Realistic Fluctuations Research Articles

A new method to simulate random dopant induced threshold voltage fluctuations in sub-50 nm MOSFET’s with non-uniform channel doping

In this paper, based on a precise and efficient analytical function of relatively realistic dopant fluctuations, a new method is proposed to simulate the threshold voltage variation of MOSFET’s with non-uniform channel doping due to random dopant fluctuations. Both the number and position fluctuations of dopants are taken into account. Using this method, 2500 microscopically different devices under certain process conditions that cover the range of channel length L from 35nm to 90nm, oxide thickness Tox from 1nm to 4nm and channel surface doping concentration NA from 1×1018 to 5×1018cm−3 are simulated to show how our method works.

Interrenal and thyroid development in red drum ( Sciaenops ocellatus): Effects of nursery environment on larval growth and cortisol concentration during settlement

Red drum settle into shallow seagrass meadows during the larval stage. Day–night cycles in these habitats result in marked diel temperature and dissolved oxygen (DO) cycles, and it is possible that extreme fluctuations influence endocrine development and growth of larvae. Here, we described red drum interrenal and thyroid ontogeny and determine responses to environmental stimuli with special emphasis on settlement to explore possible role of hormones as mediator of directive environmental factors. This study detected an early activation of thyroid and interrenal axis during the yolk-sac phase and a second activation of the thyroid starting at settlement size to the end of the larval period. Whole-body l-thyroxine (T4) and 3-5-3′-triiodo- l-thyronine (T3) showed a sharp decline at the juvenile stage. In contrast, cortisol steadily declines during the larval phase to a minimum before the end of the larval period. Older settlement-size larvae exposed to a strong stimulus increased whole body cortisol. In contrast, new settlers showed a minor cortisol rise suggesting changes on stress responsiveness during the ontogeny of the species. Additionally, settlement-size larvae exposed to various environmentally realistic temperature or DO fluctuations showed no difference in growth compared to fish grown under stable conditions (control). However, growth rate was significantly reduced in DO cycled fish with prolonged exposure to hypoxia. No differences were found in whole-body cortisol levels in the reduced growth treatment groups, suggesting that growth retardation was not related to a cortisol-mediated stress response. In moderate DO and temperature treatment groups, cortisol showed wider fluctuations than control groups during the night time that were not related to stress.

Communicating over nonstationary nonflat wireless channels

We develop the concept of joint time-frequency estimation of wireless channels. The motivation is to optimize channel usage by increasing the signal-to-noise ratio (SNR) after demodulation while keeping training overhead at a moderate level. This issue is important for single-input single-output (SISO) and multiple-input multiple-output (MIMO) systems but particularly so for the latter. Linear operators offer a general mathematical framework for symbol modulation in channels that vary both temporally and spectrally within the duration and bandwidth of one symbol. In particular, we present a channel model that assumes first-order temporal and spectral fluctuations within one symbol or symbol block. Discrete prolate spheroidal sequences (Slepian sequences) are used as pulse-shaping functions. The channel operator in the Slepian basis is almost tridiagonal, and the simple intersymbol interference pattern can be exploited for efficient and fast decoding using Viterbi's algorithm. To prove the concept, we use the acoustic channel as a meaningful physical analogy to the radio channel. In acoustic 2 /spl times/ 2 MIMO experiments, our method produced estimation results that are superior to first-order time-only, frequency-only, and zeroth-order models by 7.0, 9.4, and 11.6 db. In computer simulations of cellular wireless channels with realistic temporal and spectral fluctuations, time-frequency estimation gains us 12 to 18 db over constant-only estimation in terms of received SNR when signal-to-receiver-noise is 10 to 20 db. The bit error rate (BER) decreases by a factor of two for a binary constellation.

Compressible Wall-Injection Flows in Laminar, Transitional, and Turbulent Regimes: Numerical Prediction

Numerical simulations of compressible rocket flows are conducted in laminar, transitional, and turbulent regimes. The laminar simulation is carried out on a planar rocket flow without nozzle using the unsteady two-dimensional Navier-Stokes system. The transitional and turbulent flows are performed in three-dimensional on an extended rocket geometry with a divergent outlet using compressible large eddy simulation (LES) models. In both cases, the compressibility effect plays an important role. In the laminar case, pressure oscillation is forced at the outflow boundary. The time-averaged part of the solution is compared with the inviscid theory of compressible rocket flow of Balakrishnan et al. (Balakrishnan, G., Linan, A., and Williams F. A., Compressibility Effects in Thin Channels with Injection, AIAA Journal, Vol.29, No. 12, 1991, pp. 2149-2154) and the oscillatory part with the acoustic layer model of Majdalani and Van Moorhem (Majdalani, J., and Van Moorhem, W. K., Improved Time-Dependent Flowfield Solution for Solid Rocket Motors, AIAA Journal, Vol. 36, No. 2, 1998, pp. 241-248). The mean flow from the present numerical result is in better agreement with the compressible theory than the conventional Taylor's profiles (Taylor, G. I., Fluid Flow in Regions Bounded by Porous Surfaces, Proceedings of the Royal Society of London, Series A: Mathematical and Physical Sciences, Vol. 234, 1956, pp. 456-475), as expected. The oscillatory part of the flow agrees well in the first quarter of the axial extent, near the head end. Farther downstream, the discrepancies develop rapidly between the numerical result and the acoustic-layer model. Possible causes of the difference are the effect of compressibility, which alters the local speed of sound, hence, acoustic properties, and the interference of hydrodynamic instabilities. In the transition and turbulent regimes, the dynamic LES model is applied on different resolutions. The measurements data of Traineau et al. (Traineau, J. C., Hervat, P., and Kuentzmann, P., Cold Flow Simulation of a Two Dimensional Nozzleless Solid Rocket Motor, AIAA Paper 86-1447, June 1986) are employed for comparison purposes. The refinement study by comparison with the measurement data suggests the importance of resolving the laminar and transition region for a reliable application of LES in transitional flows. With the consideration of this aspect, LES with efficient grid size can produce resonable accuracy. Forcing hydrodynamic instabilities and a more realistic injection fluctuations model are recommended.

Inhomogeneous charging and screening effects in semiconductor quantum dot arrays

The electronic properties of quantum dot (QD) arrays are strongly influenced by the Coulomb interaction of electrons within the dot as well as with those in neighbouring dots. In this paper, we investigate this behaviour taking into account screening by a free electron gas in the vicinity of the QDs. We find pronounced effects for standard capacitance[ndash]voltage (CV) measurements of QD structures embedded in a pn-diode. In particular, we show that the three-dimensional nature of the problem is crucial for devices with low dot-density, whereas the self-consistency between electron depletion in the bulk layer and dot occupation is important for high dot-densities. The Coulomb interaction between the dots induces a broadening of the peaks in the CV characteristic which is comparable with the effect of disordered QD arrays, where we considered realistic size and position fluctuations obtained by a kinetic Monte Carlo simulation.

On the stability of wideband beam patterns with respect to weighting and envelope fluctuations

This paper presents a study on the worsening caused by a random perturbation of the weighting window (modeling, in this way, the uncertainty on the channel sensitivity), or by an alteration of the shape of the transmitted pulse, for a wideband array. In the stability evaluation, main-lobe width and signal to noise ratio evaluated at different dB levels of the beam pattern play the role of quality parameters. Two different profiles of the weighting window have been used for this analysis: a typical raised cosine, and a window optimized by a Simulated Annealing procedure. Also for the pulse envelope, two distinct shapes have been chosen: perfectly Gaussian and experimentally measured. The analysis of the obtained results provides useful hints about the amount and the kind of the beam pattern worsening, when realistic fluctuations occur. Moreover, the results show that the weighting windows provided by the simulated annealing procedure are particularly robust to such fluctuations.

Effects of fractal fluctuations in topographic relief, permittivity and conductivity on ground‐penetrating radar antenna radiation

Typical ground‐penetrating radar (GPR) transmitters and receivers are dipole antennas. These antennas have pronounced directivity properties and exhibit strong coupling to interfaces across which there are changes in electric material properties. Antenna coupling to the surface of idealized half‐space models has been the subject of intense research for several decades. In contrast, the behavior of antennas in the vicinity of interfaces with realistic topographic fluctuations and/or subsurface heterogeneities has been largely unexplored. To explore this issue, we simulate the responses of a typical surface GPR antenna system located on a suite of realistic fractal earth models using the finite‐difference time‐domain (FDTD) method. The models are characterized by topographic roughness of the air–soil interface and small‐scale heterogeneous distributions of permittivity and conductivity in the subsurface. Synthetic radiation patterns and input impedance values of the simulated GPR antenna system demonstrate that topographic roughness significantly affects the coupling of the antenna to the ground, whereas heterogeneities in the subsurface predominantly influence the antenna radiation through scattering and absorption along the propagation path.

The evaluation of screening policies for diabetic retinopathy using simulation.

To develop a model for evaluating screening strategies and to use it to determine the cost effectiveness of varying the screening method and the screening interval. A discrete event simulation was designed, validated and run for a population of 500000. Most parameters were derived from peer-reviewed publications. Standard methods of screening save up to 50% of the potential sight years lost. They give up to 85% of the sight years saved by an idealized gold standard programme using mydriatic seven-field photography reported by an ophthalmologist. The mobile camera, used for annual screening and 6-month follow-up after the detection of background retinopathy, had an estimated cost of pound 449200 per year with pound 2842 per sight year saved. It is less efficient to screen Type 2, rather than Type 1 diabetes mellitus patients, but they contributed to almost three-quarters of the sight years saved. The model can evaluate screening intervals and methods on a national or health authority basis. Results indicate that it appears more cost effective to continue to screen outside an ophthalmology clinic, until treatment is needed. Programmes with annual screening, and more frequent screening for those with background retinopathy, are robust to realistic fluctuations in compliance and screening sensitivity.

Dangling bond deflection model: growth of gel network with loop structure.

It has been shown that the closed-loop structure in the model gel networks is responsible for their stiffness. However, the creation of loops has been underestimated in most of the existing kinetic aggregation models [e.g., DLCA (diffusion-limited cluster-cluster aggregation) and derivatives]. A dangling bond deflection (DEF) mechanism is proposed to model the fluctuation of dangling branches or dead ends under thermal excitation. The random deflections of the dangling branches can create loops in the network by forming intracluster bonds, and proceed during both the gelling and aging processes. The resulting DLCADEF networks have extensive loop structure with a negligible number of dangling branches. Its growth kinetics and fractal behavior resemble those of real gels, including volume-invariant gel time and fractal dimension of about 2. The DLCADEF model is the first attempt to model the gel growth with loop formation by the physically realistic fluctuation mechanism. The mechanical properties of the resulting networks will be studied and verified by comparison with real gels.

Competition of Phytoplankton under Fluctuating Light

Light is an essential resource for phytoplankton and fluctuates on a wide range of timescales. To understand how light fluctuations affect phytoplankton community structure and diversity, we have studied a set of simple models using a combination of analytical and numerical techniques. Light fluctuations can affect community structure when species exhibit the gleaner-opportunist trade-off between competitive ability and ability to reach carrying capacity quickly. Fast fluctuations can switch the competitive dominant from a gleaner to an opportunist; slow fluctuations can cause this switch and also lead to stable coexistence. Coexistence is easiest between species that are highly differentiated along the gleaner-opportunist trade-off. Our results remain qualitatively unchanged when more realistic light fluctuations such as daily and seasonal changes in irradiance and the presence of a spatial gradient in light are considered. Seasonal change in day length may be one of the factors driving the seasonal succession of phytoplankton, from opportunist species dominant under shorter day lengths (spring and autumn) to gleaner species dominant under longer day length (summer). These results illustrate how resource fluctuations can have an important role in structuring ecological communities.

Competition of Phytoplankton under Fluctuating Light

Light is an essential resource for phytoplankton and fluctuates on a wide range of timescales. To understand how light fluctuations affect phytoplankton community structure and diversity, we have studied a set of simple models using a combination of analytical and numerical techniques. Light fluctuations can affect community structure when species exhibit the gleaner‐opportunist trade‐off between competitive ability and ability to reach carrying capacity quickly. Fast fluctuations can switch the competitive dominant from a gleaner to an opportunist; slow fluctuations can cause this switch and also lead to stable coexistence. Coexistence is easiest between species that are highly differentiated along the gleaner‐opportunist trade‐off. Our results remain qualitatively unchanged when more realistic light fluctuations such as daily and seasonal changes in irradiance and the presence of a spatial gradient in light are considered. Seasonal change in day length may be one of the factors driving the seasonal succession of phytoplankton, from opportunist species dominant under shorter day lengths (spring and autumn) to gleaner species dominant under longer day length (summer). These results illustrate how resource fluctuations can have an important role in structuring ecological communities.

Direct numerical simulation of turbulent thermal boundary layers

In this paper, a method of generating realistic turbulent temperature fluctuations at a computational inlet is proposed and direct numerical simulations of turbulent thermal boundary layers developing on a flat plate with isothermal and isoflux wall boundary conditions are carried out. Governing equations are integrated using a fully implicit fractional-step method with 352×64×128 grids for the Reynolds number of 300, based on the free-stream velocity and the inlet momentum thickness, and the Prandtl number of 0.71. The computed Stanton numbers for the isothermal and isoflux walls are in good agreement with power-law relations without transient region from the inlet. The mean statistical quantities including root-mean-square temperature fluctuations, turbulent heat fluxes, turbulent Prandtl number, and skewness and flatness of temperature fluctuations agree well with existing experimental and numerical data. A quadrant analysis is performed to investigate the coherence between the velocity and temperature fluctuations. It is shown that the behavior of the wall-normal heat flux is similar to that of the Reynolds shear stress, indicating close correlation between the streamwise velocity and temperature. The effect of different thermal boundary conditions at the wall on the near-wall turbulence statistics is also discussed.

Effects of production and marketing circumstances on economic values for beef production traits

The effect of altering production and marketing circumstances on economic values is quantified for a complete beef production system. Absolute and relative economic values were found to vary substantially with large, but realistic fluctuations in prices and costs. In addition, several examples of different management and different genotypes gave markedly different economic values than in the base situation. Also investigated were the effects of rescaling the enterprise to accommodate three alternative limitations; fixed feed available from pasture, fixed dollars available for feed or fixed amount of beef produced. The effects of rescaling were highly dependent on whether or not fixed costs were accounted for. When fixed costs were ignored (corresponding to a small positive profit) the economic value for mature size decreased while that for fertility increased, but other traits were largely unaffected by rescaling. Overall, production circumstances that reduced survival and fertility yielded the largest changes to economic values. Key words: Economic values, beef cattle, rescaling

Spin dynamics in solid dilute paramagnets

The free-induction decay signals for the two-spin clusters, the “matrix,” and the entire spin system as a whole are calculated for a real dipole-dipole interaction on the basis of a method for the cluster expansion of correlation functions previously developed by the authors. The intracluster interactions, which generate a discrete spectrum, are taken into account exactly, and the interactions outside the clusters are taken into account on the basis of the Anderson-Weiss-Kubo (AWK) theory of phase relaxation. The corresponding shape functions of the resonance line are calculated. Before the clusters are separated, an AWK analysis in the limit of slow fluctuations of the local fields is in satisfactory agreement with the exact results obtained in the Anderson model of dipolar interactions, but a similar analysis for realistic fluctuation rates and Dt?1 leads to a significant qualitative difference: exp(−Dt) transforms into \(\exp ( - \sqrt {D_1 t} )\). After the clusters are separated, this difference is obliterated. The free-induction decay is almost exponential up to Dt∼10, and nonexponential behavior of the long-time asymptote appears sooner. This finding is not altered significantly when the analysis is complicated by raising the maximum rank of the clusters to 3 or more.

Density fluctuations measured by ISEE 1-2 in the Earth's magnetosheath and the resultant scattering of radio waves

Abstract. Radio waves undergo angular scattering when they propagate through a plasma with fluctuating density. We show how the angular scattering coefficient can be calculated as a function of the frequency spectrum of the local density fluctuations. In the Earth's magnetosheath, the ISEE 1-2 propagation experiment measured the spectral power of the density fluctuations for periods in the range 300 to 1 s, which produce most of the scattering. The resultant local angular scattering coefficient can then be calculated for the first time with realistic density fluctuation spectra, which are neither Gaussian nor power laws. We present results on the variation of the local angular scattering coefficient during two crossings of the dayside magnetosheath, from the quasi-perpendicular bow shock to the magnetopause. For a radio wave at twice the local electron plasma frequency, the scattering coefficient in the major part of the magnetosheath is b(2fp) ≃ 0.5 – 4 × 10–9 rad2/m. The scattering coefficient is about ten times stronger in a thin sheet (0.1 to1RE) just downstream of the shock ramp, and close to the magnetopause.

Density fluctuations measured by ISEE 1-2 in the Earth's magnetosheath and the resultant scattering of radio waves

Radio waves undergo angular scattering when they propagate through a plasma with fluctuating density. We show how the angular scattering coefficient can be calculated as a function of the frequency spectrum of the local density fluctuations. In the Earth's magnetosheath, the ISEE 1-2 propagation experiment measured the spectral power of the density fluctuations for periods in the range 300 to 1 s, which produce most of the scattering. The resultant local angular scattering coefficient can then be calculated for the first time with realistic density fluctuation spectra, which are neither Gaussian nor power laws. We present results on the variation of the local angular scattering coefficient during two crossings of the dayside magnetosheath, from the quasi-perpendicular bow shock to the magnetopause. For a radio wave at twice the local electron plasma frequency, the scattering coefficient in the major part of the magnetosheath is b(2fp) ≃ 0.5 – 4 × 10–9 rad2/m. The scattering coefficient is about ten times stronger in a thin sheet (0.1 to1RE) just downstream of the shock ramp, and close to the magnetopause.

Modeling issues in the simulation of long-range ocean acoustic pulse transmissions

Pulse propagation from a wideband source in the ocean sound channel is being modeled theoretically and numerically in the time domain for comparison with ATOC transmissions on a 3-Mm path between California and Hawaii. The physics of pulse dispersion in a waveguide is addressed using stationary phase approximations for integrals over frequency. For a parabolic sound slowness profile the basic theory admits a closed analytic solution for an impulsive source using the WKB approximation. Inclusion of realistic SSPs with time- and range-dependent fluctuations (primarily internal waves) is accomplished by a combination of parabolic equation modeling and modal analysis. Attempts to include realistic fluctuations in the stationary phase approximation are discussed. [Work supported by NRL and ONR and by the Strategic Environmental Research and Development Program through ARPA.]

Multifragmentation of finite many-body systems under long-range forces and an exactly solvable model of fragmentation with strict mass-conversation and quantum-symmetry

During the last 2 years multifragmentation of hot nuclei has become a well established phenomenon. Model calculations suggest that this is likely connected to the the occurence of new, well defined, phase-transitions driven by the repulsive long-range Coulomb force. The origin of the transitions and the possible significance for other systems in physics is emphasised. In these transitions the critical fluctuations give further insight into the relevant physical mechanisms. For a detailed discussion of these fluctuation an exactly solvable model offragmentation is presented. The effect of mass-conservation and quantum-symmetry can rigorously be considered. As a function of a tuning parameter self similar mass-distributions with realistic power-laws and/or fluctuations which show intermittency can be generated. Thus the relation of both critical phenomena can be studied analytically. It is found that they are unliked. There are cases of the model where the mass distribution has a power-law with realistic exponents between τ = −2 and τ = −3 but no intermittency, or vice versa.

Mean values and fluctuations in the fragmentation of mesoscopic systems using an exactly solvable model with strict particle conservation and quantum symmetry

A detailed discussion of self-similarities in fragment-size distributions and fluctuations is presented using an exactly solvable model of fragmentation (the “chain model”). The effects of particle-number conservation and quantum symmetry can rigorously be considered in systems ranging from microscopic to macroscopic. Due to the analyticity of the model the various scalings can be studied free of any statistical noise. Using a tuning parameter we can generate self-similar distributions with realistic power-laws and/or fluctuations which show intermittency. Finite-size effects neither destroy nor cause intermittency. The relation of self-similarity in both the averages and the fluctuations can be studied analytically. It is found that they are unlinked - there are cases where the size-distribution is a power-law with realistic exponents τ between −2 and −3 but no intermittency. Two cases will even be shown which have indistinguishable fragment distributions but very different factorial moments. We also discuss the interpretation of both the size and slope of the factorial moments in terms of multiplicity and bin mixing. We show that while either is sufficient to produce large moments, one must have bin mixing to produce large slopes. The two types of mixing are necessarily linked in constrained systems such as described by our model.

A statistical hadron shower parametrization for sampling calorimeters

A general purpose shower parametrization algorithm for sampling calorimeters, which reproduces realistic fluctuations of the signal, is described. It makes use of multivariate statistical analysis to infer a limited number of significant factors from the data correlation matrix. Showers can be generated on the basis of those factors via a simple analytical model. Limited data samples from experiment or Monte Carlo may be equivalently used as input, allowing either the generation of experimental-like showers or the production with a negligible computer time of a large Monte Carlo-like sample.