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International audience; This study presents a hybrid non-linear unsteady vortex lattice method-vortex particle method (NL UVLM-VPM) to investigate the aerodynamics of rotor blades hovering in and out of ground effect. The method is of interest for the fast aerodynamic prediction of helicopter and smaller rotor blades. UVLM models the vorticity along the rotor blades and near field wakes with panels that are then converted into their equivalent vortex particle representations. The standard Vreman subgrid scale model is incorporated in the context of a large eddy simulation for mesh-free VPM to stabilize the wake development via particle strength exchange (PSE). The computation of the pairwise interactions in the VPM are accelerated using the fast-multipole method. Non-linear UVLM is achieved with a low computational cost viscous-inviscid alpha coupling algorithm through a stripwise 2D Reynolds-averaged Navier–Stokes (RANS) or empirical database. The aerodynamics of the scaled S76 rotor blades in and out of ground effect from the hover prediction workshop is investigated with the proposed algorithm. The results are validated with experimental data and various high-fidelity codes.

In superconducting films, the role of intrinsic disorder is typically to compete with superconductivity by fragmenting the global phase coherence and lowering the superfluid density. Nonetheless, when a transverse magnetic field is applied to the system and an Abrikosov vortex lattice form, the presence of disorder can actually strengthen the superconducting state against thermal fluctuations. By means of Monte Carlo simulations on the uniformly frustrated XY model in two dimensions, we show that while for weak pinning the superconducting critical temperature Tc increases with the applied field H, for strong enough pinning, the experimental decreasing dependence between Tc and H is recovered with a resulting more robust vortex lattice.

We study the vortex formation in coupled atomic and molecular condensates in a rotating double well trap by numerically solving the coupled Gross-Pitaevskii like equations. Starting with the atomic condensate in the double well potential we considered two-photon Raman photoassociation for coherent conversion of atoms to molecules. It is shown that the competition between atom-molecule coupling strength and repulsive atom-molecule interaction controls the spacings between atomic and molecular vortices and the rotation frequency of the trap is the key player for controlling the number of visible atomic and molecular vortices. Whereas the Raman detuning controls the spacing between atomic and molecular vortices as well as the number of atomic and molecular vortices in the trap. We have shown by considering the molecular lattices the distance between two molecular vortices can be controlled by varying the Raman detuning. In addition we have found that the Feynman rule relating the total number of vortices and average angular momentum both for atoms and molecules can be satisfied by considering the atomic and molecular vortices those are hidden in density distribution and seen as singularities in phase distribution of the coupled system except for the lattice structure where molecular vortices are overlapped with each other. It is found that although the number of visible/core vortices in atomic and molecular vortex lattices depends significantly on the system parameters the number of atomic and molecular hidden vortices remains constant in most of the cases.

In this paper, an aeroelastic framework developed in the previous works has been extended for a geometrically nonlinear aeroelastic stability analysis. In the aeroelastic stability analysis, modal mass and stiffness matrices are constructed using shell finite elements with the corotational approach, which can take into account the geometric stiffening under large deformations. A generalized aerodynamic force matrix is derived by accommodating the unsteady vortex-lattice method based on a modal approach. The derived modal aeroelastic equations are solved using the p-k method. Individual modules to calculate generalized structural matrices with the corotational approach and aerodynamic forces based on the unsteady vortex-lattice method are verified. The present geometrically nonlinear aeroelastic framework for aeroelastic stability analysis is also validated in a comparison with an experimental result in the flutter wind tunnel test showing a reasonable accuracy for a prediction of flutter speed, while there still is room for further improvement in the estimation of flutter frequency. In addition, the present method allows studying the flutter characteristics of flexible wings with different incident angles, which cannot be evaluated by traditional linear flutter simulations. Finally, effects of geometric nonlinearity and large deformation on flutter characteristics of flexible and high-aspect-ratio wings were explored by using the present method. The present method can help to understand aeroelastic stability characteristics for such high-aspect-ratio wings.

Generally, an optical vortex lattice (OVL) is generated via the superposition of two specific vortex beams. Thus far, OVL has been successfully employed to trap atoms via the dark cores. The topological charge (TC) on each optical vortex (OV) in the lattice is only ±1. Consequently, the orbital angular momentum (OAM) on the lattice is ignored. To expand the potential applications, it is necessary to rediscover and exploit OAM. Here we propose a novel high-order OVL (HO-OVL) that combines the phase multiplication and the arbitrary mode-controllable techniques. TC on each OV in the lattice is up to 51, which generates sufficient OAM to manipulate microparticles. Thereafter, the entire lattice can be modulated to desirable arbitrary modes. Finally, yeast cells are trapped and rotated by the proposed HO-OVL. To the best of our knowledge, this is the first realization of the complex motion of microparticles via OVL. Thus, this work successfully exploits OAM on OVL, thereby revealing potential applications in particle manipulation and optical tweezers.

Made available in DSpace on 2019-10-06T16:30:56Z (GMT). No. of bitstreams: 0 Previous issue date: 2019-01-01 We study numerically the vortex-lattice formation in a rapidly rotating uniform quasi-twodimensional Bose-Einstein condensate (BEC) in a box trap. We consider two types of boxes: square and circle. In a square-shaped 2D box trap, when the number of generated vortices is the square of an integer, the vortices are found to be arranged in a perfect square lattice, although deviations near the center are found when the number of generated vortices is arbitrary. In case of a circular box trap, the generated vortices in the rapidly rotating BEC lie on concentric closed orbits. Near the center, these orbits have the shape of polygons, whereas near the periphery the orbits are circles. The circular box trap is equivalent to the rotating cylindrical bucket used in early experiment(s) with liquid He II. The number of generated vortices in both cases is in qualitative agreement with Feynman's universal estimate. The numerical simulation for this study is performed by a solution of the underlying meanfield Gross-Pitaevskii (GP) equation in the rotating frame, where the wave function for the generated vortex lattice is a stationary state. Consequently, the imaginary-time propagation method can be used for a solution of the GP equation, known to lead to an accurate numerical solution. We also demonstrated the dynamical stability of the vortex lattices in real-time propagation upon a small change of the angular frequency of rotation, using the converged imaginary-time wave function as the initial state. Instituto de Física Teórica UNESP-Universidade Estadual Paulista São Paulo Instituto de Física Teórica UNESP-Universidade Estadual Paulista São Paulo

We study spontaneous-symmetry-breaking circularly-asymmetric phase separation of vortex lattices in a rapidly rotating harmonically-trapped quasi-two-dimensional (quasi-2D) binary Bose-Einstein condensate (BEC) with repulsive inter- and intra-species interactions. The phase separated vortex lattices of the components appear in different regions of space with no overlap between the vortices of the two components, which will permit an efficient experimental observation of such vortices and accurate study of the effect of atomic interaction on such vortex lattice. Such phase separation takes place when the intra-species interaction energies of the two components are equal or nearly equal with relatively strong inter-species repulsion. When the intra-species energies are equal, the two phase-separated vortex lattices have identical semicircular shapes with one being the parity conjugate of the other. When the intra-species energies are nearly equal, the phase separation is also complete but the vortex lattices have different shapes. We demonstrate our claim with a numerical solution of the mean-field Gross-Pitaevskii equation for a rapidly rotating quasi-2D binary BEC., Comment: arXiv admin note: text overlap with arXiv:1811.06816

We study spontaneous-symmetry-broken phase-separated vortex lattice in a weakly interacting uniform rapidly rotating binary Bose superfluid contained in a quasi-two-dimensional circular or square bucket. For the inter-species repulsion above a critical value, the two superfluid components separate and form a demixed phase with practically no overlap in the vortex lattices of the two components, which will permit an efficient experimental observation of such vortices and study their properties. In case of a circular bucket with equal intra-species energies of the two components, the two components separate into two non-overlapping semicircular domains for all frequencies of rotation $\Omega$ generating distinct demixed vortex lattices. In case of a binary Bose superfluid in both circular and square buckets, (a) the number of vortices increases linearly with $\Omega$ in agreement with a suggestion by Feynman, and (b) the rotational energy in the rotating frame decreases quadratically with $\Omega$ in agreement with a suggestion by Fetter., Comment: arXiv admin note: text overlap with arXiv:1908.07848

We report on the measurements of the remnant magnetization, and hence critical current, in a single crystal of YBa2Cu4O8. A peak in the temperature dependence of the critical current is observed when the external magnetic field is tilted away from the a&ndash, b planes. The observed behavior is attributed to a thermally activated instability-driven vortex-lattice splitting or vortex chain formation. The nature of the peak and the possibility of a thermally-activated dimensional crossover have been discussed.

The nearly commensurate charge density wave (CDW) excitations native to the transition-metal dichalcogenide crystal, 1T-TaS2, under ambient conditions are revealed by scanning tunneling microscopy (STM) and spectroscopy (STS) measurements of a graphene/TaS2 heterostructure. Surface potential measurements show that the graphene passivation layer prevents oxidation of the air-sensitive 1T-TaS2 surface. The graphene protective layer does not however interfere with probing the native electronic properties of 1T-TaS2 by STM/STS, which revealed that nearly commensurate CDW hosts an array of vortex-like topological defects. We find that these topological defects organize themselves to form a lattice with quasi-long-range order, analogous to the vortex Bragg glass in type-II superconductors but accessible in ambient conditions.

Penetration of a vorticity lattice formed by waves propagating on the surface at an angle to each other into the bulk of a liquid is studied experimentally. It is shown that along with the Stokes drift it is necessary to regard nonlinear wave interaction in order to explain experimental time dependences of average vorticity. Vorticity distribution over the depth of a liquid is described by a sum of two exponents considering the contribution of both mechanisms. It is determined that formation of large-scale vortex flows leads to deformation of a vortex lattice and a decrease in an average value of the amplitude of lattice vorticity as a result of its drift.

Made available in DSpace on 2021-06-25T10:45:42Z (GMT). No. of bitstreams: 0 Previous issue date: 2020-11-06 We study the vortex-lattice formation in a rotating Rashba spin-orbit (SO) coupled quasi-two-dimensional (quasi-2D) hyper-fine spin-1 spinor Bose-Einstein condensate (BEC) in the x-y plane using a numerical solution of the underlying mean-field Gross-Pitaevskii equation. In this case, the non-rotating Rashba SO-coupled spinor BEC can have topological excitation in the form of vortices of different angular momenta in the three components, e.g. the (0, +1, +2)- and (-1, 0, +1)-type states in ferromagnetic and anti-ferromagnetic spinor BEC: the numbers in the parenthesis denote the intrinsic angular momentum of the vortex states of the three components with the negative sign denoting an anti-vortex. The presence of these states with intrinsic vorticity breaks the symmetry between rotation with vorticity along the z and -z axes and thus generates a rich variety of vortex-lattice and anti-vortex-lattice states in a rotating quasi-2D spin-1 spinor ferromagnetic and anti-ferromagnetic BEC, not possible in a scalar BEC. For weak SO coupling, we find two types of symmetries of these states - hexagonal and 'square'. The hexagonal (square) symmetry state has vortices arranged in closed concentric orbits with a maximum of 6, 12, 18⋯ (8, 12, 16⋯) vortices in successive orbits. Of these two symmetries, the square vortex-lattice state is found to have the smaller energy. Instituto de Física Teórica Universidade Estadual Paulista-UNESP Sao Paulo Instituto de Física Teórica Universidade Estadual Paulista-UNESP Sao Paulo

Low-temperature scanning tunneling microscopy and spectroscopy has been used to image the vortex core and the vortex lattice in FeSe single crystals. The local tunneling spectra acquired at the center of elliptical vortex cores display a strong particle-hole asymmetry with spatial oscillation, characteristic of the quantum-limit vortex core. Furthermore, a quasihexagonal vortex lattice at low magnetic field undergoes noticeable rhombic distortions above a certain field ∼1.5 T. This field H∗ also reveals itself as a kink in the magnetic field dependence of the specific heat. The observation of a nearly hexagonal vortex lattice at low field is very surprising for materials with an orthorhombic crystal structure and it is in apparent contradiction with the elliptical shape of the vortex cores. These observations can be directly connected to the multiband nature of superconductivity in this material, provided we attribute them to the suppression of superconducting order parameter in one of the energy bands. Above the field H∗ the superconducting coherence length for this band can well exceed the intervortex distance which strengthens the nonlocal effects. Therefore, in addition to multiple-band effects, other possible sources that can contribute to the observed evolution of the vortex-lattice structure include nonlocal effects which cause the field-dependent interplay between the symmetry of the crystal and vortex lattice or the magnetoelastic interactions due to the strain field generated by vortices. © 2019 American Physical Society. Citrus Research and Development Foundation, CRDF Government Council on Grants, Russian Federation Russian Science Foundation, RSF: 17-12-01383, 18-72-10027 Ministero dellâ Istruzione, dellâ Università e della Ricerca, MIUR Foundation for the Advancement of Theoretical Physics and Mathematics: 17-11-109 Ministero dellâ Istruzione, dellâ Università e della Ricerca, MIUR Kazan Federal University Office of Science, SC Division of Materials Sciences and Engineering, DMSE Russian Foundation for Basic Research, RFBR: 17-52-12044 Ministry of Education and Science of the Russian Federation, Minobrnauka Temple University, TU Argonne National Laboratory, ANL Nanjing University of Science and Technology, NUST: K2-2017-084 Drexel University The authors would like to acknowledge fruitful discussions with V. Kogan and T. Hanaguri. We also would like to acknowledge technical support during the early stage of these measurements from S. A. Moore. The work at Temple University, where low temperature scanning tunneling measurements were performed, was supported by US Department of Energy, Office of Science, Basic Energy Science, Materials Sciences and Engineering Division under Award No. DE-SC0004556. The work at Drexel University and at the M.V. Lomonosov Moscow State University was supported by the US Civilian Research and Development Foundation (CRDF Global). The work in Russia has been supported in part by the Ministry of Education and Science of the Russian Federation in the framework of the Increase Competitiveness Program of NUST MISiS Grant K2-2017-084, by Act 211 of the Government of Russian Federation, Contracts No. 02.A03.21.0004, No. 02.A03.21.0006, and No. 02.A03.21.0011 and by the Russian Government Program of Competitive Growth of Kazan Federal University. One of the authors (C.D.G.) would like to acknowledge partial support from MIUR (Ministry of Education, Universities and Research of the Italian Government). The work in IPM RAS (Nizhny Novgorod) was supported in part by the Russian Science Foundation (the calculation of the vortex-lattice characteristics Grant No. 18-72-10027; the calculation of the vortex-core deformation and the analysis of the experimental data Grant No. 17-12-01383), the Russian Foundation for Basic Research (Grant No. 17-52-12044), and Foundation for the Advancement of Theoretical Physics and Mathematics “BASIS” (Grant No. 17-11-109). The work at Argonne National Laboratory was supported by the US Department of Energy, Office of Science, Basic Energy Science, Materials Sciences and Engineering Division.

The vortex lattice (VL) in MgB2 is characterized by the presence of long-lived metastable states (MSs), which arise from cooling or heating across the equilibrium phase boundaries. A return to the equilibrium configuration can be achieved by inducing vortex motion. Here we report on small-angle neutron scattering studies of MgB2, focusing on the structural properties of the VL as it is gradually driven from metastable to equilibrium states (ESs) by an AC magnetic field. Measurements were performed using initial MSs obtained either by cooling or heating across the equilibrium phase transition. In all cases, the longitudinal correlation length remains constant and comparable to the sample thickness. Correspondingly, the VL may be considered as a system of straight rods, where the formation and growth of ES domains only occurs in the two-dimensional plane perpendicular to the applied field direction. Spatially resolved raster scans of the sample were performed with apertures as small as 80 μm, corresponding to only 1.2 × 106 vortices for an applied field of 0.5 T. These revealed spatial variations in the metastable and equilibrium VL populations, but individual domains were not directly resolved. A statistical analysis of the data indicates an upper limit on the average domain size of approximately 50 μm., New Journal of Physics, 21, ISSN:1367-2630

Flapping, flexible insect wings deform under inertial and fluid loading. Deformation influences aerodynamic force generation and sensorimotor control, and is thus important to insect flight mechanics. Conventional flapping wing fluid–structure interaction models provide detailed information about wing deformation and the surrounding flow structure, but are impractical in parameter studies due to their considerable computational demands. Here, we develop two quasi three-dimensional reduced-order models (ROMs) capable of describing the propulsive forces/moments and deformation profiles of flexible wings. The first is based on deformable blade element theory (DBET) and the second is based on the unsteady vortex lattice method (UVLM). Both rely on a modal-truncation based structural solver. We apply each model to estimate the aeromechanics of a thin, flapping flat plate with a rigid leading edge, and compare ROM findings to those produced by a coupled fluid dynamics/finite element computational solver. The ROMs predict wing deformation with good accuracy even for relatively large deformations of 25% of the chord length. Aerodynamic loading normal to the wing's rotation plane is well captured by the ROMs, though model errors are larger for in-plane loading. We then perform a parameter sweep to understand how wing flexibility and mass affect peak deflection, mean lift and average power. All models indicate that flexible wings produce less lift but require lower average power to flap. Importantly, these studies highlight the computational efficiency of the ROMs—compared to the convention modeling approach, the UVLM and DBET ROMs solve 4 and 6 orders of magnitude faster, respectively. This article may be downloaded for personal use only. Any other use requires prior permission of the author and AIP Publishing. This article appeared in [Quasi three-dimensional deformable blade element and unsteady vortex lattice reduced-order modeling of fluid–structure interaction in flapping wings. Physics of Fluids 34, 12 p121903 (2022)] and may be found at https://doi.org/10.1063/5.0129128. Deposited by shareyourpaper.org and openaccessbutton.org. We've taken reasonable steps to ensure this content doesn't violate copyright. However, if you think it does you can request a takedown by emailing help@openaccessbutton.org.

We present the results of a study of the vortex lattice in the heavy fermion superconductor ${\mathrm{CeCu}}_{2}{\mathrm{Si}}_{2}$, using small-angle neutron scattering (SANS). In this material at temperatures well below ${T}_{\mathrm{c}}\ensuremath{\sim}0.6$ K, the value of the upper critical field ${B}_{\mathrm{c}2}\ensuremath{\sim}2.2$ T is strongly limited by the Pauli paramagnetism of the heavy fermions. In this temperature region, our SANS data show an increase in the magnetization of the flux line cores with field, followed by a rapid fall near ${B}_{\mathrm{c}2}$. This behavior is the effect of Pauli paramagnetism and we present a theory-based model, which can be used to describe this effect in a range of materials. The pairing in ${\mathrm{CeCu}}_{2}{\mathrm{Si}}_{2}$ appears to arise from the effect of magnetic fluctuations, but the evidence for a $d$-wave order parameter is rather weak. We find that the vortex lattice structure in ${\mathrm{CeCu}}_{2}{\mathrm{Si}}_{2}$ is close to regular hexagonal. There are no phase transitions to square or rhombic structures; such transitions are expected for $d$-wave superconductors and observed in ${\mathrm{CeCoIn}}_{5}$; however, the temperature dependence of the SANS intensity indicates that both large and small gap values are present, most likely due to multiband $s$-wave superconductivity, rather than a nodal gap structure.

Order-disorder transitions are widely explored in various vortex structures in condensed matter physics, i.e., in the type-II superconductors and Bose-Einstein condensates. In this study, we have investigated the ordering of the polar vortex phase in the (PZT)n/(STO)n superlattice systems through phase-field simulations. An antiorder state is discovered for short periodicity superlattice on an SSO substrate, owing to the huge interfacial coupling between PZT and STO as well as the giant in-plane polarization in STO layers due to the large tensile strain. Increasing the periodicity leads to the anti-order to disorder transition, resulting from the loss of interfacial coupling and disappearance of the polarization in STO layers. On the other hand, for short periodicity superlattices, order-disorder-antiorder transition can be engineered by mediating the substrate strain, due to the delicate competition between the depoling effect, interfacial coupling, and strain effect. We envision this study to spur further interest towards the understanding of order-disorder transition in ferroelectric topological structures., Comment: 20 Pages 5 figures

We investigate the properties of a spin-imbalanced and rotating unitary Fermi gas. Using a density functional theory (DFT), we provide insight into states that emerge as a result of a competition between Abrikosov lattice formation, spatial phase separation and the emergence of Fulde-Ferrell-Larkin-Ovchinnikov (FFLO) state. A confrontation of the experimental data [M. Zwierlein et al., Science 311, 492 (2006)] with theoretical predictions provides a remarkable qualitative agreement. In case of gas confined in a harmonic trap, the phase separation into a superfluid core populated by the Abrikosov lattice and a spin-polarized corona is the dominant process. Changing confinement to a box-like trap, reverts the spatial location of the component: gas being in the normal state is surrounded by superfluid threaded by quantum vortices. The vortex lattice no longer exhibits the triangular symmetry, and emergence of exotic geometries may be an indirect signature of the FFLO-like state formation in the system., 10 pages, 7 figures

We study numerically the formation of a vortex lattice inside a rotating bucket containing superfluid helium, paying attention to an important feature which is practically unavoidable in all experiments: the microscopic roughness of the bucket's surface. We model this using the Gross-Pitaevskii for a weakly-interacting Bose gas, a model which is idealised when applied to superfluid helium but captures the key physics of the vortex dynamics which we are interested in. We find that the vortex lattice arises from the interaction and reconnections of nucleated U-shaped vortex lines, which merge and align along the axis of rotation. We quantify the effects which the surface roughness and remanent vortex lines play in this process., 11 pages, 12 figures

This paper presents a Combined Method for the calculation of propeller forces in inhomogeneous inflow. It consists of an extended Goldstein approach based on Lifting Line Theory and a Vortex Lattice Method. After a brief overview of both methods is given, the coupling strategy is described and the additional modifications are explained. A correction factor accounting for the vortex which develops under a separated and later reattached flow on the suction side of the propeller blade is implemented as the first modification. Further, the Lamb-Oseen vortex model is used for the vortices in the free vortex system of the propeller. Finally, some results achieved with the described method are presented and compared to measured values.

The theoretical description and experimental study of a reversible vibrational vortex motion induced in a type-II superconductor by a weak low-frequency ac magnetic field are discussed. The case where the external magnetic field and vortex lines are parallel to the surface of the superconductor is considered. The equilibrium positions of vortices near a surface, the amplitude of their vibrations, and the contribution of this process to the dynamic permeability ${\ensuremath{\mu}}_{v}$ are calculated. In these calculations, the interactions of vortices with each other, with the surface, and with pinning centers are considered within the discrete vortex lattice model. The calculations in the London approximation are compared with the results obtained within a more accurate model, including the spatial variation of the order parameter in the vortex core by means of a variational function. It is shown that the structural parameters of the vortex lattice near the surface depend on the accuracy of the description of the intervortex interactions, but the contribution ${\ensuremath{\mu}}_{v}$ appears to be model independent. It is important that this conclusion is valid in the case of the exponential approximation for the interaction between vortex rows. In this approximation, the problem under consideration is analytically examined, and formulas describing the dependences of the parameters of the vortex lattice and the contribution ${\ensuremath{\mu}}_{v}$ on the external dc magnetic field and the characteristics of a superconductor are derived. The theoretical results are compared with experimental data for a $\mathrm{Y}{\mathrm{Ba}}_{2}{\mathrm{Cu}}_{3}{\mathrm{O}}_{y}$ high-${T}_{c}$ superconducting single-crystal plate. The magnetic field is parallel to the plate, i.e., to the crystallographic $ab$ plane. A strong anisotropy of the properties and shape of the crystal is considered when analyzing the experimental data. A procedure for the determination of the experimental dependence ${\ensuremath{\mu}}_{v}(H)$ is described. It is shown that the reversible vibrations of vortices occur near the planes of the single-crystal plate in the direction of the crystallographic $c$ axis, whereas vortices enter its lateral ends and leave them (along the structural layers) even at a small magnetic field amplitude of about 1 Oe. It is found that the developed theoretical description reproduces the experimental data ${\ensuremath{\mu}}_{v}(H)$ in the entire studied range of temperatures and magnetic fields under the assumptions that pinning is isotropic and the length of interaction of vortices with pinning centers is close to the coherence length.

We use large-scale Monte-Carlo simulations to study thermal fluctuations in chiral $p$-wave superconductors in an applied magnetic field in three dimensions. We consider the thermal stability of previously predicted unusual double-quanta flux-line lattice ground states in such superconductors. In previous works it was shown that, neglecting thermal fluctuations, a chiral $p$-wave superconductor forms an hexagonal lattice of doubly-quantized vortices, except extremely close to the vicinity of $H_{c2}$ where double-quanta vortices split apart. We find dissociation of double-quanta vortices driven by thermal fluctuations. However, our calculations also show that the previous predictions of hexagonal doubly-quantized vortices, where thermal fluctuations were ignored, are very robust in the considered model., Comment: 21 pages, 12 figures, submitted to Physical Review B

Using small-angle neutron scattering we have studied the superconducting vortex lattice (VL) phase diagram in MgB2 as the applied magnetic field is rotated away from the c axis and towards the basal plane. The field rotation gradually suppresses the intermediate VL phase which exists between end states aligned with two high symmetry directions in the hexagonal basal plane for H || c. Above a critical angle, the intermediate state disappears, and the previously continuous transition becomes discontinuous. The evolution towards the discontinuous transition can be parameterized by a vanishing twelvefold anisotropy term in the VL free energy., 6 pages, 4 figures; [v2]: minor revisions to text and figures; [v3] typos fixed, journal reference added

If a quantum fluid is driven with enough angular momentum, at equilibrium the ground state of the system is given by a lattice of quantised vortices whose density is prescribed by the quantization of circulation. We report on the first experimental study of the Feynman-Onsager relation in a non-equilibrium polariton fluid, free to expand and rotate. Upon initially imprinting a lattice of vortices in the quantum fluid, we track the vortex core positions on picosecond time scales. We observe an accelerated stretching of the lattice and an outward bending of the linear trajectories of the vortices, due to the repulsive polariton interactions. Access to the full density and phase fields allows us to detect a small deviation from the Feynman-Onsager rule in terms of a transverse velocity component, due to the density gradient of the fluid envelope acting on the vortex lattice., Main 6 pages, 4 figures, SM 4 pages, 5 figures

Evolution of the excess conductivity in optimally doped $$\hbox {YBa}_2\hbox {Cu}_3\hbox {O}_{7-\delta }$$ single crystals is investigated after their irradiation with fast electrons at $$T\lesssim 10\,\hbox {K}$$ at energies of 0.5–2.5 MeV and a dose of $$3\times 10^{18}\,\hbox {cm}^{-2}$$ . The measurements were performed in a magnetic field of $$15\,\hbox {kOe}$$ applied at various angles with respect to the basal plane of the crystals. The temperature dependences of the paraconductivity were analyzed within the framework of the Aslamazov–Larkin theoretical model of fluctuation conductivity and revealed two major effects of the electron irradiation. Namely, (i) the vortex-lattice-melting kinks in the resistivity temperature dependences vanish after electron irradiation and (ii) the resistivity data in the irradiated state allow for a Kouvel-Fisher-type scaling pointing to the presence of a irradiation-induced vortex Bragg-glass phase. These effects are discussed in terms of a competition between intrinsic pinning due to point defects and volume pinning induced by electron irradiation. In all, our findings are relevant for YBCO-based circuitry which is exposed to fast electrons in moderately strong magnetic fields.

Following some prehistory, we discuss the steps leading to the Abrikosov vortex lattice and type-II superconductivity. Dissipation due to flux flow and flux creep represented an important discovery and leads to the strong interest in magnetic flux pinning. High-temperature superconductors and pancake vortices started the new concepts of vortex matter. Nonlinear effects at high vortex velocities, flux-flow instabilities, and some resulting open questions are discussed.

As early as the 1930s, experiments, particularly those by Leo Vasilyevich Shubnikov in Kharkov in Soviet Ukraine, showed that the ideas of that time in the field of superconductivity needed to be expanded. Shubnikov had built a low-temperature laboratory there early on, in which experiments could also be carried out with liquid helium. (However, Shubnikov’s many foreign contacts became his undoing during the Stalin terror of the time.

ANID-Millennium Science Initiative Program ICN17_012 Comision Nacional de Investigacion Cientifica y Tecnologica (CONICYT) CONICYT FONDECYT 1180903

In this Letter we will show that, in the presence of a properly modulated Dzyaloshinskii-Moriya (DM) interaction, a U(1) vortex-antivortex lattice appears at low temperatures for a wide range of the DM interaction. Even more, in the region dominated by the exchange interaction, a standard BKT transition occurs. In the opposite regime, the one dominated by the DM interaction, a kind of inverse BKT transition (iBKT) takes place. As temperature rises, the vortex-antivortex lattice starts melting by annihilation of pairs of vortex-antivortex, in a sort of “inverse” BKT transition., Instituto de Física de Líquidos y Sistemas Biológicos

The work is supported by U.S. Department of Energy, Office of Basic Energy Sciences, Division of Materials Sciences and Engineering under Award FG02-07ER46417 (L.-Q.C., F.X., and J.B.). Z.H. acknowledges the support by NSF-MRSEC Grant DMR-1420620 and NSF-MWN Grant DMR-1210588. A.R.D. acknowledges support from the Army Research Office under Grant W911NF-14-1-0104. L.W.M. acknowledges support from the National Science Foundation under Grant DMR-1451219. A.K.Y., C.T.N., and R.R. acknowledge support from the Office of Basic Energy Sciences, U.S. Department of Energy under contract no. DE-AC02-05CH11231. L.W.M. and R.R. acknowledge support from the Gordon and Betty Moore Foundation’s EPiQS Initiative, Grant GBMF5307. A novel mesoscale state comprising of an ordered polar vortex lattice has been demonstrated in ferroelectric superlattices of PbTiO3/SrTiO3. Here, we employ phase-field simulations, analytical theory, and experimental observations to evaluate thermodynamic conditions and geometric length scales that are critical for the formation of such exotic vortex states. We show that the stability of these vortex lattices involves an intimate competition between long-range electrostatic, long-range elastic, and short-range polarization gradient-related interactions leading to both an upper and a lower bound to the length scale at which these states can be observed. We found that the critical length is related to the intrinsic domain wall width, which could serve as a simple intuitive design rule for the discovery of novel ultrafine topological structures in ferroic systems. Postprint Postprint

Thermal fluctuations always melt the vortex lattice in type-II superconductors at temperatures below the mean-field transition. The extent of the intermediate vortex-liquid state varies widely among different materials. Exploring the vortex-lattice melting in the magnetic iron arsenide RbEuFe${}_{4}$As${}_{4}$ via calorimetry and transport, the authors demonstrate here that the vortex liquid occupies a significant portion of the phase diagram and that the location of the melting line agrees with theoretical predictions without fitting parameters. Comparison with other materials reveals the universality of melting behavior.

We observe a hysteretic reorientation of the magnetic vortex lattice in the noncentrosymmetric superconductor ${\mathrm{Ru}}_{7}{\mathrm{B}}_{3}$, with the change in orientation driven by altering the magnetic field below ${T}_{\mathrm{c}}$. Normally a vortex lattice chooses either a single or degenerate set of orientations with respect to a crystal lattice at any given field or temperature, a behavior well described by prevailing phenomenological and microscopic theories. Here, in the absence of any typical VL structural transition, we observe a continuous rotation of the vortex lattice which exhibits a pronounced hysteresis and is driven by a change in magnetic field. We propose that this rotation is related to the spontaneous magnetic fields present in the superconducting phase, which are evidenced by the observation of time-reversal symmetry breaking, and the physics of broken inversion symmetry. Finally, we develop a model from the Ginzburg-Landau approach which shows that the coupling of these to the vortex lattice orientation can result in the rotation we observe.

The generation and decay of a vortex lattice formed by standing gravity waves of 3 Hz frequency on the water surface have been studied experimentally using the particle image velocimetry technique. During the process of excitation the formation of large-scale vortices as a result of interaction of eddies from a vortex lattice is observed. Immediately after the pumping had been turned off in the initial time interval of about 30 s, the time dependence of vortex motion energy E vort close to an exponential decrease was observed with a characteristic time of τ w ≈ 7 s , which is mainly due to the viscous losses of the surface waves. A decrease in wave amplitude leads to reduction in the Stokes drift of floating particles. In the second time interval from 30 to 200 s there is no exponential dependence E vort ( t ) , that is related, apparently, to an energy relaxation of different vortex scales: from the bath size to the pumping wavelength. In this time interval the vortex interaction is also recorded leading to redistribution of energy from the pumping region to large scales. It should be emphasized that the vortex motion generated by the interaction of nonlinear waves is conserved after the complete decay of the wave motion on the water surface, and there is a transfer of energy from small scales to large ones as a result of the process of unification of several lattice vortices into a large-scaled vortex – reverse energy flow. In the time interval above 200 s there is an exponential energy relaxation of vortices of the maximum scale because of the viscous bath bottom and sides friction.

We utilize a combination of vector magnetic field and scanning tunneling microscopy to elucidate the three-dimensional field based electronic phase diagram of a correlated iron-based superconductor, LiFeAs. We observe, under a zero-field-cooled method, an ordered hexagonal vortex lattice ground state in contrast to the disordered lattice observed under a field-cooled method. It transforms to a fourfold-symmetric state by increasing the c-axis field and distorts elliptically upon tilting the field in-plane. The vortex lattice transformations correlate with the field-dependent superconducting gap that characterizes the Cooper pairing strength. The anisotropy of the vortex lattice agrees with the field-enhanced Bogoliubov quasiparticle scattering channel that is determined by the pairing symmetry in respect to its Fermi surface structure. Our systematic tuning of the vortex lattice symmetry and study of its correlation with Cooper pairing demonstrates the many-body interplay between the superconducting order parameter and emergent vortex matter.

A perturbed system relaxes towards an equilibrium given by a minimum in the potential-energy landscape. This often occurs by thermally activated jumps over metastable states. The corresponding dynamics is called creep and follows Arrhenius' law. Here we consider the situation where the equilibrium position itself depends on temperature. We show that this effect occurs in the vortex lattice of the anisotropic superconductor 2H-NbSe2 when the magnetic field is tilted away from the principal axes, and that it leads to the peculiar appearance of creep when cooling the sample. Temperature determines the system's equilibrium state and at the same time brings the system back to equilibrium, playing a dual and antagonistic role. We expect that cooling-induced creep occurs in correlated systems with many degrees of freedom, allowing one to tune the equilibrium state via heat treatment., Physical Review Research, 2 (1), ISSN:2643-1564

Building on the discovery that a Weyl superconductor in a magnetic field supports chiral Landau level motion along the vortex lines, we investigate its transport properties out of equilibrium. We show that the vortex lattice carries an electric current $I=\tfrac{1}{2}(Q_{\rm eff}^2/h)(\Phi/\Phi_0) V$ between two normal metal contacts at voltage difference $V$, with $\Phi$ the magnetic flux through the system, $\Phi_0$ the superconducting flux quantum, and $Q_{\rm eff}, Comment: 13 pages, 6 figures

The full H-T phase diagram in the nematic superconductor FeSe is mapped out using specific-heat and thermal-expansion measurements down to 0.7 K and up to 30 T for both field directions. A clear thermodynamic signal of an underlying vortex-melting transition is found in both datasets and could be followed down to low temperatures. The existence of significant Gaussian thermal superconducting fluctuations is demonstrated by a scaling analysis, which also yields the mean-field upper critical field Hc2(T). For both field orientations, Hc2(T) shows Pauli-limiting behavior. Whereas the temperature dependence of the vortex-melting line is well described by the model of Houghton et al., Phys. Rev. B 40, 6763 (1989) down to the lowest temperatures for H $\perp$ FeSe layers, the vortex-melting line exhibits an unusual behavior for fields parallel to the planes, where the Pauli limitation is much stronger. Here, the vortex-melting anomaly is only observed down to T*= 2-3 K, and then merges with the Hc2(T) line as predicted by Adachi and Ikeda, Phys. Rev. B 68 184510 (2003). Below T*, Hc2(T) also exhibits a slight upturn possibly related to the occurence of a Fulde-Ferrell-Larkin-Ovchinnikov (FFLO) state., 7 figures, 11 pages

In this Letter we will show that, in the presence of a properly modulated Dzyaloshinskii-Moriya (DM) interaction, a $U(1)$ vortex-antivortex lattice appears at low temperatures for a wide range of the DM interaction. Even more, in the region dominated by the exchange interaction, a standard BKT transition occurs. In the opposite regime, the one dominated by the DM interaction, a kind of inverse BKT transition (iBKT) takes place. As temperature rises, the vortex-antivortex lattice starts melting by annihilation of pairs of vortex-antivortex, in a sort of "inverse" BKT transition., Comment: 6 pages, 5 figures

In the Ginzburg-Landau limit, a possible texture without singular cores in a rotating neutron spin-triplet superfluid is studied. It is constructed the lattice of non-singular vortices with a vorticity diffusely distributed over the entire unit cell. The upper limit of the free energy associated with this structure is estimated, and the result shows that non-singular vortices are more preferable in the core of rotating neutron stars than ordinary linear singular vortices. The order parameter of the studied neutron system belongs to the unitary class. This implies that the superfluid under consideration does not have spin polarization and the core-less $^{3}$P$_{2}$ vortices do not have magnetization, which makes electron scattering by vortices ineffective. For the same reason, pinning with flux tubes is also suppressed. This can affect the hydrodynamics of superfluid neutron stars., Comment: 7 pages, 5 figures

The behaviour of a harmonically trapped dipolar Bose-Einstein condensate with its dipole moments rotating at angular frequencies lower than the transverse harmonic trapping frequency is explored in the co-rotating frame. We obtain semi-analytical solutions for the stationary states in the Thomas-Fermi limit of the corresponding dipolar Gross-Pitaevskii equation and utilise linear stability analysis to elucidate a phase diagram for the dynamical stability of these stationary solutions with respect to collective modes. These results are verified via direct numerical simulations of the dipolar Gross-Pitaevskii equation, which demonstrate that dynamical instabilities of the co-rotating stationary solutions lead to the seeding of vortices that eventually relax into a triangular lattice configuration. Our results illustrate that rotation of the dipole polarization represents a new route to vortex formation in dipolar Bose-Einstein condensates., 11 pages, 5 figures; updated with responses to referee reports and minor typographical corrections

We have studied non-equilibrium phase transitions in the vortex lattice in superconducting MgB2, where metastable states are observed in connection with an intrinsically continuous rotation transition. Using small-angle neutron scattering and a stop-motion technique, we investigated the manner in which the metastable vortex lattice returns to the equilibrium state under the influence of an ac magnetic field. This shows a qualitative difference between the supercooled case which undergoes a discontinuous transition, and the superheated case where the transition to the equilibrium state is continuous. In both cases the transition may be described by an an activated process, with an activation barrier that increases as the metastable state is suppressed, as previously reported for the supercooled vortex lattice [E. R. Louden et al., Phys. Rev. B 99, 060502(R) (2019)]. Separate preparations of superheated metastable vortex lattices with different domain populations showed an identical transition towards the equilibrium state. This provides further evidence that the vortex lattice metastability, and the kinetics associated with the transition to the equilibrium state, is governed by nucleation and growth of domains and the associated domain boundaries., Comment: 27 pages, 10 figures. arXiv admin note: text overlap with arXiv:1812.05970

We investigate vortex lattice solutions in a holographic superconductor model in asymptotically $AdS_4$ spacetime which includes the gravitational backreaction of the vortex. The circular cell approximation, which is known to give a good result for several physical quantities in the Ginzburg-Landau model, is used. The critical magnetic fields and the magnetization curve are computed. The vortex lattice profiles are compared to expectations from the Abrikosov solution in the regime nearby the upper critical magnetic field $H_{2c}$ for which superconductivity is lost., Comment: 14 pages 4 figures, matches version published in JHEP

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 I c 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*/I c that is approaching 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 the nanoscale.

We investigate the vortex lattice and vortex bound states in ${\mathrm{CsFe}}_{2}{\mathrm{As}}_{2}$ single crystals by scanning tunneling microscopy/spectroscopy (STM/STS) under various magnetic fields. A possible structural transition or crossover of vortex lattice is observed with the increase of magnetic field, i.e., the vortex lattice changes from a distorted hexagonal lattice to a distorted tetragonal one at the magnetic field near 0.5 T. It is found that a mixture of stripelike hexagonal and square vortex lattices emerges in the crossover region. The vortex bound state is also observed in the vortex center. The tunneling spectra crossing a vortex show that the bound-state peak position holds near zero bias with the STM tip moving away from the vortex core center. The Fermi energy estimated from the vortex bound state energy is very small. Our investigations provide experimental information to both the vortex lattice and the vortex bound states in this iron-based superconductor.

This paper describes an analysis of the wake flow behind a horizontal axis wind turbine rotor by means of a vortex lattice method using a free wake model. Rotor blades and their wakes are represented by vortex lattices, which can fit arbitrary blade shapes. The configuration of the wake develops downstream with rolling up, because of the largely decelerated axial flow near the rotor plane and also of the interaction effects within highly skewed vortex sheets. Therefore, a free wake model is adapted with a time marching scheme, where the position of the vortex lattice in the wake is calculated at every time step, using the local velocity, which includes the component induced by the wake. The comparison between the calculated results and the experimental data shows good agreement, especially for the axial velocity distribution in the near wake field.

Gold nanoparticles moving in an aqueous solution under an optical vortex lattice are shown to present a complex collective optofluidic dynamics. Above a critical field intensity and concentration, the system presents a spontaneous transition toward synchronized motion, driven by the interplay between nonconservative optical forces, thermal fluctuations, and hydrodynamic pairing. Above criticality, the system exhibits swarm behavior with strong unidirectional currents of nanoparticles reaching speeds of centimeters per second. This relatively simple optofluidic setup offers an alternative way to control nanoparticle transport in aqueous environments.