Polarization Current Effects Research Articles

Effects of electric field and light on resistivity switching of Eu0.7Sr0.3MnO3 thin films.

Based on the excellent piezoelectric properties of 0.7Pb(Mg1/3Nb2/3)O3-0.3PbTiO3 (PMN-PT) single crystals, a hole-doped manganite film/PMN-PT heterostructure has been constructed to achieve electric-field and light co-control of physical properties. Here, we report the resistivity switching behavior of Eu0.7Sr0.3MnO3/PMN-PT(111) multiferroic heterostructures under different in-plane reading currents, temperatures, light stimuli and electric fields, and discuss the underlying coupling mechanisms of resistivity change. The transition from the electric-field induced lattice strain effect to polarization current effect can be controlled effectively by decreasing the in-plane reading current at room temperature. With the decrease of temperature, the interfacial charge effect dominates over the lattice strain effect due to the reduced charge carrier density. In addition, light stimulus can lead to the delocalization of eg carriers, and thus enhance the lattice strain effect and suppress the interfacial charge effect. This work helps to understand essential physics of magnetoelectric coupling and also provides a potential method to realize energy-efficient multi-field control of manganite thin films.

Polarization current effect, strain effect and ferroelectric field effect on electrical transport properties of Eu0.7Sr0.3MnO3/PMN-PT multiferroic heterostructure

Abstract By constructing Eu0.7Sr0.3MnO3 thin films/0.7Pb(Mg1/3Nb2/3)O3-0.3PbTiO3 (001) multiferroic heterostructures, the electrical transport properties of the Eu0.7Sr0.3MnO3 thin film under electro-photo dual control is studied. At room temperature, as the in-plane reading current increases from 1 μA to 100 μA, the polarization current effect gradually weakens, while the strain effect becomes more evident. As the temperature decreases from 300 K to 240 K, the ferroelectric field effect is observed to strengthen progressively. In addition, by applying light illumination, the ferroelectric field effect observed at 240 K can be suppressed, which proves that the light field can obviously control the tunable mechanism of electric field. Our results demonstrate that the temperature and light field play an important role in the switching between the electric field induced multiple tunable effects, such as polarization current effect, lattice strain effect and ferroelectric field effect. Meanwhile, the sensitivity of the Eu0.7Sr0.3MnO3 film to lattice strain enhances its application in tunable electronics.

Analysis of the polarization effects in the gyrokinetic theory of magnetized plasmas.

By adopting the hybrid coordinates, in which the nonlinearity of polarization displacement is included in the configuration space variables compared to the conventional gyrocenter coordinates, the polarization effects are analyzed by using the modern gyrokinetic (GK) theory of magnetized plasmas. Based on the invariant property, the velocity transformation between the gyrocenter and hybrid coordinates is calculated, and the phase-space velocity in terms of the hybrid coordinates is obtained. The linear and nonlinear polarization distribution functions are defined, and the evolutions for the polarization distribution functions are derived. It is well known that the polarization density is important in the GK calculation of particle density. Analogously, it is shown that the polarization current should be considered in the GK calculation of current density. In the case with electrostatic fluctuations, the roles of the polarization current are illustrated in the derivations of the Hasegawa-Mima equation and the dispersion relation for geodesic acoustic mode. In the case with magnetic fluctuations, the procedure for the GK calculation of perpendicular current is clarified, the dispersion relation for compressional Alfvén wave is derived, in which the effect of polarization current is discussed.

Electric field tuning resistance switching behavior of SrRuO3/Pb(Mg1/3Nb2/3)O3–PbTiO3 heterostructures at various temperatures

The resistance switching behavior induced by in-plane read current in SrRuO3/Pb(Mg1/3Nb2/3)O3–PbTiO3 heterostructures is investigated at different temperatures. With decreasing in-plane read current from 10 mA to 0.01 mA, the symmetrical butterfly-like shape of resistance is gradually converted to an antisymmetrical shape at different temperatures, which is resulted from the enhancement of polarization current effect. Specifically, non-volatile resistance behaviors induced by asymmetric bipolar sweeping of electric field and pulsed electric field are achieved at different temperatures. Our results suggests resistance switching behavior dependence of in-plane read current, which is crucial for further application of complex oxide magnetoelectric and spintronic devices.

Polarization-induced resistive switching behaviors in complex oxide heterostructures

Complex oxide heterostructures are fabricated by growing La0.67Ca0.33MnO3 films on ferroelectric 0.71Pb(Mg1/3Nb2/3)O3-0.29PbTiO3 (011) single-crystal substrates. The nonvolatile or pulsed resistive switching behaviors induced by an electric field are achieved simultaneously. Further analyses indicate that the different resistive switching behaviors are resulted from co-control of piezostrain and polarization current effects. With decreasing in-plane read current from 0.1 mA to 0.001 mA, the polarization current effect gradually begins to play a more important role than the piezostrain effect. Consequently, the nonvolatile resistive switching behavior is converted to pulse resistive switching behavior. The results further enhance the application of complex oxides in multifunctional memory devices.

Effect of Finite-Ion-Banana-Width on the Polarization Contribution to the Neoclassical Tearing Modes Evolution

In the previous analytical description of the neoclassical polarization current effect on the neoclassical tearing modes (NTMs), it is usually assumed that the magnetic island is much larger than the finite-ion-banana-width (FBW). This assumption is questionable when the experimentally observed seed island width of the NTMs is comparable to the FBW. We introduce a simple and direct theoretical method to investigate the FBW effect on the neoclassical polarization contribution to the NTM evolution in collisional plasmas. The results show that, the FBW effect can strongly modify the neoclassical polarization current profile near the island separatrix, and thus weaken its probably stabilizing effect on the NTMs.

Symmetry breaking and self-consistent rotation of magnetic islands in neoclassical viscous regimes

Classical or neoclassical tearing modes (NTMs) perturb the ideal axisymmetry of tokamaks. As a consequence of symmetry breaking a neoclassical toroidal viscosity (NTV) appears, that depends on the island amplitude. This work shows that in the low collisionality regimes NTV has a key role in determining self-consistently the magnetic island velocity and at the same time modifies significantly the ion polarization current effects on NTM instability. This finding can provide a better understanding of the mechanism of onset of NTMs, observed experimentally, and improve the concepts for their control or avoidance.

Mechanism of neoclassical tearing modes triggering by a magnetic perturbation

A new mechanism whereby Neoclassical Tearing Modes (NTMs) can be triggered through toroidal mode coupling to a magnetic perturbation is proposed. The physical picture is the presence of a relatively small “pre-NTM” magnetic island whose frequency is modified by the perturbation, changing polarization current effects from stabilizing to destabilizing.

Parametric study of two-dimensional potential structures induced by radio-frequency sheaths coupled with transverse currents in front of the Ion Cyclotron Resonance Heating antenna

For the first time, a two-dimensional (2D) fluid model and an analytical expression for the rectified potential with respect to the transverse polarization current are established and verified by a 2D PIC (particle in cell) code over the validity domain of our model. Then the model is extended to the overall ion cyclotron frequency range used in different heating and current drive scenarios. First, the models demonstrate that these transverse polarization currents add some inertia in the temporal dynamic. Due to the nonlinear behavior of the I-V sheath characteristic, the time average amplitude (dc potential) of the rectified potential structure is increased compared to the time average rf potential. Second, they induce only a slight widening of the potential structure. Such modifications are quantified using a “test map” initially characterized by a Gaussian shape. The map is assumed to remain Gaussian near its summit. The time behavior of the peak can be estimated analytically in the presence of polarization current as a function of its width r0 and amplitude ϕ0 (normalized to local temperature and to a characteristic length for transverse transport). A potential peaking criterion has been built to determine the peaking zone of the dc potential structure induced by the rf field. Computations made for typical parameters of the edge plasma in front of the antenna of the Tokamak Tore Supra show that the dc rectified potential is up to 50% higher than the previous computations neglecting polarization current effects. The weak diffused and high dc potential structures computed can explain the hot spot formation induced by convective cells associated to high energetic ion fluxes on the corners of the Ion Cyclotron Resonance Heating antenna.

Role of anomalous transport in onset and evolution of neoclassical tearing modes

A key role in the evolution of the neoclassical tearing modes (NTMs) belongs to the radial profiles of the perturbed plasma flow, temperature and density which are determined by the conjunction of the longitudinal and cross-field transport arising from thermal conduction, particle diffusion and viscosity. In a tokamak, the perpendicular transport of particles, heat and momentum is typically anomalous. In this paper the results of theoretical studies on the influence of anomalous perpendicular heat transport and anomalous ion perpendicular viscosity on early stages of NTM evolution are presented. Several parallel transport mechanisms competitive with anomalous cross-island heat transport in the formation of the perturbed electron and ion temperature profiles within the island are considered. The perturbed electron temperature profile is established in competition between anomalous perpendicular electron heat conductivity and parallel electron heat convection. The formation of the ion perturbed temperature profile was found to be dependent on the island rotation frequency. The perpendicular ion heat conductivity is balanced by the parallel transport associated with the ion inertia for an island rotating with subsonic frequency or with island rotation with respect to the plasma for supersonic islands. The partial contributions from the plasma electron and ion temperature perturbations in the bootstrap drive of the mode and magnetic curvature effect were taken into account in construction of a generalized transport threshold model of NTMs. This model gives more favourable predictions for NTM stability and qualitatively modifies the scaling law for βonset. The anomalous perpendicular ion viscosity is shown to modify the collisionality dependence of the polarization current effect, reducing it to the low collisionality limit. In its turn a viscous contribution to the bootstrap drive of NTMs is found to be of the same order as a conventional bootstrap drive for the islands of width close to the characteristic one of the transport threshold model. A viscous contribution to the perturbed bootstrap current is destabilizing for the island rotating in the ion diamagnetic drift direction. In this case, an alternative threshold mechanism should be considered.

Theory of magnetic islands in tokamaks with accenting neoclassicaltearing modes

AbstractKey aspects of existing theory of magnetic islands in tokamaks are overviewed. The threshold models of neoclassical tearing modes (NTMs) are discussed. It is found that though the NTM theory is often used for interpreting MHD data observed in experimental devices and for designing next‐step machines, the existing NTM theory is imperfect and the use of it is questionable. The main reason for such conclusion is a tangle of problems related to polarization current effect. Analytical approaches in the magnetic island theory are presented.

Electron drift effects on magnetic islands

Electron drift effects on magnetic islands in a hot-electron plasma are studied. The slab approximation is used. The electron temperature gradient is neglected. The authors analyze the properties of the electrostatic potential profile function and the density profile function in the presence of the electron drift effects and the problem of regularization of these profile functions near the island separatrix. It is emphasized that the electron drift frequency depends on the island magnetic flux and, as a result of this dependence, the resonant interaction of magnetic islands with the electron drift waves is possible. It is shown that, allowing for the finiteness of the “effective” ion Larmor radius ρs (the ion sound radius), the polarization current effect proves to be dependent on the electron drift effects.

Possible resolution of the “main intrigue” of the neoclassical tearing mode theory

The dilemma is discussed whether the polarization current effect on the neoclassical tearing modes is stabilizing or destabilizing. It is suggested that, if the island rotation frequency is formed due to the perpendicular viscosity and if one uses the correct expression for the viscosity allowing for the heat flux derivatives and takes into account the perpendicular heat flux in the ion heat balance equation, this effect should be stabilizing.

Analysis of the ac Current Properties of a PAN-Based Carbon-Fiber/ Polycarbonate Composite

Some electrical properties of a polycarbonate/PAN-based carbon-fiber composite are reported. Impedance measurements performed at the frequency range of 1Hz-100 kHz on various composite specimens showed frequency, filler concentration, and temperature dependence. The observed results are similar to those reported on most dielectric solid materials. The complex dielectric constant determined from the impedance data analysis yields the dielectric constant, which shows strong dependence on the fiber concentration. The real part varies by 1/f at frequencies below 300 Hz, indicating that the mterfacial polarization dominates and enhances the dielectnc constant at low frequencies. The loss factor shows some oscillatory behavior, which indicates that different relaxation processes operate in the composite bulk. The loss factor increases with increasing filler concentration as a result of enhancement of the electrical conductivity. The observed decrease in the impedance with temperature may be attributed to tunnelling and polariza tion current effects.

Electrophysiological and ultrastructural characterization of the cornea during in vitro perfusion

Functional and ultrastructural changes in rabbit cornea during in vitro perfusion were studied by means of electrical measurements and electron microscopy. Electrical measurements were used to provide information on membrane potential, resistance, and capacitance. Membrane potential reflects cellular metabolic activity and thus is used to monitor tissue viability. On the other hand, since electrical resistance and capacitance indicate charge flow and the ability of the membrane dielectric to store charges, they can be used to detect structural changes in the aqueous transport pathway, i.e., intercellular space, and membrane surface, respectively. Studies on the current-voltage relationship of the cornea indicate self-generated membrane potential and non-ohmic resistive behavior of the cornea. However, the electrical resistance remains constant and is invariant to the external applied voltage. Problems associated with the use of ohmic resistance as an indication for membrane resistance are discussed. During in vitro perfusion, the cornea undergoes a biphasic change in resistivity with an initial rise and subsequent fall. The initial rise of the resistance appears to be induced by a change in membrane potential while the declining portion, at longer time periods, correlates well with the change in tissue morphology, i.e., expansion of the aqueous intercellular spaces. With the exception of this structural change, the overall integrity of the cornea is preserved and its viability is maintained for at least 6 h in vitro. The effect of polarization current on the d.c. electrical resistance was investigated by means of discharging kinetic measurements. Results indicate that under direct current, the cornea, due to its capacitive property, undergoes rapid, exponential-like, charging and discharging with an average time constant of approx. 50 μs. The polarization current, however, was found to have no significant effect on determination of membrane resistance. In addition to the kinetic study, the capacitive property of the cornea was further investigated by phase difference and impedance measurements. The use of a range of frequencies of measuring current results in a model of the cornea as electrically equivalent to a parallel RC circuit. Unlike resistance, the capacitance of the cornea remains remarkably constant during in vitro perfusion. This finding is in agreement with electron microscopic studies of the tissue. The use of electrical methods to study membrane transport properties as well as to assess tissue viability and integrity is discussed.

Magnetostatic modes in a sphere and polarization current corrections

Resonance frequencies of magnetostatic modes are obtained for a ferrite sphere. Magnetic field and magnetization are obtained in the same approximation ( rot h = 0 ). Polarization current effects are then taken into account for modes of the type ( n, n, 0) and ( n, n − 1, 0) by a perturbation method correct to the order ( kR) 2. The frequency shift at resonance is obtained; it is shown to be of order 4πM sϵ( kR n ) 2 , in good agreement with experiments. Coupling of modes due to the magnetic field induced by the polarization current can also occur in definite situations. The selection rules are derived and the modes repulsion near degeneracy is obtained in agreement with experiment.