The leapfrogging of coaxial vortex rings is a famous effect which has been noticed since the times of Helmholtz. Recent advances in ultra-cold atomic gases show that the effect can now be studied in quantum fluids. The strong confinement which characterises these systems motivates the study of leapfrogging of vortices within narrow channels. Using the two-dimensional point vortex model, we show that in the constrained geometry of a two-dimensional channel the dynamics is richer than in an unbounded domain: alongside the known regimes of standard leapfrogging and the absence of it, we identify new regimes of image-driven leapfrogging and periodic orbits. Moreover, by solving the Gross-Pitaevskii equation for a Bose-Einstein condensate, we show that all four regimes exist for quantum vortices too. Finally, we discuss the differences between classical and quantum vortex leapfrogging which appear when the quantum healing length becomes significant compared to the vortex separation or the channel size, and when, due to high velocity, compressibility effects in the condensate becomes significant.

We present here an innovative photon detector based on the proximity junction array device (PAD) working at long wavelengths. We show that the vortex dynamics in PAD undergoes a transition from a Mott insulator to a vortex metal state by application of an external magnetic field. The PAD also evidences a Josephson I-V characteristic with the external field dependent tunneling current. At high applied currents, we observe a dissipative regime in which the vortex dynamics is dominated by the quasi-particle contribution from the normal metal. The PAD has a relatively high photo-response even at frequencies below the expected characteristic frequency while, its superconducting properties such as the order parameter and the Josephson characteristic frequency can be modulated via external fields to widen the detection band. This device represents a promising and reliable candidate for new high-sensitivity long-wavelength detectors.

We have realized YBa2Cu3O7-delta (YBCO) nanorings and measured the magnetoresistance R(B) close to the superconducting transition. The large oscillations that we have measured can be interpreted in terms of vortex dynamics triggering the nanowires to the resistive state. The Fast Fourier Transform spectrum of the magnetoresistance oscillations shows a single sharp peak for nanorings with narrower loop arm width: this peak can be univocally associated to a h/2e periodicity as predicted for optimally doped YBCO. Moreover it is a clear evidence of a uniform vorticity of the order parameter inside the rings, confirming a high degree of homogeneity of our nanostructures. This result gives a boost to further investigations of YBCO nanorings at different dopings within the superconducting dome, where in the underdoped regime a R(B) periodicity different from the conventional h/2e has been predicted. (C) 2014 Elsevier B.V. All rights reserved.

We have realized YBa2Cu3O7-delta nanowires and nano Superconducting Quantum Interference Devices (nanoSQUID). The measured temperature dependence of the wire resistances below the superconducting transition temperature has been analyzed using a thermally activated vortex entry model valid for wires wider than the superconducting coherence length. The extracted zero temperature values of the London penetration depth, lambda(0) similar or equal to 270 +/- 15 nm, are in good agreement with the value obtained from critical current modulations as a function of an externally applied magnetic field in a nanoSQUID implementing two nanowires. (C) 2014 Elsevier B.V. All rights reserved.

Air water interaction phenomena taking place during the breaking of ocean waves are investigated here. The study is carried out by exploiting the combination between a potential flow method, which is used to describe the evolution of the wave system up to the onset of the modulational instability, and a two-fluids Navier-Stokes solver which describes the strongly non-linear air-water interaction taking place during breaking events. The potential flow method is based on a fully non-linear mixed Eulerian-Lagrangian approach, whereas the two-fluid model uses a level-set method for the interface capturing. The method is applied to study the evolution of a modulated wave train composed by a fundamental wave component with two side band disturbances. It is shown that breaking occurs when the initial steepness exceed a threshold value. Once the breaking starts, it is not just a single event but it is recurrent with a period associated to the group velocity. Results are presented in terms of free surface shapes, velocity and vorticity fields, energy and viscous dissipation. The analysis reveals the formation of large vortex structures in the air domain which are originated by the separation of the air flow at the crest of the breaking wave. The form drag associated to the flow separation process significantly contributes to the dissipation of the energy content of the wave system. The energy fraction dissipated by each breaking event is distinguished. © 2014 Elsevier Inc.

We combine wideband (1-20 GHz) Corbino Disk and Dielectric Resonator (8.2 GHz) techniques to study the microwave properties in Nb/PdNi/Nb trilayers, grown by UHV dc magnetron sputtering, composed by Nb layers of nominal thickness d S =15 nm, and a ferromagnetic PdNi layer of thicknesses d F =1, 2, 8 and 9 nm. We focus on the vortex state. Magnetic fields up to H c2 were applied. The microwave resistivity at fixed H/H c2 increases with d F, eventually exceeding the Bardeen-Stephen flux-flow value. Vortex pinning is very weak at large d F . © 2012 Springer Science+Business Media New York.

A numerical study of turbulence seeded with light particles is presented. We analyze the statistical properties of coherent, small-scale structures by looking at the trapping events of light particles inside vortex filaments. We study the properties of particles attracting set, measuring its fractal dimension and the probability that the separation between two particles remains within the dissipative scale, even for time lapses as long as the large-scale correlation time, T(L). We show how to estimate the vortex lifetime by studying the moment of inertia of bunches of particles, showing the presence of an exponential lifetime distribution, with events up to T(L). (C) 2010 American Institute of Physics. [doi:10.1063/1.3431660]

We study the influence of artificial pinning centers on the vortex critical velocity in Al thin films deposited on top of a periodic array of Permalloy (FeNi) square rings. We demonstrate that the field dependence of the flux flow velocity strongly depends on the particular magnetic state of the rings. In particular, we find that, even when the rings are in a flux closure state, i.e. with little stray field, the vortex critical velocity shows a non-monotonic magnetic field dependence. This behaviour is in sharp contrast with the results obtained in a reference plain film, with no rings underneath. A comparison with the intrinsic strong pinning Nb films previously studied, suggests an interpretation in terms of a channel-like motion of vortices, here induced by the artificial pinning structure.

The flux dynamics in LaO0.92F0.08FeAs polycrystalline samples has been investigated looking at the first and higher harmonics of the AC magnetic susceptibility. The investigation of the real and imaginary part of the first harmonic shows that the critical temperature is reduced when the applied DC field increases. Moreover, the intergrain and intragrain contributions persist up to the highest applied DC field, suggesting that the electromagnetic connection among the superconducting grains is strong in spite of the estimated very short coherence length. Concerning the higher harmonics, a comparison between the absolute value of the real part of the first harmonic and the sum of the higher harmonics clearly indicates the activation of a linear regime below the superconducting critical temperature. Furthermore, the temperature range where this linear regime is activated is smaller than the one found in cuprate perovskite superconductors.

The flux dynamics in LaO0.92F0.08FeAs polycrystalline samples has been investigated looking at the first and higher harmonics of the AC magnetic susceptibility. The investigation of the real and imaginary part of the first harmonic shows that the critical temperature is reduced when the applied DC field increases. Moreover, the intergrain and intragrain contributions persist up to the highest applied DC field, suggesting that the electromagnetic connection among the superconducting grains is strong in spite of the estimated very short coherence length. Concerning the higher harmonics, a comparison between the absolute value of the real part of the first harmonic and the sum of the higher harmonics clearly indicates the activation of a linear regime below the superconducting critical temperature. Furthermore, the temperature range where this linear regime is activated is smaller than the one found in cuprate perovskite superconductors.

The velocity induced by a plane, uniform vortex is investigated through the use of an integral relation between Schwarz function of the vortex boundary and conjugate of the velocity. The analysis is restricted to a certain class of vortices, the boundaries of which are described through conformal maps onto the unit circle and the corresponding Schwarz functions possess two poles in the plane of the circle. The dependence of the velocity field on the vortex shape is investigated by comparing velocity and streamfunction with the ones of the equivalent Rankine vortex (which has the same vorticity, area, and center of vorticity). By changing the parameters of the Schwarz function (poles and corresponding residues), rather complicated vortex shapes can be easily analyzed, some of them mimicing an incipient filamentation of the vortex boundary.

The air entraiment induced by vorticity-free-surface interaction is here numerically investigated using a two-fluid model which describes the flow in air and water as that of a single incompressible fluid whose density and viscosity vary smoothly across the interface. The numerical approach is used for the simulation of a viscous vortex pair vertically rising toward the free surface. Several flow conditions are studied aimed at understanding the role played by vortex intensity, surface tension and gravity forces on the amount of entrained air and the mechanisms forits entrainment.

The flow about a lentil at large Reynolds number is investigated. A potential flow is considered and the shed vorticity from the lentil edges form two wakes which are discretized using point vortices. The analysis is carried out by employing the conformal mapping technique and two different vorticity generation mechanisms are compared. The results obtained with several lentil geometries (thickness and length) and incidence are shown.

A comparative numerical study of two-dimensional wake dynamics is presented. An application to the free wake motion behind an elliptical loaded wing on the Treffts plane is approximated either by the dynamics of a set of vortices (vortex method) or by the dynamics of a piecewise linear curve (boundary element method). In both approaches some considerations about the time integration accuracy control are made in terms of the most sensitive flow first integral: the Hamiltonian. Two types of the vorticity generation mechanism are tested in the second part of the paper where the application to the flow around a lentil at large incidence in an uniform stream is analysed. Some aspects of the coupling between the vorticity production and the wake interaction are discussed in order to explain the periodic vortex shedding.

In this work we develop an analysis of the two dimensional inviscid flow past a lentil, focusing our attention on the vorticity production and convection. The study, carried out numerically, shows several interesting features related to the wakes interaction which induces a periodic vorticity shedding (see fig.1). Due to the flow direction on the back side of the lentil two different breaking mechanisms may be observed for the upper and lower wake. In fact, for the case presented in fig. 2, the upper wake breaking is forced by a massive penetration of the lower wake into the region between the first wake and the body (fig. 2a), while the lower wake breaking is caused by a convection of a few vortices detached by the upper wake (fig. 2b). This behaviour of the wakes leads to a significant perturbation in the vorticity generation mechanism. As a consequence several wiggles appear in the time history of d Gamma/dt (see fig. 3), while a negligible influence is observed for the time history of Gamma and for the Strouhal number. The difference between the upper and the lower wake seems to disappear for the lentil thickness going to zero, as experimented for thinner lentils and for a flat plate at the same incidence. A comparison performed with a solution obtained by a conformal mapping technique shows a quite satisfactory agreement, at least for the global flow structure, even if a completely different method for the vorticity generation is adopted. With regard to the numerical model, we use for the velocity in the body frame of reference a Poincare representation that gives an integral equation in terms of the tangential velocity on the body boundary, to be solved according to the Kelvin theorem. The shed vorticity is discretized by the vortex method which facilitates the study of the wakes interaction with respect to a boundary elements approach. The production of vorticity is actually modelled in the following approximate way. Between two consecutive generation times we follow a neutral particle which leaves the edge with an initial velocity given by the vectorial mean on the two sides of the edge. At the new generation time we assign to this particle a circulation given by the conservation of the vorticity flux across the edge. A deeper comprehension of the local flow structure and its influence on the observed dGamma/dt oscillations is required and a more satisfactory physical modelling of the vorticity shedding should be provided. A different approach accounting locally for the mass, vorticity and momentum fluxes (from the edge to the wake) is discussed together with its main effects on the numerical results.