Showing papers on "Drift velocity published in 2007"

Longitudinal structure of the vertical E × B drift and ion density seen from ROCSAT‐1

Abstract: [1] We investigate the longitudinal distribution of the vertical E × B drift velocity and ion density in the low-latitude ionosphere using the first Republic of China Satellite (ROCSAT-1) data acquired during 1999–2004. The ROCSAT-1 observations during daytime demonstrate the presence of the longitudinally periodic patterns of the vertical E × B drift and plasma density on the topside F region (600 km). The four longitude sectors where the peaks in the plasma density are found are coincident with the peaks in the E × B drift. This observation may indicate the association of the large-scale longitudinal density structure with the daytime E-region dynamo electric field. The density structure exists before the occurrence of the pre-reversal enhancement (PRE) and therefore the PRE is not directly related to this phenomenon.

Internal motions of a quasiparticle governing its ultrafast nonlinear response

Abstract: This paper reports a study of polarons in a GaAs crystal subject to a strong electric field. In addition to the overall drift motion of the polaron, an oscillatory internal motion is observed in which the electron is impulsively moved away from the centre of the surrounding lattice distortion. Such quantum coherent processes directly affect high-frequency transport in nanostructures. A charged particle modifies the structure of the surrounding medium: examples include a proton in ice1, an ion in a DNA molecule2, an electron at an interface3, or an electron in an organic4 or inorganic crystal5,6,7. In turn, the medium acts back on the particle. In a polar or ionic solid, a free electron distorts the crystal lattice, displacing the atoms from their equilibrium positions. The electron, when considered together with its surrounding lattice distortion, is a single quasiparticle5,6, known as the Frohlich polaron8,9. The basic properties of polarons and their drift motion in a weak electric field are well known10,11,12. However, their nonlinear high-field properties—relevant for transport on nanometre length and ultrashort timescales—are not understood. Here we show that a high electric field in the terahertz range drives the polaron in a GaAs crystal into a highly nonlinear regime where, in addition to the drift motion, the electron is impulsively moved away from the centre of the surrounding lattice distortion. In this way, coherent lattice vibrations (phonons) and concomitant drift velocity oscillations are induced that persist for several hundred femtoseconds. They modulate the optical response at infrared frequencies between absorption and stimulated emission. Such quantum coherent processes directly affect high-frequency transport in nanostructures and may be exploited in novel terahertz-driven optical modulators and switches.

Suppression of Particle Drifts by Turbulence

Abstract: We present results from direct numerical simulations showing the suppression of the large-scale drift motion of an ensemble of charged particles in a nonuniform turbulent magnetic field. We find that when scattering is negligible, the ensemble average drift velocity is in the direction predicted by the usual guiding center theory. When scattering is very strong, we find that all large-scale drift motions vanish. For an intermediate amount of scattering we find that the antisymmetric drift velocity is typically suppressed by a larger amount than the antisymmetric drift coefficient. We show that the total drift motion of the ensemble is not necessarily completely contained in the antisymmetric part of the diffusion tensor. Because of the occurrence of scattering, knowledge of the spatial variation of the symmetric part of the diffusion tensor is also needed to fully describe the total drift motion of the ensemble.

Drift-compensated data acquisition performed at room temperature with frequency modulation atomic force microscopy

Abstract: The authors have performed distortionless atom imaging and force mapping experiments, under a large thermal drift condition at room temperature (RT), using frequency modulation atomic force microscopy (FM-AFM) that had been done previously only at low temperature. In the authors’ experimental scheme, three-dimensional position feedback with atom tracking detects the thermal drift velocity that is constant for a period of time at RT. The detected velocity is then used as the model for implementing the feedforward in order to compensate for the thermal drift. This technique can be expected to be used for precise positioning of the tip-sample in atom manipulation experiments using the FM-AFM at RT.

Observation of resonant behavior in the energy velocity of diffused light.

Abstract: In this Letter we demonstrate Mie resonances mediated transport of light in randomly arranged, monodisperse dielectric spheres packed at high filling fractions. By means of both static and dynamic optical experiments we show resonant behavior in the key transport parameters and, in particular, we find that the energy transport velocity, which is lower than the group velocity, also displays a resonant behavior.

Potential around a charged dust particle in a collisional sheath

Abstract: By employing a self-consistent kinetic approach, an analytical expression is derived for the potential of a test charge in a weakly ionized plasma with ion drift. The drift is assumed to be due to an external electric field, with the velocity being mobility-limited and much larger than the thermal velocity of neutrals. The derived expression is proven to be in excellent agreement with the measurements by Konopka et al. [Phys. Rev. Lett. 84, 891 (2000)] performed in the sheath region of a rf discharge.

Hot phonon effect on electron velocity saturation in GaN: A second look

Abstract: A theoretical model is developed for electron velocity saturation in high power GaN transistors. It is shown that electron velocity at high electric fields is reduced due to heating of electron gas since the high density of nonequilibrium LO phonons cannot efficiently transfer heat to the lattice. However, the resulting degradation of electron velocity is found to be weaker than previously reported. The results are compared with experimental data, and the ways to improve the efficiency of cooling the electron gas to increase the drift velocity are discussed.

Persistence of direction increases the drift velocity of run and tumble chemotaxis.

Abstract: Escherichia coli is a motile bacterium that moves up a chemoattractant gradient by performing a biased random walk composed of alternating runs and tumbles. Previous models of run and tumble chemotaxis neglect one or more features of the motion, namely (a) a cell cannot directly detect a chemoattractant gradient but rather makes temporal comparisons of chemoattractant concentration, (b) rather than being entirely random, tumbles exhibit persistence of direction, meaning that the new direction after a tumble is more likely to be in the forward hemisphere, and (c) rotational Brownian motion makes it impossible for an E. coli cell to swim in a straight line during a run. This paper presents an analytic calculation of the chemotactic drift velocity taking account of (a), (b) and (c), for weak chemotaxis. The analytic results are verified by Monte Carlo simulation. The results reveal a synergy between temporal comparisons and persistence that enhances the drift velocity, while rotational Brownian motion reduces the drift velocity.

Local Hanle-effect studies of spin drift and diffusion in n:GaAs epilayers and spin-transport devices

Abstract: In electron-doped GaAs, we use scanning Kerr-rotation microscopy to locally probe and spatially resolve the depolarization of electron spin distributions by transverse magnetic fields. The shape of these local Hanle- effect curves provides a measure of the spin lifetime as well as spin transport parameters including drift velocity, mobility and diffusion length. Asymmetries in the local Hanle data can be used to reveal and map out the effective magnetic fields due to spin-orbit coupling. Finally, using both spin imaging and local Hanle effect studies, we investigate the drift and diffusion of electrically-injected spins in lateral Fe/GaAs spin-detection devices, both within and outside the current path.

Electron impact ionization and attachment, drift velocities and longitudinal diffusion in CF3I and CF3I–N2 mixtures

Abstract: This paper reports the measurement of the electron drift velocity ve, the longitudinal diffusion coefficient NDL and the density-normalized effective ionization coefficient (α − η)/N in pure CF3I and in the CF3I–N2 mixtures, where α and η are the electron impact ionization and attachment coefficients, respectively, and N is the gas density. The E/N range covered was 100–850 Td (1 Td = 10−17 V cm2). The present results were derived from a pulsed Townsend experiment. For pure CF3I, the values of ve and (α − η)/N were found to increase linearly with E/N. Moreover, the E/N value at which ionization equals attachment, commonly referred to as the limiting field strength, was found to be E/Nlim = 437 Td, which is greater than that of SF6 (360 Td), a widely used insulating gas. For the CF3I–N2 mixture with 70% CF3I, this E/Nlim value was found to be essentially the same as that for pure SF6.

Electron and Hole Transport in Bulk ZnO: A Full Band Monte Carlo Study

Abstract: Electron and hole transport in wurtzite phase ZnO is studied using an ensemble full band Monte Carlo method. The model includes an accurate description of the electronic structure obtained with the nonlocal pseudopotential method and numerically calculated impact ionization transition rates based on a wavevector-dependent dielectric function. Results of transport simulations at both low and high electric fields are presented. It is found that the low field electron mobility is close to 300 cm 2 V -1 s -1 at room temperature, and the peak electron drift velocity is 2.2 × 10 7 cm/s at a field of 275 kV/cm. The determination of the ionization coefficients is affected by some uncertainties due to the incomplete knowledge of the high energy phonon scattering rates. Nevertheless, the present calculations of the ionization coefficients provide a reasonably accurate estimate of the impact ionization process.

Simulation and optimization of GaN-based metal-oxide-semiconductor high-electron-mobility-transistor using field-dependent drift velocity model

Abstract: Undoped GaN-based metal-oxide-semiconductor high-electron-mobility-transistors (MOS-HEMTs) with atomic-layer-deposited Al2O3 gate dielectrics are fabricated with gate lengths from 1 μm up to 40 μm. With a two-dimensional numerical simulator, we report simulation results of the GaN-based MOS-HEMTs using field-dependent drift velocity model. A developed model, taking into account polarization-induced charges and defect-induced traps at all of the interfaces and process-related trap levels of bulk traps measured from experiments, is built. The simulated output characteristics are in good agreement with reported experimental data. The effect of the high field at the drain-side gate edge and bulk trap density of GaN on the output performance is discussed in detail for the device optimization. AlGaN/GaN/AlN quantum-well (QW) MOS-HEMTs have been proposed and demonstrated based on numerical simulations. The simulation results also link the current collapse with electrons spreading into the bulk, and confirm that a better electron localization can dramatically reduce the current collapse for the QW-MOS-HEMTs. Due to the large band edge discontinuity and effective quantum confinement of the AlGaN/GaN/AlN quantum well, the parasitic conduction in the bulk is completely eliminated.

On the post‐sunset rise of the equatorial F layer and superposed upwellings and bubbles

Abstract: [1] Usual practice has been to describe the post-sunset rise (PSSR) of the equatorial F layer with the vertical drift velocity measured overhead, as a function of local time, by a ground-based sensor. However, with the finding that large-scale wave structure (LSWS) develops with its own eastward polarization electric field, during a PSSR, the measured upward velocity must now be thought of as containing two components, one associated with PSSR, and the other with LSWS. This distinction may hold the key to understanding the lack of correlation, on a day-to-day basis, between measured vertical drift and ensuing plasma structure in the form of plasma bubbles. Measurements of backscatter from the bottomside of the F layer, using a three-beam radar at Pohnpei, are presented in this paper to further validate this interpretation.

Exact solution for the generalized Bohm criterion in a two-ion-species plasma.

Abstract: For a weakly collisional two-ion species plasma, it is shown that the minimum phase velocity of ion acoustic waves (IAWs) at the sheath-presheath boundary is equal to twice the phase velocity in the bulk plasma. This condition provides a theoretical basis for the experimental results that each ion species leaves the plasma with a drift velocity equal to the IAW phase velocity in the bulk plasma [D. Lee et al., Appl. Phys. Lett. 91, 041505 (2007)]. It is shown that this result is a consequence of the generalized Bohm criterion and fluid expressions for the IAW phase velocities.

Large parallel electric fields, currents, and density cavities in dispersive Alfvén waves above the aurora

Abstract: [1] Large parallel electric fields (E∥ > 10 mV/m) are sometimes observed with intense filamentary field-aligned currents (J∥ > 10 μA/m2 mapped to 100-km altitude) on transverse scales where kxc/ωpe ≈ 1 above the auroral oval. The ratio of the transverse electric (Ex) and magnetic field (By) variations is approximately the local Alfven speed and increases with decreasing transverse scale in a manner consistent with inertial Alfven wave dispersion. These fields are typically observed in association with density cavities. Statistically, it is shown that the depth of the cavities observed over the altitude range from 350 to 4175 km by the Fast Auroral Snapshot (FAST) satellite typically lies between 30 and 50% of the background plasma with 90% depletions being observed for 1% of the recorded events. The wave currents embedded within these cavities exceed 100 μA/m2 for 2% of the ensemble and are typically found over transverse widths of ∼2πλe. Using a fluid description of the plasma, we show statistically that electron pressure gradients provide a probable means for balancing the large E∥ found in these structures with a likely but unknown contribution from anomalous resistivity. Furthermore, we find that the ratio of the average drift speed of the bulk electron plasma (vd) to the electron thermal speed (ve) inside these structures is vd/ve ≥ 0.1 and that in at least 2% of cases veh05d/ve ≥ 1.0. On the basis of recent simulation results and the Bohm criterion for double layer formation, these statistics indicate that the large E∥ sometimes observed in density cavities may occur in double layers driven by the Alfven wave current.

Effects of Al doping on the electromigration performance of damascene Cu interconnects

Abstract: We report the effects of Al doping on electromigration in damascene Cu interconnects. Al doping was performed by thermal diffusion from a CuAl seed layer positioned underneath the Cu interconnects. To investigate the dependencies of the Al concentration, the seed CuAl layer thickness was increased from 40to90nm on a planar surface. The effects of Al doping on the incubation time, drift velocity, and critical product of electromigration were investigated. The drift velocity of Cu mass transport in CuAl alloys decreases with an increase in the concentration of Al atoms. The observed critical product of electromigration is 1500A∕cm, and it is independent of the Al concentration. The measured activation energies of the normalized drift velocities for CuAl seed layer thicknesses of 40, 60, and 90nm are 1.2, 1.4, and 1.5eV, respectively. The Al concentrations at Cu∕SiCN interface, grain boundary, Ta∕Cu interface, and bulk were investigated along the length of a line by the electron microprobe technique. The tim...

Analysis of equatorial plasma bubble zonal drift velocities in the Pacific sector by imaging techniques

Abstract: . Using 1024 nights of data from 2002–2005 taken by the Cornell Narrow Field Imager (CNFI), we examine equatorial plasma bubble (EPB) zonal drift velocity characteristics. CNFI is located at the Maui Space Surveillance Site on the Haleakala Volcano (geographic: 20.71° N, 203.83° E; geomagnetic: 21.03° N, 271.84° E) on the island of Maui, Hawaii. The imager is set up to view in a magnetic field-aligned geometry in order to maximize its resolution. We calculate the zonal drift velocities using two methods: a correlation routine and an EPB west-wall intensity gradient tracking routine. These two methods yield sizeable differences in the evenings, suggesting strong pre-local midnight EPB development. An analysis of the drift velocities is also performed based on the three influencing factors of season, geomagnetic activity, and solar activity. In general, our data match published trends and drift characteristics from past studies. However, we find that the drift magnitudes are much lower than results from other imagers at similar latitude sectors but at different longitude sectors, suggesting that zonal drift velocities have a longitudinal dependence.

The effect of jet velocity profile on the characteristics of thickness and velocity of the liquid sheet formed by two impinging jets

Abstract: In this study, the effect of jet velocity profile on the thickness and velocity of the liquid sheet formed by two low-speed impinging jets was investigated. In addition to the constant jet velocity and the Poiseuille parabolic profile, the jet velocity profile measured experimentally was considered to account for the real nonuniform jet velocity profile. For three jet velocity profiles, the distributions of the thickness and velocity of the liquid sheet were analytically predicted by solving conservation equations for mass, momentum, and energy. The predicted results were compared with the previous experimental results. The jet velocity profile affected the resulting thickness and velocity characteristics of the liquid sheet. The distributions of the thickness and velocity of the liquid sheet predicted by using the measured jet velocity profile produced more acceptable results, which agreed better with the experimental observations, than those obtained by using the constant jet velocity, which has been commonly used in previous theoretical works.

Electron swarm parameters in water vapour

Abstract: Electron swarm parameters, such as the drift velocity and the ionization coefficient, in water vapour have been measured for relatively wide ranges in reduced electric fields (E/N) at room temperature. The drift velocity (Wm) was obtained based upon the arrival-time spectra of electrons by using a double-shutter drift tube for the E/N from 60 to 1000 Td, while the first and second ionization coefficients (α and γ) were determined by the steady-state Townsend method from 50 to 3000 Td. A comparison between the results and other data in the literature shows that our results for both the drift velocity and the effective ionization coefficient are lower than those of the other data in the above ranges.

Drift of Levitated/Suspended Body in High- $T_{c}$ Superconducting Levitation Systems Under Vibration—Part II: Drift Velocity for Gap Varying With Time

Abstract: To study levitation drift further, i.e., the gap between a superconductor and a permanent magnet varying with time in high-Tc superconducting levitation systems, drift velocity is introduced. Based on the numerical simulations of the dynamic response of a levitated body, and according to the essential reasons for drift, the drift velocity is first divided into two regimes: Vff [related to flux flow (FF)] and Vtc [related to flux creep (FC)]. The drift velocity is shown to be mainly dependent upon properties of superconductivity (such as the critical current density of superconductors), initial disturbances, and applied excitations (such as the amplitude and the frequency of external excitations). Furthermore, the corresponding influences of the drift velocities Vff and Vfc have been investigated quantitatively in this paper.

The Effects of the Injection-Channel Velocity on the Gate Leakage Current of Nanoscale MOSFETs

Abstract: A model is established to describe how the injection-channel electron velocity of nanometer MOSFETs affects the gate leakage current in the regime of diffusive to ballistic transition. The results show that the coupling between the transverse and the longitudinal components of electron motion largely affects the gate leakage current when the electron velocity is higher than the thermal injection velocity (1times107 cm/s). It suggests that due to the ballistic transport, the coupling effect should be considered for the gate leakage current of nanoscale MOSFETs, if the channel electrons reach a higher velocity

A Monte Carlo study of backscattering effects in the photoelectron emission from CsI into CH 4 and Ar-CH 4 mixtures

Abstract: Monte Carlo simulation is used to investigate photoelectron backscattering effects in the emission from a CsI photocathode into CH4 and Ar-CH4 mixtures for incident monochromatic photons with energies Eph in the range 6.8 eV to 9.8 eV (182 nm to 127 nm), and photons from a continuous VUV Hg(Ar) lamp with a spectral distribution peaked at Eph = 6.7 eV (185 nm), considering reduced applied electric fields E/N in the 0.1 Td to 40 Td range. The addition of CH4 to a noble gas efficiently increases electron transmission and drift velocity, due to vibrational excitation of the molecules at low electron energies. Results are presented for the photoelectron transmission efficiencies f, where f is the fraction of the number of photoelectrons emitted from CsI which are transmitted through the gas as compared to vacuum. The dependence of f on Eph, E/N, and mixture composition is analyzed and explained in terms of electron scattering in the different gas media, and results are compared with available measurements. Electron drift parameters are also calculated and compared with experimental data, confirming the choice of electron scattering cross-sections used in the simulations.

Generation of Alfvén Waves by a Plasma Inhomogeneity Moving in the Earth's Magnetosphere

Abstract: The generation of an Alfven wave by an azimuthally drifting cloud of high-energy particles injected in the Earth's magnetosphere is studied analytically. In contrast to the previous studies where the generation mechanisms associated with the resonant wave-particle interaction were considered, a nonresonant mechanism is investigated in which the wave is excited by the alternating current produced by drifting particles. It is shown that, at a point with a given azimuthal coordinate, a poloidally polarized wave, in which the magnetic field lines oscillate predominantly in the radial direction, is excited immediately after the passage of the particle cloud through this point. As the cloud moves away from that point, the wave polarization becomes toroidal (the mag- netic field lines oscillate predominantly in the azimuthal direction). The azimuthal wavenumber m is defined as the ratio of the wave eigenfrequency to the angular velocity of the cloud (the drift velocity of the particles). It is shown that the amplitudes of the waves so generated are close to those obtained under realistic assumptions about the density and energy of the particles.

A Numerical Simulation Code for Electronic and Ionic Transients From a Time-Resolved Pulsed Townsend Experiment

Abstract: A numerical code which calculating the spatio-temporal development of the discharge current in the Townsend regime has been developed. This code was written in Fortran 77. The individual contribution of electrons, positive, and negative ions to the total measurable current in the pulsed discharge is calculated by solving the corresponding set of coupled continuity equations for each species, for which boundary and initial conditions are set adequately. This simulation code is also capable of considering a more realistic scheme of photoelectron emission, such as that produced using a pulsed ultraviolet laser. In addition, the processes of ion and electron drift and longitudinal diffusion, electron detachment, negative-ion stabilization, and ion conversion have been calculated successfully. Some of these effects are extremely difficult to model analytically, and the simultaneous inclusion of two or more of them proves practically impossible. The power of this method in simulating complex discharge situations is discussed in this paper by means of examples considering the processes of electron and ion drift, diffusion, electron impact ionization and attachment, electron detachment, and negative-ion stabilization.

Spin lifetime in silicon in the presence of parasitic electronic effects

Abstract: A hybrid ferromagnet/semiconductor device is used to determine a lower bound on the spin lifetime for conduction electrons in silicon. We use spin precession to self-consistently measure the drift velocity vs. drift field of spin-polarized electrons and use this electronic control to change the transit time between electron injection and detection. A measurement of normalized magnetocurrent as a function of drift velocity is used with a simple exponential-decay model to argue that the lifetime obtained (~2 ns) is artificially lowered by electronic effects and is likely orders of magnitude higher.

A new experimental technique for positive ion drift velocity measurements in noble gases: Results for xenon ions in xenon

Abstract: A new technique is described for measuring the drift velocities and mobilities of noble gas ions in noble gases that makes use of a gaseous electron multiplier (GEM) and a UV flash lamp to produce the ions in a thin, well-defined, planar region. The drift velocities are determined from measurements of the transit time of the ions in crossing the drift space. We present experimental results for xenon ions in xenon. For these ions, the reduced mobility values measured at various pressures when E/N→0 fall within the interval [0.70, 0.74] cm2 V−1 s−1. A discussion of the results concerning the existence of Xe+, Xe 2 + , Xe 3 + , etc., ions is also made.

Heat transport properties of clean spin ladders coupled to phonons: Umklapp scattering and drag

Abstract: We study the low-temperature heat transport in clean two-leg spin-ladder compounds coupled to threedimensional phonons. We argue that the very large heat conductivities observed in such systems can be traced back to the existence of approximate symmetries and corresponding weakly violated conservation laws of the effective gapful low-energy model, namely, pseudomomenta. Depending on the ratios of spin gaps and Debye energy and on the temperature, the magnetic contribution to the heat conductivity mag can be positive or negative and can exhibit an activated or antiactivated behavior. In most regimes, mag is dominated by the spin-phonon drag: the excitations of the two subsystems have almost the same drift velocity, and this allows for an estimate of the ratio mag/ph of the magnetic and phononic contributions to the heat conductivity.