Dynamic aggregation of chiral spinners

An object spinning at the surface of a liquid creates a chiral vortex. If the spinning object is itself chiral, its shape modifies the characteristics of the vortex; interactions between that vortex and other vortices then depend on the chirality of the objects that produce them. This paper describes the aggregation of millimeter-sized, chiral magnetized plates floating at a liquid-air interface and rotating under the influence of a rotating external magnetic field. This external field confines all the plates at densities that cause the vortices they generate to interact strongly. For one set of plates investigated, plates of one chirality attract one another, and plates of the other chirality repel other plates of both chiralities.     

Magnetic flux-line lattices and vortices in the copper oxide superconductors

A variety of recent experiments on both the static and the dynamic properties of vortices and flux-line lattices in the mixed state of the copper oxide superconductors are discussed. The experiments are of two basic types: (i) experiments that image the magnetic flux patterns either with magnetic decoration or neutrons and give information about static structures, and (ii) experiments that explore the dynamics of vortices either through the resistivity or other electrodynamic responses of the material. Results of these experiments argue in favor of the existence of a true phase transition in the high-field vortex state from a low-temperature superconducting vortex glass phase into a disordered high-temperature vortex fluid phase. The vortex glass phase transition model does a good job of explaining high-precision measurements of the dynamics at the transition. At low fields and temperatures, very long range hexatic order in the flux-line lattice is observed.     

Fish exploiting vortices decrease muscle activity

Fishes moving through turbulent flows or in formation are regularly exposed to vortices. Although animals living in fluid environments commonly capture energy from vortices, experimental data on the hydrodynamics and neural control of interactions between fish and vortices are lacking. We used quantitative flow visualization and electromyography to show that trout will adopt a novel mode of locomotion to slalom in between experimentally generated vortices by activating only their anterior axial muscles. Reduced muscle activity during vortex exploitation compared with the activity of fishes engaged in undulatory swimming suggests a decrease in the cost of locomotion and provides a mechanism to understand the patterns of fish distributions in schools and riverine environments.     

Spins in the vortices of a high-temperature superconductor

Neutron scattering is used to characterize the magnetism of the vortices for the optimally doped high-temperature superconductor La(2-x)Sr(x)CuO4 (x = 0.163) in an applied magnetic field. As temperature is reduced, low-frequency spin fluctuations first disappear with the loss of vortex mobility, but then reappear. We find that the vortex state can be regarded as an inhomogeneous mixture of a superconducting spin fluid and a material containing a nearly ordered antiferromagnet. These experiments show that as for many other properties of cuprate superconductors, the important underlying microscopic forces are magnetic.     

贯流泵站(前池灯泡式)进口淹没深度的试验研究

本文通过观察同一水位下各种不同的流量及保持流量不变改变淹没深度时的流态、旋涡及吸气状况,研究了泵站进水口的旋涡特性及旋涡发生、发展的影响因素,得出了旋涡类型随淹没深度的变化规律,就后壁距等边界条件对临界淹没深度的影响进行了试验研究,並导出了临界淹没深度的经验公式。     

Observation of vortex lattices in Bose-Einstein condensates

Quantized vortices play a key role in superfluidity and superconductivity. We have observed the formation of highly ordered vortex lattices in a rotating Bose-condensed gas. These triangular lattices contained over 100 vortices with lifetimes of several seconds. Individual vortices persisted up to 40 seconds. The lattices could be generated over a wide range of rotation frequencies and trap geometries, shedding light on the formation process. Our observation of dislocations, irregular structure, and dynamics indicates that gaseous Bose-Einstein condensates may be a model system for the study of vortex matter.     

Spin transfer torques in MnSi at ultralow current densities

Spin manipulation using electric currents is one of the most promising directions in the field of spintronics. We used neutron scattering to observe the influence of an electric current on the magnetic structure in a bulk material. In the skyrmion lattice of manganese silicon, where the spins form a lattice of magnetic vortices similar to the vortex lattice in type II superconductors, we observe the rotation of the diffraction pattern in response to currents that are over five orders of magnitude smaller than those typically applied in experimental studies on current-driven magnetization dynamics in nanostructures. We attribute our observations to an extremely efficient coupling of inhomogeneous spin currents to topologically stable knots in spin structures.     

High-performance high-TC superconducting wires

We demonstrated short segments of a superconducting wire that meets or exceeds performance requirements for many large-scale applications of high-temperature superconducting materials, especially those requiring a high supercurrent and/or a high engineering critical current density in applied magnetic fields. The performance requirements for these varied applications were met in 3-micrometer-thick YBa2Cu3O(7-delta) films epitaxially grown via pulsed laser ablation on rolling assisted biaxially textured substrates. Enhancements of the critical current in self-field as well as excellent retention of this current in high applied magnetic fields were achieved in the thick films via incorporation of a periodic array of extended columnar defects, composed of self-aligned nanodots of nonsuperconducting material extending through the entire thickness of the film. These columnar defects are highly effective in pinning the superconducting vortices or flux lines, thereby resulting in the substantially enhanced performance of this wire.     

Resonant amplification of magnetic domain-wall motion by a train of current pulses

The current-induced motion of magnetic domain walls confined to nanostructures is of interest for applications in magnetoelectronic devices in which the domain wall serves as the logic gate or memory element. The injection of spin-polarized current below a threshold value through a domain wall confined to a pinning potential results in its precessional motion within the potential well. We show that by using a short train of current pulses, whose length and spacing are tuned to this precession frequency, the domain wall's oscillations can be resonantly amplified. This makes possible the motion of domain walls with much reduced currents, more than five times smaller than in the absence of resonant amplification.     

Polariton superfluids reveal quantum hydrodynamic solitons

A quantum fluid passing an obstacle behaves differently from a classical one. When the flow is slow enough, the quantum gas enters a superfluid regime, and neither whirlpools nor waves form around the obstacle. For higher flow velocities, it has been predicted that the perturbation induced by the defect gives rise to the turbulent emission of quantized vortices and to the nucleation of solitons. Using an interacting Bose gas of exciton-polaritons in a semiconductor microcavity, we report the transition from superfluidity to the hydrodynamic formation of oblique dark solitons and vortex streets in the wake of a potential barrier. The direct observation of these topological excitations provides key information on the mechanisms of superflow and shows the potential of polariton condensates for quantum turbulence studies.     

Inclined-Field Structure, Morphology, and Pinning of the Vortex Lattice in Microtwinned YBa2Cu3O7

A detailed small-angle neutron scattering study of the vortex lattice in a single crystal of YBa(2)Cu(3)O(7) was made for a field of 0.5 tesla inclined at angles between 0 and 80 degrees to the crystalline c axis. The vortex lattice is triangular for all angles, and for angles less than or equal to 70 degrees its orientation adjusts itself to maximize the pinning energy to densely and highly regularly spaced twin planes. These observations have important implications for the microscopic flux-pinning mechanism, and hence for the critical current achievable in YBa(2)Cu(3)O(7). For large angles (about 80 degrees) the vortex lattice consists of independent chains in the orientation predicted by anisotropic London theory.     

Vortex core-driven magnetization dynamics

Time-resolved x-ray imaging shows that the magnetization dynamics of a micron-sized pattern containing a ferromagnetic vortex is determined by its handedness, or chirality. The out-of-plane magnetization in the nanometer-scale vortex core induces a three-dimensional handedness in the planar magnetic structure, leading to a precessional motion of the core parallel to a subnanosecond field pulse. The core velocity was an order of magnitude higher than expected from the static susceptibility. These results demonstrate that handedness, already well known to be important in biological systems, plays an important role in the dynamics of microscopic magnets.     

Dynamics of Saturn's south polar vortex

The camera onboard the Cassini spacecraft has allowed us to observe many of Saturn's cloud features. We present observations of Saturn's south polar vortex (SPV) showing that it shares some properties with terrestrial hurricanes: cyclonic circulation, warm central region (the eye) surrounded by a ring of high clouds (the eye wall), and convective clouds outside the eye. The polar location and the absence of an ocean are major differences. It also shares properties with the polar vortices on Venus, such as polar location, cyclonic circulation, warm center, and long lifetime, but the Venus vortices have cold collars and are not associated with convective clouds. The SPV's combination of properties is unique among vortices in the solar system.     

不可压缩粘性流体旋涡运动积分方程的建立

将导出的旋涡运动积分恒等式应用于涡动力学微分方程,分别建立了不可压缩粘性流体在外力有势条件下的流体质量体(系统)及控制体的旋涡运动积分方程,并对不可压缩粘性流体、外力有势条件下质量体的旋涡运动积分方程及其涡强度和矢量的随体导数予以讨论。其结论可对旋涡运动宏观动力学特性的进一步研究提供参考。     

Dynamical superconducting order parameter domains in Sr2RuO4

We present direct evidence for complex p-wave order parameter symmetry and the presence of dynamical chiral order parameter domains of the form px +/- ipy in the ruthenate superconductor Sr2RuO4. The domain structure creates differences in the magnetic field modulation of the critical current of Josephson junctions fabricated on orthogonal faces of Sr2RuO4 single crystals. Transitions between the chiral states of a domain or the motion of domain walls separating them generates telegraph noise in the critical current as a function of magnetic field or time and is responsible for hysteresis observed in field sweeps of the critical current. The presence of such domains confirms the p-wave triplet spin and complex (broken time-reversal symmetry) nature of the superconducting pairing state in Sr2RuO4.     

Surface pinning as a determinant of the bulk flux-line lattice structure in copper oxide superconductors

Direct knowledge of crystal defects and their perturbation of magnetic flux lines is essential to understanding pinning and to devising approaches that enhance critical currents in superconductors with high critical temperatures (T(c)). Atomic force microscopy was used to simultaneously characterize crystal defects and the magnetic flux-line lattice in single crystals of Bi(2)Sr(2)CaCu(2)O(8). Images show that surface defects, which are present on all real samples, pin the flux-line lattice. Above a critical height, the pinning interaction is sufficiently strong to form grain boundaries in the bulk flux-line lattice. These results elucidate the structure of the defects that pin flux lines and demonstrate that surface pinning, through the formation of grain boundaries, can determine the bulk flux-line lattice structure in high-T(c) materials. The implications of these results to the bulk flux-line lattice structure observed in previous experiments and to enhancing critical currents are discussed.     

Earth's core and the geodynamo

Earth's magnetic field is generated by fluid motion in the liquid iron core. Details of how this occurs are now emerging from numerical simulations that achieve a self-sustaining magnetic field. Early results predict a dominant dipole field outside the core, and some models even reproduce magnetic reversals. The simulations also show how different patterns of flow can produce similar external fields. Efforts to distinguish between the various possibilities appeal to observations of the time-dependent behavior of the field. Important constraints will come from geological records of the magnetic field in the past.     

Triple vortex ring structure in superfluid helium II

Superfluids such as helium II consist of two interpenetrating fluids: the normal fluid and the superfluid. The helium II vortex ring has generally been considered merely as a superfluid object, neglecting any associated motion of the normal fluid. We report a three-dimensional calculation of the coupled motion of the normal-fluid and superfluid components, which shows that the helium II vortex ring consists of a superfluid vortex ring accompanied by two coaxial normal-fluid vortex rings of opposite polarity. The three vortex rings form a coherent, dissipative structure.