308 research outputs found
Dynamics of Charge Flow in the Channel of a Thin-Film Field-Effect Transistor
The local conductivity in the channel of a thin-film field-effect transistor
is proportional to the charge density induced by the local gate voltage. We
show how this determines the frequency- and position-dependence of the charge
induced in the channel for the case of "zero applied current": zero
drain-source voltage with charge induced by a square-wave voltage applied to
the gate, assuming constant mobility and negligible contact impedances. An
approximate expression for the frequency dependence of the induced charge in
the center of the channel can be conveniently used to determine the charge
mobility. Fits of electro-optic measurements of the induced charge in organic
transistors are used as examples.Comment: 9 pages including table + 3 figures; submitted to Jnl. Appl. Phy
Heavy Water Obtained by the Combustion of Dried Biological Specimens
Dry, finely divided plant and animal tissues were oxidized by passage over hot copper oxide; the aqueous condensates were first distilled from potassium permanganate and then alone. If necessary, oxidation and distillation were repeated until there was no significant change in density. Densities were determined by a modification of the submerged magnetic float procedure, and the accuracy was one part per million. The specimens examined were practically all of the chief organs of cattle; yeasts; dairy products; and some parts of plants like corn and melons. The study is being extended to other biological specimens, normal and pathological
The shape of invasion perclation clusters in random and correlated media
The shape of two-dimensional invasion percolation clusters are studied
numerically for both non-trapping (NTIP) and trapping (TIP) invasion
percolation processes. Two different anisotropy quantifiers, the anisotropy
parameter and the asphericity are used for probing the degree of anisotropy of
clusters. We observe that in spite of the difference in scaling properties of
NTIP and TIP, there is no difference in the values of anisotropy quantifiers of
these processes. Furthermore, we find that in completely random media, the
invasion percolation clusters are on average slightly less isotropic than
standard percolation clusters. Introducing isotropic long-range correlations
into the media reduces the isotropy of the invasion percolation clusters. The
effect is more pronounced for the case of persisting long-range correlations.
The implication of boundary conditions on the shape of clusters is another
subject of interest. Compared to the case of free boundary conditions, IP
clusters of conventional rectangular geometry turn out to be more isotropic.
Moreover, we see that in conventional rectangular geometry the NTIP clusters
are more isotropic than TIP clusters
Nematic Ordering of Rigid Rods in a Gravitational Field
The isotropic-to-nematic transition in an athermal solution of long rigid
rods subject to a gravitational (or centrifugal) field is theoretically
considered in the Onsager approximation. The new feature emerging in the
presence of gravity is a concentration gradient which coupled with the nematic
ordering. For rodlike molecules this effect becomes noticeable at centrifugal
acceleration g ~ 10^3--10^4 m/s^2, while for biological rodlike objects, such
as tobacco mosaic virus, TMV, the effect is important even for normal
gravitational acceleration conditions. Rods are concentrated near the bottom of
the vessel which sometimes leads to gravity induced nematic ordering. The
concentration range corresponding to phase separation increases with increasing
g. In the region of phase separation the local rod concentration, as well as
the order parameter, follow a step function with height.Comment: Full article http://prola.aps.org/abstract/PRE/v60/i3/p2973_
On the Microscopic Origin of Cholesteric Pitch
We present a microscopic analysis of the instability of the nematic phase to
chirality when molecular chirality is introduced perturbatively. We show that
previously neglected short-range biaxial correlations play a crucial role in
determining the cholesteric pitch. We propose an order parameter which
quantifies the chirality of a molecule.Comment: RevTeX 3.0, 4 pages, one included eps figure. Published versio
Order parameter for two-dimensional critical systems with boundaries
Conformal transformations can be used to obtain the order parameter for
two-dimensional systems at criticality in finite geometries with fixed boundary
conditions on a connected boundary. To the known examples of this class (such
as the disk and the infinite strip) we contribute the case of a rectangle. We
show that the order parameter profile for simply connected boundaries can be
represented as a universal function (independent of the criticality model)
raised to the power eta/2. The universal function can be determined from the
Gaussian model or equivalently a problem in two-dimensional electrostatics. We
show that fitting the order parameter profile to the theoretical form gives an
accurate route to the determination of eta. We perform numerical simulations
for the Ising model and percolation for comparison with these analytic
predictions, and apply this approach to the study of the planar rotor model.Comment: 10 pages, 14 figures. Revisions: Removed many typos, improved
presentation of most of the figure
Glassiness Vs. Order in Densely Frustrated Josephson Arrays
We carry out extensive Monte Carlo simulations on the Coulomb gas dual to the
uniformly frustrated two dimensional XY model, for a sequence of frustrations f
converging to the irraltional (3-sqrt 5)/2. We find in these systems a sharp
first order equilibrium phase transition to an ordered vortex structure at a
T_c which varies only slightly with f. This ordered vortex structure remains in
general phase incoherent until a lower pinning transition T_p(f) that varies
with f. We argue that the glassy behaviors reported for this model in earlier
simulations are dynamic effects.Comment: 4 pages, 4 eps figure
Markov chain analysis of random walks on disordered medium
We study the dynamical exponents and for a particle diffusing
in a disordered medium (modeled by a percolation cluster), from the regime of
extreme disorder (i.e., when the percolation cluster is a fractal at )
to the Lorentz gas regime when the cluster has weak disorder at and
the leading behavior is standard diffusion. A new technique of relating the
velocity autocorrelation function and the return to the starting point
probability to the asymptotic spectral properties of the hopping transition
probability matrix of the diffusing particle is used, and the latter is
numerically analyzed using the Arnoldi-Saad algorithm. We also present evidence
for a new scaling relation for the second largest eigenvalue in terms of the
size of the cluster, , which provides a
very efficient and accurate method of extracting the spectral dimension
where .Comment: 34 pages, REVTEX 3.
The role of tyrosine M210 in the initial charge separation in the reaction center of Rhodobacter sphaeroides
Phase Separation of Crystal Surfaces: A Lattice Gas Approach
We consider both equilibrium and kinetic aspects of the phase separation
(``thermal faceting") of thermodynamically unstable crystal surfaces into a
hill--valley structure. The model we study is an Ising lattice gas for a simple
cubic crystal with nearest--neighbor attractive interactions and weak
next--nearest--neighbor repulsive interactions. It is likely applicable to
alkali halides with the sodium chloride structure. Emphasis is placed on the
fact that the equilibrium crystal shape can be interpreted as a phase diagram
and that the details of its structure tell us into which surface orientations
an unstable surface will decompose. We find that, depending on the temperature
and growth conditions, a number of interesting behaviors are expected. For a
crystal in equilibrium with its vapor, these include a low temperature regime
with logarithmically--slow separation into three symmetrically--equivalent
facets, and a higher temperature regime where separation proceeds as a power
law in time into an entire one--parameter family of surface orientations. For a
crystal slightly out of equilibrium with its vapor (slow crystal growth or
etching), power--law growth should be the rule at late enough times. However,
in the low temperature regime, the rate of separation rapidly decreases as the
chemical potential difference between crystal and vapor phases goes to zero.Comment: 16 pages (RevTex 3.0); 12 postscript figures available on request
([email protected]). Submitted to Physical Review E. SFU-JDSDJB-94-0
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