Dipolar Dynamics Research Articles

Spin-torque-induced rotational dynamics of a magnetic vortex dipole

We study, both experimentally and by numerical modeling, the magnetic dynamics that can be excited in a magnetic thin-film nanopillar device using the spin torque from a spatially localized current injected via a tens-of-nanometer-diameter aperture. The current-driven magnetic dynamics can produce large-amplitude microwave emission at zero magnetic field, with a frequency well below that of the uniform ferromagnetic resonance mode. Micromagnetic simulations indicate that the physical origin of this efficient microwave nano-oscillator is the nucleation and subsequent steady-state rotational dynamics of a magnetic vortex dipole driven by the localized spin torque. The results show that this implementation of a spintronic nano-oscillator is a promising candidate for microwave technology applications.

Dipolar Solvation Dynamics in Room Temperature Ionic Liquids: An Effective Medium Calculation Using Dielectric Relaxation Data

Solvation dynamics in four imidazolium cation based room temperature ionic liquids (RTIL) have been calculated by using the recently measured dielectric relaxation data [ J. Phys. Chem. B 2008, 112, 4854 ] as an input in a molecular hydrodynamic theory developed earlier for studying solvation energy relaxation in polar solvents. Coumarin 153 (C153), 4-aminophthalimide (4-AP), and trans-4-dimethylamino-4'-cyanostilbene (DCS) have been used as probe molecules for this purpose. The medium response to a laser-excited probe molecule in an ionic liquid is approximated by that in an effective dipolar medium. The calculated decays of the solvent response function for these RTILs have been found to be biphasic and the decay time constants agree well with the available experimental and computer simulation results. Also, no probe dependence has been found for the average solvation times in these ionic liquids. In addition, dipolar solvation dynamics have been predicted for two other RTILs for which experimental results are not available yet. These predictions should be tested against experiments and/or simulation studies.

Dielectric and conductivity relaxation in mixtures of glycerol with LiCl

We report a thorough dielectric characterization of the alpha relaxation of glass-forming glycerol with varying additions of LiCl. Nine salt concentrations from 0.1 to 20mol% are investigated in a frequency range of 20Hz-3GHz and analyzed in the dielectric loss and modulus representation. Information on the dc conductivity, the dielectric relaxation time (from the loss) and the conductivity relaxation time (from the modulus) is provided. Overall, with increasing ion concentration, a transition from reorientationally to translationally dominated behavior is observed and the translational ion dynamics and the dipolar reorientational dynamics become successively coupled. This gives rise to the prospect that, by adding ions to dipolar glass formers, dielectric spectroscopy may directly couple to the translational degrees of freedom determining the glass transition, even in frequency regimes where usually strong decoupling is observed.

Accelerated 3-D Numerical Scheme for the Evaluation of Hysteresis in Ferromagnetic Grains

The magnetic behavior of iron based ferromagnetic materials is important for the improvement of performances of electromagnetic devices. This paper deals with an accelerated 3-D model that computes the magnetic behavior of an iron grain under a varying applied field starting from its microscopic material parameters. The dynamics of the magnetic dipoles M in each space point of the grain is computed by integrating the Landau-Lifshitz equation using a semi-analytical predictor-corrector time stepping scheme. In the numerical computations, the spatial discretization is obtained by finite-different techniques. Evaluation of the magnetostatic field is done using fast Fourier transforms. This is the most time consuming part of the computations and in this paper we will demonstrate that this can be accelerated by computing the magnetostatic field on a higher discretization level. The values of the magnetostatic field in each finite-difference cell are determined through interpolation, still preserving an excellent precision.

Exploring a free energy landscape by means of multidimensional infrared and terahertz spectroscopies

A model for the dipolar crystal system is employed to explore a role of free energy landscape (FEL), in which dipolar molecules are posted on two-dimensional lattice sites with two-state libratinal dynamics. All dipole-dipole interactions are included to have frustrated interactions among the dipoles. For the regular and distorted lattice cases, the FEL is calculated from the interaction energies and the total polarizations for all possible dipolar states at various temperatures. At high temperatures, the shape of the calculated FEL is smooth and parabolic, while it becomes bumpy at low temperatures exhibiting multiple local minima. To study dynamical aspects of the system, the single flip dynamics and the single-double mixed flips dynamics of dipoles are examined from a master equation approach. As the observables of linear absorption and two-dimensional (2D) infrared, far infrared, and terahertz spectroscopies, the first- and third-order response functions of polarization are calculated for different physical conditions characterized by the FEL. While the linear absorption signals decay in time in a similar manner regardless of the FEL profiles, the 2D signals exhibit prominent differences for those profiles. This indicates that we may differentiate the FEL profiles by changing two-time valuables in 2D spectroscopy. As illustrated in the single-double flips case, the FEL study by means of 2D spectroscopy, however, relies on the dynamics which is set independently from the FEL. The Smoluchowski equation is applied to examine the description of the collective dynamics on the microscopically calculated FEL. We found that the one-dimensional and 2D signals calculated from the Smoluchowski equation agree with those from master equation only at temperatures where the FEL becomes parabolic shape.

A Single-Dipole Model of Surface Relief Grating Formation on Azobenzene Polymer Films

A new model that keeps track of the dynamics of single dipoles is suggested for the photoinduced formation of a surface relief grating on azobenzene polymer films by two optical fields. Interfering optical fields provide a single dipole of an azobenzene molecule with two dynamical resources: rotation of the molecule caused by the torque, leading to an induced dipole, and its subsequent migration because of the electric force. Explicit development of the induced dipole of the molecule and its real-time migration, which depend on the details of the optical fields such as polarization and wavelength, can be understood self-consistently within the model.

Soft-to-hard mode transition in the dynamical dipole mode induced in dissipative heavy-ion collisions

The collective dipole dynamics evolving in intermediate dinuclear systems with exotic shape and charge distributions formed in charge asymmetric fusion entrance channels are systematically studied by solving a microscopic transport Boltzmann-Nordheim-Vlasov (BNV) equation and analyzing the obtained dipole signal. The BNV analysis shows a typical transition in the dipole oscillation frequency related to a transition from soft-to-hard mode in the dynamical dipole mode. The relative pre-equilibrium {gamma}-ray emission probability is also evaluated by using the classical bremsstrahlung approach. In particular, by using this approach an analytical formula that gives the time-dependent {gamma} rate during the equilibration of the collective mode versus the statistical GDR is extrapolated. The results are in agreement with other models and the few existing experimental data.

Structure and dynamics of water at a clay surface from molecular dynamics simulation

We report a molecular dynamics study of the structure and dynamics of water at a clay surface. The negative charge of the surface and the presence of surface oxygen atoms perturbs water over two to three molecular layers, while the nature of the counterions (Na(+)or Cs(+)) has only a small effect. In the first molecular layer, approximately half of the water molecules are H-bonded to the surface. We also analyze the H-bond network between surface water molecules. The diffusion of water molecules along the surface is slowed down compared to the bulk case. As far as the orientational order and dynamics of the water dipole are concerned, only the component normal to the clay surface is perturbed. We investigate the surface H-bond formation and dissociation dynamics and their coupling to the release of molecules from the first molecular layer. We introduce a simple kinetic model in the spirit of Luzar and Chandler [Nature, 1996, 379, 55] to allow for a comparison with bulk water dynamics. This model semi-quantitatively reproduces the molecular simulation results and suggests that H-bond formation is faster with the surface than in the bulk, while H-bond dissociation is slower.

Enhanced ordered structure and relaxor behaviour of0.98Pb(Mg1/3Nb2/3)O3–0.02La(Mg2/3Nb1/3)O3 single crystals

High quality Pb(Mg1/3Nb2/3)O3 (PMN) and La-doped PMN (PLMN) single crystals were grown via a top-seededsolution growth method. At a doping level as low as 2 at.% of La, the 1:1 B-sitecation (chemical) order was significantly enhanced as revealed by the presenceof superlattice peaks in the x-ray diffraction pattern and comparatively large(∼100 nm) chemically ordered regions (CORs) in the transmission electron microscopy dark field images.The average chemical order parameter was calculated (from the intensity of superlattice peaks) tobe S≈0.44. Besides, x-ray diffraction revealed the presence of a low-symmetry (presumablyrhombohedral) phase. The effect of La doping on the dielectric relaxation was notsubstantial. Though the magnitude of the dielectric constant peak in PLMN was reducedby half as compared to pure PMN, the peak temperature and diffuseness as well as theshape of the dielectric relaxation spectra remained almost unchanged. The analysis of thetemperature dependence of the characteristic time for the main relaxation process in termsof the Vögel–Fulcher law revealed the freezing of dipole dynamics at temperatureTf = 192 K, which was onlyslightly lower than the Tf = 213 K in PMN. To reconcile the structural and dielectric data we suggest that therhombohedral phase forms inside CORs while the chemically disordered matrix remainscubic and is populated by dynamic polar nanoregions that give rise to the characteristicdielectric response.

Impossibility of pressure-induced crossover from ferroelectric to nonergodic relaxor state in aPb(Mg1∕3Nb2∕3)0.7Ti0.3O3crystal: Dielectric spectroscopic study

Relaxor behavior induced by hydrostatic pressure up to $0.95\phantom{\rule{0.3em}{0ex}}\mathrm{GPa}$ in the $\mathrm{Pb}({\mathrm{Mg}}_{1∕3}{\mathrm{Nb}}_{2∕3}{)}_{0.7}{\mathrm{Ti}}_{0.3}{\mathrm{O}}_{3}$ (PMN-30PT) ferroelectric crystal was studied using dielectric spectroscopy. With increasing pressure we observed the decrease of the ferroelectric phase transition temperature $({T}_{C})$, the suppression and smearing of the dielectric anomaly at ${T}_{C}$, and the appearance of strong relaxorlike dielectric dispersion below the temperature of the permittivity maximum $({T}_{m})$. Such kinds of pressure-induced alteration are inherent in compositionally disordered perovskite ferroelectrics. It is usually believed to signify a crossover from the ferroelectric ground state to the nonergodic relaxor ground state in which the dipole moments of polar nanoregions (PNRs) are frozen in a way characteristic of dipole glasses. Surprisingly, our analysis of the dielectric spectra in PMN-30PT at high pressure did not reveal any glassy freezing of dipole dynamics. This means that the nature of the high-pressure-induced ground state is different from the nonergodic relaxor state observed in canonical relaxors at ambient pressure. At $T>{T}_{C}$ the dielectric spectra measured in PMN-30PT under different pressures are qualitatively similar. They are composed of two contributions that follow the Kohlrausch-Williams-Watts (KWW) and the Curie--von Schweidler (CS) relaxation patterns, respectively. The dielectric susceptibility related to the KWW relaxation provides the major contribution to the total dielectric constant. The shapes of the frequency and temperature dependences of this susceptibility remain practically unaffected by pressure. Contrary to the canonical relaxors the KWW relaxation time does not obey the Vogel-Fulcher law. On the other hand the CS-related susceptibility, which is significant only at low frequencies, considerably increases with increasing pressure and the shapes of its frequency and temperature dependences change radically. At $T<{T}_{C}$ the KWW and CS relaxation processes are not observed at ambient pressure, but persist at $0.8\phantom{\rule{0.3em}{0ex}}\mathrm{GPa}$. The KWW characteristic relaxation time varies with temperature according to the Arrhenius law. We propose that the observed variation of properties results from the pressure-induced crossover from the sharp order-disorder-type ferroelectric phase transition, which is triggered by the cooperative interactions among dynamic (in the high-temperature phase) PNRs to the diffuse displacive-type ferroelectric transition, which is related to the growth of PNR dimensions.

The physical mechanisms of relaxation times distribution in inorganic and organic disordered ferroelectrics

We discuss the physical background for relaxation times distribution in organic and inorganic disordered ferroelectrics. We show that such background is a disorder which is believed to be a reason for relaxor behavior. Due to this disorder the internal electric and elastic fields are of a random nature being characterized by certain distribution function. Having the distribution function f(E) of random field E, we can obtain the distribution function of relaxation times F(τ) with the help of relation between τ and E. Latter relation can be obtained considering the dynamics of a single electric dipole in a random field created by the ensemble of defects and impurities in a sample. We show that in our approach the known empirical relaxation laws like Cole–Cole, Davidson–Cole, Havriliak–Negami etc correspond to different concentrations and kind of defects in the above substances. The character of interaction between them also plays an important role. The comparison of our theory with experiment shows that our approach captures the main relaxation properties of disordered ferroelectrics. Our formalism can be easily generalized on the other nonferroelectric disordered substances, both organic and inorganic.

Phase transition in the conductive and dielectric regimes of liquid crystals

A phase transition is the transformation of a thermodynamic system from one phase to another. The distinguishing characteristic of a phase transition is an abrupt sudden change in one or more physical properties. Typically the phase transition is driven by a change in the temperature of the system. However, change of state may be executed by a change in an intensive variable like, for instance, the voltage. For liquid crystals in a sandwich cell the two dynamics can be executed. In the conduction regime at low excitation frequencies, one observes the director and flow fields changes (pattern dynamics). In the high-frequency, dielectric regime, we investigate the molecular dipole dynamics. So, in the conductive regime (detected via electroconvection measurements) the phase transition can be executed by voltage while in the dielectric regime (detected via dielectric spectroscopy) by temperature. The Low Frequency Dielectric Spectroscopy diagram can be used as a classical thermogram. In planarly and homeotropically oriented liquid crystal cells electroconvection experiment results in the morphological phase diagram. In this article we demonstrate that for certain excitation (ac voltage and/or temperature) the liquid crystal film system's dynamic response can by interpreted in terms of the phase transition.

Emergence from nonrelativistic strong-field rescattering to ultrastrong-field laser-atom physics: A semiclassical analysis

We discuss a rescattering deflection parameter to gauge the progression from strong- to ultrastrong-field rescattering dynamics. Our analytical and numerical treatments of rescattering quantify the change from nonrelativistic, dipole approximation strong-field dynamics to an ultrastrong-field interaction where $v∕c\ensuremath{\ne}0$ and ${\mathbf{B}}_{\text{laser}}\ensuremath{\ne}0$. At the threshold between these two regimes the Lorentz deflection reduces the returning electron flux at the core and suppresses rescattering related phenomena. Both radiation and nonsequential ionization from rescattering are studied. We show the ultrastrong-field regime can occur at intensities as low as ${10}^{14}\phantom{\rule{0.3em}{0ex}}\mathrm{W}∕{\mathrm{cm}}^{2}$ for mid-IR $(2\phantom{\rule{0.3em}{0ex}}\ensuremath{\mu}\mathrm{m}\phantom{\rule{0.3em}{0ex}}\text{to}\phantom{\rule{0.3em}{0ex}}5\phantom{\rule{0.3em}{0ex}}\ensuremath{\mu}\mathrm{m})$ wavelengths or as high as ${10}^{18}\phantom{\rule{0.3em}{0ex}}\mathrm{W}∕{\mathrm{cm}}^{2}$ for UV radiation.

Turbulence modelling of statistically periodic flows: Synthetic jet into quiescent air

Computations of a 2D synthetic jet are performed with usual RANS equations solved in time-accurate mode (URANS), with the standard k– ε model and the Rotta + IP second moment closure. The purpose of the present work is to investigate the ability of these standard turbulence models to close the phase-averaged Navier–Stokes equations. Results are compared with recent experiments by Yao et al. made available to the CFD Validation of Synthetic Jets and Turbulent Separation Control workshop held in Williamsburg in 2004. Comparisons of the performance of the models with experimental data show that the evolution of the vortex dipole generated by inviscid mechanisms is not correctly reproduced by the k– ε model. The Reynolds-stress model gives much more realistic predictions. However, several characteristics are not well predicted, as for instance the convection velocity. A detailed analysis shows that the vortex dipole dynamics is essentially inviscid during the early blowing phase, when the flow is more transitional than fully turbulent. Turbulence develops and influences the dynamics of the vortices only at a later stage of the blowing phase. Consequently, it is of importance that the turbulence models do not predict erroneously high levels of turbulence. In particular, the present study shows that the correct prediction of the region of negative production that appears during the deceleration of the blowing velocity, due to the misalignment of the strain and anisotropy tensors, is crucial. Therefore, linear eddy-viscosity models must be discarded for this type of pulsed flows, in particular for flow control using synthetic jets.

Polar Nano-Clusters in Relaxors on the Atomic Level*

207Pb, 93Nb and 45Sc quadrupole pertubed NMR spectra in PMN and PSN relaxor single crystals show that a significant fraction of the polar clusters are static and randomly oriented on the NMR time scale 10−4s even far above the freezing temperature. At the freezing temperature Tf the volume fraction of anisotropic static polar clusters increases and reaches the percolation threshold resulting in the formation of macroscopic ferroelectric domains. The clusters become oriented parallel to the polar ferroelectric direction and an orientational percolation transition takes place in the field cooled case in PMN and even in the zero field cooled case in PSN. The intra—cluster reorientational dynamics of the individual ionic dipoles still persists below Tf though it is biased due to random fields and chemical disorder. * Originally presented at IMF-11, Foz do Iguaçu, Brasil, September 2005.

Electrical polarization and charge dynamics of cellular polypropylene ferroelectret films

In this work, the polarization of the ferroelectret was first estimated based on the experimental results of charging current by a corona charging with a grid in combination with the reverse-polarity corona charging compensation method. The experimental results indicated that the polarization drastically increases with the rise of grid voltage accompanying the breakdown of gas in the void. Also, by means of the charging method mentioned above and analyzing the thermally stimulated discharging spectra, the charge dynamics of macroscope dipoles with space charges and their compensation charges near the free surface of the film during thermal depolarization were discussed. A geometric distribution of trap levels trapping two kinds of charges was discovered, that is, the space charges formed in the macroscopic dipoles were trapped in shallow- and deep- energy level traps respectively, while the space charges located near the free surface of the film were trapped in medium energy level traps.

A theory for cell microtubule wall in external field and pseudo-spin wave excitation

The cytoskeleton of eukaryotic cells contains networks of protein polymers called microtubules (MTs), which provide a wide range of microskeletal and micromuscular functionalities. Evidences from a number of directions suggest that they can also serve as a medium for intracellular signal processing. For the inherent symmetry structures and the electric properties of the microtubule (MT), we treat the MT wall as a one-dimension ferroelectric system and describe the nonlinear dynamics of the dimer electric dipoles with the double-well potential, and then map the physical problems onto the pseudo-spin system. By using the random phase approximation, the effect from the external electric field has been taken into account. We have developed an expression for the Hamiltonian in the pseudo-spin system and obtained the coupled motion equations for the disturbed pseudo-spin wave, and there appears to be a transverse collective excitation with an intrinsic frequency.

Neutron tagging of quasielastic [formula omitted] photoproduction off nucleus in ultraperipheral heavy ion collisions at RHIC energies

We compare the coherent and quasielastic J/ψ photoproduction in the peripheral heavy ion collisions in kinematics of Relativistic Heavy Ion Collider (RHIC) at sNN=200 GeV. Our improved estimate of the total coherent cross section is a factor of two smaller than the earlier ones. We find that the counting rate of quasielastic J/ψ photoproduction tagged by neutrons emitted due to cascading of the recoiled nucleon within the residual nucleus exceeds the rate of the coherent events. We argue that measurements of this process can be used to learn about the dynamics of color dipole–nucleon interactions in nuclei in the wide effective range of energies of γA interactions.

BOUND STATES IN THE DYNAMICS OF A DIPOLE IN THE PRESENCE OF A CONICAL DEFECT

In this work we investigate the quantum dynamics of an electric dipole in a (2+1)-dimensional conical spacetime. For specific conditions, the Schrödinger equation is solved and bound states are found with the energy spectrum and eigenfunctions determined. We find that the bound states spectrum extends from minus infinity to zero with a point of accumulation at zero. This unphysical result is fixed when a finite radius for the defect is introduced.

Pseudo-spin model for the microtubule wall in external field

Microtubules (MTs) in the cytoskeletons of eukaryotic cells provide a wide range of microskeletal and micromuscular functionalities. Some evidence has indicated that they can serve as a medium for intracellular signaling processing. In this paper, for the inherent symmetry structures and the electric properties of tubulin dimers, the microtubule (MT) is treated as a one-dimensional ferroelectric system. The nonlinear dynamics of the dimer electric dipoles is described by virtue of the double-well potential and the physical problem is further mapped onto the pseudo-spin system. In addition, the effect of the external electric field on the MT has been taken into account.