Azimuthal Spin Research Articles

Imaging of Molecular Complexes on the Surfaces of Living Cells by Uniform Total Internal Reflection-Based Fret Microscopy

AbstractFluorescence Resonance Energy Transfer (FRET) microscopy measures the interaction between donor and acceptor labeled proteins in living cells. Currently, these measurements are limited by out-of-focus contributions, which degrade the image. Total internal reflection (TIR) microscopy can selectively excite molecules within about 150 nanometers of the glass-water interface thereby eliminating out-of-focus fluorescence. However, the interference fringing of the coherent laser illumination used in TIR creates artifacts that prohibit quantitative imaging methods that require multiple laser illuminations such as FRET. Axelrod and collogues demonstrated that for through-the-lens TIR that rapid azimuthal spinning of a collimated beam in the back focal plane eliminates interference fringes. Here we describe multicolor, depth-matched, 360-degree spinning TIR illumination to enable quantitative, high-resolution FRET imaging of molecular complexes near the plasma membranes of living cells. We have devised new methods for normalizing spatial variations in illumination and calibrating this field for quantitative FRET analysis of protein interactions on the plasma membrane. Initial applications to Rac1 activation demonstrate a novel spatial organization of these signaling activities during adhesion of macrophages to the coverglass.

More evidence for azimuthal ion spin in HiPIMS discharges

The velocity and energy distribution functions of ions escaping radially from the magnetic trap region of a HiPIMS discharge have been measured using a retarding field analyzer (RFA). Spatially and angularly resolved measurements recorded at a representative time show more energetic ions detected along a line-of-sight coincident with an oncoming rotating ion fluid, which circulates above the racetrack in the same direction as the electron E × B drift. The difference in the mean ion energies between measurements made into and against the direction of rotation is ∼5 eV. Numerical solutions of the equation of motion for the ions accounting for azimuthal acceleration (modified two-stream instability model used by Lundin et al) have been found. The centripetal force caused by the radial electric field and a drag force term accounting for ion collisions revealed that only a small fraction (typically <5%) of the circulating ion flux can leave the discharge tangentially. Operating the discharge at different background pressures revealed an interplay between the azimuthal acceleration of ions, dominating under low pressure conditions and the scattering of ions into the RFA at higher pressure.

Spin excitation frequencies in magnetostatically coupled arrays of vortex state circular Permalloy dots

Broadband ferromagnetic resonance in square arrays of Permalloy circular dots with different interdot separations was measured in the vortex ground state. The detected spin excitations show a complicated dependence of their frequencies on the interdot coupling strength. A considerable influence of the interdot separation on the gyrotropic vortex frequency and splitting of the azimuthal spin wave frequencies was detected. The gyrotropic frequency and the first azimuthal doublet frequency splitting depend nonmonotonously on the interdot spacing, whereas the dependence of the second doublet frequency splitting on this parameter is monotonous. The observed effects are explained by the influence of both the dipolar and quadrupolar contributions to the dynamic magnetostatic interactions.

Azimuthal Spin Wave Modes Excited in an Elliptical Nanomagnet With Vortex Pair States

The ferromagnetic thin-film elements can be prepared to form a ?diamond? remanent state that contains two vortices with opposite circulation and with the same or opposite core polarities. The vortex polarity could influence their normal modes through the magnetostatic interactions between two vortices. In this paper, micromagnetic simulations, combined with Fourier techniques, are employed to study the spin-wave (SW) modes excited in a submicron Permalloy ellipse with a vortex pair configuration. After the application of an inplane Gaussian field pulse, it is found that a series of azimuthal SW modes is excited and the high-order harmonics of SW modes are strongly dependent of the core polarities due to the dynamic magnetic interactions between the two vortices.

Precise probing spin wave mode frequencies in the vortex state of circular magnetic dots

We report on detailed broadband ferromagnetic resonance measurements of azimuthal and radial spin wave excitations in circular Permalloy dots in the vortex ground state. Dots with aspect ratio (β=height over radius) varied from 0.03 to 0.1 were explored. The frequency splitting of two lowest azimuthal modes was observed. The experimentally observed dependence of the frequency splitting on β was reasonably well described by dynamic splitting model accounting the spin waves and vortex gyrotropic mode interaction.

THE SPIN STRUCTURE OF THE NUCLEON IN LIGHT-CONE QUARK MODELS

The quark spin densities related to generalized parton distributions in impact-parameter space and to transverse-momentum dependent parton distributions are reviewed within a light-cone quark model, with focus on the role of the different spin-spin and spin-orbit correlations of quarks. Results for azimuthal spin asymmetries in semi-inclusive deep inelastic scattering due to T -even transverse-momentum dependent parton distributions are also discussed.

Azimuthal spin asymmetries in light-cone constituent quark models

We present results for all leading-twist azimuthal spin asymmetries in semi-inclusive lepton-nucleon deep-inelastic scattering due to T-even transverse-momentum dependent parton distribution functions on the basis of a light-cone constituent quark model. Attention is paid to discuss the range of applicability of the model, especially with regard to the scale dependence of the observables and the transverse-momentum dependence of the distributions. We find good agreement with available experimental data and present predictions to be further tested by future CLAS, COMPASS, and HERMES data.

A Tool to Study Azimuthal Standing and Spinning Modes in Annular Combustors

A methodology for the computation of azimuthal combustion instabilities which can occur in annular combustors is proposed in this work. A thermoacoustic numerical tool using the n – τ model for the coupling of acoustic and combustion is required to solve the Helmholtz equation in reactive media. The methodology is based on the Independence Sector Assumption in Annular Combustor (ISAAC) which states that the heat release fluctuations in a given sector are driven only by the fluctuating mass flow rates due to the velocity perturbations through its own swirler. This assumption is first discussed with respect to a Large Eddy Simulation of an annular combustor. The methodology is then applied to an academic annular test case which exhibits amplified or damped, standing or rotating azimuthal eigenmodes depending on parameters n and τ.

Transverse momentum dependent distribution functionh1T⊥and the single spin asymmetryAUTsin(3ϕ−3S)

The leading twist transverse momentum dependent parton distribution function h_1T^perp, which is sometimes called ``pretzelosity,'' is studied. We review the theoretical properties of this function, and present bag model predictions. We observe an interesting relation valid in a large class of relativistic models: The difference between helicity and transversity distributions, which is often said to be a 'measure of relativistic effects' in nucleon, is nothing but the pretzelosity distribution. Pretzelosity is chirally odd and can be accessed in combination with the Collins effect in semi-inclusive deep inelastic scattering, where it gives rise to an azimuthal single spin asymmetry proportional to sin(3phi-phi_S). We discuss the preliminary deuteron target data from COMPASS, on that observable and make predictions for future experiments on various targets at JLab, COMPASS and HERMES.

Transversity from two pion interference fragmentation

We present calculation on the azimuthal spin asymmetries for pion pair production in semi-inclusive deep inelastic scattering (SIDIS) process at both HERMES and COMPASS kinematics, with transversely polarized proton, deuteron, and neutron targets. We calculate the asymmetry by adopting a set of parametrization of the interference fragmentation functions and two different models for the transversity. We find that the result for the proton target is insensitive to the approaches of the transversity but more helpful to understand the interference fragmentation functions. However, for the neutron target, which can be obtained through using deuteron and $^{3}\mathrm{He}$ targets, we find different predictions for different approaches to the transversity. Thus probing the two pion interference fragmentation from the neutron can provide us more interesting information on the transversity.

Spin dynamics of a magnetic antivortex: Micromagnetic simulations

We report on a study of the dynamics of a magnetic antivortex in a submicrometer, asteroid-shaped, permalloy ferromagnet using micromagnetic simulations. As with vortex states in disk and square geometries, a gyrotropic mode was found in which a shifted antivortex core orbits about the center of the asteroid. Pulsed magnetic fields were used to generate azimuthal or radial spin wave modes, depending on the field orientation. The degeneracy of low-frequency azimuthal mode frequencies is lifted by gyrotropic motion of the antivortex core, and restored by inserting a hole in the center of the particle to suppress this motion. We briefly compare the dynamics of the vortex state of the asteroid to the antivortex. The size dependence of the antivortex modes is reported.

Single target-spin asymmetry in semi-inclusive deep inelastic scattering on a transversely polarized nucleon target

We use a new set of Collins functions to update a previous prediction on the azimuthal asymmetries of pion production in a semi-inclusive deep-inelastic scattering process on a transversely polarized nucleon target. We find that the calculated results can give a good explanation to the HERMES experiment with the new parametrization, and this can enrich our knowledge of the fragmentation process. Furthermore, with two different approaches of distribution and fragmentation functions, we present a prediction on the azimuthal asymmetries of pion and kaon production at the kinematics region of the experiments E06010 and E06011 planned at Jefferson Lab. It is shown that the results are insensitive to the models for the pion case. However, the results for kaon production are sensitive to different approaches of distribution and fragmentation functions. This is helpful to clarify some points in the study of the azimuthal spin asymmetries and fragmentation functions in hadronization processes.

Neutron single spin asymmetries from semi-inclusive deep inelastic scattering off transversely polarizedHe3

A study of semi-inclusive deep inelastic scattering off transversely polarized $^{3}\mathrm{He}$ is presented. The formal expressions of the Collins and Sivers contributions to the azimuthal single spin asymmetry for the production of leading pions are derived, in impulse approximation, and estimated in the kinematics of forth-coming experiments at JLab. The AV18 interaction has been used for a realistic description of the nuclear dynamics; the nucleon structure has been described by proper parametrizations of data or suitable model calculations. The initial transverse momentum of the struck quark has been properly included in the calculation. The crucial issue of extracting the neutron information from $^{3}\mathrm{He}$ data, planned to shed some light on the puzzling experimental scenario arisen from recent measurements for the proton and the deuteron, is thoroughly discussed. It is found that a model independent procedure, widely used in inclusive deep inelastic scattering to take into account the momentum and energy distributions of the bound nucleons in $^{3}\mathrm{He}$, can be applied also in the kinematics of the planned JLab experiments, although fragmentation functions, not only parton distributions, are involved. The possible role played by final state interactions in the process under investigation is addressed.

Collins effect in semiinclusive deeply inelastic scattering and in electron-positron-annihilation

The Collins fragmentation function is extracted from HERMES data on azimuthal single spin asymmetries in semi-inclusive deeply inelastic scattering, and BELLE data on azimuthal asymmetries in electron positron annihilations. A Gaussian model is assumed for the distribution of transverse parton momenta and predictions are used from the chiral quark-soliton model for the transversity distribution function. We find that the HERMES and BELLE data yield a consistent picture of the Collins fragmentation function which is compatible with COMPASS data and the information previously obtained from an analysis of DELPHI data. Estimates for future experiments are made.

Single spin asymmetries in hadron-hadron collisions

We study weighted azimuthal single spin asymmetries in hadron-hadron scattering using the diagrammatic approach at leading order and assuming factorization. The effects of the intrinsic transverse momenta of the partons are taken into account. We show that the way in which $T$-odd functions, such as the Sivers function, appear in these processes does not merely involve a sign flip when compared with semi-inclusive deep inelastic scattering, such as in the case of the Drell-Yan process. Expressions for the weighted scattering cross sections in terms of distribution and fragmentation functions folded with hard cross sections are obtained by introducing modified hard cross sections, referred to as gluonic pole cross sections.

Polarized structure functions

We discuss the spin structure of quarks in hadrons, in particular the transverse spin polarization or transversity. The most direct way to probe transversity appears to be via azimuthal spin asymmetries. This brings in the role of intrinsic transverse momenta of quarks in hadrons and the study of T -odd phenomena, which are connected to single spin asymmetries.

Instanton-induced azimuthal spin asymmetry in deep inelastic scattering

It is by now well understood that spin asymmetry in deep inelastic scattering (DIS) can appear if two things are both present: (i) a chirality flip of the struck quark; (ii) a nonzero T-odd phase due to its final state interaction. So far (i) was attributed to a new structure/wave function of the nucleon and (ii) to some gluon exchanges. We propose a new mechanism utilizing strong vacuum fluctuations of the gluon field described semiclasically by instantons, and show that both (i) and (ii) are present. The magnitude of the effect is estimated using known parameters of the instanton ensemble in the QCD vacuum and known structure and fragmentation functions, without any new free parameters. The result agrees in sign and (roughly) in magnitude with the available data on single particle inclusive DIS. Furthermore, our predictions uniquely relate effects for longitudinally and transversely polarized targets.

Rotational stability of plasma blobs

The stability of plasma blobs which have both density and temperature higher than the surrounding plasma, and can transport heat as well as particles, is considered. It is shown that the internal blob temperature profile Te(r) can drive azimuthal rotation or spin vθ(r) about the blob axis, which produces a robust m=2 rotational instability in the interchange limit (k∥=0). The theory includes the effects of the centrifugal and Coriolis forces, the sheared velocity vθ(r), and the axial sheath boundary condition. Estimates show that finite-Larmor-radius stabilization is ineffective, but the sheath conductivity can be strongly stabilizing. The blob rotational instability has only a small direct impact on the particle and energy transport, but it serves as a useful diagnostic for the underlying blob spin, which is an important variable in determining the blob’s radial velocity. A separate branch of temperature-gradient-driven sheath instabilities, predicted in the eikonal limit, is not observed for low mode numbers.

Sivers function in light-cone quark model and azimuthal spin asymmetries in pion electroproduction

We perform a calculation of Sivers function in a light-cone SU(6) quark–diquark model with both scalar diquark and vector diquark spectators. We derive the transverse momentum dependent light-cone wave function of the proton by taking into account the Melosh–Wigner rotation. By adopting one-gluon exchange, we obtain a non-vanishing Sivers function of down quark from interference of proton spin amplitudes. We analyze the |P h⊥| M weighted Sivers asymmetries in π +, π − and π 0 electroproduction off transverse polarized proton target, averaged and not averaged by the kinematics of HERMES experiment.

Azimuthal single spin asymmetries in SIDIS in the light of chiral symmetry breaking

An attempt is made to understand the z-dependence of the azimuthal single spin asymmetries observed by the HERMES Collaboration in terms of chiral models based on effective quark and Goldstone boson degrees of freedom. The effects of respectively neglecting and considering Gaussian intrinsic parton transverse momenta and the Sivers effect are explored. Predictions for the transverse target polarization experiment at HERMES are presented.