Dynamic Polarization Effects Research Articles

Analytical theory of magnetization of fine particles in external alternating field

Nonlinear dynamics of a spherical magnetic particle and an ensemble of such particles in an external oscillating magnetic field have been studied analytically and numerically in terms of the Landau-Lifshitz-Gilbert equation. The exact analytical formulas have been obtained allowing the calculation of the projections of the mean-in-time magnetization of the system on the frequency of an external field and initial orientation of magnetic moments. The behavior of the system in the limiting cases of low and high frequencies has been considered. The analytical solutions obtained agree well with the direct numerical calculations of the Landau-Lifshitz-Gilbert equation. The analytical theory of the effect of the nonlinear dynamic polarization has been developed, and its applicability for designing devises of magnetic memory has been discussed.

Dynamical nuclear polarization and confinement effects in ZnO quantum dots

AbstractThe spatial distribution of the electronic wave function of a shallow donor (SD) in a ZnO semiconductor quantum dots (QD's) has been determined in the regime of quantum confinement by using the nuclear spins as probes. Hyperfine (HF) interactions as monitored by electron nuclear double resonance spectroscopy quantitatively reveal the transition from semiconductor to molecular properties upon reduction of the size of the nanoparticles. Influence of confinement effect on g‐factor value of SD's in ZnO and CdS QD's was displayed. The almost complete dynamic nuclear polarization (DNP) of nuclear spins has been demonstrated can be achieved in ZnO QD's by saturating the EPR transition of the SD present in the QD's with using high‐frequency at low temperatures. Polarization of 67Zn nuclear spins in ZnO core and of 1H nuclear spins in the Zn(OH)2 capping layer have been obtained which manifests itself via the creation of a hole and an antihole in the EPR absorption line of the SD in QD's and a shift of the hole (antihole). The enhancement of the nuclear polarization opens the possibility to study semiconductor nanostructures with NMR techniques

The donut and dynamic polarization effects in proton channeling through carbon nanotubes

We investigate the angular and spatial distributions of protons with an energy of 0.223 MeV after channeling them through an (11, 9) single-wall carbon nanotube of 0.2 μm length. The proton incident angle is varied between 0 and 10 mrad, being close to the critical angle for channeling. We show that, as the proton incident angle increases and approaches the critical angle for channeling, a ring-like structure is developed in the angular distribution—the donut effect. We demonstrate that it is the rainbow effect. If the proton incident angle is between zero and half of the critical angle for channeling, the image force affects considerably the number and positions of the maxima of the angular and spatial distributions. However, if the proton incident angle is close to the critical angle for channeling, its influence on the angular and spatial distributions is considerably decreased. We demonstrate that an increase of the proton incident angle can lead to a significant rearrangement of the propagating protons within the nanotube. This effect may be used to locate atomic impurities in nanotubes as well as for creating nanosized proton beams to be used in materials science, biology and medicine.

Dynamical polarization effect of nuclear spin bath dragged by electron spin resonance in double quantum dot integrated with micro-magnet

We studied on Overhauser shift of electron dipole spin resonance (EDSR) peaks by using a double quantum dot integrated with a micro-magnet. Two EDSR peaks are well resolved, reflecting electron spin flip events at different resonance conditions between two dots, which depend on the in-plane field at the two dots produced by a micro magnet. One of the two peaks is significantly higher than the other and shows a larger Overhauser shift, indicating that an electron spin flip process local to the dot causes dynamical polarization of local nuclear spins to the same dot. After the nuclear spin polarization is saturated, we observed the decay of the Overhauser shift by repeatedly measuring the EDSR peak with a minimum microwave power. The decay time constant is much longer than by other groups. We discuss the possible reason for this difference.

Nonlinear stopping power for ions moving in magnetized two-component plasmas

Energy losses of test particles in magnetized two-component plasmas are investigated by means of particle-in-cell (PIC) simulations, taking into account the dynamic polarization effects of both the plasma ions and electrons. The influences of the magnetic field, the angle between the test particle velocity and magnetic field, and certain plasma parameters on the nonlinear stopping power are studied. Comparisons are made between the PIC simulations and the linearized dielectric theory to show the nonlinear effects on the stopping power. Simulation results show that the dynamic polarization effects of the plasma ions become significant and contribute mainly to the nonlinear stopping power for low particle velocities and strong magnetic field. The nonlinear effects are found to enhance the stopping power in low particle velocity regions.

Dynamic polarization in the Coulomb breakup of loosely bound 17F

Angular distributions of the Coulomb breakup of radioactive 17F were measured by impinging a 10 MeV/nucleon beam on 208Pb and on 58Ni to study the dynamic polarization effects. The breakup products, oxygen and a proton, were detected in coincidence. First-order perturbation theory significantly overpredicts the breakup cross section for the 208Pb target. Dynamical calculations with a dynamic polarization as the leading order correction were performed. The calculations reproduce the data for 17F on 58Ni but overpredict the breakup of 17F on 208Pb by a factor of two at forward angles.

Dynamic polarization and energy dissipation for charged particles moving in magnetized two-component plasmas

Energy losses of test particles in magnetized two-component plasmas are investigated within the framework of the linearized Vlasov-Poisson theory, taking into account the dynamic polarization effects of both the plasma ions and electrons. General expressions of the potential and stopping power are obtained and calculations are performed for protons in a magnetized hydrogen plasma. The influences of the magnetic field, the angle between the proton velocity and magnetic field, and certain plasma parameters on the stopping power are studied. Numerical results show that for low particle velocities and strong magnetic field the dynamic polarization effects of the plasma ions become obvious and contribute mainly to the stopping power.

Channeling of protons through carbon nanotubes embedded in dielectric media

We investigate how the dynamic polarization of the carbon atom valence electrons affectsthe spatial distributions of protons channeled in (11, 9) single-wall carbon nanotubesplaced in vacuum and embedded in various dielectric media. The initial protonspeed is varied between 3 and 8 a.u., corresponding to the energies between 0.223and 1.59 MeV, respectively, while the nanotube length is varied between 0.1 and0.8 µm. The spatial distributions of channeled protons are generated using a computer simulationmethod, which includes the numerical solving of the proton equations of motion in thetransverse plane. We show that the dynamic polarization effect can strongly affect therainbow maxima in the spatial distributions, so as to increase the proton flux at distancesfrom the nanotube wall of the order of a few tenths of a nanometer at the expense of theflux at the nanotube center. While our findings are connected to the possible applicationsof nanosized ion beams created with the nanotubes embedded in various dielectric mediafor biomedical research and in materials modification, they also open prospects of applyingion channeling for detecting and locating atoms and molecules intercalated inside thenanotubes.

Dynamical polarization processes in double quantum dots coupled in series

The polarization processes in a system of double quantum dots coupled in series withelectrodes are studied in the limit of sequential tunnelling. The Coulomb interactions ofaccumulated charges on both quantum dots and the competition between dot–dot anddot–electrode coupling are responsible for the non-monotonic filling of each dot, whichleads to a change of the sign of the polarization as a function of bias voltage. We show thatthis dynamical switching polarization effect is common and observable in various multidotsystems.

Relativistic multichannel treatment of autoionization Rydberg series of 4s2nf(n = 4 − 23) Jπ = (7/2)° for scandium

Based on relativistic multichannel theory, this paper calculates the energy levels of autoionization Rydberg series 4s2nf(n = 4 − 23)Jπ = (7/2)° of scandium at different levels of approximation within the framework of multichannel quantum defect theory. The present results show that the dipole polarizations play an important role. Considering the dynamical dipole polarization effects, this paper finds that the difference between calculated and experimental quantum defects for the 4s2nf(n = 4 − 23)Jπ = (7/2)° series is generally about 0.01–0.03. Furthermore, the reason that 4s216f is obscured in experimental spectra is suggested to be the interaction with the neighbouring resonance state converged to 3d2(1G4)of Sc+.

Dynamic polarization effects on the angular distributions of protons channeled through carbon nanotubes in dielectric media

The best level of ordering and straightening of carbon nanotube arrays is often achieved when they are grown in a dielectric matrix, so such structures present the most suitable candidates for future channeling experiments with carbon nanotubes. Consequently, we investigate here how the dynamic polarization of carbon valence electrons in the presence of various surrounding dielectric media affects the angular distributions of protons channeled through (11,9) single-wall carbon nanotubes. Proton speeds between 3 and 10 a.u., corresponding to energies of 0.223 and 2.49 MeV, are chosen with the nanotube's length varied between 0.1 and $1\text{ }\ensuremath{\mu}\text{m}$. We describe the repulsive interaction between a proton and the nanotube's atoms in a continuum-potential approximation based on the Doyle-Turner potential, whereas the attractive image force on a proton is calculated using a two-dimensional hydrodynamic model for the dynamic response of the nanotube valence electrons, while assigning to the surrounding medium an appropriate (frequency dependent) dielectric function. The angular distributions of channeled protons are generated using a computer simulation method which solves the proton equations of motion in the transverse plane numerically. Our analysis shows that the presence of a dielectric medium can strongly affect both the appearance and positions of maxima in the angular distributions of channeled protons.

Dynamic Lateral Polarization in CdZnTe Under High Flux X-Ray Irradiation

The dynamic lateral polarization and charge steering effect was studied in 2D pixilated CdZnTe monolithic detector arrays designed for high flux X-ray imaging applications. While these detectors have shown the ability to work at 15 times 106 counts s-1 mm-2 and higher count-rates in pulse mode, we observed some detectors that exhibited a dynamic lateral polarization and charge steering effect causing non-uniform spatial response to the radiation field. The dynamic nature of the effect is shown by its flux dependence and reversibility upon changing the X-ray flux without a requirement to turn off the bias voltage. The effect causes the induced charge that would normally move from the cathode towards the anode to instead move laterally causing counts to shift away from a flux boundary. We show that the effect is not related to the physical boundary of the detector but rather related to the boundary of the irradiated area of the device. The dynamic polarization and charge steering effect can be attributed to the limited hole transport in the bulk material causing a buildup of a dynamic space-charge region under the irradiated area. The resulting lateral (perpendicular to the irradiation direction) electric field causes the lateral drift (steering) of the X-ray injected charge clouds. The static version of such lateral steering is often observed for charged structural defects in CdZnTe crystals. The studied 2D CdZnTe monolithic arrays were 16 times16 pixel devices having 0.4 mm times 0.4 mm area pixels on a 0.5 mm pitch and were fabricated using 8.7 mm times 8.7 mm times3.0 mm CdZnTe single crystals grown by the high-pressure electro-dynamic gradient freeze technique. The devices were probe tested in a system consisting of a custom 16 times16 pin probe head, 256 channel read-out electronics utilizing 8-channel fast bipolar ASIC chips, and a computer controlled 120 kVp X-ray source.

Influence of the dynamic polarization effect on the angular distributions of protons channeled in double-wall carbon nanotubes

We investigate the influence of the effect of dynamic polarization of the carbon atom valence electrons on the angular distributions of protons channeled in (6, 4)@(11, 9) double-wall carbon nanotubes. The proton velocities are 3 and 5a.u., corresponding to energies of 0.223 and 0.621MeV, while the nanotube lengths are 0.1–0.2μm. The interaction between a proton and the nanotube atoms is described by the Doyle–Turner potential whereas the image force acting on the proton is calculated by a two-dimensional hydrodynamic model of the dynamic response of the nanotube valence electrons. The angular distributions of channeled protons are generated by a computer simulation method using the numerical solution of the proton equations of motion in the transverse plane. Our analysis shows that the image force induces additional extrema in the proton deflection functions, giving rise to the rainbow maxima in the angular distributions that do not exist when the proton is subject to the atomic force only. These maxima could be used to probe the total interaction potential in proton channeling in nanotubes.

Saturated Ionization of Fullerenes in Intense Laser Fields

We investigate the ionization of icosahedral fullerenes (C20, C60, C80, and C180) in an intense laser pulse using the S-matrix theory. The results obtained are in excellent agreement with the recent observations of unexpectedly high saturation intensities of the Buckminster fullerene and its multiply charged ions. Our analysis strongly suggests that the related phenomenon of suppressed ionization of these complex fullerenes is due to the finite cage size and the "multislit" interference effect between partial waves emitted from the different nuclei rather than to a dynamical multielectron polarization effect.

Dynamic polarization effects in ion channeling through single-wall carbon nanotubes

Ion channeling through a single-wall carbon nanotube is simulated by solving Newton’s equations for ion motion at intermediate energies, under the action of both the surface-atom repulsive forces and the polarization forces due to the dynamic perturbation of the nanotube electrons. The atomic repulsion is described by a continuum potential based on the Thomas-Fermi-Moliere model, whereas the dynamic polarization of the nanotube electrons is described by a two-dimensional hydrodynamic model, giving rise to the transverse dynamic image force and the longitudinal stopping force. In the absence of centrifugal forces, a balance between the image force and the atomic repulsion is found to give rise to ion trajectories which oscillate over peripheral radial regions in the nanotube, provided the ion impact position is not too close to the nanotube wall, the impact angle is sufficiently small, and the incident speed is not too high. Otherwise, the ion is found to oscillate between the nanotube walls, passing over a local maximum of the potential in the center of the nanotube, which results from the image interaction. The full statistical analysis of 10 3 ion trajectories has been made to further demonstrate the actual effect of dynamic polarization on the ion channeling.

Absolute Ionization Rates of Multielectron Transition Metal Atoms in Strong Infrared Laser Fields

We report on nonresonant strong field ionization of the multielectron transition metal atoms V, Nb, Ta, Ni, and Pd. Operating in the adiabatic regime (lambda = 1.5 microm), we quantitatively determined both (i) the first charge state saturation intensities and (ii) the absolute ionization rates for intensities ranging from threshold up to 3 x 10(14) W/cm2. We observed a dramatic suppression of ionization relative to single active electron approximation expectations. We suggest that this derives from dynamic polarization or screening effects within the multielectron atom, stressing a need for many-body theories of strong field ionization.

Dynamic polarization effects in Coulomb excitation

In this paper we discuss the excitation of a quantum harmonic system, such as a particle bound in a harmonic oscillator potential, by means of dipole and quadrupole electric fields, focusing on effects of their interference. We obtain an exact solution to this problem with methods that date back to the ideas of Wigner. We discuss the rich class of dynamic polarization effects that are involved in such excitation processes and which are relevant to stopping theory, and electromagnetic excitation of atomic and nuclear systems.

Dynamic polarization in the Coulomb dissociation of8B

The Coulomb dissociation of ${}^{8}\mathrm{B}$ on high-$Z$ targets can be described by first-order perturbation theory at high beam energies but the far-field approximation, which is commonly used, becomes inaccurate at impact parameters less than $\ensuremath{\sim}25\mathrm{fm}.$ The leading-order correction at lower beam energies is a dynamic polarization effect, which reduces the dissociation probability. The relative significance of the effect scales roughly as $Z/E$ in terms of the target charge Z and beam energy E. The reduction due to a destructive Coulomb-nuclear interference, on the other hand, is rather modest.

Role ofE1−E2interplay in multiphonon Coulomb excitation

In this work we study the problem of a charged particle, bound in a harmonic-oscillator potential, being excited by the Coulomb field from a fast charged projectile. Based on a classical solution to the problem and using the squeezed-state formalism we are able to treat exactly both dipole and quadrupole Coulomb field components. Addressing various transition amplitudes and processes of multiphonon excitation we study different aspects resulting from the interplay between E1 and E2 fields, ranging from classical dynamic polarization effects to questions of quantum interference. We compare exact calculations with approximate methods. Results of this work and the formalism we present can be useful in studies of nuclear reaction physics and in atomic stopping theory.

Higher-order effects in the two-body breakup of 17F

The two-body breakup 17 F→ 16 O+ p on Ni and Pb targets at 10–40 MeV/u is calculated by solving the time-dependent Schrödinger equation numerically. Both Coulomb and the nuclear potentials are important in these reactions, but the Coulomb contribution cannot be calculated reliably in first-order perturbation theory. The most dramatic feature is a dynamic polarization effect, which reduces the Coulomb dissociation probability compared to first-order perturbation theory. Over most of the energy range, the correction is proportional to Z 3, the cube of the target charge, and the relative importance is nearly inversely proportional to the beam energy. At the lowest energies in the range considered, higher-order effects (⩾ Z 3) become significant as well. At the impact parameters where nuclear induced breakup is dominant, the diffraction dissociation is reduced whereas stripping is enhanced compared to the eikonal approximation. Here too the deviations decrease inversely with beam energy over most of the range considered.