- Emergence of the Vortex State in Confined Ferroelectric Heterostructures. [Journal Article]
- AMAdv Mater 2019 Jul 19; :e1901014
- The manipulation of charge and lattice degrees of freedom in atomically precise, low-dimensional ferroelectric superlattices can lead to exotic polar structures, such as a vortex state. The role of i…
The manipulation of charge and lattice degrees of freedom in atomically precise, low-dimensional ferroelectric superlattices can lead to exotic polar structures, such as a vortex state. The role of interfaces in the evolution of the vortex state in these superlattices (and the associated electrostatic and elastic boundary conditions they produce) has remained unclear. Here, the toroidal state, arranged in arrays of alternating clockwise/counterclockwise polar vortices, in a confined SrTiO3 /PbTiO3 /SrTiO3 trilayer is investigated. By utilizing a combination of transmission electron microscopy, synchrotron-based X-ray diffraction, and phase-field modeling, the phase transition as a function of layer thickness (number of unit cells) demonstrates how the vortex state emerges from the ferroelectric state by varying the thickness of the confined PbTiO3 layer. Intriguingly, the vortex state arises at head-to-head domain boundaries in ferroelectric a1 /a2 twin structures. In turn, by varying the total number of PbTiO3 layers (moving from trilayer to superlattices), it is possible to manipulate the long-range interactions among multiple confined PbTiO3 layers to stabilize the vortex state. This work provides a new understanding of how the different energies work together to produce this exciting new state of matter and can contribute to the design of novel states and potential memory applications.
- Vortex lattice instability at the nanoscale in a parallel magnetic field. [Journal Article]
- NNanotechnology 2019 Jul 17
- In superconducting materials a dynamical rearrangement of the vortex lattice occurs by forcing vortices at high velocities, until the system can become unstable. This phenomenon is known as vortex la…
In superconducting materials a dynamical rearrangement of the vortex lattice occurs by forcing vortices at high velocities, until the system can become unstable. This phenomenon is known as vortex lattice instability, in which a sudden transition drives the superconducting system abruptly to the normal state. We present an experimental study on submicron bridges of NbN and NbTiN ultra-thin films with a thickness of few nanometers. The nanoscale effect on vortex lattice instability is investigated not only by the ultra-thin thickness in wide bridges, but also by changing the direction of the external magnetic field applied parallel and perpendicular to the c-axis epitaxial films. Indeed, measurements are performed for both orientations and show the vortex lattice instability, regardless of the superconducting material. Critical currents Ic as well as instability currents I* have been compared. However, only in the parallel configuration an unusual "flying birds" feature appears in the magnetic field dependence of current switching, as a consequence of the ratio I*/Ic that is approaching to 1. This amazing tendency becomes relevant for practical applications involving nanostructures, since by scaling down sample thickness and rotating the external field towards the in-plane orientation, the ultra-thin film geometry can mimic the bridge narrowing down to nanoscale.
- Quenching active swarms: effects of light exposure on collective motility in swarming Serratia marcescens. [Journal Article]
- JRJ R Soc Interface 2019 Jul 26; 16(156):20180960
- Swarming colonies of the light-responsive bacteria Serratia marcescens grown on agar exhibit robust fluctuating large-scale flows that include arrayed vortices, jets and sinuous streamers. We study t…
Swarming colonies of the light-responsive bacteria Serratia marcescens grown on agar exhibit robust fluctuating large-scale flows that include arrayed vortices, jets and sinuous streamers. We study the immobilization and quenching of these collective flows when the moving swarm is exposed to intense wide-spectrum light with a substantial ultraviolet component. We map the emergent response of the swarm to light in terms of two parameters-light intensity and duration of exposure-and identify the conditions under which collective motility is impacted. For small exposure times and/or low intensities, we find collective motility to be negligibly affected. Increasing exposure times and/or intensity to higher values suppresses collective motility but only temporarily. Terminating exposure allows bacteria to recover and eventually reestablish collective flows similar to that seen in unexposed swarms. For long exposure times or at high intensities, exposed bacteria become paralysed and form aligned, jammed regions where macroscopic speeds reduce to zero. The effective size of the quenched region increases with time and saturates to approximately the extent of the illuminated region. Post-exposure, active bacteria dislodge immotile bacteria; initial dissolution rates are strongly dependent on duration of exposure. Based on our experimental observations, we propose a minimal Brownian dynamics model to examine the escape of exposed bacteria from the region of exposure. Our results complement studies on planktonic bacteria, inform models of patterning in gradated illumination and provide a starting point for the study of specific wavelengths on swarming bacteria.
- A complex flow phantom for medical imaging: ring vortex phantom design and technical specification. [Journal Article]
- JMJ Med Eng Technol 2019 Jul 15; :1-12
- Cardiovascular fluid dynamics exhibit complex flow patterns, such as recirculation and vortices. Quantitative analysis of these complexities supports diagnosis, leading to early prediction of patholo…
Cardiovascular fluid dynamics exhibit complex flow patterns, such as recirculation and vortices. Quantitative analysis of these complexities supports diagnosis, leading to early prediction of pathologies. Quality assurance of technologies that image such flows is challenging but essential, and to this end, a novel, cost-effective, portable, complex flow phantom is proposed and the design specifications are provided. The vortex ring is the flow of choice because it offers patterns comparable to physiological flows and is stable, predictable, reproducible and controllable. This design employs a piston/cylinder system for vortex ring generation, coupled to an imaging tank full of fluid, for vortex propagation. The phantom is motor-driven and by varying piston speed, piston displacement and orifice size, vortex rings with different characteristics can be produced. Two measurement methods, namely Laser-PIV and an optical/video technique, were used to test the phantom under a combination of configurations. Vortex rings with a range of travelling velocities (approximately 1-80 cm/s) and different output-orifice diameters (10-25 mm) were produced with reproducibility typically better than ±10%. Although ultrasound compatibility has been demonstrated, longer-term ambitions include adapting the design to support comparative studies with different modalities, such as MRA and X-ray-CTA.
- A quasi three-dimensional visualization of unsteady wake flow in human undulatory swimming. [Journal Article]
- JBJ Biomech 2019 Jul 04
- Human undulatory underwater swimming (UUS) is an underwater propelling technique in competitive swimming and its propulsive mechanism is poorly understood. The purpose of this study was to visualize …
Human undulatory underwater swimming (UUS) is an underwater propelling technique in competitive swimming and its propulsive mechanism is poorly understood. The purpose of this study was to visualize the three-dimensional (3D) flow field in the wake region during human UUS in a water flume. A national level male swimmer performed 41 UUS trials in a water flume. A motion capture system and stereo particle image velocimetry (PIV) equipment were used to investigate the 3D coordinates of the swimmer and 3D flow fields in the wake region. After one kick cycle was divided into eight phases, we conducted coordinate transformations and phase averaging method to construct quasi 3D flow fields. At the end of the downward kick, the lower limbs external rotations of the lower limbs were observed, and the feet approached towards each other. A strong downstream flow, i.e. a jet was observed in the wake region during the downward kick, and the paired vortex structure was accompanied by a jet. In the vortex structure, a cluster of vortices and a jet were generated in the wake during the downward kick, and the vortices were subsequently shed from the feet by the rotated leg motion. This suggested that the swimmer gained a thrust by creating vortices around the foot during the downward kick, which collided to form a jet. This paper describes, illustrates, and explains the propulsive mechanism of human UUS.
- Speeding up the extended Kalman filter approach for denoising XMCD movies of fast magnetization dynamics. [Journal Article]
- UUltramicroscopy 2019 Jul 04; 206:112810
- The Kalman filter is a well-established approach to get information on the time-dependent state of a system from noisy observations. In former times it has been used for systems with only a few degre…
The Kalman filter is a well-established approach to get information on the time-dependent state of a system from noisy observations. In former times it has been used for systems with only a few degrees of freedom (typically about 10). The fast magnetization dynamics is often investigated by x-ray magnetic circular dichroism movies (XMCD movies) where the number of components of the state vector is very large (typically about 105). For such systems the Jacobian matrix which is required in the extended Kalman filter approach cannot be calculated numerically (as it is done in former papers) by use of the Landau-Lifschitz-Gilbert equation of motion in a finite time because of the many degrees of freedom of the state vector. In the present paper it is shown that the calculation of the Jacobian matrix can be much speeded up by using good analytical approximations, which are derived by a tensorial Green's function method to solve the linearized Landau-Lifschitz-Gilbert equation of motion. This makes it possible to investigate the dynamics of magnetic vortices and spin waves in circular discs of Permalloy, which are initiated by time-dependent external magnetic fields.
- Particlelike Behavior of Topological Defects in Linear Wave Packets in Photonic Graphene. [Journal Article]
- PRPhys Rev Lett 2019 Jun 14; 122(23):233905
- Topological defects, such as quantum vortices, determine the properties of quantum fluids. Their study has been at the center of activity in solid state and BEC communities. In parallel, the nontrivi…
Topological defects, such as quantum vortices, determine the properties of quantum fluids. Their study has been at the center of activity in solid state and BEC communities. In parallel, the nontrivial behavior of linear wave packets with complex phase patterns was investigated by singular optics. Here, we study the formation, evolution, and interaction of optical vortices in wave packets at the Dirac point in photonic graphene. We show that while their exact behavior goes beyond the Dirac equation and requires a full account of the lattice properties, it can be still approximately described by an effective theory considering the phase singularities as "particles". These particles are capable of mutual interaction, with their trajectory obeying the laws of dynamics.
- Spatial variation of vector vortex beams with plasmonic metasurfaces. [Journal Article]
- SRSci Rep 2019 Jul 10; 9(1):9969
- The spatial variation of vector vortex beams with arbitrary polarization states and orbital angular momentum (OAM) values along the beam propagation is demonstrated by using plasmonic metasurfaces wi…
The spatial variation of vector vortex beams with arbitrary polarization states and orbital angular momentum (OAM) values along the beam propagation is demonstrated by using plasmonic metasurfaces with the initial geometric phase profiles determined from the caustic theory. The vector vortex beam is produced by the superposition of deflected right- and left-handed circularly polarized component vortices with different helical phase charges, which are simultaneously generated off-axially by the single metasurface. Besides, the detailed evolution processes of intensity profile, polarization distribution and OAM value along the beam propagation distance is analyzed. The demonstrated arbitrary space-variant vector vortex beam will pave the way to many promising applications related to spin-to-orbital angular momentum conversion, spin-orbit hybrid entanglement, particle manipulation and transportation, and optical communication.
- Hydrodynamics of Vortex Generation during Bell Contraction by the Hydromedusa Eutonina indicans (Romanes, 1876). [Journal Article]
- BBiomimetics (Basel) 2019 Jul 05; 4(3)
- Swimming bell kinematics and hydrodynamic wake structures were documented during multiple pulsation cycles of a Eutonina indicans (Romanes, 1876) medusa swimming in a predominantly linear path. Bell …
Swimming bell kinematics and hydrodynamic wake structures were documented during multiple pulsation cycles of a Eutonina indicans (Romanes, 1876) medusa swimming in a predominantly linear path. Bell contractions produced pairs of vortex rings with opposite rotational sense. Analyses of the momentum flux in these wake structures demonstrated that vortex dynamics related directly to variations in the medusa swimming speed. Furthermore, a bulk of the momentum flux in the wake was concentrated spatially at the interfaces between oppositely rotating vortices rings. Similar thrust-producing wake structures have been described in models of fish swimming, which posit vortex rings as vehicles for energy transport from locations of body bending to regions where interacting pairs of opposite-sign vortex rings accelerate the flow into linear propulsive jets. These findings support efforts toward soft robotic biomimetic propulsion.
New Search Next
- Edge Wave and Boundary Layer of Vortex Matter. [Journal Article]
- PRPhys Rev Lett 2019 May 31; 122(21):214505
- We show that vortex matter, that is, a dense assembly of vortices in an incompressible two-dimensional flow, such as a fast rotating superfluid or turbulent flows with signlike eddies, exhibits (i) a…
We show that vortex matter, that is, a dense assembly of vortices in an incompressible two-dimensional flow, such as a fast rotating superfluid or turbulent flows with signlike eddies, exhibits (i) a boundary layer of vorticity (vorticity layer) and (ii) a nonlinear wave localized within the vorticity layer, the edge wave. Both are solely an effect of the topological nature of vortices. Both are lost if vortex matter is approximated as a continuous vorticity patch. The edge wave is governed by the integrable Benjamin-Davis-Ono equation, exhibiting solitons with a quantized total vorticity. Quantized solitons reveal the topological nature of the vortices through their dynamics. The edge wave and the vorticity layer are due to the odd viscosity of vortex matter. We also identify the dynamics with the action of the Virasoro-Bott group of diffeomorphisms of the circle, where odd viscosity parametrizes the central extension. Our edge wave is a hydrodynamic analog of the edge states of the fractional quantum Hall effect.