Spl Beta Research Articles

On modeling biomedical ultrasound RF echoes using a power-law shot-noise model.

We propose a new model for the RF ultrasound echo, namely the power-law shot-noise process. Based on this model, the in-phase and quadrature components of the echo are shown to exhibit 1/f beta-type spectral behavior, in a sense that is defined in the paper. The envelope also exhibits this type of spectral behavior, but with a different exponent. This result explains the experimental observations by other researchers of the power-law trend of the RF echo spectrum. Although the shot-noise model has been used in the past for modeling the RF echo, this is the first time that a power-law impulse response filter is used and that the resulting 1/f beta-type spectral behavior of the RF echo has been investigated. The model parameters are linked to tissue characteristics, such as scatterer density and attenuation; thus, they have the potential to be used as tissue characterization features. The validity of the proposed model is tested based on a database of 100 clinical ultrasound images of the breast.

Magnetic field and thermal reversal properties of exchange-bias recording films

Effect of exchange-bias coupling between ferromagnetic (FM) layer and antiferromagnetic (AFM) underlayer on the H-field and thermal reversal properties was studied analytically by minimizing the free energy E(/spl alpha/,/spl beta/) of the bilayer system. Correlation between the effective-thermal-stability-factor (K/sub u/V*/k/sub B/T)/sub eff/, coercivity H/sub C/, and parameters of the bilayer system is presented. Emphasis is on analyzing the energy-barrier E/sub B/ influence upon the moment reversal properties.

Semiparametric ARX neural-network models with an application to forecasting inflation

We examine semiparametric nonlinear autoregressive models with exogenous variables (NLARX) via three classes of artificial neural networks: the first one uses smooth sigmoid activation functions; the second one uses radial basis activation functions; and the third one uses ridgelet activation functions. We provide root mean squared error convergence rates for these ANN estimators of the conditional mean and median functions with stationary beta-mixing data. As an empirical application, we compare the forecasting performance of linear and semiparametric NLARX models of US inflation. We find that all of our semiparametric models outperform a benchmark linear model based on various forecast performance measures. In addition, a semiparametric ridgelet NLARX model which includes various lags of historical inflation and the GDP gap is best in terms of both forecast mean squared error and forecast mean absolute deviation error.

Universal discriminant for continuous-wave operation of laser diodes

We propose a simple and powerful criterion for defining continuous-wave (CW) operation of laser diodes. Our criterion is represented by a quasi-triangular region in the plain spanned by dimensionless parameters /spl alpha/ and /spl beta/. These parameters, deduced from the characteristics of the laser diode operating under pulsed current injection, represent the effects of the turn-on voltage and the series resistance on the increase in the CW threshold. We demonstrate the validity of our criterion using blue-green lasers made from II-VI semiconductors as examples.

Effects of leakage flux on magnetic fields of interior permanent magnet synchronous motors

This paper presents a magnetic circuit model for an interior permanent magnet (IPM) synchronous machine. The model includes each portion of magnetic flux paths, especially the leakage flux paths around the magnets in the rotor. The magnetic field quantities are expressed analytically in terms of the reluctance ratio /spl beta/ and the leakage flux ratios /spl eta/ and /spl lambda/. It is shown that the width and the iron saturation level of the iron bridge have an important effect on the magnetic field distribution within the motor. Numerical results confirm the validity of the analytical model.

Coupling model explanation of salient dynamic properties of glass-forming substances

Most of the important and general dynamic properties of structural glass-forming substances have been classified into twelve different categories in a recent review. Within the same property in each category, by examining a large number of glass-formers, a pattern of variation is found which correlates with the departure of the structural relaxation of the glass-former from exponential decay or the quantity 1-/spl beta/, where /spl beta/ is the fractional exponent in the Kohlrausch expression exp[-(t//spl tau/)/sup /spl beta//], that well describes the time dependence of the structural at relaxation. The correlation of the patterns or trends with 1-/spl beta/ suggests that the salient dynamic properties, or phenomenology, of glass-forming substances, all are governed by the non-exponential nature of the structural relaxation, i.e. the quantity 1-/spl beta/. Therefore, a theory or model capable of relating the structural relaxation time /spl tau/ to 1-/spl beta/ has the potential of explaining most, if not all, of the salient dynamic properties of glass-forming substances. The coupling model of the author has this relation. In this accompanying paper we show the coupling model can either explain or interpret all these properties.

Extraction of eleven model parameters for consistent reproduction of lateral bipolar snapback high-current I-V characteristics in NMOS devices

A series of literature models originally devoted to the second breakdown trigger current I/sub t2/ in a grounded-gate NMOS transistor can further find promising potential in handling high-current I-V due to lateral bipolar snapback. This is achieved primarily by building significant linkage between bipolar current-gain /spl beta/-related parameters: 1) the collector-to-base junction voltage dependencies A/sub 1/ and A/sub 2/ of the medium-level injection /spl beta/ roll-off factor; 2) the high-level /spl beta/ roll-off factor A/sub 3/; and 3) the collector-to-base junction voltage dependencies A/sub 4/ and A/sub 5/ of the collector corner current at the onset of high-level /spl beta/ roll-off. The new parameters A/sub 1/ to A/sub 5/ enable a consistent I-V solution along with other existing six model parameters such as the substrate resistance R/sub sub/ and its conductivity modulation factor A/sub /spl tau//, the impact ionization coefficients K/sub 1/ and K/sub 2/, and the emitter series resistance R/sub e/ and collector series resistance R/sub c/. Parameter extraction except R/sub c/ is thoroughly performed using only the parametric analyzer, and opposed to the traditional procedure, impact ionization coefficients and current gains are all assessed without entering the snapback regime. Remarkably, not only excellent agreements are obtained, but also bipolar snapback I-V measured under the current pulsing condition can be separated into two distinct parts: medium- and high-level injection region. This is quite effective under R/sub e/=R/sub c/. Series resistance, although having very low value, is not to be absent under the high-level injection conditions.

Complex dynamics of hydrogen bonded self-assembling polymers

Supramolecular polymers, which are non-covalently bonded and formed by self association of di or trifunctional monomers exhibit, by virtue of quadruple hydrogen bonds, many of the properties of normal high molecular weight polymers, e.g. a dynamic rubber plateau. We focus on the molecular and cooperative dynamics of self-assembled linear polymers and networks, studied by broadband dielectric relaxation spectroscopy (DRS) in the frequency range from 10/sup -2/ to 10/sup 6/ Hz. The DRS analysis was backed up by dynamic mechanical and rheological experiments. In the high temperature region two loss processes (/spl alpha/ and /spl alpha/*) show up, the relaxation times of which obey the Vogel-Fulcher-Tammann (VFT) law. The dielectric /spl alpha/ process is related to the dynamic glass-rubber transition and is slightly faster than the corresponding mechanical /spl alpha/ process. A slower (high-temperature) /spl alpha/* relaxation is identified as chemical, corresponding to the mean lifetime of the hydrogen bonded linkages in the supramolecular chains. Its relaxation time /spl tau//sub /spl alpha/*/ was found to be 1 to 2 decades larger than the terminal flow relaxation time, indicating that the relaxation of an entire chain is dominated by the joint dynamics of many hydrogen bonds. The /spl beta/ relaxation, observed in both DRS and dynamic mechanical analysis (DMA) at temperatures below T/sub g/, arises from the local junction dynamics of hydrogen bonded units in the glassy state. Details of the temperature dependence and the shape of the loss peaks of the /spl alpha/ and /spl alpha/* relaxations will be discussed in terms of temporary physical networks and cooperativity.

Can charge cause dielectric hysteresis? Observations on non-ferroelectric α-PVDF

Under suitable conditions, hysteresis of the current or of the polarization can be observed as a function of the electric field during poling of ferroelectric polymer films. This behavior has been investigated extensively, and was explained for various ferroelectric polymers with the well-known theory of ferroelectrics. The present work, however, indicates that poling of polymers that contain excess charges but no polar crystallites, sometimes leads to a similar hysteresis behavior. In support of this finding, we present and discuss current measurements during the electric poling of /spl alpha/-polyvinylidenefluoride (PVDF) films and compare the results to the respective behavior of ferroelectric /spl beta/-PVDF films in which charge effects play quite a different role.

Model-based characterization of electromagnetic induction signatures obtained with the MTADS electromagnetic array

A sensor response model based on empirically determined orthogonal response coefficients /spl beta/ is presented for the analysis of electromagnetic induction (EMI) sensor data. The model is applied to high-quality survey data from the multi-sensor towed array detection system (MTADS) program. Data from both a prepared test range and a live-site survey are analyzed. At the test field, the authors show that the model successfully reproduces the measured signatures of ordnance, ordnance simulants, flat plates, and clutter. At the live site, fit statistics were collected showing that for the most abundant ordnance target, 81-mm mortars, the three beta response parameters are distributed log-normally. This allows for a simple method of discrimination. A receiver operating characteristic (ROC) analysis based on this discrimination method is performed and the results discussed.

Current gain dependence on subcollector and etch-stop doping in InGaP/GaAs HBTs

The DC current gain dependence of InGaP/GaAs heterojunction bipolar transistors (HBTs) on subcollector and etch-stop doping is examined. Samples of InGaP/GaAs HBTs having various combinations of subcollector doping and etch-stop doping are grown, and large area 60 /spl mu/m/spl times/60 (/spl mu/) HBTs are then fabricated for DC characterization. It is found that the DC current gain has a strong dependence on the doping concentration in the subcollector and the subcollector etch-stop. Maximum gain is achieved when the subcollector is doped at 6/spl sim/7/spl times/10/sup 18/ cm/sup -3/ while the subcollector etch-stop is doped either above 6/spl times/10/sup 18/ cm/sup -3/ (current gain/sheet resistance ratio, /spl beta//R/sub b/=0.435 at I/sub c/=1 mA) or below 3.5/spl times/10/sup 17/ cm/sup -3/ (/spl beta//R/sub b/=0.426/spl sim/0.438 at I/sub c/=1 mA). The data show that it is not necessary to heavily dope the subcollector etch-stop to reduce the conduction barrier and to obtain high current gain. The high current gain obtained with the low InGaP etch-stop doping concentration is attributed to the reduction of the effective energy barrier thickness due to band bending at the heterojunction between the InGaP etch-stop and the GaAs subcollector. These results show that the /spl beta//R/sub b/ of InGaP/GaAs HBTs can improve as much as 69% with the optimized doping concentration in subcollector and subcollector etch-stop.

Measurements of the InGaAs hole impact ionization coefficient in InAlAs/InGaAs pnp HBTs

The hole multiplication factor in pnp InAlAs/InGaAs single heterojunction bipolar transistors (HBTs) has been measured as a function of the base-collector bias. The hole impact ionization coefficient /spl beta//sub p/ has been estimated taking into account the Early effect, I/sub CBO/, and thermal effects. Numerical corrections for dead space were made. The importance of considering second order effects is highlighted, showing that rough approximations can lead to an overestimation of the coefficient /spl beta//sub p/. At low electric fields, the extracted coefficient agrees with the most recent photomultiplication measurements available in the literature. At high electric fields, hole impact ionization coefficient is estimated up to values previously not reported in the literature (/spl beta//sub p//spl ap/10/sup 4/ cm/sup -1/).

Three-dimensional pulse-width modulation technique in three-level power inverters for three-phase four-wired system

Shunt-connected trilevel power inverter in three-phase four-wired system as an active filter or individual current supply (peak-load supply) is studied by a novel technique: three-dimensional (3-D) voltage vectors pulse width modulation (PWM). In past decades, almost all the study for PWM is limited to the two-dimensional (2-D) domain, /spl alpha/ and /spl beta/ frames, in a three-phase three-wired system. However, in practical operation, there are many three-phase four-wired systems in distribution sites. The generalized study of 3-D two-level and three-level inverters is achieved in this paper so as to perform the basic theory of 3-D multilevel space vector switching PWM technique. The sign cubical hysteresis control strategy is proposed and studied with simulation results in 3-D aspect. The 3-D PWM technique in three-level inverters is accomplished.

Using porous silicon as semi-insulating substrate for β-SiC high temperature optical-sensing devices

This work demonstrates the availability of using porous silicon as semi-insulating substrate for /spl beta/-SiC high temperature optical sensing devices. An MSM structure was fabricated both on a porous silicon substrate and a conventional silicon substrate, respectively. Experimental results show the optical current ratio can be improved up to 400% at room temperature and 3000% at 200/spl deg/C operating temperature, respectively, with the porous silicon substrate.

A new metastable phase with T/sub c/ of 32 K in La/sub 2/CuO/sub 4+δ/ system

A metastable superconducting phase with T/sub c/ of 32 K and /spl delta/=0.084 in La/sub 2/CuO/sub 4+/spl delta// system is reported in this paper. It is prepared by electrochemical oxidation for more than 2 months using a small constant current of 10 /spl mu/A. Its structure and physical properties are studied by X-ray diffraction, magnetic susceptibility and resistance measurements. This phase (we call it /spl beta/-phase) is different from the previously known one (we call it /spl alpha/-phase) that has the same T/sub c/ but 0/spl les//spl delta//spl les/0.055. After heat treatment at temperature T=80/spl deg/C, the /spl beta/-phase decomposes into two phases with T/sub c/'s of /spl sim/17 K and /spl sim/30 K, respectively. This kind of behavior is similar to the phase with T/sub c/ of 45 K. Our study shows that T/sub c/ increases with the intercalation rate, and oxygen distribution is an important factor for controlling superconducting properties.

The convergence properties of a clipped Hopfield network and its application in the design of keystream generator

We first present a modified Hopfield network, the clipped Hopfield network, with synaptic weights assigned to three values {-1,0,+1}. We give the necessary conditions under which a set of 2n binary vectors can be stored as stable points of the network. We show that in the parallel updating mode, for most of the state vectors, the network will always converge to these 2n stable points. We further demonstrate that these 2n stable points can be divided into two groups, the alpha group and the beta group, each with n stable points. It is shown that the basins of attraction of the stable points in the alpha group are evenly distributed, and the basins of attraction of the stable points in the beta group are also evenly distributed. By ways of application, we show that this class of Hopfield network can be used to build a cryptographically secure keystream generator.

Estimation of the Nb-Ti-Ta phase diagram

The Nb-Ti-Ta ternary alloy system has been studied experimentally using x-ray diffraction, electron microprobe analysis, and measurements of the superconducting critical temperature. This information, together with additional analysis of the thermodynamics of the system, has been used to develop an improved picture of the Nb-Ti and Ta-Ti binary phase diagrams, and the Nb-Ti-Ta ternary phase diagram. The experimental phase diagrams differ in several respects from published versions of the binaries, and the kinetics of the /spl alpha/-Ti precipitation reaction is invoked to explain some of these inconsistencies. The principle differences lie in the position of the (/spl alpha/+/spl beta/)//spl beta/ phase boundary in the Nb-Ti binary, which appears to be pushed to higher Nb contents than in the presently accepted phase diagram. An improved ternary phase diagram can be used to suggest alternative alloys and processing to provide high field, high current density Nb-Ti-Ta superconductors for magnet applications.

Progress of LHC low-/spl beta/ quadrupole magnets at KEK

Development of the LHC low-/spl beta/ insertion quadrupole magnet has been in progress at KEK since 1995 as a cooperative program between CERN and KEK. Five 1-m short model magnets have been fabricated and three of them have been tested. From the various test results of the first two models, the coil configuration was further optimized to reduce the higher magnetic field harmonic coefficients. The cold test of the third model showed satisfactory performances of the field harmonics. After this R&D work, the authors are at a stage for the fabrication of two prototype magnets which have the same scale as the production magnets. The status of the R&D for the LHC low-beta insertion quadrupole magnet at KEK is described.

An implanted-emitter 4H-SiC bipolar transistor with high current gain

An epi-base, implanted-emitter, npn bipolar transistor which showed a maximum common emitter current gain (/spl beta/) of /spl sim/40, the highest current gain reported for BJT in any polytype of SiC has been experimentally demonstrated in 4H-SiC. The forward drop was /spl sim/1 V at forward current density of 50 A/cm/sup 2/. The current gain decreases hence specific on-resistance increases with increasing temperature. The negative temperature coefficient of /spl beta/ makes the device attractive for paralleling and for preventing thermal runaways.

YBa/sub 2/Cu/sub 3/O/sub 7-δ/ bicrystal grain boundary T/sub c/ microstructure

We use an S/sub /spl alpha//S/sub /spl beta//IS/spl beta//sub S/spl alpha// tunneling/proximity-effect model with measured YBa/sub 2/Cu/sub 3/O/sub 7-/spl delta// (YBCO) grain boundary function (GBJ) normalized critical current temperature dependence /spl eta/(T)=I/sub c/(T)I/sub c/(O) to infer microstructural transition temperature profiles T/sub c/(x) for bicrystal GBJs with various misorientation angles /spl theta/. We discuss the rationale for selecting an optimum value for a numerical analysis parameter /spl epsi/ (associated with the S/sub /spl alpha//S/sub /spl beta// interface T/sub c/ discontinuity). We determine the extent to which inferred T/sub c/(x) profiles vary with /spl plusmn/20% rms deviations from nominal values for the fixed input parameters whose a priori values are not precisely known. This sensitivity analysis shows that specified T/sub c/ locations can be determined with /spl les//spl plusmn/1 nm precision for T/sub c/<60 K. Finally, we present new inferred T/sub c/(x) profiles for bicrystal junctions with /spl theta/=24/spl deg/ and 32/spl deg/ and compare these profiles with those of bicrystal JJs with /spl theta/=19/spl deg/ and 36.8/spl deg/ which we previously reported. Our inferred T/sub c/(x) profiles for these functions exhibit a clear correlation with /spl theta/: as /spl theta/ increases so does the extent of the grain boundary region with suppressed T/sub c/. This expected result gives us confidence that our approach should prove useful as a tool for elucidating the structure and properties of high temperature superconductors.