Showing papers on "Electric field published in 1999"

Dipole correction for surface supercell calculations

Abstract: When performing density-functional calculations of surfaces using a plane-wave pseudopotential code, it is necessary to embed a slab with two surfaces in a periodic supercell. In many situations, it is desirable to study an asymmetric slab with a net surface dipole density. The periodic boundary conditions imposed on the electrostatic potential then give rise to an artificial electric field across the slab. We present a dipole correction that cancels the artificial field, and show how this correction can be incorporated in the density-functional theory total-energy expression. The results are supported by total-energy calculations of water-molecule layers.

Liquid-Crystal Photonic-Band-Gap Materials: The Tunable Electromagnetic Vacuum

Abstract: We demonstrate that when an optically birefringent nematic liquid crystal is infiltrated into the void regions of an inverse opal, photonic-band-gap (PBG) material, the resulting composite material exhibits a completely tunable PBG. In particular, the three-dimensional PBG can be completely opened or closed by applying an electric field which rotates the axis of the nematic molecules relative to the inverse opal backbone. Tunable light localization effects may be realized by controlling the orientational disorder in the nematic.

Acceleration of relativistic electrons via drift‐resonant interaction with toroidal‐mode Pc‐5 ULF oscillations

Abstract: There has been increasing evidence that Pc-5 ULF oscillations play a fundamental role in the dynamics of outer zone electrons. In this work we examine the adiabatic response of electrons to toroidal-mode Pc-5 field line resonances using a simplified magnetic field model. We find that electrons can be adiabatically accelerated through a drift-resonant interaction with the waves, and present expressions describing the resonance condition and half-width for resonant interaction. The presence of magnetospheric convection electric fields is seen to increase the rate of resonant energization, and allow bulk acceleration of radiation belt electrons. Conditions leading to the greatest rate of acceleration in the proposed mechanism, a nonaxisymmetric magnetic field, superimposed toroidal oscillations, and strong convection electric fields, are likely to prevail during storms associated with high solar wind speeds.

The diffusion region in collisionless magnetic reconnection

Abstract: The structure of the dissipation region in collisionless magnetic reconnection is investigated by means of kinetic particle-in-cell simulations and analytical theory. Analyses of simulations of reconnecting current sheets without guide magnetic field, which keep all parameters fixed with the exception of the electron mass, exhibit very similar large scale evolutions and time scales. A detailed comparison of two runs with different electron masses reveals very similar large scale parameters, such as ion flow velocities and magnetic field structures. The electron-scale phenomena in the reconnection region proper, however, appear to be quite different. The scale lengths of these processes are best organized by the trapping length of bouncing electrons in a field reversal region. The dissipation is explained by the electric field generated by nongyrotropic electron pressure tensor effects. In the reconnection region, the relevant electron pressure tensor components exhibit gradients which are independent of t...

Temperature- and field-dependent electron and hole mobilities in polymer light-emitting diodes

Abstract: We have studied the transport properties of electron- and hole-dominated MEH-PPV, poly(2-methoxy,5-(2′-ethyl-hexoxy)-p-phenylene vinylene), devices in the trap-free limit and have derived the temperature-dependent electron and hole mobilities (μ=μ0eγ√E) from the space-charge-limited behavior at high electric fields. Both the zero-field mobility μ0 and electric-field coefficient γ are temperature dependent with an activation energy of the hole and electron mobility of 0.38±0.02 and 0.34±0.02 eV, respectively. At 300 K, we find a zero-field mobility μ0 on the order of 1±0.5×10−7 cm2/V s and an electric-field coefficient γ of 4.8±0.3×10−4 (m/V)1/2 for holes. For electrons, we find a μ0 an order of magnitude below that for holes but a larger γ of 7.8±0.5×10−4 (m/V)1/2. Due to the stronger field dependence of the electron mobility, the electron and hole mobilities are comparable at working voltages in the trap-free limit, applicable to thin films of MEH-PPV.

A novel ultrathin elevated channel low-temperature poly-Si TFT

Abstract: A novel ultrathin elevated channel thin-film transistor (UT-ECTFT) made using low-temperature poly-Si is proposed. The structure has an ultrathin channel region (300 /spl Aring/) and a thick drain/source region. The thin channel is connected to the heavily doped drain/source through a lightly doped overlapped region. The lightly doped overlapped region provides an effective way to spread out the electric field at the drain, thereby reducing significantly the lateral electric field there at high drain bias. Thus, the UT-ECTFT exhibits excellent current saturation characteristics even at high bias (V/sub ds/=30 V, V/sub gs/=20 V). Moreover, the UT-ECTFT has more than two times increase in on-state current and 3.5 times reduction in off-state current compared to conventional thick channel TFT's.

Electric field dependence of the exchange-correlation potential in molecular chains

Abstract: Density functional calculations on the (non)linear optical properties of conjugated molecular chains using currently popular exchange-correlation (xc) potentials give overestimations of several orders of magnitude. By analyzing ``exact'' and Krieger-Li-Iafrate xc potentials, the error is traced back to an incorrect electric field dependence of the ``response part'' of the xc potential in local and gradient-corrected density approximations, which lack a linear term counteracting the applied electric field.

Built-in electric-field effects in wurtzite AlGaN/GaN quantum wells

Abstract: AlGaN/GaN quantum well (QW) structures are grown on c-plane sapphire substrates by molecular beam epitaxy. Control at the monolayer scale of the well thickness is achieved and sharp QW interfaces are demonstrated by the low photoluminescence linewidth. The QW transition energy as a function of the well width evidences a quantum-confined Stark effect due to the presence of a strong built-in electric field. Its origin is discussed in terms of piezoelectricity and spontaneous polarization. Its magnitude versus the Al mole fraction is determined. The role of the sample structure geometry on the electric field is exemplified by changing the thickness of the AlGaN barriers in multiple-QW structures. Straightforward electrostatic arguments well account for the overall trends of the electric-field variations.

Radiation Force Exerted on Subwavelength Particles near a Nanoaperture

Abstract: We report the first numerical analysis of the radiation force exerted on a subwavelength particle by an evanescent field localized near a subwavelength aperture. The radiation force is calculated by using the Maxwell stress tensor through the electric field distribution obtained with the finite-difference time-domain method. The result indicates that a particle moves towards the aperture. We found that if two particles exist the first particle is trapped and the second one is also attracted to the first one. The radiation force is found to be larger than the forces due to thermal fluctuations and to gravity.

Atmospheric pressure ion focusing in a high-field asymmetric waveform ion mobility spectrometer

Abstract: The focusing of ions at atmospheric pressure and room temperature in a high-field asymmetric waveform ion mobility spectrometer (FAIMS) has been investigated. FAIMS operates with the application of a high-voltage, high-frequency asymmetric waveform across parallel plates. This establishes conditions wherein an ion migrates towards one of the plates because of a difference in the ion mobility at the low and high electric field conditions during application of the waveform. The migration can be stopped by applying a dc compensation voltage (CV) which serves to create a “balanced” condition wherein the ion experiences no net transverse motion. This method has also been called “transverse field compensation ion mobility spectrometry” and “field ion spectrometry®.” If this experiment is conducted using a device with cylindrical geometry, rather than with flat plates, an ion focusing region can exist in the annular space between the two concentric cylinders. Ion trajectory modeling showed that the behavior of t...

Brownian Ratchets: Molecular Separations in Lipid Bilayers Supported on Patterned Arrays

Abstract: Brownian ratchets use a time-varying asymmetric potential that can be applied to separate diffusing particles or molecules. A new type of Brownian ratchet, a geometrical Brownian ratchet, has been realized. Charged, fluorescently labeled phospholipids in a two-dimensional fluid bilayer were driven in one direction by an electric field through a two-dimensional periodic array of asymmetric barriers to lateral diffusion fabricated from titanium oxide on silica. Diffusion spreads the phospholipid molecules in the orthogonal direction, and the asymmetric barriers rectify the Brownian motion, causing a directional transport of molecules. The geometrical ratchet can be used as a continuous molecular sieve to separate mixtures of membrane-associated molecules that differ in electrophoretic mobility and diffusion coefficient.

Effects of pulsed electric fields on inactivation kinetics of Listeria innocua.

Abstract: The effects of pulsed electric field (PEF) treatment and processing factors on the inactivation kinetics of Listeria innocua NCTC 11289 were investigated by using a pilot plant PEF unit with a flow rate of 200 liters/h. The electric field strength, pulse length, number of pulses, and inlet temperature were the most significant process factors influencing the inactivation kinetics. Product factors (pH and conductivity) also influenced the inactivation kinetics. In phosphate buffer at pH 4.0 and 0.5 S/m at 40°C, a 3.0-V/μm PEF treatment at an inlet temperature of 40°C resulted in ≥6.3 log inactivation of strain NCTC 11289 at 49.5°C. A synergistic effect between temperature and PEF inactivation was also observed. The inactivation obtained with PEF was compared to the inactivation obtained with heat. We found that heat inactivation was less effective than PEF inactivation under similar time and temperature conditions. L. innocua cells which were incubated for a prolonged time in the stationary phase were more resistant to the PEF treatment, indicating that the physiological state of the microorganism plays a role in inactivation by PEF. Sublethal injury of cells was observed after PEF treatment, and the injury was more severe when the level of treatment was increased. Overall, our results indicate that it may be possible to use PEF in future applications in order to produce safe products.

Dielectrophoresis of Submicrometer Latex Spheres. 1. Experimental Results

Abstract: A nonuniform electric field exerts a force on a polarizable particle through the Coulomb interaction with the electric dipole induced in the particle, resulting in a motion termed dielectrophoresis. The magnitude of the force depends on the dielectric properties of both the particle and the medium it is suspended in. As a result, measurement of the dielectrophoretic force provides information about the internal and surface dielectric properties of the particle. This paper presents the first detailed measurements of the dielectrophoretic response of submicrometer particles as a function of electrolyte composition and conductivity, applied field frequency, and particle size. Comparisons are made between the experimental results and the classical theory of the dielectrophoretic force derived from Maxwell−Wagner interfacial polarization. For particles of 557 nm diameter, good agreement is obtained between the experimental results and theory of interfacial polarization taking into account the effects of surfac...

Near-field to near/far-field transformation for arbitrary near-field geometry utilizing an equivalent electric current and MoM

Abstract: Presented here is a method for computing near- and far-field patterns of an antenna from its near-field measurements taken over an arbitrarily shaped geometry. This method utilizes near-field data to determine an equivalent electric current source over a fictitious surface which encompasses the antenna. This electric current, once determined, can be used to ascertain the near and the far field. This method demonstrates the concept of analytic continuity, i.e., once the value of the electric field is known for one region in space, from a theoretical perspective, its value for any other region can be extrapolated. It is shown that the equivalent electric current produces the correct fields in the regions in front of the antenna regardless of the geometry over which the near-field measurements are made. In this approach, the measured data need not satisfy the Nyquist sampling criteria. An electric field integral equation is developed to relate the near field to the equivalent electric current. A moment method procedure is employed to solve the integral equation by transforming it into a matrix equation. A least-squares solution via singular value decomposition is used to solve the matrix equation. Computations with both synthetic and experimental data, where the near field of several antenna configurations are measured over various geometrical surfaces, illustrate the accuracy of this method.

Direct Observation of Pinning and Bowing of a Single Ferroelectric Domain Wall

Abstract: We have made a direct optical observation of pinning and bowing of a single 180\ifmmode^\circ\else\textdegree\fi{} ferroelectric domain wall under a uniform applied electric field using a collection mode near-field scanning optical microscope. The domain wall is observed to curve between the pinning defects, with a radius of curvature determined by the material parameters and the applied electric field. The change in birefringence with applied field is used to infer the orientation of the internal field at the domain wall.

Nonlinear piezoelectric behavior of ceramic bending mode actuators under strong electric fields

Abstract: The nonlinear electromechanical behavior of cantilevered piezoelectric ceramic bimorph, unimorph, and reduced and internally biased oxide wafer actuators is studied in a wide electric field and frequency range. It is found that under quasistatic condition, linear relationships between actuator tip displacement-electric field, and blocking force-electric field are only valid under weak field driving. With increasing the driving field, electromechanical nonlinearity begins to contribute significantly to the actuator performance because of ferroelectric hysteresis behavior associated with piezoelectric lead zirconate titanate (PZT)-type ceramic materials. The bending resonance frequencies of all these actuators vary with the magnitude of the electric field. The decrease of resonance frequency with electric field is explained by the increase of elastic compliance of PZT ceramic due to elastic nonlinearity. Mechanical quality factors of the actuators also depend on the magnitude of electric field strength. No significant temperature increase is observed when actuators are driven near resonance frequency under high electric field.

FAST Observations of Inertial Alfven Waves in the Dayside Aurora

Abstract: Bursts of large amplitude impulsive electric and magnetic field oscillations are a common feature observed from FAST when crossing the dayside auroral oval at altitudes from 1500–2500 km. The oscillations have transverse amplitudes of up to 1 V/m and 100nT and exhibit a parallel electric field component with amplitudes which may be as large as 100mV/m. Calculation of E1/B1 over 100 events yields an average value four times the local Alfven speed. The wave period is usually less than 0.25s with ‘perpendicular wavelengths’ which average to 7.1 electron skin depths (c/ωpe∼80m). Poynting flux calculations indicate predominately downward fluxes with magnitude up 10−2 Wm−2 usually accompanied by a smaller upwards component. Invariably these waves are accompanied by field-aligned fluxes of down going and sometimes counterstreaming suprathermal electrons. Comparison with theoretical studies indicate that these observations are consistent with the characteristics of a shear Alfven wave with k⟂∼ωpe/c propagating in the inertial dispersive regime and interfering with a reflected component. However the observed large parallel electric field component, if real, has yet to be explained

Electric field dependence of piezoelectric properties for rhombohedral 0.955Pb(Zn1/3Nb2/3)O3– 0.045PbTiO3 single crystals

Abstract: The electric field dependence of the piezoelectric properties of rhombohedral 0.955Pb(Zn1/3Nb2/3)O3–0.045PbTiO3 crystals were investigated as a function of orientation with respect to the prototypic (cubic) axes. For 〈111〉 oriented fields, depolarization and subsequent domain reorientation resulted in an apparent maximum in the piezoelectric coefficients occurring at ∼5 kV/cm, followed by nonhysteretic dij saturation, indicating a single domain state under bias. By extrapolation, single domain values for the piezoelectric coefficients d33 and d31 were determined to be 125 and −35 pC/N, respectively. The hydrostatic piezoelectric coefficient dh for single domain crystals was calculated to be ∼55 pC/N, coincident with the experimentally determined values under hydrostatic pressure. For 〈001〉 oriented fields, piezoelectric coefficients d33〈001〉 and d31〈001〉 as high as 2250 and −1000 pC/N were determined, respectively. Although a high value of dh〈001〉 (∼250 pC/N) was expected, the experimentally determined va...

Simultaneous trapping of low-index and high-index microparticles observed with an optical-vortex trap

Abstract: We report the first observation of the simultaneous three-dimensional confinement of both a low-index particle and a high-index particle within a single-beam optical trap by using a strongly focused laser beam containing an optical vortex. Experimental and theoretical investigations of the trap stability are described.

Acoustic and optical methods for measuring electric charge distributions in dielectrics

Abstract: Much progress had been made during the last two decades in acoustic and optical methods for measuring charge distributions in dielectrics. A review on this topic is given, which mainly covers the past research activities associated with that at our laboratory. For acoustic methods, we will discuss andoompare the pulsed electroacoustic (PEA) and pressure wave propagation (pwP) methods and present some of the results that enabled us to gain physical insights into the charge dynamics within solid plate samples and coaxial cables. For optical methods, we will discuss the Pockels effect technique that is used for the dynamic measurement of surface charge distributions, and the Kerr effect technique that is developed for measuring electric field distributions within liquid dielectrics.

Dielectric constant of water at high electric fields: Molecular dynamics study

Abstract: Molecular dynamics computer simulations have been carried out for water enclosed between Pt(111) surfaces at high external electric fields. The dielectric constant of water as a function of electric fields has been calculated. Two-dimensional Ewald summation technique has been used for the calculation of long-range Coulombic forces. Simulations with a larger distance between walls, different surfaces, and bulk water have been done to confirm the macroscopic nature of the dielectric constant. Calculated dielectric constants have been compared with those obtained by a theoretical prediction and a recent simulation study.

Vibrational Stark Spectroscopy in Proteins: A Probe and Calibration for Electrostatic Fields

Abstract: We report the first measurement of the vibrational Stark effect in a protein, providing quantitative information on the sensitivity of a vibrational transition to an applied electric field. This can be used to interpret changes in the vibrational frequency that are often observed when amino acids are changed or when a protein undergoes a structural change in terms of the change in the internal or matrix electric field associated with the perturbation. The vibrational Stark effect has been measured for the vibration of CO bound to the heme iron in myoglobin. The vibrational Stark effect is surprisingly large, giving a Stark tuning rate of (2.4/f) cm-1/(MV/cm), where f is the local field correction; this is nearly 4 times larger than for free CO. It is also found that the change in dipole moment is parallel to the transition moment; that is, the change in dipole moment is in the direction perpendicular to the heme plane. Vibrational Stark effect data are also reported as a function of pH, for various mutant...

Mechanisms of electron field emission from diamond, diamond-like carbon, and nanostructured carbon

Abstract: It is argued that the facile electron field emission from carbon systems occurs primarily because surface groups such as C–H can produce large changes in local electron affinity, so that electric fields from the anode can be focused toward unhydrogenated surface areas of high affinity, the fields ending on negative charges in an underlying depletion layer. The resulting downward band bending creates large surface fields which allow Fowler–Nordheim emission, while not exceeding the material’s breakdown field.

Giant electric fields in unstrained GaN single quantum wells

Abstract: We demonstrate that, even in unstrained GaN quantum wells with AlGaN barriers, there exist giant electric fields as high as 1.5 MV/cm. These fields, resulting from the interplay of the piezoelectric and spontaneous polarizations in the well and barrier layers due to Fermi level alignment, induce large redshifts of the photoluminescence energy position and dramatically increase the carrier lifetime as the quantum well thickness increases.

Hot Exciton Dissociation in a Conjugated Polymer

Abstract: A model has been developed to treat field-assisted ultrafast on-chain dissociation of optical excitations in a conjugated polymer Conversion of the excess photon energy above the singlet exciton into the vibrational heat bath of a polymer segment is considered as a main source of the energy required for charges to escape from a potential well formed by a superposition of the Coulomb and external electric fields The calculated yield of carrier photogeneration increases strongly with increasing field but is independent of the temperature in good quantitative agreement with data obtained by Barth et al [Chem Phys Lett 288, 147 (1998)] on weakly disordered conjugated ladder-type poly-phenylene

Manipulating Molecules via Combined Static and Laser Fields

Abstract: Interaction of the strong electric field of an intense laser beam with the anisotropic polarizability of a linear molecule creates pendular states, superpositions of the field-free rotational states, in which the molecular axis librates about the field direction. Angular motion in the low-lying pendular states is thereby restricted by a double-well potential, governed by the laser intensity. The pendular energy levels occur as pairs of opposite parity, with separations corresponding to the frequency for tunneling between the wells. If the molecule is polar or paramagnetic, introducing a static electric or magnetic field connects the nearly degenerate pendular levels and thus induces strong pseudo-first-order Stark or Zeeman effects. This can be exploited in many schemes to control and manipulate molecular trajectories.