ had little effect on the transition temperature. The transition temperature of the CaC₆ prepared from PGS was slightly lower than that of the CaC₆ prepared from HOPG and Grafoil.     

Out-of-equilibrium dynamics of the vortex glass in superconductors

We study the relaxational dynamics of flux lines in high-temperature superconductors with random pinning using Langevin dynamics. At high temperatures the dynamics is stationary and the fluctuation dissipation theorem (FDT) holds. At low temperatures the system does not equilibrate with its thermal bath: a simple multiplicative aging is found, the FDT is violated, and we find that an effective temperature characterizes the slow modes of the system. The generic features of the evolution--scaling laws--are dictated by those of the single elastic line in a random environment.     

Quantum dynamics of a Bose superfluid vortex

We derive a fully quantum-mechanical equation of motion for a vortex in a 2-dimensional Bose superfluid in the temperature regime where the normal fluid density ρ(n)(T) is small. The coupling between the vortex "zero mode" and the quasiparticles has no term linear in the quasiparticle variables--the lowest-order coupling is quadratic. We find that as a function of the dimensionless frequency Ω=?Ω/k(B)T, the standard Hall-Vinen-Iordanskii equations are valid when Ω?1 (the "classical regime"), but elsewhere, the equations of motion become highly retarded, with significant experimental implications when Ω?1.     

Nonintegrability of two problems in vortex dynamics

An analysis is presented of two problems in vortex dynamics whose equations can be written in the Hamiltonian form. They are: the interaction of three coaxial vortex rings and the motion of four point vortices on a sphere. The nonintegrability of these problems in the restricted formulation is demonstrated analytically by the method of split separatrices, using a small parameter. (c) 1997 American Institute of Physics.