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The levitation of a cylindrical permanent magnet over a high-temperature superconductor cooled by liquid nitrogen can be accompanied by spontaneous oscillations and rotation. The reason for spontaneous rotation of the magnet is magnetization inhomogeneity induced by the temperature gradient. An experiment was carried out on the levitation of Nd-Fe-B magnets over a composite high-temperature superconductor. The experimental results confirm that the rotation frequency depends on the difference in the magnetization values in the upper and lower halves of the magnet. Methods for controlling the rotation frequency of a levitating magnet are proposed., Comment: 8 pages, 3 figures

A misalignment of anisotropic crystallites causes small values of anisotropy and decreases the critical current density of textured polycrystalline superconductors. To relate the crystallite misalignment and out-plane anisotropy, the magnetic properties of the textured Bi2223 polycrystalline superconductor were investigated. A distribution of orientation angles of crystallites was determined using different data: scanning electron microscopy images and hysteresis magnetization loops when an external magnetic field was applied at different angles with respect to the texturing plane of the sample. It was demonstrated that the standard deviation of the distribution and the magnetic disorder angle of crystallites in textured samples can be determined from the magnetization data in perpendicular directions. These data may be either the irreversible magnetization measured for two different orientations of the sample or the simultaneously measured magnetization projections parallel and perpendicular to the magnetic field., Comment: 11 pages, 6 figures

In this paper, we study a stable optomechanical system based on a nanoparticle chain coupled to a waveguide mode. Under the plane wave excitation the nanoparticles form a stable self-organized periodic chain array along the direction of the waveguide through the transverse binding effect. We show that owing to the long-range interaction between the nanoparticles the trapping potential for each nanoparticle in the chain increases linearly with the system size making the formation of long chains more favourable. We propose that this effect can be observed with an optical nanofiber which is a versatile platform for achieving optical binding of atoms and nanoparticles. Our calculations show that binding energy for two nanoparticles is in the range of $9\div13~kT$ reaching the value of 100 $kT$ when the size of the chain is increased to 20 nanoparticles that makes the potential experimental observation of the effect possible. We also suggest the geometry of the two counter-propagating plane waves excitation, which will allow trapping the nanoparticles close to the optical nanofiber providing efficient interaction between the nanoparticles and the nanofiber.

We examine acoustic radiation force and torque on a small (subwavelength) absorbing isotropic particle immersed in a monochromatic (but generally inhomogeneous) sound-wave field. We show that by introducing the monopole and dipole polarizabilities of the particle, the problem can be treated in a way similar to the well-studied optical forces and torques on dipole Rayleigh particles. We derive simple analytical expressions for the acoustic force (including both the gradient and scattering forces) and torque. Importantly, these expressions reveal intimate relations to the fundamental field properties introduced recently for acoustic fields: the canonical momentum and spin angular momentum densities. We compare our analytical results with previous calculations and exact numerical simulations. We also consider an important example of a particle in an evanescent acoustic wave, which exhibits the mutually-orthogonal scattering (radiation-pressure) force, gradient force, and torque from the transverse spin of the field., Comment: 7 pages, 3 figures, Supplemental Material, to appear in Phys. Rev. Lett

Anisotropic nanophotonic structures can couple the levels of a quantum emitter through the quantum interference effect. In this paper we study the coupling of quantum emitters excited states through the modes of a fully anisotropic structure: a structure for which all directions are physically nonequivalent. We consider an anisotropic metasurface as an illustrative example of such a structure. We point out a novel degree of freedom in controlling the temporal dynamics and spectral profiles of quantum emitters: namely, we show that a combination of the metasurface anisotropy and tilt of the emitter quantization axis with respect to the metasurface normal results in nonsymmetric dynamics between the transitions of electrons from left-circular state to the right-circular states and the inverse process. Our findings give an additional mechanism for control over the light emission by quantum systems and, vice versa, can be utilized for probing active transitions of quantum emitters.

We study the force of light on a two-level atom near an ultrathin optical fiber using the mode function method and the Green tensor technique. We show that the total force consists of the driving-field force, the spontaneous-emission recoil force, and the fiber-induced van der Waals potential force. Due to the existence of a nonzero axial component of the field in a guided mode, the Rabi frequency and, hence, the magnitude of the force of the guided driving field may depend on the propagation direction. When the atomic dipole rotates in the meridional plane, the spontaneous-emission recoil force may arise as a result of the asymmetric spontaneous emission with respect to opposite propagation directions. The van der Waals potential for the atom in the ground state is off-resonant and opposite to the off-resonant part of the van der Waals potential for the atom in the excited state. Unlike the potential for the ground state, the potential for the excited state may oscillate depending on the distance from the atom to the fiber surface.

In this paper we develop a microscopic analysis of the light scattering on a periodic two-level atomic array coupled to an optical nanofiber. We extend the scattering matrix approach for two-level system interaction with nanofiber fundamental waveguiding mode HE_{11}, that allows us modeling the scattering spectra. We support these results considering the dispersion of the polaritonic states formed by the superposition of the fundamental mode of light HE_{11} and the atomic chain states. To illustrate our approach we start with considering a simple model of light scattering over atomic array in the free space. We discuss the Bragg diffraction at the atomic array and show that the scattering spectrum is defined by the non-symmetric coupling of two-level system with nanofiber and vacuum modes. The proposed method allows considering two-level systems interaction with full account for dipole-dipole interaction both via near fields and long-range interaction owing to nanofiber mode coupling., Comment: 11 pages, 9 figures

We demonstrate both analytically and numerically the existence of optical pulling forces acting on particles located near plasmonic interfaces. Two main factors contribute to the appearance of this negative reaction force. The interference between the incident and reflected waves induces a rotating dipole with an asymmetric scattering pattern while the directional excitation of surface plasmon polaritons (SPP) enhances the linear momentum of scattered light. The strongly asymmetric SPP excitation is determined by spin-orbit coupling of the rotating dipole and surface plasmon polariton. As a result of the total momentum conservation, the force acting on the particle points in a direction opposite to the incident wave propagation. We derive analytical expressions for the force acting on a dipolar particles placed in the proximity of plasmonic surfaces. Analytical expressions for this pulling force are derived within the dipole approximation and are in excellent agreement with results of electromagnetic numerical calculations. The forces acting on larger particles are analyzed numerically, beyond the dipole approximation., Comment: 18 pages, 6 figures, supplementary materials (7 pages, movie)

Over the last decade, plasmonic antireflecting nanostructures have been extensively studied to be utilized in various optical and optoelectronic systems such as lenses, solar cells, photodetectors, and others. The growing interest to all-dielectric photonics as an alternative optical technology along with plasmonics motivates us to compare antireflection properties of all-dielectric and plasmonic nanoparticle coatings based on silver and crystalline silicon. Our results of numerical simulations for periodic arrays of spherical nanoparticles on top of amorphous silicon show that both silicon and silver nanoparticle coatings demonstrate strong anti-reflective properties in the visible spectral range. In this work, we show for the first time that blooming effect, that is zero reflection from the structure, with silicon coatings originates from the interference of electric- and magnetic-dipole responses of nanoparticles with the wave reflected from the substrate, and we refer to it as substrate-mediated Kerker effect. For the silver coating, our results agree with previously observed substrate-induced bi-anisotropy and blooming, caused by substrate-induced magnetic response. Finally, we numerically show high effectiveness of silicon and silver coatings for the application in thin-film photovoltaic elements, which is related to the suppression of reflection from the high-index substrate and increased light absorbance in the active layer with coating. Depending on the nanoparticle size, either silicon or silver coating is more efficient, and overall increase of absorption up to 30% can be achieved.

The current-voltage characteristics of a porous superconductor Bi2Sr2Ca2Cu3Ox (Bi2223) have been measured at temperature range from 10 to 90 K. The experimental dependences have been analyzed within the model allowing for pinning by clusters of a normal phase with fractal boundaries, as well as the model taking into account phase transformations of vortex matter. It has been found that the electrical resistance of the superconductor material significantly increases at temperatures of 60-70 K over the entire range of magnetic fields under consideration without changing in the sign of the curvature of the R(I) dependence. The melting of the vortex structure occurs at these temperatures. It has been assumed that this behavior is associated with the specific feature of the pinning in a highly porous high-temperature superconductor, which lies in the fractal distribution of pinning centers in a wide range of self-similarity scales., Comment: 6 pages, 6 figures, a brief version of the article published in Physics of the Solid State, field values are corrected

Inhomogeneous distribution of the pinning force in superconductor results in a magnetization asymmetry. A model considering the field distribution in superconductor was developed and symmetric and asymmetric magnetization loops of porous and textured Bi_{1.8}Pb_{0.3}Sr_{1.9}Ca_{2}Cu_{3}O_{x} were fitted. It is found that the thermal equilibrium magnetization realizes in crystals smaller than some size depending on temperature and magnetic field., Comment: 11 pages, 5 figures

Context: A recent new approach to apply a simple dynamical mass estimate of tidally limited star clusters is based on the identification of the tidal radius in a King profile with the dynamical Jacobi radius. The application to an unbiased open cluster catalogue yields significantly higher cluster masses compared to the classical methods. Aims: We quantify the bias in the mass determination as function of projection direction and cluster age by analysing a simulated star cluster. Methods: We use direct $N$-body simulations of a star cluster including stellar evolution in an analytic Milky Way potential and apply a best fit to the projected number density of cluster stars. Results: We obtain significantly overestimated star cluster masses which depend strongly on the viewing direction. The overestimation is typically in the range of 10-50 percent and reaches a factor of 3.5 for young clusters. Mass segregation reduces the derived limiting radii systematically., Comment: 9 pages, 10+1 figures, accepted by Astronomy and Astrophysics

The dependence of magnetization on magnetic field M(H) was measured for textured BSCCO samples for H || c, H || ab at different temperatures. Inclusion of the pinning parameter in the Valkov-Khrustalev model [V.V. Valkov, B.P. Khrustalev, JETP 80 (1995) 680] allowed to describe successfully the asymmetric M(H) dependencies. The temperature and magnetic field dependencies of critical current and pinning parameter for H || c and H || ab were estimated., Comment: 4 pages, 1 figure, will be published in Physica C

(abridged) In this paper we derive observed and modelled shape parameters (apparent ellipticity and orientation of the ellipse) of 650 Galactic open clusters identified in the ASCC-2.5 catalogue. We provide the observed shape parameters of Galactic open clusters, computed with the help of a multi-component analysis. For the vast majority of clusters these parameters are determined for the first time. High resolution ("star by star") N-body simulations are carried out with the specially developed $\phi$GRAPE code providing models of clusters of different initial masses, Galactocentric distances and rotation velocities. The comparison of models and observations of about 150 clusters reveals ellipticities of observed clusters which are too low (0.2 vs. 0.3), and offers the basis to find the main reason for this discrepancy. The models predict that after $\approx 50$ Myr clusters reach an oblate shape with an axes ratio of $1.65:1.35:1$, and with the major axis tilted by an angle of $q_{XY} \approx 30^\circ$ with respect to the Galactocentric radius due to differential rotation of the Galaxy. Unbiased estimates of cluster shape parameters require reliable membership determination in large cluster areas up to 2-3 tidal radii where the density of cluster stars is considerably lower than the background. Although dynamically bound stars outside the tidal radius contribute insignificantly to the cluster mass, their distribution is essential for a correct determination of cluster shape parameters. In contrast, a restricted mass range of cluster stars does not play such a dramatic role, though deep surveys allow to identify more cluster members and, therefore, to increase the accuracy of the observed shape parameters., Comment: 13 pages, 12 figures, accepted for publication in Astronomy and Astrophysics

Tidal tails of star clusters are not homogeneous but show well defined clumps in observations as well as in numerical simulations. Recently an epicyclic theory for the formation of these clumps was presented. A quantitative analysis was still missing. We present a quantitative derivation of the angular momentum and energy distribution of escaping stars from a star cluster in the tidal field of the Milky Way and derive the connection to the position and width of the clumps. For the numerical realization we use star-by-star $N$-body simulations. We find a very good agreement of theory and models. We show that the radial offset of the tidal arms scales with the tidal radius, which is a function of cluster mass and the rotation curve at the cluster orbit. The mean radial offset is 2.77 times the tidal radius in the outer disc. Near the Galactic centre the circumstances are more complicated, but to lowest order the theory still applies. We have also measured the Jacobi energy distribution of bound stars and showed that there is a large fraction of stars (about 35%) above the critical Jacobi energy at all times, which can potentially leave the cluster. This is a hint that the mass loss is dominated by a self-regulating process of increasing Jacobi energy due to the weakening of the potential well of the star cluster, which is induced by the mass loss itself., Comment: 14 pages, 17 figures; accepted by MNRAS

The intragrain pinning in high-$T_c$ superconductor compounds Y$_{1-x}$RE$_{x}$Ba$_{2}$Cu$_{3}$O$_{7-\delta}$ with low concentration of RE (La, Ce, Pr) was investigated. Magnetic and transport measurements reveal that the pinning is maximal for the concentration of heterovalent RE such that the average distance between the impurity ions in the plane of rare-earth elements close to the diameter of Abrikosov vortices in YBCO., Comment: 11 pages, 6 figures, will be published in SUST

The current-voltage ($I$-$V$) characteristics of break junctions of polycrystalline La$_{1.85}$Sr$_{0.15}$CuO$_4$, Y$_{0.75}$Lu$_{0.25}$Ba$_2$Cu$_3$O$_{7-\delta}$, Bi$_{1.8}$Pb$_{0.3}$Sr$_{1.9}$Ca$_2$Cu$_3$O$_x$ and composite YBa$_2$Cu$_3$O$_{7-\delta}$ + Ag are investigated. The experimental $I$-$V$ curves exhibit the specific peculiarities of superconductor/normal-metal/superconductor junctions. The relation between an $I$-$V$ characteristic of network of weak links and $I$-$V$ dependencies of typical weak links is suggested to describe the experimental data. The $I$-$V$ curves of typical weak links are calculated by the K\"{u}mmel - Gunsenheimer - Nicolsky model considering the multiple Andreev reflections., Comment: 9 pages, 5 figures, 2 Tables

We have monitored two bright galaxies, M85 and NGC7331, on timescales as short as 0.01s. In the optical, we discovered an unusual burst coincident with the galaxy M85. We registered a sudden onset with a characteristic time of less than 10ms with subsequent quasi-exponential decay within approximately 1s and an amplitude of 2.5mag in the V-band. In the course of high-speed monitoring with two Crimean telescopes operated synchronously, in both independent instruments we have registered one coincident event occurring in NGC7331 with a duration of ~0.6s. The amplitudes range from ~3mag to ~0.3mag in the U-band and I-band, respectively. Merging of an intermediate-mass black hole with a small black hole or normal star seems to be the most plausible mechanism responsible for short bursts. Our observations support the hypothesis concerning the existence of intermediate-mass black holes in the centers of galaxies and in dense globular clusters., Comment: 4 pages, 3 figures, proceedings of the conference "AGN Variability from X-rays to Radio Waves", Nauchny, Ukraine, June 2004, to be published in ASP Conference Series, Vol XX, 2004

The magnetization of porous $Bi_{1.8}Pb_{0.3}Sr_2Ca_2Cu_3O_x$ have been investigated. The experimental magnetization hysteretic loops of $M(H)$ were described in the frames of Val'kov -- Khrustalev model developed for type II granular superconductors., Comment: 3 pages, 1 figure

In this paper we study the process of the subsequent (runaway) fragmentation of the rotating isothermal Giant Molecular Cloud (GMC) complex. Our own developed Smoothed Particle Hydrodynamics (SPH) gas-dynamical model successfully reproduce the observed Cloud Mass-distribution Function (CMF) in our Galaxy (even the differences between the inner and outer parts of our Galaxy). The steady state CMF is established during the collapse within a free-fall timescale of the GMC. We show that one of the key parameters, which defines the observed slope of the present day CMF, is the initial ratio of the rotational (turbulent) and gravitational energy inside the fragmented GMC., Comment: 8 pages, 9 EPS figures, special forma.cls class file is used

The current-voltage characteristic (CVC) of break junction made from polycrystalline Bi_{1.8}Pb_{0.3}Sr_{1.9}Ca_2Cu_3O_x is investigated. The experimental CVC has hysteretic feature that reflects a part of a curve with negative differential resistance. The CVC is discussed within the framework of the Kummel-Gunsenheimer-Nicolsky theory considering multiple Andreev reflections in superconductor/normal-metal/superconductor junctions., Comment: 5 pages, 1 figure