Showing papers on "Electric field published in 2007"

Biased Bilayer Graphene: Semiconductor with a Gap Tunable by the Electric Field Effect

Abstract: We demonstrate that the electronic gap of a graphene bilayer can be controlled externally by applying a gate bias. From the magnetotransport data (Shubnikov-de Haas measurements of the cyclotron mass), and using a tight-binding model, we extract the value of the gap as a function of the electronic density. We show that the gap can be changed from zero to midinfrared energies by using fields of less, approximately < 1 V/nm, below the electric breakdown of SiO2. The opening of a gap is clearly seen in the quantum Hall regime.

Biased bilayer graphene: semiconductor with a gap tunable by electric field effect

Abstract: We demonstrate that the electronic gap of a graphene bilayer can be controlled externally by applying a gate bias. From the magnetotransport data (Shubnikov-de Haas measurements of the cyclotron mass), and using a tight-binding model, we extract the value of the gap as a function of the electronic density. We show that the gap can be changed from zero to midinfrared energies by using fields of less, approximately < 1 V/nm, below the electric breakdown of SiO2. The opening of a gap is clearly seen in the quantum Hall regime.

Electric Field-Induced Modification of Magnetism in Thin-Film Ferromagnets

Abstract: A large electric field at the surface of a ferromagnetic metal is expected to appreciably change its electron density. In particular, the metal's intrinsic magnetic properties, which are commonly regarded as fixed material constants, will be affected. This requires, however, that the surface has a strong influence on the material's properties, as is the case with ultrathin films. We demonstrated that the magnetocrystalline anisotropy of ordered iron-platinum (FePt) and iron-palladium (FePd) intermetallic compounds can be reversibly modified by an applied electric field when immersed in an electrolyte. A voltage change of -0.6 volts on 2-nanometer-thick films altered the coercivity by -4.5 and +1% in FePt and FePd, respectively. The modification of the magnetic parameters was attributed to a change in the number of unpaired d electrons in response to the applied electric field. Our device structure is general and should be applicable for characterization of other thin-film magnetic systems.

Electric field effect tuning of electron-phonon coupling in graphene.

Abstract: Gate-modulated low-temperature Raman spectra reveal that the electric field effect (EFE), pervasive in contemporary electronics, has marked impacts on long-wavelength optical phonons of graphene. The EFE in this two-dimensional honeycomb lattice of carbon atoms creates large density modulations of carriers with linear dispersion (known as Dirac fermions). Our EFE Raman spectra display the interactions of lattice vibrations with these unusual carriers. The changes of phonon frequency and linewidth demonstrate optically the particle-hole symmetry about the charge-neutral Dirac point. The linear dependence of the phonon frequency on the EFE-modulated Fermi energy is explained as the electron-phonon coupling of massless Dirac fermions.

Coherent Control of a Single Electron Spin with Electric Fields

Abstract: Manipulation of single spins is essential for spin-based quantum information processing Electrical control instead of magnetic control is particularly appealing for this purpose, because electric fields are easy to generate locally on-chip We experimentally realized coherent control of a single-electron spin in a quantum dot using an oscillating electric field generated by a local gate The electric field induced coherent transitions (Rabi oscillations) between spin-up and spin-down with 90 degrees rotations as fast as approximately 55 nanoseconds Our analysis indicated that the electrically induced spin transitions were mediated by the spin-orbit interaction Taken together with the recently demonstrated coherent exchange of two neighboring spins, our results establish the feasibility of fully electrical manipulation of spin qubits

Corona of Magnetars

Abstract: We develop a theoretical model that explains the formation of hot coronae around strongly magnetized neutron stars—magnetars. The starquakes of a magnetar shear its external magnetic field, which becomes nonpotential and threaded by an electric current. Once twisted, the magnetosphere cannot untwist immediately because of its self-induction. The self-induction electric field lifts particles from the stellar surface, accelerates them, and initiates avalanches of pair creation in the magnetosphere. The created plasma corona maintains the electric current demanded by ∇ × and regulates the self-induction EMF by screening. This corona persists in dynamic equilibrium: it is continually lost to the stellar surface on the light crossing time ~10-4 s and replenished with new particles. In essence, the twisted magnetosphere acts as an accelerator that converts the toroidal field energy to particle kinetic energy. Using a direct numerical experiment, we show that the corona self-organizes quickly (on a millisecond timescale) into a quasi-steady state, with voltage 108-109 V along the magnetic lines. The voltage is maintained near the threshold for e± discharge. The heating rate of the corona is ~1036 ergs s-1, in agreement with the observed persistent, high-energy output of magnetars. We deduce that a static twist that is suddenly implanted into the magnetosphere will decay on a timescale of 1-10 yr. The particles accelerated in the corona impact the solid crust, knock out protons, and regulate the column density of the hydrostatic atmosphere of the star. The transition layer between the atmosphere and corona may be hot enough to create additional e± pairs. This layer can be the source of the observed 100 keV emission from magnetars. The corona emits curvature radiation and can supply the observed IR-optical luminosity.

Bioelectric Effects of Intense Nanosecond Pulses

Abstract: Electrical models for biological cells predict that reducing the duration of applied electrical pulses to values below the charging time of the outer cell membrane (which is on the order of 100 ns for mammalian cells) causes a strong increase in the probability of electric field interactions with intracellular structures due to displacement currents. For electric field amplitudes exceeding MV/m, such pulses are also expected to allow access to the cell interior through conduction currents flowing through the permeabilized plasma membrane. In both cases, limiting the duration of the electrical pulses to nanoseconds ensures only nonthermal interactions of the electric field with subcellular structures. This intracellular access allows the manipulation of cell functions. Experimental studies, in which human cells were exposed to pulsed electric fields of up to 300 kV/cm amplitude with durations as short as 3 ns, have confirmed this hypothesis and have shown that it is possible to selectively alter the behavior and/or survival of cells. Observed nanosecond pulsed effects at moderate electric fields include intracellular release of calcium and enhanced gene expression, which could have long term implications on cell behavior and function. At increased electric fields, the application of nanosecond pulses induces a type of programmed cell death, apoptosis, in biological cells. Cell survival studies with 10 ns pulses have shown that the viability of the cells scales inversely with the electrical energy density, which is similar to the "dose" effect caused by ionizing radiation. On the other hand, there is experimental evidence that, for pulses of varying durations, the onset of a range of observed biological effects is determined by the electrical charge that is transferred to the cell membrane during pulsing. This leads to an empirical similarity law for nanosecond pulse effects, with the product of electric field intensity, pulse duration, and the square root of the number of pulses as the similarity parameter. The similarity law allows one not only to predict cell viability based on pulse parameters, but has also been shown to be applicable for inducing platelet aggregation, an effect which is triggered by internal calcium release. Applications for nanosecond pulse effects cover a wide range: from a rather simple use as preventing biofouling in cooling water systems, to advanced medical applications, such as gene therapy and tumor treatment. Results of this continuing research are leading to the development of wound healing and skin cancer treatments, which are discussed in some detail.

Will zigzag graphene nanoribbon turn to half metal under electric field

Abstract: At B3LYP level of theory, we predict that the half-metallicity in zigzag edge graphene nanoribbon (ZGNR) can be realized when an external electric field is applied across the ribbon. The critical electric field decreases with the increase of the ribbon width to induce the half-metallicity. Both the spin polarization and half-metallicity are removed when the edge state electrons fully transferred from one side to the other under very strong electric field. The electric field range under which ZGNR remains half-metallic increases with the ribbon width. Our study demonstrates a rich field-induced spin polarization behavior, which may lead to some important applications in spinstronics.

Direct measurement of the electric-field distribution in a light-emitting electrochemical cell.

Abstract: The interplay between ionic and electronic charge carriers in mixed conductors offers rich physics and unique device potential. In light-emitting electrochemical cells (LEECs), for example, the redistribution of ions assists the injection of electronic carriers and leads to efficient light emission. The mechanism of operation of LEECs has been controversial, as there is no consensus regarding the distribution of electric field in these devices. Here, we probe the operation of LEECs using electric force microscopy on planar devices. We show that obtaining the appropriate boundary conditions is essential for capturing the underlying device physics. A patterning scheme that avoids overlap between the mixed-conductor layer and the metal electrodes enabled the accurate in situ measurement of the electric-field distribution. The results show that accumulation and depletion of mobile ions near the electrodes create high interfacial electric fields that enhance the injection of electronic carriers.

Electrically assisted magnetic recording in multiferroic nanostructures.

Abstract: We demonstrate the room-temperature control of magnetization reversal with an electric field in an epitaxial nanostructure consisting of ferrimagnetic nanopillars embedded in a ferroelectric matrix. This was achieved by combining a weak, uniform magnetic field with the switching electric field to selectively switch pillars with only one magnetic configuration. On the basis of these experimental results, we propose to use an electric field to assist magnetic recording in multiferroic systems with high perpendicular magnetic anisotropy.

Coherent spin transport through a 350 micron thick silicon wafer.

Abstract: We use all-electrical methods to inject, transport, and detect spin-polarized electrons vertically through a 350-micron-thick undoped single-crystal silicon wafer. Spin precession measurements in a perpendicular magnetic field at different accelerating electric fields reveal high spin coherence with at least 13pi precession angles. The magnetic-field spacing of precession extrema are used to determine the injector-to-detector electron transit time. These transit time values are associated with output magnetocurrent changes (from in-plane spin-valve measurements), which are proportional to final spin polarization. Fitting the results to a simple exponential spin-decay model yields a conduction electron spin lifetime (T1) lower bound in silicon of over 500 ns at 60 K.

Sensitivity of coherent oscillations in rat hippocampus to AC electric fields

Abstract: The sensitivity of brain tissue to weak extracellular electric fields is important in assessing potential public health risks of extremely low frequency (ELF) fields, and potential roles of endogenous fields in brain function. Here we determine the effect of applied electric fields on membrane potentials and coherent network oscillations. Applied DC electric fields change transmembrane potentials in CA3 pyramidal cell somata by 0.18 mV per V m−1 applied. AC sinusoidal electric fields have smaller effects on transmembrane potentials: sensitivity drops as an exponential decay function of frequency. At 50 and 60 Hz it is ∼0.4 that for DC fields. Effects of fields of ≤ 16 V m−1 peak-to-peak (p-p) did not outlast application. Kainic acid (100 nm) induced coherent network oscillations in the beta and gamma bands (15–100 Hz). Applied fields of ≥ 6 V m−1 p-p (2.1 V m−1 r.m.s.) shifted the gamma peak in the power spectrum to centre on the applied field frequency or a subharmonic. Statistically significant effects on the timing of pyramidal cell firing within the oscillation appeared at distinct thresholds: at 50 Hz, 1 V m−1 p-p (354 mV m−1 r.m.s.) had statistically significant effects in 71% of slices, and 0.5 V m−1 p-p (177 mV m−1 r.m.s.) in 20%. These threshold fields are consistent with current environmental guidelines. They correspond to changes in somatic potential of ∼70 μV, below membrane potential noise levels for neurons, demonstrating the emergent properties of neuronal networks can be more sensitive than measurable effects in single neurons.

Electrically tunable negative permeability metamaterials based on nematic liquid crystals

Abstract: An electrically tunable negative permeability metamaterial consisting of a periodic array of split ring resonators infiltrated with nematic liquid crystals is demonstrated. It shows that the transmitted resonance dip of the metamaterial can be continuously and reversibly adjusted by an applied electric field, and the maximum shift is about 210MHz with respect to the resonance frequency around 11.08GHz. Numerical simulation shows that the permeability is negative near the resonance frequency, and the frequency range with negative permeability can be dynamically adjusted and widened by about 200MHz by the electric field. It provides a convenient means to design adaptive metamaterials.

Electronic structure and nonlinear optical rectification in a quantum dot: effects of impurities and external electric field

Abstract: The electronic structure of a spherical quantum dot with parabolic confinement that contains a hydrogenic impurity and is subjected to a DC electric field is studied. In our calculations we vary the position of the impurity and the electric field strength. The calculated electronic structure is further used for determining the nonlinear optical rectification coefficient of the quantum dot structure. We show that both the position of the impurity and the strength of the electric field influence the nonlinear optical rectification process.

Spike timing amplifies the effect of electric fields on neurons: implications for endogenous field effects.

Abstract: Despite compelling phenomenological evidence that small electric fields (<5 mV/mm) can affect brain function, a quantitative and experimentally verified theory is currently lacking. Here we demonstrate a novel mechanism by which the nonlinear properties of single neurons "amplify" the effect of small electric fields: when concurrent to suprathreshold synaptic input, small electric fields can have significant effects on spike timing. For low-frequency fields, our theory predicts a linear dependency of spike timing changes on field strength. For high-frequency fields (relative to the synaptic input), the theory predicts coherent firing, with mean firing phase and coherence each increasing monotonically with field strength. Importantly, in both cases, the effects of fields on spike timing are amplified with decreasing synaptic input slope and increased cell susceptibility (millivolt membrane polarization per field amplitude). We confirmed these predictions experimentally using CA1 hippocampal neurons in vitro exposed to static (direct current) and oscillating (alternating current) uniform electric fields. In addition, we develop a robust method to quantify cell susceptibility using spike timing. Our results provide a precise mechanism for a functional role of endogenous field oscillations (e.g., gamma) in brain function and introduce a framework for considering the effects of environmental fields and design of low-intensity therapeutic neurostimulation technologies.

Estimations of electric field effects on the oxygen reduction reaction based on the density functional theory.

Abstract: By varying the external electric field in density functional theory (DFT) calculations we have estimated the impact of the local electric field in the electric double layer on the oxygen reduction reaction (ORR). Potentially, including the local electric field could change adsorption energies and barriers substantially, thereby affecting the reaction mechanism predicted for ORR on different metals. To estimate the effect of local electric fields on ORR we combine the DFT results at various external electric field strengths with a previously developed model of electrochemical reactions which fully accounts for the effect of the electrode potential. We find that the local electric field only slightly affects the output of the model. Hence, the general picture obtained without inclusion of the electric field still persists. However, for accurate predictions at oxygen reduction potentials close to the volcano top local electric field effects may be of importance.

Novel electric field effects on Landau levels in graphene.

Abstract: A new effect in graphene in the presence of crossed uniform electric and magnetic fields is predicted. Landau levels are shown to be modified in an unexpected fashion by the electric field, leading to a collapse of the spectrum, when the value of electric to magnetic field ratio exceeds a certain critical value. Our theoretical results, strikingly different from the standard 2D electron gas, are explained using a "Lorentz boost," and as an "instability of a relativistic quantum field vacuum." It is a remarkable case of emergent relativistic type phenomena in nonrelativistic graphene. We also discuss few possible experimental consequence.

Measurement of Rashba and Dresselhaus spin-orbit magnetic fields

Abstract: Spin–orbit coupling is a manifestation of special relativity. In the reference frame of a moving electron, electric fields transform into magnetic fields, which interact with the electron spin and lift the degeneracy of spin-up and spin-down states. In solid-state systems, the resulting spin–orbit fields are referred to as Dresselhaus and Rashba fields, depending on whether the electric fields originate from bulk or structure inversion asymmetry, respectively. Yet, it remains a challenge to determine the absolute value of both contributions in a single sample. Here, we show that both fields can be measured by optically monitoring the angular dependence of the electrons’ spin precession on their direction of motion with respect to the crystal lattice. Furthermore, we demonstrate spin resonance induced by the spin–orbit fields. We apply our method to GaAs/InGaAs quantum-well electrons, but it should be universally useful to characterize spin–orbit interactions in semiconductors, and therefore could facilitate the design of spintronic devices.

Reconstruction of Equivalent Currents Distribution Over Arbitrary Three-Dimensional Surfaces Based on Integral Equation Algorithms

Abstract: A technique for the determination of the equivalent currents distribution from a known radiated field is described. This Inverse Radiation Problem is solved through an Integral Equation algorithm that allows the characterization of antennas of complex geometry both for near field to far field (NF-FF) transformation purposes as well as for diagnostic tasks. The algorithm is based on the representation of the radiating structure by means of a set of equivalent currents over a three-dimensional (3D) surface that can be fitted to the arbitrary geometry of the antenna. The innovative formulation uses an integral equation involving the electric field due to the currents tangential components to the represented antenna 3D surface. For that purpose, both the magnetic and electric equivalent currents are considered in the integral equations. Regularization techniques are also introduced to improve the convergence of the proposed iterative solution. The paper concludes with several results related to the practical verification of the Equivalence Principle and the characterization of a horn antenna.

Electron beams and microwave vacuum electronics

Abstract: PREFACE. Introduction. I.1 Outline of the Book. I.2 List of Symbols. I.3 Electromagnetic Fields and Potentials. I.4 Principle of Least Action. Lagrangian. Generalized Momentum. Lagrangian Equations. I.5 Hamiltonian. Hamiltonian Equations. I.6 Liouville Theorem. I.7 Emittance. Brightness. PART I ELECTRON BEAMS. 1 Motion of Electrons in External Electric and Magnetic Static Fields. 1.1 Introduction. 1.2 Energy of a Charged Particle. 1.3 Potential-Velocity Relation (Static Fields). 1.4 Electrons in a Linear Electric Field e0E kx. 1.5 Motion of Electrons in Homogeneous Static Fields. 1.6 Motion of Electrons in Weakly Inhomogeneous Static Fields. 1.6.1 Small Variations in Electromagnetic Fields Acting on Moving Charged Particles. 1.7 Motion of Electrons in Fields with Axial and Plane Symmetry. Busch's Theorem. 2 Electron Lenses. 2.1 Introduction. 2.2 Maupertuis's Principle. Electron-Optical Refractive Index. Differential Equations of Trajectories. 2.3 Differential Equations of Trajectories in Axially Symmetric Fields. 2.4 Differential Equations of Paraxial Trajectories in Axially Symmetric Fields Without a Space Charge. 2.5 Formation of Images by Paraxial Trajectories. 2.6 Electrostatic Axially Symmetric Lenses. 2.7 Magnetic Axially Symmetric Lenses. 2.8 Aberrations of Axially Symmetric Lenses. 2.9 Comparison of Electrostatic and Magnetic Lenses. Transfer Matrix of Lenses . 2.10 Quadrupole lenses. 3 Electron Beams with Self Fields. 3.1 Introduction. 3.2 Self-Consistent Equations of Steady-State Space-Charge Electron Beams. 3.3 Euler's Form of a Motion Equation. Lagrange and Poincare' Invariants of Laminar Flows. 3.4 Nonvortex Beams. Action Function. Planar Nonrelativistic Diode. Perveance. Child-Langmuir Formula. r- and T-Modes of Electron Beams. 3.5 Solutions of Self-Consistent Equations for Curvilinear Space-Charge Laminar Beams. Meltzer Flow. Planar Magnetron with an Inclined Magnetic Field. Dryden Flow. 4 Electron Guns. 4.1 Introduction. 4.2 Pierce's Synthesis Method for Gun Design. 4.3 Internal Problems of Synthesis. Relativistic Planar Diode. Cylindrical and Spherical Diodes. 4.4 External Problems of Synthesis. Cauchy Problem. 4.5 Synthesis of Electrode Systems for Two-Dimensional Curvilinear Beams with Translation Symmetry (Lomax-Kirstein Method). Magnetron Injection Gun. 4.6 Synthesis of Axially Symmetric Electrode Systems. 4.7 Electron Guns with Compressed Beams. Magnetron Injection Gun. 4.8 Explosive Emission Guns. 5 Transport of Space-Charge Beams. 5.1 Introduction. 5.2 Unrippled Axially Symmetric Nonrelativistic Beams in a Uniform Magnetic field. 5.3 Unrippled Relativistic Beams in a Uniform External Magnetic Field. 5.4 Cylindrical Beams in an Infinite Magnetic Field. 5.5 Centrifugal Electrostatic Focusing. 5.6 Paraxial-Ray Equations of Axially Symmetric Laminar Beams. 5.7 Axially Symmetric Paraxial Beams in a Uniform Magnetic Field with Arbitrary Shielding of a Cathode Magnetic Field. 5.8 Transport of Space-Charge Beams in Spatial Periodic Fields. PART II MICROWAVE VACUUM ELECTRONICS. 6 Quasistationary Microwave Devices. 6.1 Introduction. 6.2 Currents in Electron Gaps. Total Current and the Shockley-Ramo Theorem. 6.3 Admittance of a Planar Electron Gap. Electron Gap as an Oscillator. Monotron. 6.4 Equation of Stationary Oscillations of a Resonance Self-Excited Circuit. 6.5 Effects of a Space-Charge Field. Total Current Method. High-Frequency Diode in the r-Mode. Llewellyn-Peterson Equations. 7 Klystrons. 7.1 Introduction. 7.2 Velocity Modulation of an Electron beam. 7.3 Cinematic (Elementary) Theory of Bunching. 7.4 Interaction of a Bunched Current with a Catcher Field. Output Power of A Two-Cavity Klystron. 7.5 Experimental Characteristics of a Two-Resonator Amplifier and Frequency-Multiplier Klystrons. 7.6 Space-Charge Waves in Velocity-Modulated Beams. 7.7 Multicavity and Multibeam Klystron Amplifiers. 7.8 Relativistic Klystrons. 7.9 Reflex Klystrons. 8 Traveling-Wave Tubes and Backward-Wave Oscillators (O-Type Tubes). 8.1 Introduction. 8.2 Qualitative Mechanism of Bunching and Energy Output in a TWTO. 8.3 Slow-Wave Structures. 8.4 Elements of SWS Theory. 8.5 Linear Theory of a Nonrelativistic TWTO. Dispersion Equation, Gain, Effects of Nonsynchronism, Space Charge, and Loss in a Slow-Wave Structure. 8.6 Nonlinear Effects in a Nonrelativistic TWTO. Enhancement of TWTO Efficiency (Velocity Tapering, Depressed Collectors). 8.7 Basic Characteristics and Applications of Nonrelativistic TWTOs. 8.8 Backward-Wave Oscillators. 8.9 Millimeter Nonrelativistic TWTOs, BWOs, and Orotrons. 8.10 Relativistic TWTOs and BWOs. 9 Crossed-Field Amplifiers and Oscillators (M-Type Tubes). 9.1 Introduction. 9.2 Elementary Theory of a Planar MTWT. 9.3 MTWT Amplification. 9.4 M-type Injected Beam Backward-Wave Oscillators (MWO, M-Carcinotron). 9.5 Magnetrons. 9.6 Relativistic Magnetrons. 9.7 Magnetically Insulated Line Oscillators. 9.8 Crossed-Field Amplifiers. 10 Classical Electron Masers and Free Electron Lasers. 10.1 Introduction. 10.2 Spontaneous Radiation of Classical Electron Oscillators. 10.3 Stimulated Radiation of Excited Classical Electron Oscillators. 10.4 Examples of Electron Cyclotron Masers. 10.5 Resonators of Gyromonotrons (Free and Forced Oscillations). 10.6 Theory of a Gyromonotron. 10.7 Subrelativistic Gyrotrons. 10.8 Elements of Gyrotron Electron Optics. 10.9 Mode Interaction and Mode Selection in Gyrotrons. Output Power Systems. 10.10 Gyroklystrons. 10.11 Gyro-Traveling-Wave Tubes. 10.12 Applications of Gyrotrons. 10.13 Cyclotron Autoresonance Masers. 10.14 Free Electron Lasers. Appendixes. 1. Proof of the 3/2 Law for Nonrelativistic Diodes in the r-Mode. 2. Synthesis of Guns for M-Type TWTS and BWOS. 3. Magnetic Field in Axially Symmetric Systems. 4. Dispersion Characteristics of Interdigital and Comb Structures. 5. Electromagnetic Field in Planar Uniform Slow-Wave Structures. 6. Equations of Free Oscillations of Gyrotron Resonators. 7. Derivation of Eqs. (10.66) and (10.67). 8. Calculation of Fourier Coefficients in Gyrotron Equations. 9. Magnetic Systems of Gyrotrons. References. Index.

AgNbO3: A lead-free material with large polarization and electromechanical response

Abstract: Polarization measurements reveal that AgNbO3 has an extremely large polarization, which can reach a value of 52μC∕cm2 in polycrystals. Experiments also show that the large internal atom distortion in AgNbO3 is also strongly coupled to the electric field, indicating that high piezoelectric performance can be realized in AgNbO3 system. This finding opens the way to designing a new class of lead-free, high-performance piezoelectric materials based on AgNbO3.

Equatorial Ionospheric Electric Fields During the November 2004 Magnetic Storm

Abstract: [1] We use radar measurements from the Jicamarca Radio Observatory, magnetometer observations from the Pacific sector and ionosonde data from Brazil to study equatorial ionospheric electric fields during the November 2004 geomagnetic storm. Our data show very large eastward and westward daytime electrojet current perturbations with lifetimes of about an hour (indicative of undershielding and overshielding prompt penetration electric fields) in the Pacific equatorial region during the November 7 main phase of the storm, when the southward IMF, the solar wind and reconnection electric fields, and the polar cap potential drops had very large and nearly steady values. This result is inconsistent with the recent suggestion that solar wind electric fields penetrate without attenuation into the equatorial ionosphere for several hours during storm main phase. The largest daytime prompt penetration electric fields (about 3 mV/m) ever observed over Jicamarca occurred during the November 9 storm main phase, when large equatorial electrojet current and drift perturbations were also present in the Pacific and Brazilian equatorial regions. The rise and decay times of these equatorial electric fields were about 20 min longer than of the corresponding solar wind electric fields. The ratios of prompt penetration electric fields and corresponding solar wind electric field changes were highly variable even during the day, and had largest values near dawn. Also, the prompt penetration electric fields did not show polar cap potential drop saturation effects. Our results clearly highlight that the relationships of prompt penetration and solar wind electric fields, and polar cap potentials are far more complex than implied by simple proportionality factors.

Calculation of MRI-induced heating of an implanted medical lead wire with an electric field transfer function.

Abstract: Purpose To develop and demonstrate a method to calculate the temperature rise that is induced by the radio frequency (RF) field in MRI at the electrode of an implanted medical lead. Materials and Methods The electric field near the electrode is calculated by integrating the product of the tangential electric field and a transfer function along the length of the lead. The transfer function is numerically calculated with the method of moments. Transfer functions were calculated at 64 MHz for different lengths of model implants in the form of bare wires and insulated wires with 1 cm of wire exposed at one or both ends. Results Heating at the electrode depends on the magnitude and the phase distribution of the transfer function and the incident electric field along the length of the lead. For a uniform electric field, the electrode heating is maximized for a lead length of approximately one-half a wavelength when the lead is terminated open. The heating can be greater for a worst-case phase distribution of the incident field. Conclusion The transfer function is proposed as an efficient method to calculate MRI-induced heating at an electrode of a medical lead. Measured temperature rises of a model implant in a phantom were in good agreement with the rises predicted by the transfer function. The transfer function could be numerically or experimentally determined. J. Magn. Reson. Imaging 2007;26:1278–1285. © 2007 Wiley-Liss, Inc.

Manipulating exciton fine structure in quantum dots with a lateral electric field

Abstract: The fine structure of the neutral exciton in a single self-assembled InGaAs quantum dot is investigated under the effect of a lateral electric field. Stark shifts up to 1.5meV, an increase in linewidth, and a decrease in photoluminescence intensity were observed due to the electric field. The authors show that the lateral electric field strongly affects the exciton fine-structure splitting due to active manipulation of the single particle wave functions. Remarkably, the splitting can be tuned over large values and through zero.

Axisymmetric deformation and stability of a viscous drop in a steady electric field

Abstract: We consider a neutrally buoyant and initially uncharged drop in a second liquid subjected to a uniform electric field. Both liquids are taken to be leaky dielectrics. The jump in electrical properties creates an electric stress balanced by hydrodynamic and capillary stresses. Assuming creeping flow conditions and axisymmetry of the problem, the electric and flow fields are solved numerically with boundary integral techniques. The system is characterized by the physical property ratios R (resistivities), Q (permitivities) and λ (dynamic viscosities). Depending on these parameters, the drop deforms into a prolate or an oblate spheroid. The relative importance of the electric stress and of the drop/medium interfacial tension is measured by the dimensionless electric capillary number, CaE .F orλ = 1, we present a survey of the various behaviours obtained for a wide range of R and Q. We delineate regions in the (R, Q)-plane in which the drop either attains a steady shape under any field strength or reaches a fold-point instability past a critical CaE. We identify the latter with linear instability of the steady shape to axisymmetric disturbances. Various break-up modes are identified, as well as more complex behaviours such as bifurcations and transition from unstable to stable solution branches. We also show how the viscosity contrast can stabilize the drop or advance break-up in the different situations encountered for λ =1 .

Effect of field direction on electrowetting in a nanopore

Abstract: We manifest a significant influence of field direction and polarity on surface wetting, when the latter is tuned by application of an external electric field. Thermodynamics of field-induced filling of hydrocarbon-like nanopores with water is studied by open ensemble molecular simulation. Increased field strength consistently results in water-filling and electrostriction in hydrophobic nanopores. A threshold field commensurate with surface charge density of about one elementary charge per 10 nm2 suffices to render prototypical paraffin surfaces hydrophilic. When a field is applied in the direction perpendicular to the confining walls, the competition between orientational polarization and angle preferences of interfacial water molecules relative to the walls results in an asymmetric wettability of opposing surfaces (Janus interface). Reduction of surface free energy observed upon alignment of confinement walls with field direction suggests a novel mechanism whereby the applied electric field can operate s...

Continuous-Flow Particle Separation by 3D Insulative Dielectrophoresis Using Coherently Shaped, dc-Biased, ac Electric Fields

Abstract: We present the development of a continuous-flow, "dielectrophoretic spectrometer" based on insulative DEP techniques and three-dimensional geometric design. Hot-embossed thermoplastic devices allow for high-throughput analysis and geometric control of electric fields via ridged microstructures patterned in a high width-to-depth aspect ratio (250:1) channel. We manipulate particles with dc-biased, ac electric fields and generate continuous-output streams of particles with a transverse outlet position specified by linear and nonlinear particle mobilities. We show, with simulation and experiment, that characteristic shape factors can be defined that capture the effects of constrictions in channel depth and that modulating the angle of these constrictions changes the resulting local DEP force. Microdevices are fabricated with an insulative constriction in channel depth, whose angle of incidence with the direction of flow varies continuously across the channel width. The resulting electric field gradients enable demonstration of a dielectrophoretic spectrometer that separates particles and controls their transverse channel position.

Electric field gating with ionic liquids

Abstract: The authors show that ionic liquids are well suited to specialized electric field gating applications in which large surface charge densities can be induced on the surfaces of low-carrier density thin-film metals. Using either coplanar or overlay gate configurations, they demonstrate field-induced resistance changes on the order of a factor of 104 for thin conducting InOx films. The areal capacitances and field effect mobilities noticeably exceed those that can be achieved using AlOx dielectrics. In addition, the charge state can be frozen in by reducing the temperature, thus providing an opportunity for electric field tuning of metal-insulator transitions in a variety of thin-film systems.