Vortex Configurations Research Articles

Terahertz bound states in the continuum on-and-off-Г point of a moiré photonic superlattice.

Moiré metasurfaces exhibit high optical tuneabilities and versatile light manipulation capabilities. Both infinite quality factor (Q factor) and topological vortex configurations in momentum space (k-space) of the bound state in the continuum (BIC) have introduced new dimensions for light modulation. Herein, we propose a moiré metasurface comprising two identical square photonic lattices superimposed with a commensurate angle of 12.68°. By tuning the incidence angle, the symmetric-protected BICs, Friedrich-Wintgen BIC, and accidental BIC can be achieved simultaneously in our moiré metasurfaces. It is found that the quasi-BICs maintain an ultrahigh Q factor beyond 107. The photonic band structures manifest that the three types of BICs are at the center of far-field polarization vortices in k-space, which have their own topological charges. We experimentally show that these BICs exhibit high sensitivity to subtle changes in analyte refractive index for thin-film sensor application. Our discovery predicts an approach to a highly sensitive multi-channel terahertz biosensor.

Dynamics of Vortex Matter in 2D Gapless Superconducting Materials with Impurities.

The recent interest in using ultrafast single-photon detectors in research and commercial applications has garnered significant attention from the scientific community. The dynamic event in the detection process consists of a photon causing local destruction of the order parameter, and then the applied current dissipates heat, bringing the material even more out of the superconducting state and then spiking a voltage peak in a measurement device. We investigated the role of superconducting and thermal parameters of the Generalized Time-Dependent Ginzburg-Landau (GTDGL) theory within the event of the first vortex penetration and the thermal dissipation in superconductors near the critical temperature. Moreover, in the vicinity of the Bogomolny point of the static Ginzburg-Landau theory, where κ ≈ 1/√2 and T → Tc, it has been found severely different vortex profiles. We point out that within the GTDGL theory, the presence of impurities (usual or paramagnetic) influences flux penetration and enhances the dissipation of heat, causing anomalous vortex configurations.

Investigations into Hydraulic Instability during the Start-Up Process of a Pump-Turbine under Low-Head Conditions

To investigate the hydraulic characteristics during the start-up process of a full-flow pumped storage unit under low-head conditions, numerical simulations were conducted to study the dynamic characteristics during the process, providing a detailed analysis of the dynamic behavior of the internal flow field during the transition period as well as the associated variation in external performance parameters. Study results revealed a vortex-shedding phenomenon during the initial phase of the start-up process. These vortices restrict the flow, initiating a water hammer effect that abruptly elevates the upstream pressure within the runner. As the high-pressure water hammer dissipated, the flow rate rapidly increased, leading to a secondary but relatively weaker water hammer effect, which caused a momentary drop in pressure. This series of events ultimately resulted in significant oscillations in the unit’s head. After the guide vanes stop opening, the vortex structures at the runner inlet and outlet gradually weaken. As the runner torque continues to decline, the unit gradually approaches a no-load condition and enters the S-shaped region. Concurrently, pressure pulsations intensify, and unstable vortex formations reemerge along the leading and trailing edges of the runner blades. The escalated flow velocity at the runner’s exit contributes to the elongation of the vortex band within the draft tube, ultimately configuring a double-layer vortex structure around the central region and the pipe walls. This configuration of vortices precipitates the no-load instability phenomenon experienced by the unit.

Effect of void geometry and magnetic anisotropy in controlling the vortex in sub-micron annular Permalloy disc

Ferromagnetic rings, particularly asymmetric Permalloy (Py) rings are recognized as promising configurations for spintronic devices, offering additional degrees of freedom for manipulating magnetic states, especially in vortex configurations. Through micromagnetic simulations, our study explores the impact on magnetization states and spin configuration concerning ring symmetry, aligning with the interest in controlling vortex states for information storage. We initially obtained zero-field spin configurations by varying ring thickness (t), observing a 360° domain wall in rings with t < 12 nm and bi-vortex wall in rings with t ∼36 nm during magnetization reversal. Notably, an extended stability of the global-vortex state was observed in rings with t > 36 nm, indicating the dominance of global-vortex nucleation in thick asymmetric rings during domain wall movement. We investigate the hysteresis loops and spin configurations by varying the in-plane and out-of-plane anisotropy values. Our findings reveal the presence of multiple vortex cores with different polarities and sense of rotations in the ring for the in-plane anisotropy ∼30 to ∼40 kJ m−3. Additionally, a global-vortex with two vortex cores was formed due to demagnetization energy. We analysed the energy profile of stable magnetization states for various t and anisotropy values. Interestingly, the shape of the hysteresis loop changes significantly for the disc containing different shapes of void. Circular and square-shaped geometries suggest that the bi-vortex state is a stable configuration during magnetization reversal in both cases. The study also indicates the stability of the vortex with a square-shaped void geometry up to a sufficiently large field. For the case of triangular-shaped voids, the global-vortex state was favored with even the small fields. The estimated spin canting angles are found to be correlated with the presence of vortex spin configurations. Overall, these results are important for the development of magnetization vortex-based spintronics devices.

Influence of magnetic anisotropy on the vortex stability in circular Permalloy nanodots using energy analysis

We thoroughly investigated the stability of vortex magnetization in Permalloy (Py) circular nanodots with and without magnetocrystalline anisotropy using micromagnetic simulations. This study explores a wide-ranging understanding of influence of anisotropy on hysteresis loops, and spin-configurations, and provides a detailed analysis of the energy profile. The ground state vortex configuration considered of size 64 × 20-nm2 Py-nanodot at zero out-of-plane anisotropy had been modulated till the anisotropy 250 kJ/m3. We analyzed different energy terms at nucleation field, annihilation field and remanent states to gain deeper insights into the magnetization reversal mechanism. The energy analysis revealed an interplay between exchange and demagnetization energies, resulting in the stability of vortex up to a critical anisotropy (CK) 170 kJ/m3. Beyond this CK, the vortex destabilizes, switching the magnetization to a single-domain state. Additionally, we estimated the energy barrier involved in single domain to vortex state transformation. These findings provide valuable insights for designing vortex-based magnetic data storage and memory devices.

Superfluid excitations in rotating two-dimensional ring traps

We studied a rotating Bose–Einstein condensate confined in ring trap configurations that can be produced starting with a bubble trap confinement, approximated by a Mexican hat and shift harmonic oscillator potentials. Using a variational technique and perturbation theory, we determined the vortex configurations in this system by varying the interparticle interaction and the angular velocity of the atomic cloud. We found that the phase diagram of the system has macrovortex structures for small positive values of the interaction parameter, and the charge of the central vortex increases with rotation. Strengthening the atomic interaction makes the macrovortex unstable, and it decays into multiple singly charged vortices that arrange themselves in a lattice configuration. We also look for experimentally realizable methods to determine the vortex configuration without relying upon absorption imaging since the structures are not always visible in the latter. More specifically, we study how the vortex distribution affects the collective modes of the condensate by solving the Gross–Pitaevskii equation numerically and by analytical predictions using the sum-rule approach for the frequencies of the modes. These results reveal important signatures to characterize the macrovortices and vortex lattice transitions in the experiments.

Observations of airflow around a supertall curved building and its impact on temperature and humidity in Houston's urban center

Characterization of realistically shaped skyscrapers embedded in non-uniform neighbourhoods experiencing intricate weather patterns remains inadequately investigated. Aiming to close this gap, the Center for Multiscale Applied Sensing team deployed its mobile observatory in the street canyons around the curved Wells Fargo Plaza skyscraper in downtown Houston, TX. Three deployments allowed airflow observations under different inflow wind and thermodynamic stability conditions. Doppler lidar measurements reveal that when inflow hits the curved wall of the skyscraper, perpendicular canyons experience similar vortex configurations creating two windward and two leeward circulations. Windsond measurements support that buoyancy within the deep street canyons can generate thermal updrafts as strong as 2 m s−1 which is sufficient to overturn the mechanical downwash under gentle wind conditions. Canyons experiencing venting during the daytime were observed to be more thermodynamically stable at night while thermodynamically stable canyons during the day were observed to be more thermodynamically unstable at night owing to the accumulation of heat near street-level. Fourier decomposition of the vertical velocity measurements shows that in all cases flow exhibited high Reynolds numbers and was composed of turbulent eddies of predominantly 6 and 15 min periods. This observational dataset provides insights to assess wind load, pedestrian comfort, urban air mobility, and natural ventilation and may be used as a benchmark for numerical model and wind tunnel studies attempting to represent realistically complex urban conditions.

Numerical simulations of bio-inspired approaches to enhance underwater swimming efficiency

The present study discusses the numerical simulation results of swimming similar to manta rays. The complex three-dimensional kinematics of manta rays were implemented to unravel the intricacies of its propulsion mechanisms by using the discrete vortex method (DVM). The DVM replaces the requirement for a structured grid across the computational domain with a collection of vortex elements. This method simplifies grid generation, especially for intricate geometries, resulting in time and effort savings in meshing complex shapes. By modeling the pectoral fins with discrete panels and utilizing vortex rings to represent circulation and wake, the study accurately computes the pressure distribution, circulation distribution, lift coefficient, and thrust coefficient of the manta ray. This study focuses on the modulation of aerodynamic performance by altering the span length and the length change ratio during the downstroke and upstroke motion (SV). The manta ray's three-dimensional vortex configurations comprise a combination of vortex rings, vortex contrails, and horseshoe vortices. Analysis of the three-dimensional vortex structure indicates the presence of multiple vortex rings and horseshoe vortex rings at higher SV values, while adequate formation of horseshoe vortices is not observed at lower SV values. In terms of propulsive performance, both lift and thrust increase with SV, while the propulsive efficiency demonstrates its peak at SV = 1.75. The analysis reveals that at higher SV values, the net thrust generated primarily originates from the tip of the fins. Moreover, the study illustrates a significant enhancement in propulsive efficiency, particularly in association with optimal Strouhal numbers ranging between 0.3 and 0.4. The key findings of this study may be used in efficient design of agile autonomous underwater vehicles for marine exploration and surveillance applications.

Decay of time correlations in point vortex systems

The dynamics of a large point vortex system whose initial configuration consists in uniformly distributed independent positions is investigated. Time correlations of local observables of the vortex configuration are shown to be compatible with power law decay 1/t, providing additional insight on ergodicity and mixing properties of equilibrium dynamics in point vortex models.

Stability of vortices in exciton-polariton condensates with spin–orbital-angular-momentum coupling

The existence and dynamics of stable quantized vortices is an important subject of quantum many-body physics. Spin–orbital-angular-momentum coupling (SOAMC), a special type of spin–orbit coupling, has been experimentally achieved to create vortices in atomic Bose–Einstein condensates (BEC). Here, we generalize the concept of SOAMC to a two-component polariton BEC and analyze the emergence and configuration of vortices under a finite-size circular pumping beam. We find that the regular configuration of vortex lattices induced by a finite-size circular pump is significantly distorted by the spatially dependent Raman coupling of SOAMC, even in the presence of a repulsive polariton interaction which can assist the forming of stable vortex configuration. Meanwhile, a pair of vortices induced by SOAMC located at the center of polariton cloud remains stable. When the Raman coupling is sufficiently strong and interaction is weak, the vortices spiraling in from the edge of polariton cloud will disrupt the polariton BEC.

Spectrum of Gyrotropic Modes and Energy Transfer between Dipolar‐Coupled Magnetic Vortices in a Square Lattice via Triggered Vortex Gyration: A Promise for Spintronics Technology

AbstractThe atoms in a crystal lattice vibrate together and generate vibrational modes known as acoustic and optical phonon modes. Similarly, collective vortex gyration motion can be generated in artificial magnetic vortex lattices, created from periodically arranged nanodisks of specific dimensions and aspect ratios. This study uses micromagnetic simulations to characterize the spectra of gyrotropic modes in two‐dimensional magnetic vortex‐based, tailor‐made square lattice arrangements. The shifting of mode frequencies and mode splitting is witnessed depending on the vortex configurations in the lattice. The obtained gyrotropic modes are analogous to two‐dimensional square lattice structures exhibiting acoustic and optical phonon modes. The dynamics of magnetic vortices are also discussed in the transfer of magnetic energy at three points along the three high symmetry directions of the lattice when the central magnetic vortex is excited by a spin‐polarized current with an excitation frequency equal to the collective gyrotropic frequencies of the vortices. Similar to the higher contribution of acoustic phonon modes in the heat transmission process in a crystal lattice, acoustic‐like gyrotropic modes transfer more energy than optical‐like gyration modes in the vortex lattices. These magnetic metastructures hold promise for novel applications in magnetic waveguides and information processing devices, as well as for advancing spintronics.

Magnetic vortex configuration in Fe3O4 nanorings/nanotubes for aspect ratio and magnetic orientation regulated microwave absorption

Morphological structure can greatly affect the magnetic domain of magnetic materials, which plays a significant role on regulating the magnetic characteristics and electromagnetic properties. Herein, Fe3O4 nanorings/nanotubes with adjustable aspect ratio were fabricated through a hydrothermal route followed by reduction annealing treatment. The magnetic flux distributions in radial and axial direction were characterized by electron holography, suggesting that the Fe3O4 nanorings/nanotubes show magnetic vortex structure. The magnetic vortex-domain structure is favored to break the Snoek’s limit of Fe3O4, improving the high-frequency permeability and boosting the natural resonance band to 1 ∼ 10 GHz. Meanwhile the complex permittivity is highly sensitive to the aspect ratio. The Fe3O4 nanorings/nanotubes perform excellent comprehensive microwave absorption properties, owing to the special magnetic vortex structure, tunable complex permittivity, big complex permeability, large magnetization and high natural resonance. The Fe3O4 nanorings with the smallest aspect ratio show a minimum reflection loss (RL) value of −47.9 dB at 2.6 mm and EAB of 4 GHz at 3 mm. Besides, rotational orientation by an external magnetic field can cause the parallel arrangement of nanorings/nanotubes, which can increase both the complex permittivity and broad the resonance band. Rotational orientation can realize effective absorption efficiency at a broader frequency range with a thinner thickness, further improving the comprehensive microwave absorption. These findings not only suggest that designing particular magnetic-domain structures by morphologies can break the Snoek’s limit of magnetic materials, but also demonstrate that aspect ratio and magnetic field-assisted rotational orientation is promising ways to regulate the electromagnetic parameters, resulting in high-performance microwave absorption.

Reconstruction of Electron Vortex in Space Plasmas

Space plasmas are turbulent and maintain different types of critical points or flow nulls. Electron vortex, as one type of flow null structure, is crucial in the energy cascade in turbulent plasmas. However, due to the limited time resolution of the spacecraft observations, one can never analyze the three-dimensional properties of the electron vortex. In the present study, with the advancement of the FOTE-V method and the unprecedented high-resolution measurements from four Magnetospheric Multiscale spacecraft, we successfully identify the electron vortex and then reconstruct its three-dimensional topology of the surrounding electron flow field. The results of the reconstruction show that the configuration of the electron vortex is elliptical. Comparison between the observation and reconstruction scales of the vortex indicates the reliable reconstruction of the flow velocity. Our study sheds light on the understanding of the topology and property of the electron vortex and its relationship with kinetic-scale magnetic holes.

Moduli spaces of instantons in flag manifold sigma models. Vortices in quiver gauge theories

In this paper, we discuss lumps (sigma model instantons) in flag manifold sigma models. In particular, we focus on the moduli space of BPS lumps in general Kähler flag manifold sigma models. Such a Kähler flag manifold, which takes the form Un1+⋯+nL+1Un1×⋯×UnL+1\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$ \\frac{\ extrm{U}\\left({n}_1+\\cdots +{n}_{L+1}\\right)}{\ extrm{U}\\left({n}_1\\right)\ imes \\cdots \ imes \ extrm{U}\\left({n}_{L+1}\\right)} $$\\end{document}, can be realized as a vacuum moduli space of a U(N1) × ··· × U(NL) quiver gauged linear sigma model. When the gauge coupling constants are finite, the gauged linear sigma model admits BPS vortex configurations, which reduce to BPS lumps in the low energy effective sigma model in the large gauge coupling limit. We derive an ADHM-like quotient construction of the moduli space of BPS vortices and lumps by generalizing the quotient construction in U(N) gauge theories by Hanany and Tong. As an application, we check the dualities of the 2d models by computing the vortex partition functions using the quotient construction.

Magnetic interactions in vortex-state nanodisk arrays characterized by gradient magnetic vortex echo

Magnetic vortices have potential applications in the field of spintronics and medicine and studying their magnetic interactions is crucial for future applications. This work introduces a new method based on obtaining the gradient magnetic vortex echo (GMVE) using micromagnetic simulations following a magnetic resonance imaging protocol. The results show that it is possible to characterize the magnetic interaction of arrays of nanodisks, having equal diameter and vortex configuration, as a function of disk separation. This characterization was performed by creating an inhomogeneity in the system through the application of a magnetic field gradient perpendicular to the plane of the nanodisk array. The inhomogeneity allows refocusing the magnetization in a time-controlled way by inverting the sign of the gradient and obtaining the characteristic transverse relaxation time T2∗ from the GMVE that contains the information on the magnetic interaction.

Cross-Component Energy Transfer in Superfluid Helium-4

The reciprocal energy and enstrophy transfers between normal fluid and superfluid components dictate the overall dynamics of superfluid 4He including the generation, evolution and coupling of coherent structures, the distribution of energy among lengthscales, and the decay of turbulence. To better understand the essential ingredients of this interaction, we employ a numerical two-way model which self-consistently accounts for the back-reaction of the superfluid vortex lines onto the normal fluid. Here we focus on a prototypical laminar (non-turbulent) vortex configuration which is simple enough to clearly relate the geometry of the vortex line to energy injection and dissipation to/from the normal fluid: a Kelvin wave excitation on two vortex anti-vortex pairs evolving in (a) an initially quiescent normal fluid, and (b) an imposed counterflow. In (a), the superfluid injects energy and vorticity in the normal fluid. In (b), the superfluid gains energy from the normal fluid via the Donnelly–Glaberson instability.

Trapped vortex dynamics implemented in composite Bessel beams

The divergence-free nature of Bessel beams can be harnessed to effectively trap optical vortices in free space laser propagation. We show how to generate arbitrary vortex configurations in Bessel traps to investigate few-body vortex interactions within a dynamically evolving fluid of light, which is a formal analog to a non-interacting Bose gas. We implement—theoretically and experimentally—initial conditions of vortex configurations first predicted in harmonically trapped quantum fluids, in the limit of weak atomic interactions, and model and measure the resultant dynamics. These hard trap dynamics are distinct from the harmonic trap predictions due to the non-local interactions that occur among the hard-wall boundary and steep phase gradients that nucleate other vortices. By simultaneously presenting experimental demonstrations with the theoretical proposal, we validate the potential application of using Bessel hard-wall traps as testing grounds for engineering few-body vortex interactions within trapped, two-dimensional compressible fluids.

Dominant higher-order vortex gyromodes in circular magnetic nanodots.

The transition to the third dimension enables the creation of spintronic nanodevices with significantly enhanced functionality compared to traditional 2D magnetic applications. In this study, we extend common two-dimensional magnetic vortex configurations, which are known for their efficient dynamical response to external stimuli without a bias magnetic field, into the third dimension. This extension results in a substantial increase in vortex frequency, reaching up to 5 GHz, compared to the typical sub-GHz range observed in planar vortex oscillators. A systematic study reveals a complex pattern of vortex excitation modes, explaining the decrease in the lowest gyrotropic mode frequency, the inversion of vortex mode intensities, and the nontrivial spatial distribution of vortex dynamical magnetization noted in previous research. These phenomena enable the optimization of both oscillation frequency and frequency reproducibility, minimizing the impact of uncontrolled size variations in those magnetic nanodevices.

The Type-II/Type-I Crossover in Dirty Ferromagnetic Superconductors.

In this work, we investigate the intertype (IT) domain in strongly disordered ferromagnetic superconductors with a Curie temperature lower than the superconducting critical temperature. In such unique materials, the coexistence of superconductivity and ferromagnetism allows for the exploration of both unconventional superconductivity and the interplay between magnetism and superconductivity. The study utilizes an extended Ginzburg-Landau model for the dirty limit to calculate the boundaries of the IT domain, which is characterized by a complex vortex-vortex interaction and exotic vortex configurations. The analysis reveals that the IT domain in dirty ferromagnetic superconductors is not qualitatively different from that in the clean case and remains similarly large, suggesting that disorder does not hinder the exploration of the rich variety of IT superconductivity in ferromagnetic superconductors. This work expands our understanding of the interplay between superconductivity and magnetism in complex materials and could guide future experimental studies.

Magnetic vortex states manipulation in overlapping ferromagnetic disks

We present the study of vortex magnetic states in a system of two overlapping ferromagnetic disks by Lorenz microscopy (L-TEM) and micromagnetic modeling. It is shown that in the case of small overlaps in this system there are two magnetic configurations Vortex–Antivortex–Vortex (VAV) state and Vortex–Vortex (VV) state. They are promising for phase locking in vortex spin-transfer nanooscillators. However, our experiments show that the nucleation of VV and VAV states is random and, as a result, the arrays consisting of overlapping disks are inhomogeneous. To control the nucleation of vortices with a certain vorticity, we changed the shape of overlapping disks by cutting them off from different edges. Disks of one type have been cut at the same edge (SD), while disks of another type have been cut at different edges (AD). In situ L-TEM experiments showed that after the sample magnetizing in the magnetic field applied in the plane of disks, the VV states have been realized in both SD and AD arrays. On the other hand, when samples were magnetized in a perpendicular magnetic field, the VAV state was realized in SD array, while the VV state was still realized in the AD array. The peculiarities of vortex nucleation in SD and AD in an external magnetic field are analyzed by means of micromagnetic simulations. The proposed method for controlling the configuration of magnetic vortices can be applied to create large arrays of overlapping disks that are in the same magnetic state.