19,445 research outputs found
Development of GaAs and GaAs/1-x/P/x/ thin-film bipolar transistors Final report
Development of GaAs and GaAs/1-xPx thin film bipolar transistor
Renormalization of Drift and Diffusivity in Random Gradient Flows
We investigate the relationship between the effective diffusivity and
effective drift of a particle moving in a random medium. The velocity of the
particle combines a white noise diffusion process with a local drift term that
depends linearly on the gradient of a gaussian random field with homogeneous
statistics. The theoretical analysis is confirmed by numerical simulation. For
the purely isotropic case the simulation, which measures the effective drift
directly in a constant gradient background field, confirms the result
previously obtained theoretically, that the effective diffusivity and effective
drift are renormalized by the same factor from their local values. For this
isotropic case we provide an intuitive explanation, based on a {\it spatial}
average of local drift, for the renormalization of the effective drift
parameter relative to its local value. We also investigate situations in which
the isotropy is broken by the tensorial relationship of the local drift to the
gradient of the random field. We find that the numerical simulation confirms a
relatively simple renormalization group calculation for the effective
diffusivity and drift tensors.Comment: Latex 16 pages, 5 figures ep
Development of GaAs and GaAs sub /1-x/ P sub x thin-film bipolar transistors Final report
Fabrication and electrical properties of GaAs type thin film bipolar transistor
The unreasonable effectiveness of equilibrium-like theory for interpreting non-equilibrium experiments
There has been great interest in applying the results of statistical
mechanics to single molecule experiements. Recent work has highlighted
so-called non-equilibrium work-energy relations and Fluctuation Theorems which
take on an equilibrium-like (time independent) form. Here I give a very simple
heuristic example where an equilibrium result (the barometric law for colloidal
particles) arises from theory describing the {\em thermodynamically}
non-equilibrium phenomenon of a single colloidal particle falling through
solution due to gravity. This simple result arises from the fact that the
particle, even while falling, is in {\em mechanical} equilibrium (gravitational
force equal the viscous drag force) at every instant. The results are
generalized by appeal to the central limit theorem. The resulting time
independent equations that hold for thermodynamically non-equilibrium (and even
non-stationary) processes offer great possibilities for rapid determination of
thermodynamic parameters from single molecule experiments.Comment: 6 page
Sound propagation in and radiation from acoustically lined flow ducts: A comparison of experiment and theory
The results of an experimental and theoretical study of many of the fundamental details of sound propagation in hard wall and soft wall annular flow ducts are reported. The theory of sound propagation along such ducts and the theory for determining the complex radiation impedance of higher order modes of an annulus are outlined, and methods for generating acoustic duct modes are developed. The results of a detailed measurement program on propagation in rigid wall annular ducts with and without airflow through the duct are presented. Techniques are described for measuring cut-on frequencies, modal phase speed, and radial and annular mode shapes. The effects of flow velocity on cut-on frequencies and phase speed are measured. Comparisons are made with theoretical predictions for all of the effects studies. The two microphone method of impedance is used to measure the effects of flow on acoustic liners. A numerical study of sound propagation in annular ducts with one or both walls acoustically lined is presented
First Experiences Integrating PC Distributed I/O Into Argonne's ATLAS Control System
First Experiences Integrating PC Distributed I/O Into Argonne's ATLAS Control
System The roots of ATLAS (Argonne Tandem-Linac Accelerator System) date back
to the early 1960s. Located at the Argonne National Laboratory, the accelerator
has been designated a National User Facility, which focuses primarily on
heavy-ion nuclear physics. Like the accelerator it services, the control system
has been in a constant state of evolution. The present real-time portion of the
control system is based on the commercial product Vsystem [1]. While Vsystem
has always been capable of distributed I/O processing, the latest offering of
this product provides for the use of relatively inexpensive PC hardware and
software. This paper reviews the status of the ATLAS control system, and
describes first experiences with PC distributed I/O.Comment: ICALEPCS 2001 Conference, PSN WEAP027, 3 pages, 1 figur
Perturbation theory for the effective diffusion constant in a medium of random scatterer
We develop perturbation theory and physically motivated resummations of the
perturbation theory for the problem of a tracer particle diffusing in a random
media. The random media contains point scatterers of density uniformly
distributed through out the material. The tracer is a Langevin particle
subjected to the quenched random force generated by the scatterers. Via our
perturbative analysis we determine when the random potential can be
approximated by a Gaussian random potential. We also develop a self-similar
renormalisation group approach based on thinning out the scatterers, this
scheme is similar to that used with success for diffusion in Gaussian random
potentials and agrees with known exact results. To assess the accuracy of this
approximation scheme its predictions are confronted with results obtained by
numerical simulation.Comment: 22 pages, 6 figures, IOP (J. Phys. A. style
On the distribution of estimators of diffusion constants for Brownian motion
We discuss the distribution of various estimators for extracting the
diffusion constant of single Brownian trajectories obtained by fitting the
squared displacement of the trajectory. The analysis of the problem can be
framed in terms of quadratic functionals of Brownian motion that correspond to
the Euclidean path integral for simple Harmonic oscillators with time dependent
frequencies. Explicit analytical results are given for the distribution of the
diffusion constant estimator in a number of cases and our results are confirmed
by numerical simulations.Comment: 14 pages, 5 figure
Continuum Derrida Approach to Drift and Diffusivity in Random Media
By means of rather general arguments, based on an approach due to Derrida
that makes use of samples of finite size, we analyse the effective diffusivity
and drift tensors in certain types of random medium in which the motion of the
particles is controlled by molecular diffusion and a local flow field with
known statistical properties. The power of the Derrida method is that it uses
the equilibrium probability distribution, that exists for each {\em finite}
sample, to compute asymptotic behaviour at large times in the {\em infinite}
medium. In certain cases, where this equilibrium situation is associated with a
vanishing microcurrent, our results demonstrate the equality of the
renormalization processes for the effective drift and diffusivity tensors. This
establishes, for those cases, a Ward identity previously verified only to
two-loop order in perturbation theory in certain models. The technique can be
applied also to media in which the diffusivity exhibits spatial fluctuations.
We derive a simple relationship between the effective diffusivity in this case
and that for an associated gradient drift problem that provides an interesting
constraint on previously conjectured results.Comment: 18 pages, Latex, DAMTP-96-8
The western economy in the 80s
Federal Reserve District, 12th ; Economic conditions - West (U.S.)
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