Sweep Direction Research Articles

Global stability of swept flow around a parabolic body: connecting attachment-line and crossflow modes

The global linear stability of a three-dimensional compressible flow around a yawed parabolic body of infinite span is investigated using an iterative eigenvalue method in conjunction with direct numerical simulations. The computed global spectrum shows an unstable branch consisting of three-dimensional boundary layer modes whose amplitude distributions exhibit typical characteristics of both attachment-line and crossflow modes. In particular, global eigenfunctions with smaller phase velocities display a more pronounced structure near the stagnation line, reminiscent of attachment-line modes while still featuring strong crossflow vortices further downstream. This analysis establishes a link between the two prevailing instability mechanisms on a swept parabolic body which, so far, have only been studied separately and locally. A parameter study shows maximum modal growth for a spanwise wavenumber of β = 0.213, suggesting a preferred disturbance length scale in the sweep direction.

Quasi-Static Capacitance-Voltage Characterizations of Mobile Ion Accumulation and Depletion Phenomena in Organic Thin-Film Transistors

By conducting C-V profile measurement, we analyzed the charge transport behavior in polycrystalline pentacene OTFTs. We found that carrier accumulation and depletion processes in the semiconductor channel layer is caused by a trap and de-trap mechanism. We apply such processes to explain C-V profiles at various gate voltages, ramp rates and sweep directions. At faster ramp rates, hysteresis can be observed from C-V profile. The phenomenon can be attributed to the asymmetry of the relaxation time of carrier trap and detrap mechanism. At the slower ramp rate, most carriers are able to interact with the gate stress, so the hysteresis disappears. And beyond this ramp rate, the corresponding threshold voltages are then kept at constant values, despite the existence of traps. Furthermore, low-mobility mobile ions or impurities participate in the charge accumulation and depletion when the ramp rate is much slower, which results in a skew of C-V profiles.

Impact of in-plane currents on magnetoresistance properties of an exchange-biased spin valve with an insulating antiferromagnetic layer

The impact of in-plane alternating currents on the exchange bias, resistance, and magnetoresistance of a ${\mathrm{Co}}_{85}{\mathrm{Fe}}_{15}∕{\mathrm{Ni}}_{0.85}{\mathrm{Co}}_{0.15}\mathrm{O}∕{\mathrm{Co}}_{85}{\mathrm{Fe}}_{15}∕\mathrm{Cu}∕{\mathrm{Co}}_{85}{\mathrm{Fe}}_{15}$ spin valve is studied. With increasing current, the resistance is increased while the maximum magnetoresistance ratio decreases. Noticeably, the reversal field of the pinned layer is systematically suppressed in both field sweeping directions. Since ${\mathrm{Ni}}_{0.85}{\mathrm{Co}}_{0.15}\mathrm{O}$ is a good insulator, it is expected that the ac flows only in the ${\mathrm{Co}}_{85}{\mathrm{Fe}}_{15}∕\mathrm{Cu}∕{\mathrm{Co}}_{85}{\mathrm{Fe}}_{15}$ top layers, thus ruling out any presence of spin-transfer torque acting on the spins in the antiferromagnetic layer. Instead, our measurements show clear evidence of the influence of Joule heating caused by the current. Moreover, results from temperature-dependent measurements very much resemble those of the current dependence, which indicates that the effect of Joule heating plays a major role in the current-in-plane spin-valve configurations. The results also suggest that spin-transfer torques between ferromagnetic layers might still exist and compete with the exchange bias at sufficiently high currents.

Frequency‐swept HMQC sequences for high‐throughput NMR analysis

We describe here new versions of the DEPT phase-encoded HMQC experiment that offer robust performance and improved sensitivity. The new sequences rely on frequency-swept proton and carbon pulses to minimize signal losses from miscalibrated pulses while providing 'J compensation' to optimize the signal strength over a range of heteronuclear coupling constants. By including both proton and carbon-swept pulses, the new sequences also offer an additional signal gain of roughly 10% over well-calibrated hard-pulse experiments. The new sequences also demonstrate that one can construct a sequence that incorporates both 90 degrees and 180 degrees frequency-swept pulses. Although individual pulses in the sequence cause severe phase roll, the phase roll can be eliminated by the proper choice of pulse lengths and sweep directions.

Effects of the task of categorizing FM direction on auditory evoked magnetic fields in the human auditory cortex

We examined effects of the task of categorizing linear frequency-modulated (FM) sweeps into rising and falling on auditory evoked magnetic fields (AEFs) from the human auditory cortex, recorded by means of whole-head magnetoencephalography. AEFs in this task condition were compared with those in a passive condition where subjects had been asked to just passively listen to the same stimulus material. We found that the M100-peak latency was significantly shorter for the task condition than for the passive condition in the left but not in the right hemisphere. Furthermore, the M100-peak latency was significantly shorter in the right than in the left hemisphere for the passive and the task conditions. In contrast, the M100-peak amplitude did not differ significantly between conditions, nor between hemispheres. We also analyzed the activation strength derived from the integral of the absolute magnetic field over constant time windows between stimulus onset and 260 ms. We isolated an early, narrow time range between about 60 ms and 80 ms that showed larger values in the task condition, most prominently in the right hemisphere. These results add to other imaging and lesion studies which suggest a specific role of the right auditory cortex in identifying FM sweep direction and thus in categorizing FM sweeps into rising and falling.

Characteristics and mechanisms of hysteresis in polymer field‐effect transistors

AbstractHysteresis effects do occur usually in polymer field‐effect devices. It has been suggested that trap recharging or mobile ions, or formation/dissociation of bipolarons (BP) in the accumulation layer can cause this effect. Here, at first a literature survey on the hysteresis in field‐effect transistors is given. Then examples of measured hysteresis in field‐effect transistors and in metal‐insulator‐semiconductor (MIS) capacitors are presented. The typical peculiarity is the dependency of the flat band voltage on the sweep direction of the gate voltage. Our recent numerical simulations on the trap recharging mechanism in MIS capacitors are continued and extended to transistors. Energetically distributed traps can lead to hysteresis. But the form of the hysteresis deviates from the observed one and extreme parameter values are needed. Thus it is more likely that trap recharging can modify a hysteresis caused by another mechanism. For discussing the bipolaron mechanism the equilibrium between polarons and doubly charged states of the polymer chains was analyzed anew. With a rate constant for the bipolaron formation determined recently by Salleo and Street relaxation times for formation/dissociation are determined. They indicate that these processes can cause the hysteresis. A further possible mechanism influencing the hysteresis is connected with complexes between polarons, bipolarons and mobile counter ions. The estimated rate constants indicate processes on the time scale of the measurements. Numerical simulations of these processes as well as of the role of mobile ions and their reactions are required for a full description of the hysteresis. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Capacitance‐voltage characteristics of MIS capacitors using polymeric insulators

In this study, we investigate the capacitance‐voltage (C‐V) characteristics of metal‐insulator‐semiconductor (MIS) capacitors consisting of pentacene, as an organic semiconductor, and polymeric insulators such as poly(4‐vinylphenol) (PVP) or polystyrene (PS) prepared by spin‐coating process, to analyze the interfacial characteristics between pentacene and polymeric insulators. Compared with the device with PS, the MIS capacitor with PVP exhibited a pronounced shift in the flat‐band voltage according to the bias sweep direction. This hysteric feature in the C‐V characteristics is thought to be attributed to the trapped charges at the interface between pentacene and PVP owing to the hydrophilicity of PVP. From the experimental results, we can conclude that surface polarity of polymeric insulator has a critical effect on the interfacial properties, thereby affecting the bias stability of organic thin‐film transistors.

New PMOS LTPS–TFT pixel for AMOLED to suppress the hysteresis effect on OLED current by employing a reset voltage driving

New PMOS LTPS (low temperature polycrystalline silicon)–thin film transistor (TFT) pixel circuit, which can suppress an OLED current error caused by the hysteresis of LTPS–TFT for active matrix organic light emitting diode (AMOLED) display, is proposed and fabricated. The proposed pixel circuit employs a reset voltage driving so that the sweep direction of gate voltage in the current driving TFT is not altered by the gate voltage in the previous frame. Our experimental results show that OLED current error of the proposed pixel is successfully suppressed because a reset voltage can enable the starting gate voltage for a desired one not to be varied, while that of the conventional 2-TFT pixel exceeds over 15% due to the hysteresis of LTPS–TFT.

Investigation of the hysteresis phenomenon of an a‐Si:H TFT at an elevated temperature for AMOLED displays

Abstract— The temperature dependence of the hysteresis of an a‐Si:H TFT has been investigated. An a‐Si:H TFT pixel driving scheme has been proposed and investigated. This scheme can eliminate changes in the organic light‐emitting diode (OLED) current caused by hysteresis of an a‐Si:H TFT. The VTH of the a‐Si:H TFT was changed according to the gate‐voltage sweep direction because of the hysteresis of the a‐Si:H TFT. The variation of VTH for a a‐Si:H TFT decreased from 0.41 to 0.17 V at an elevated temperature of 60°C because the sub‐threshold slope (s‐slope) of the a‐Si:H TFT, in the reverse voltage sweep direction, increased more than in the forward voltage sweep direction due to a greater increase in the initial electron trapped charges than the hole charges. Although the OLED current variation caused by hysteresis decreased (∼14%) as the temperature increased, the error in the OLED current needed to be improved in order to drive the pixel circuit of AMOLED displays. The proposed pixel circuit can apply the reset voltage (−10 V) before the data voltage for the present frame that was written to fix the sweep direction of the data voltage. The variation in the OLED current caused by hysteresis of the a‐Si:H TFT was eliminated by the fixed voltage sweep direction in the proposed pixel circuit regardless of operating temperature.

Quasi-static capacitance–voltage characterizations of carrier accumulation and depletion phenomena in pentacene thin film transistors

We analyze charge accumulation and depletion behaviors from C–V curves at various gate voltages, ramp rates and sweep directions in pentacene OTFTs. The polycrystalline property of the pentacene layer has lead to carrier charge and discharge processes in the channel layer by a trap-controlled transport mechanism. We apply such processes to describe C–V profiles at various ramp rates. Hysteresis can be observed from C–V curves at faster ramp rates. The phenomenon can be attributed to the difference of the relaxation time of carrier trap and detrap processes. As the ramp rate becomes slower, hysteresis disappears since most carriers are able to interact with the gate stress. And beyond this ramp rate, the corresponding threshold voltages are then kept at constant values, despite the existence of traps. Furthermore, low-mobility carriers (mobile ions or impurities) participate in the charge accumulation and depletion when the ramp rate is even slower, which results in a skew of C–V profiles. Finally, we extend the C–V measurement to devices with different channel lengths.

Effect of Vibroseis Arrays on Ground Vibrations: A Numerical Study

Vibroseis trucks are commonly used for the acquisition of seismic reflection data for petroleum exploration because they produce low levels of ground vibration. When operating in an urban setting, vibrations are typically monitored using particle velocity meters at the ground surface, and vibration measurements are quantified in terms of peak particle velocity (PPV). Distance from the Vibroseis trucks is generally used as a parameter to identify potential points of high PPV. However, numerical modeling of PPV fields surrounding multiple-truck Vibroseis arrays reveals the presence of complex zones of constructive interference where PPV can double within as little as [Formula: see text] [Formula: see text]. Vibroseis sweep direction and duration is shown to have little effect on PPV. The results presented herein are specific to a certain set of modeling parameters, but nevertheless demonstrate that it may be necessary to perform pre-survey numerical simulation of surface vibrations to identify points relative to the Vibroseis array that are susceptible to high vibration levels, and monitor vibrations at those points.

The Hysteresis Phenomenon in a-Si:H TFT and Poly-Si TFT in AMOLED

The hysteresis phenomenon of both a-Si and poly-Si TFT has been studied in order to investigate the hystersis phenomenon effect on residual image of AMOLED display. The transfer characteristics of both a-Si:H TFT and poly-Si TFT were varied according to the gate voltage sweep direction due to the different initial interface trapped charges in the gate insulator. The threshold voltage in a-Si:H TFT was altered by 0.23V due to hysteresis of device according to gate sweep direction while that in poly-Si TFT was varied by 0.21V due to the different gate insulator. In order to suppress hystersis effect on OLED current error we proposed and fabricated a new pixel circuit. By applying the reset voltage in pixel circuit, the data voltage sweep direction could be fixed and hysteresis effect on OLED current error was successfully suppressed. We showed that the OLED current error was successfully suppressed by experiment.

Tunneling induced electron transfer in SiN /AlGaN/GaN based metal–insulator–semiconductor structures

Abstract Tunneling induced electron transfer in SiN x /Al 0.22 Ga 0.78 N/GaN based metal–insulator–semiconductor (MIS) structures has been investigated by means of capacitance–voltage (C–V) measurements at various temperatures. Large clock-wise hysteresis window in C–V profiles indicates the injection of electrons from the two-dimensional electron gas (2DEG) channel to the SiN x layer. Depletion of the 2DEG at positive bias in the negative sweeping direction indicates that the charges injected have a long decay time, which was also observed in the recovery process of the capacitance after injection. The tunneling induced electron transfer effect in SiN x /Al 0.22 Ga 0.78 N/GaN based MIS structure opens up a way to design Al x Ga 1− x N/GaN based variable capacitors and memory devices.

Impact of Positive Charges on NBTI Degradation and Characterization in Ultra-Thin SiON Gate Dielectrics

Impact of positive oxide charges on NBTI degradation and characterization was studied in this work. It was found that the subthreshold swing shifts extracted by Id-Vg characteristic measurements are significantly affected by the gate voltage sweep directions. Further experimental results demonstrate that positive oxide charge is responsible for the observed phenomena, simultaneously providing new evidence for the importance of positive oxide charges in NBTI degradation and recovery. Neutralization (and re-charge) of positive charges during the measurements is considered to have significant influence on NBTI characterization. Our results show that the impact of positive oxide charges on the NBTI degradation and characterization should be taken into consideration seriously.

Half-Integer Flux Quantization in a Superconducting Loop with a Ferromagnetic π-Junction

Superconducting loops containing a π-junction are predicted to show a spontaneous magnetic moment in zero external magnetic field. In order to confirm this longstanding prediction experimentally, we performed magnetization measurements on individual mesoscopic superconducting niobium loops with a ferromagnetic (PdNi) π-junction. The loops are prepared on top of the active area of a micro Hall-sensor based on high mobility GaAs/AlGaAs heterostructures. We observe switching of the loop between different magnetization states at very low-magnetic fields, which is asymmetric for positive and negative sweep direction. This is evidence for a spontaneous current induced by the intrinsic phase shift of the π-junction. In addition, the presence of the spontaneous current at zero applied field is directly revealed by an increase of the magnetic moment with decreasing temperature, which results in a half integer flux quantization in the loop at low temperatures.

Lateralization of linear and logarithmic frequency-modulated (FM) sweeps at high frequencies

The ability to lateralize linear and logarithmic frequency‐modulated (FM) sweeps at high frequencies was investigated using a 2IFC adaptive block design. The FM stimulus swept from 3 kHz to 8 kHz with the onset frequency roved by 15% on each presentation. The cue to be detected was an interaural delay in the waveform’s fine‐structure (i.e., delay in instantaneous frequency) that varied on each trial according to a 2‐down 1‐up adaptive rule. The waveforms to the left and right channels had simultaneous onset/offset envelopes. The stimulus duration (0.5, 1, 5, 10, 25, or 50 ms) and thus the sweep rate, as well as the sweep direction (up or down), were additional parameters of the study. Thresholds were higher than expected from studies of sinusoidal FM and ranged from approximately 100 μs for the 0.5 ms sweep to near chance for durations exceeding 10 ms. Sweep direction and type (log versus linear) did not substantially affect thresholds. Implications of these findings for models of binaural interaction at high frequencies and within‐channel mechanisms of FM‐to‐AM transduction will be discussed. [Work supported by NSF.]

Electrochemical determination of activation energies for methanol oxidation on polycrystalline platinum in acidic and alkaline electrolytes

The oxidation pathways of methanol (MeOH) have been the subject of intense research due to its possible application as a liquid fuel in polyelectrolyte membrane (PEM) fuel cells. The design of improved catalysts for MeOH oxidation requires a deep understanding of these complex oxidation pathways. This paper will provide a discussion of the literature concerning the extensive research carried out in acidic and alkaline electrolytes. It will highlight techniques that have proven useful in the determination of product ratios, analysis of surface poisoning, anion adsorption, and oxide formation processes, in addition to the effects of temperature on the MeOH oxidation pathways at bulk polycrystalline platinum (Pt(poly)) electrodes. This discussion will provide a framework with which to begin the analysis of activation energy (E(a)) values. This kinetic parameter may prove useful in characterizing the rate-limiting step of the MeOH oxidation at an electrode surface. This paper will present a procedure for the determination of E(a) values for MeOH oxidation at a Pt(poly) electrode in acidic and alkaline media. Values from 24-76 kJ mol(-1) in acidic media and from 36-86 kJ mol(-1) in alkaline media were calculated and found to be a function of applied potential and direction of the potential sweep in a voltammetric experiment. Factors that influence the magnitude of the calculated E(a) include surface poisoning from MeOH oxidation intermediates, anion adsorption from the electrolyte, pH effects, and oxide formation processes. These factors are all potential, and temperature, dependent and must clearly be addressed when citing E(a) values in the literature. Comparison of E(a) values must be between systems of comparable electrochemical environment and at the same potential. E(a) values obtained on bulk Pt(poly), compared with other catalysts, may give insight into the superiority of other Pt-based catalysts for MeOH oxidation and lead to the development of new catalysts which lower the E(a) barrier at a given potential, thus driving MeOH oxidation to completion.

Scanning capacitance microscopy and the role of localized charges in dielectric films: Infering or challenging?

Scanning capacitance microscopy (SCM) is considered a suitable technique to examine dopant profiles and to provide images of the charge carrier distribution in real device structures. A drawback of the application is the sensitivity of the method to different kinds of charges in the surface passivation layer. For a systematic study silicon structures with stripe patterns of different doping concentrations, covered either by a native oxide or by a thermally grown silicondioxide layer, were examined. The dependence of the d C/d V characteristic on the sweep rate of the tip bias and on the bias sweep direction was analyzed. Local charge trapping in the passivation layer was estimated from shifts of the peak position of the d C/d V versus V curves. It is demonstrated as to how features related to the charge trapping in the passivation layer and induced by a certain dc bias at the tip superimpose the dopant related features in the SCM image.

Bipolarons or polaron pairs in conducting polymers: Equilibrium and kinetics

Equilibrium and kinetics are compared for conjugated polymers, supposing the charged states are either polarons (Ps) and bipolarons (BPs) or polarons and polaron pairs (PPs). For low concentrations equilibrium and kinetics are virtually indistinguishable. Both are essentially different for high concentrations due to the fact that the extension of the bipolaron is almost the same as that of one polaron whereas the polaron pair has the extension of two polarons. As a result, the polaron concentration, i.e. the spin concentration, shows for a system with bipolarons a maximum and decreases for continuing increase of the potential. This dependence corresponds to results of ESR spectroelectrochemistry. In contrast, for the system with PPs both P and PP concentrations saturate at high potentials. Thus, the PP model is only reasonable if an additional subsequent second oxidation step is possible. The kinetics of the formation and dissociation of PPs or BPs, is essentially the same for concentrations that can be achieved in accumulation layers in organic MOS devices. The estimated relaxation times point to the formation and dissociation of either BPs or PPs as processes that can either cause or influence hysteresis effects in polymeric MOS capacitors, which can be described by an interface charge, depending on the sweep direction in the capacitance-voltage measurements.

Electro-Catalytic Oxidation of Methanol on a Ni–Cu Alloy in Alkaline Medium

The electro-catalytic oxidation of methanol on a Ni–Cu alloy (NCA) with atomic ratio of 60/40 having previously undergone 50 potential sweep cycles in the range 0–600 mV vs. (Ag/AgCl) in 1 m NaOH was studied by cyclic voltammetry (CV), chronoamperometry (CA) and impedance spectroscopy (EIS). The electro-oxidation was observed as large anodic peaks both in the anodic and early stages of the cathodic direction of potential sweep around 420 mV vs. (Ag/AgCl). The electro-catalytic surface was at least an order of magnitude superior to a pure nickel electrode for methanol oxidation. The diffusion coefficient and apparent rate constant of methanol oxidation were found to be 2.16 × 10−4 cm2 s−1 and 1979.01 cm3 mol−1 s−1, respectively. EIS studies were employed to unveil the charge transfer rate as well as the electrical characteristics of the catalytic surface. For the electrochemical oxidation of methanol at 5.0 m concentration, charge transfer resistance of nearly 111 Ω was obtained while the resistance of the electro-catalyst layer was ca. 329 Ω.