Azimuthal Spin Research Articles

Transverse Spin Hall Effect and Twisted Polarization Ribbons at the Sharp Focus

Transverse Spin Hall Effect and Twisted Polarization Ribbons at the Sharp Focus

Cooling the optical-spin driven limit cycle oscillations of a levitated gyroscope

Birefringent microspheres, trapped in vacuum and set into rotation by circularly polarised light, demonstrate remarkably stable translational motion. This is in marked contrast to isotropic particles in similar conditions. Here we demonstrate that this stability is obtained because the fast rotation of these birefringent spheres reduces the effect of azimuthal spin forces created by the inhomogeneous optical spin of circularly polarised light. At reduced pressures, the unique profile of these rotationally averaged, effective azimuthal forces results in the formation of nano-scale limit cycles. We demonstrate feedback cooling of these non-equilibrium oscillators, resulting in effective temperatures on the order of a milliKelvin. The principles we elaborate here can inform the design of high-stability rotors carrying enhanced centripetal loads or result in more efficient cooling schemes for autonomous limit cycle oscillations. Ultimately, this latter development could provide experimental access to non-equilibrium quantum effects within the mesoscopic regime.

Static and dynamic magnetic characteristics in concentric permalloy nanorings

Magnetic nanodisks and nanorings have been focus of attention for quite some time due to their applicability in magnetic memory devices. In this paper, static and dynamic magnetic properties of Permalloy concentric thin ring nanostructures are reported as a function of the number of rings by using micromagnetic simulations. The maximum outer radius (R) and thickness (t) of the concentric rings are fixed at 100 nm and 20 nm respectively. The distance between any two neighbouring rings is always maintained equal to the width of each ring while the number of nanorings is increased starting from one to five. In-plane magnetic hysteresis loops show that in all cases, the remanence and coercivity values are found to be almost zero while vortex state is exhibited with opposite helicity in adjacent nanorings. When the number of nanorings reach five, the narrowest hysteresis loop is observed with almost no hysteresis at all. Dynamic susceptibility spectra was computed by relaxing the system from a out of plane saturated state followed by an in-plane excitation using a small field pulse. In most of the cases, the in plane magnetic field pulse excites azimuthal spin wave modes. The resonance frequency corresponding to these excitation modes depends on the remanent configuration and the number of peaks increase with increment in the number of rings. The ferromagnetic resonance mode that is common for all the samples considered, moves towards lower frequency with increase in number of concentric rings.

Type-II quantum spin Hall effect in two-dimensional metals

The quantum spin Hall (QSH) effect has been observed in topological insulators and long quantum wells using spin–orbit coupling as the probe, but it has not yet been observed in a metal. An experiment is proposed to measure the different Type-II QSH effect of an electron or hole in a two-dimensional (2D) metal by using the previously unexplored but relativistically gauge-invariant form of the generated 2D QSH Hamiltonian. Instead of using the electric field in the surface of the spin-polarized bands of a topological insulator or across the quantum well width as the probe, ones uses an applied azimuthal vector potential and an applied radial electric field as the tools to generate a spontaneously quantized spin current in an otherwise spin unpolarized 2D metal. A long cylindrical solenoid lies normally through the inner radius of a 2D metallic Corbino disk. The current surrounding the solenoid produces an azimuthal magnetic vector potential but no magnetic field in the disk. In addition, a radial electric field is generated across the disk by imposing either a potential difference or a radial charge current I across its inner and outer radii. Combined changes in and in either or I generate spontaneously quantized azimuthal charge and spin currents. The experiment is designed to measure these quantized azimuthal charge and spin currents in the disk consistently. The quantum Hamiltonians for both experiments are solved exactly. A method to control the Joule heating is presented, which could potentially allow the Type-II QSH measurements to be made at room temperature.

Shear-Induced Spin Polarization in Heavy-Ion Collisions.

We study the spin polarization generated by the hydrodynamic gradients. In addition to the widely studied thermal vorticity effects, we identify an undiscovered contribution from the fluid shear. This shear-induced polarization (SIP) can be viewed as the fluid analog of strain-induced polarization observed in elastic and nematic materials. We obtain the explicit expression for SIP using the quantum kinetic equation and linear response theory. Based on a realistic hydrodynamic model, we compute the differential spin polarization along both the beam direction z[over ^] and the out-plane direction y[over ^] in noncentral heavy-ion collisions at sqrt[s_{NN}]=200 GeV, including both SIP and thermal vorticity effects. We find that SIP contribution always shows the same azimuthal angle dependence as experimental data and competes with thermal vorticity effects. In the scenario that Λ inherits and memorizes the spin polarization of a strange quark, SIP wins the competition, and the resulting azimuthal angle dependent spin polarization P_{y} and P_{z} agree qualitatively with the experimental data.

Experimental study on self-excited thermoacoustic instabilities and intermittent switching of azimuthal and longitudinal modes in an annular combustor

For annular combustors, the interactions between different modal structures (longitudinal, azimuthal spinning, standing, and slanted) remain a challenge in ongoing studies of thermoacoustic instabilities. The present work develops an annular combustor named TurboCombo, similar to the multiple-injector annular combustor named MICCA, but with a convergent turbine section connected to the combustor outlet. A Helmholtz solver provides a proper estimate of the modal frequencies and reveals the spatial structures for both two rigs. The experimental results present the consistencies of frequency domains between the acoustic pressure signals and the hydroxyl (OH*) radicals emission signal. The high-speed flame images are processed by the dynamic mode decomposition method, statistically showing longitudinal, azimuthal spinning, or standing nature in different cases. The stability maps of the two combustors show slight differences under atmospheric conditions. In the TurboCombo rig, there are two types of interaction behaviors between the 1/4 wave longitudinal mode of the chamber and the 1st order azimuthal mode of the plenum. Unlike the prevalent “dual-mode coexistence” characteristics, an interesting behavior of “intermittent switching” is revealed, which manifests as conspicuous two-way jumps between the two modes with different frequencies. Such intermittent switching occurs randomly between two stable limit cycles in a long-time observation without any external excitation, but is driven by the turbulence-induced stochastic process. Moreover, there still exist some weak azimuthal oscillations while the longitudinal mode is dominated. The standing or spinning bias of the azimuthal mode also features intermittent transition, corresponding to the occasions dominated by longitudinal mode or azimuthal mode, respectively.

Calibrating Electrostatic Deflection of Charged Particle Sensors Using Ambient Plasma Measurements.

As space‐based charged particle measurement pushes the technical envelope, resolution, both spatially and temporally, is ever improving. As such, the knowledge of the associated error must also improve. We present a method for correlating data collected from multiple sensors at different times in order to estimate the pointing error of each sensor. The method is demonstrated using flight data from the Dual Ion Spectrometer suite, part of the Fast Plasma Investigation on the NASA's Magnetospheric Multiscale mission. By looking at signals with sharp features in the direction of spacecraft spin, the relative error in look direction between sensors can be estimated with sub‐degree precision, roughly 20 times better than the native resolution in the azimuthal (spin) direction. These sharp features appear in nature often enough that a sufficiently large sample size can be identified, using an automated filter of routine science data, to calibrate the system, or post correct measured data. The relative pointing error can then be trended over time to monitor the evolution/aging of the measurement system. These data inform calibration/correction methods, should the error grow to a point where science quality is adversely affected.

Spin wave modes of skyrmioniums in the presence of Dzyaloshinskii-Moriya interaction

A skyrmionium or 2π skyrmion is an exotic magnetic configuration that can be stabilized in ferromagnetic nanostructures, e.g., nanodisks. Differently from the standard (or π) skyrmion, it can move without suffering deflections due to the skyrmion Hall effect (SkHE), a property that is desirable for potential applications in spintronics. In this work, using micromagnetic simulations, it is investigated the influence of the Dzyaloshinskii-Moriya interaction on the azimuthal and radial spin wave spectra of the skyrmionium. First, the ground state of the ferromagnetic disk is obtained for several values of Dzyaloshinskii-Moriya exchange constant Dint and a fixed value of perpendicular anisotropy constant Kz. The spin wave modes of the skyrmionium were obtained using in-plane and out-of-plane magnetic pulses. The results show that the values of the frequencies of the spin wave modes depend on the values of Dint. Additionally, it is shown that it is possible to create a skyrmionium using in-plane rotating magnetic fields, by tuning parameters such as the intensity of the applied perpendicular magnetic fields, and their duration.

The internal dynamic modes of an antiskyrmion in ultrathin ferromagnetic nanodisks

We report a numerical study on the internal resonant modes of an antiskyrmion structure in thin-film ferromagnetic nanodisks by using micromagnetic simulations. As the samples are excited by an oscillating magnetic field along different directions, we are able to resolve several sets of resonant eigenmodes: gyrotropic mode and azimuthal and radial spin wave modes. At the in-plane field excitation, the gyrotropic mode at the low frequency behaves consistently with Thiele’s analytic theory, while a pair of twin azimuthal spin wave modes at higher frequencies demonstrate that their splitting is strongly relevant to the coexistent gyrotropic mode. At the out-of-plane field excitation, several breathing-like radial spin wave modes have been found. In addition, we carry out a brief comparison between these modes and their counterpart modes of an isolated skyrmion structure. We also quantitatively investigate the dependence of the eigenfrequencies of these modes on the different parameter sets of the sample size and internal interactions, which exhibit complex relationships between the internal dynamic modes of an antiskrymion and the geometry and material of the sample.

Spin momentum-dependent orbital motion

We present a theoretic analysis on (azimuthal) spin momentum-dependent orbital motion experienced by particles in a circularly-polarized annular focused field. Unlike vortex phase-relevant (azimuthal) orbital momentum flow whose direction is specified by the sign of topological charge, the direction of (azimuthal) spin momentum flow is determined by the product of the field’s polarization ellipticity and radial derivative of field intensity. For an annular focused field with a definite polarization ellipticity, the intensity’s radial derivative has opposite signs on two sides of the central ring (intensity maximum), causing the spin momentum flow to reverse its direction when crossing the central ring. When placed in such a spin momentum flow, a probe particle is expected to response to this flow configuration by changing the direction of orbital motion as it traversing from one side to the other. The reversal of the particle’s orbital motion is a clear sign that spin momentum flow can affect particles’ orbital motion alone even without orbital momentum flow. More interestingly, for dielectric particles the spin momentum-dependent orbital motion tends to be ‘negative’, i.e., in the opposite direction of the spin momentum flow. This arises mainly because of spin–orbit interaction during the scattering process. For the purpose of experimental observation, we suggest the introduction of an auxiliary radially-polarized illumination to adjust the particle’s radial equilibrium position, for the radial gradient force of the circularly-polarized annular focused field tends to constrain the particle at the ring of intensity maximum.

Spin Angular Momentum of Guided Light Induced by Transverse Confinement and Intrinsic Helicity

In recent years, extraordinary spin angular momenta have been investigated in a variety of structured electromagnetic waves, being of especial interest in sub-wavelength evanescent fields. Here we demonstrate analytically that, in planar and cylindrical waveguides supporting transverse electric/magnetic modes, transverse spin density arises inside the waveguide (different from the spin induced in the evanescent region outside the waveguide), carrying indeed longitudinal extraordinary (so-called) Belinfante's spin momentum. Such contribution depends linearly on the mode transverse wave vector, and is thus induced by mode confinement. Cylindrical waveguides support in addition hybrid modes that exhibit a richer phenomenology with not only azimuthal (confinement-related) spin, but also an intrinsic helicity which leads to longitudinal spin density and transverse helicity-dependent spin momentum. Results are indeed presented for configurations relevant to spin-orbit coupling in nanophotonic waveguides and to manipulating optical forces in IR-to-microwave water-filled channels. Thus guided modes intrinsically carrying confinement-induced transverse spin, combined with intrinsic-helicity-induced longitudinal spin (when hybrid), hold promise of superb devices to control spin-orbit interaction and optical forces within confined geometries throughout the electromagnetic spectra.

Artifact-free objective-type multicolor total internal reflection fluorescence microscopy with light-emitting diode light sources-Part I.

Total internal reflection fluorescence excitation (TIRF) microscopy allows the selective observation of fluorescent molecules in immediate proximity to an interface between different refractive indices. Objective-type or prism-less TIRF excitation is typically achieved with laser light sources. We here propose a simple, yet optically advantageous light-emitting diode (LED)-based implementation of objective-type TIRF (LED-TIRF). The proposed LED-TIRF condenser is affordable and easy to set up at any epifluorescence microscope to perform multicolor TIRF and/or combined TIRF-epifluorescence imaging with even illumination of the entire field of view. Electrical control of LED light sources replaces mechanical shutters or optical modulators. LED-TIRF microscopy eliminates safety burdens that are associated with laser sources, offers favorable instrument lifetime and stability without active cooling. The non-coherent light source and the type of projection eliminate interference fringing and local scattering artifacts that are associated with conventional laser-TIRF. Unlike azimuthal spinning laser-TIRF, LED-TIRF does not require synchronization between beam rotation and the camera and can be monitored with either global or rolling shutter cameras. Typical implementations, such as live cell multicolor imaging in TIRF and epifluorescence of imaging of short-lived, localized translocation events of a Ca2+ -sensitive protein kinase C α fusion protein are demonstrated.

Spin dynamics of anisotropic azimuthal modes in heterogeneous magnetic nanodisks

It is well known that azimuthal spin wave modes of magnetic vortex state in permalloy nanodisks have circular symmetry. Intuitively, magnetic materials having magnetocrystalline anisotropy is not compatible with the circular symmetry of the azimuthal modes. In this article, however, we report cubic azimuthal modes in heterogeneous nanodisks consisting of a permalloy core and a Fe shell. The fourfold symmetry of azimuthal modes is due to the exchange, and magneto-static, interactions between the permalloy core and the Fe shell. In comparison to the results of circular azimuthal mode, the vortex switching occurs considerably faster under the excitation of cubic azimuthal mode. The gyration path of vortex core turns into square under the influence of induced cubic anisotropy in the permalloy region. We find out periodic oscillation of the vortex core size and the gyration speed as well. Our findings may offer a new route for spintronic applications using heterogeneous magnetic nanostructures.

Role of transverse momentum dependence of unpolarized parton distribution and fragmentation functions in the analysis of azimuthal spin asymmetries

Information on the Sivers distribution and the Collins fragmentation functions and their transverse momentum dependence is mainly based on fitting single spin asymmetry data from semi-inclusive deep inelastic scattering (SIDIS). Independent information, respectively on the Sivers distribution and the Collins fragmentation, can be obtained from Drell-Yan and $e^+e^-$ annihilation processes. In the SIDIS case, the transverse momentum of the final observed hadron, which is the quantity measured, is generated both by the average transverse momentum in the distribution and in the fragmentation functions. As a consequence, these are strongly correlated and a separate extraction is made difficult. In this paper we investigate, in a simple kinematical Gaussian configuration, this correlation, its role on the transverse single spin asymmetries in SIDIS and the consequences for predictions of the Sivers asymmetry in Drell-Yan processes and for the Collins asymmetry in $e^+e^-$ annihilation. We find that, in some cases, these effects can be relevant and must be carefully taken into account.

Flame dynamics of azimuthal forced spinning and standing modes in an annular combustor

Azimuthal forcing has been applied to flames in a laboratory scale annular combustor in order to accurately control the azimuthal mode of excitation. A new forcing configuration permitted not only the pressure amplitude, but also the spin ratio and mode orientation to be accurately controlled, in order to generate standing modes and for the first time strong spinning modes in both a clockwise (CW) and anti-clockwise (ACW) direction. The phase averaged heat release dynamics of these modes was compared and a number of differences observed depending on the direction of pressure wave propagation, demonstrating characteristic ACW and CW heat release patterns. A new spin compensating averaging method was then introduced to analyse the flame dynamics, and it was shown that through the application of this method the dynamics of standing wave oscillations could be decomposed to recover the characteristic ACW and CW heat release responses. The global heat release response was also assessed during strongly spinning modes, and the magnitude of the response was shown to depend strongly on the direction of propagation, demonstrating the importance of the local swirl direction on the global heat release response, with important implications for the modelling of such flows.

Azimuthal Spin Asymmetries in SIDIS

We have presented the results for the single and double spin asymmetries in semi-inclusive deep inelastic scatterings for proton in a light front quark–diquark model. The asymmetries are found to satisfy certain inequalities. The model wavefunctions are modified to generate complex phase which is essential to produce asymmetries with Sivers and Boer-Mulders functions. The model predictions for Collins, Sivers and Boer-Mulders asymmetries are found to be consistent with data.

Model predictions for azimuthal spin asymmetries for HERMES and COMPASS kinematics

We have presented the results for the single and double spin asymmetries in semi-inclusive deep inelastic scatterings for proton in a light front quark-diquark model. The asymmetries generated by the T-even TMDs are discussed here. The model predictions are found to agree with the available data. We also present our model predictions for the Collins asymmetry for the future electron-ion collider experiments.

Analysis and Experiment of the Compressive Sensing Approach for Duct Mode Detection

This paper describes a new mode detection method based on the compressive sensing approach, which has been developed in information technology to reduce samples required by the classical Nyquist–Shannon sampling theory. A revised approach is proposed here to detect azimuthal spinning modes from aeroengine fan noise by using a microphone array outside the bypass duct. A number of numerical simulations is prepared to examine the associated test performance, such as the reconstruction accuracy, robustness, background noise interference, and coherence impact. The mode sparsity is usually much smaller than the highest order of the modes, so the azimuthal mode is sparse at the frequency of interest to ensure a satisfactory compressive sensing-based mode detection. The current simulations show log sensors shall be generally sufficient rather than sensors required by the Shannon–Nyquist sampling theory. An experimental system is designed and implemented to demonstrate the proposed method. The test system contains a straightened and rigid duct with an enclosed spinning mode synthesizer to emulate fan noise propagation inside and scattering from a bypass duct. An outer sensor array is employed to demonstrate that the compressive sensing-based mode detection method can significantly reduce the required number of sensors, especially for modes of high order, which results in a much simplified sensor array design for aerospace noise tests.

Spin excitation spectrum in a magnetic nanodot with continuous transitions between the vortex, Bloch-type skyrmion, and Néel-type skyrmion states

We study spin-wave excitations in a circular ferromagnetic nanodot in different inhomogeneous, topologically non-trivial magnetization states, specifically, vortex and skyrmion states. Gradual change in the strength of the out-of-plane magnetic anisotropy and the Dzyaloshinskii-Moriya exchange interaction leads to continuous phase transitions between different stable magnetic configurations and allows for mapping of dynamic spin modes in and between the vortex, Bloch-type skyrmion and N\'eel-type skyrmion states. Our study elucidates the connections between gyrotropic modes, azimuthal spin waves and breathing modes in various stable magnetization states and helps to understand the rich spin excitation spectrum on the skyrmion background.

Polarization evolution of vector beams generated by q-plates

The polarization evolution of vector beams (VBs) generated by q-plates is investigated theoretically and experimentally. An analytical model is developed for the VB created by a general quarter-wave q-plate based on vector diffraction theory. It is found that the polarization distribution of VBs varies with position and the value q. In particular, for the incidence of circular polarization, the exit vector vortex beam has polarization states that cover the whole surface of the Poincare sphere, thereby constituting a full Poincare beam. For the incidence of linear polarization, the VB is not cylindrical but specularly symmetric, and exhibits an azimuthal spin splitting. These results are in sharp contrast with those derived by the commonly used model, i.e., regarding the incident light as a plane wave. By implementing q-plates with dielectric metasurfaces, further experiments validate the theoretical results.