50,721 research outputs found
Thermodynamics and phase behavior of the lamellar Zwanzig model
Binary mixtures of lamellar colloids represented by hard platelets are
studied within a generalization of the Zwanzig model for rods, whereby the
square cuboids can take only three orientations along the , or axes.
The free energy is calculated within Rosenfeld's ''Fundamental Measure Theory''
(FMT) adapted to the present model. In the one-component limit, the model
exhibits the expected isotropic to nematic phase transition, which narrows as
the aspect ratio ( is the width and the thickness of the
platelets) increases. In the binary case the competition between nematic
ordering and depletion-induced segregation leads to rich phase behaviour.Comment: 9 pages, 6 figure
Macroion adsorption: The crucial role of excluded volume and coions
The adsorption of charged colloids (macroions) onto an oppositely charged
planar substrate is investigated theoretically. Taking properly into account
the finite size of the macroions, unusual behaviors are reported. It is found
that the role of the coions (the little salt-ions carrying the same sign of
charge as that of the substrate) is crucial to understand the mechanisms
involved in the process of macroion adsorption. In particular, the coions can
accumulate near the substrate's surface and lead to a counter-intuitive {\it
surface charge amplification}.Comment: 11 pages - 4 figures. To appear in JC
The plasmatron: Advanced mode thermionic energy conversion
A theory of the plasmatron was developed. Also, a wide range of measurements were obtained with two versatile, research devices. To gain insight into plasmatron performance, the experimental results are compared with calculations based on the theoretical model of plasmatron operation. Results are presented which show that the plasma arc drop of the conventional arc (ignited) mode converter can be suppressed by use of an auxiliary ion source. The improved performance, however, is presently limited to low current densities because of voltage losses due to plasma resistance. This resistance loss could be suppressed by an increase in the plasma electron temperature or a decrease in spacing. Plasmatron performance characteristics for both argon and cesium are reported. The argon plasmatron has superior performance. Results are also presented for magnetic cutoff effects and for current distributing effects. These are shown to be important factors for the design of practical devices
Conditional quantum state engineering in repeated 2-photon down conversion
The U(1,1) and U(2) transformations realized by three-mode interaction in the
respective parametric approximations are studied in conditional measurement,
and the corresponding non-unitary transformation operators are derived. As an
application, the preparation of single-mode quantum states using an optical
feedback loop is discussed, with special emphasis of Fock state preparation.
For that example, the influence of non-perfect detection and feedback is also
considered.Comment: 17 pages, 4 figures, using a4.st
Self-Affinity in the Gradient Percolation Problem
We study the scaling properties of the solid-on-solid front of the infinite
cluster in two-dimensional gradient percolation. We show that such an object is
self affine with a Hurst exponent equal to 2/3 up to a cutoff-length
proportional to the gradient to the power (-4/7). Beyond this length scale, the
front position has the character of uncorrelated noise. Importantly, the
self-affine behavior is robust even after removing local jumps of the front.
The previously observed multi affinity, is due to the dominance of overhangs at
small distances in the structure function. This is a crossover effect.Comment: 4 pages, 4 figure
Tolerance and Sensitivity in the Fuse Network
We show that depending on the disorder, a small noise added to the threshold
distribution of the fuse network may or may not completely change the
subsequent breakdown process. When the threshold distribution has a lower
cutoff at a finite value and a power law dependence towards large thresholds
with an exponent which is less than , the network is not sensitive
to the added noise, otherwise it is. The transition between sensitivity or not
appears to be second order, and is related to a localization-delocalization
transition earlier observed in such systems.Comment: 12 pages, 3 figures available upon request, plain Te
Performance characteristics of the 12 GHz, 200 watt transmitter experiment package for CTS
The experiment package consists of a 200 W output stage tube (OST) powered by a power processing system (PPS). Descriptions of both the PPS and OST are given. The PPS provides the necessary voltages with a measured dc/dc conversion efficiency of 89 percent. The OST, a traveling wave tube with multiple collectors, has a saturated rf output power of 224 W and operates at an overall efficiency exceeding 40 percent over an 85 MHz bandwidth at 12 GHz. OST performance given includes frequency response, saturation characteristics, group delay, AM to PM conversion, inter-modulation distortion, and two channel gain suppression. Single and dual channel FM video performance is presented. It was determined that for 12 MHz peak to peak frequency deviation on each channel, dual channel FM television signals can be transmitted through the TEP at 60 W, each channel, with 40 MHz channel spacing (center to center)
Scaled Particle Theory for Hard Sphere Pairs. I. Mathematical Structure
We develop an extension of the original Reiss-Frisch-Lebowitz scaled particle
theory that can serve as a predictive method for the hard sphere pair
correlation function g(r). The reversible cavity creation work is analyzed both
for a single spherical cavity of arbitrary size, as well as for a pair of
identical such spherical cavities with variable center-to-center separation.
These quantities lead directly to prediction of g(r). Smooth connection
conditions have been identified between the small-cavity situation where the
work can be exactly and completely expressed in terms of g(r), and the
large-cavity regime where macroscopic properties become relevant. Closure
conditions emerge which produce a nonlinear integral equation that must be
satisfied by the pair correlation function. This integral equation has a
structure which straightforwardly generates a solution that is a power series
in density. The results of this series replicate the exact second and third
virial coefficients for the hard sphere system via the contact value of the
pair correlation function. The predicted fourth virial coefficient is
approximately 0.6 percent lower than the known exact value. Detailed numerical
analysis of the nonlinear integral equation has been deferred to the sequel
(following paper
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