2,881 research outputs found
Diffusion of small light particles in a solvent of large massive molecules
We study diffusion of small light particles in a solvent which consists of
large heavy particles. The intermolecular interactions are chosen to
approximately mimic a water-sucrose (or water-polysaccharide) mixture. Both
computer simulation and mode coupling theoretical (MCT) calculations have been
performed for a solvent-to-solute size ratio five and for a large variation of
the mass ratio, keeping the mass of the solute fixed. Even in the limit of
large mass ratio the solute motion is found to remain surprisingly coupled to
the solvent dynamics. Interestingly, at intermediate values of the mass ratio,
the self-intermediate scattering function of the solute, F_{s}(k,t) (where k is
the wavenumber and t the time), develops a stretching at long time which could
be fitted to a stretched exponential function with a k-dependent exponent,
\beta. For very large mass ratio, we find the existence of two stretched
exponentials separated by a power law type plateau. The analysis of the
trajectory shows the coexistence of both hopping and continuous motions for
both the solute and the solvent particles. It is found that for mass ratio
five, the MCT calculations of the self-diffusion underestimates the simulated
value by about 20 %, which appears to be reasonable because the conventional
form of MCT does not include the hopping mode. However, for larger mass ratio,
MCT appears to breakdown more severely. The breakdown of the MCT for large mass
ratio can be connected to a similar breakdown near the glass transition.Comment: RevTex4, 9 pages, 10 figure
Orientational relaxation in a dispersive dynamic medium : Generalization of the Kubo-Ivanov-Anderson jump diffusion model to include fractional environmental dynamics
Ivanov-Anderson (IA) model (and an earlier treatment by Kubo) envisages a
decay of the orientational correlation by random but large amplitude molecular
jumps, as opposed to infinitesimal small jumps assumed in Brownian diffusion.
Recent computer simulation studies on water and supercooled liquids have shown
that large amplitude motions may indeed be more of a rule than exception.
Existing theoretical studies on jump diffusion mostly assume an exponential
(Poissonian) waiting time distribution for jumps, thereby again leading to an
exponential decay. Here we extend the existing formalism of Ivanov and Anderson
to include an algebraic waiting time distribution between two jumps. As a
result, the first and second rank orientational time correlation functions show
the same long time power law, but their short time decay behavior is quite
different. The predicted Cole-Cole plot of dielectric relaxation reproduces
various features of non-Debye behaviour observed experimentally. We also
developed a theory where both unrestricted small jumps and large angular jumps
coexist simultaneously. The small jumps are shown to have a large effect on the
long time decay, particularly in mitigating the effects of algebraic waiting
time distribution, and in giving rise to an exponential-like decay, with a time
constant, surprisingly, less than the time constant that arises from small
amplitude decay alone.Comment: 14 figure
Tracking down localized modes in PT-symmetric Hamiltonians under the influence of a competing nonlinearity
The relevance of parity and time reversal (PT)-symmetric structures in
optical systems is known for sometime with the correspondence existing between
the Schrodinger equation and the paraxial equation of diffraction where the
time parameter represents the propagating distance and the refractive index
acts as the complex potential. In this paper, we systematically analyze a
normalized form of the nonlinear Schrodinger system with two new families of
PT-symmetric potentials in the presence of competing nonlinearities. We
generate a class of localized eigenmodes and carry out a linear stability
analysis on the solutions. In particular, we find an interesting feature of
bifurcation charaterized by the parameter of perturbative growth rate passing
through zero where a transition to imaginary eigenvalues occurs.Comment: 10pages, To be published in Acta Polytechnic
Orientational relaxation in a discotic liquid crystal
We investigate orientational relaxation of a model discotic liquid crystal,
consists of disc-like molecules, by molecular dynamics simulations along two
isobars starting from the high temperature isotropic phase. The two isobars
have been so chosen that (A) the phase sequence isotropic (I)-nematic
(N)-columnar (C) appears upon cooling along one of them and (B) the sequence
isotropic (I)-columnar (C) along the other. While the orientational relaxation
in the isotropic phase near the I-N phase transition in system (A) shows a
power law decay at short to intermediate times, such power law relaxation is
not observed in the isotropic phase near the I-C phase boundary in system (B).
In order to understand this difference (the existence or the absence of the
power law decay), we calculated the the growth of the orientational pair
distribution functions (OPDF) near the I-N phase boundary and also near the I-C
phase boundary. We find that OPDF shows a marked growth in long range
correlation as the I-N phase boundary is approached in the I-N-C system (A),
but such a growth is absent in the I-C system, which appears to be consistent
with the result that I-N phase transition in the former is weakly first order
while the the I-C phase transition in the later is not weak. As the system
settles into the nematic phase, the decay of the single-particle second-rank
orientational OTCF follows a pattern that is similar to what is observed with
calamitic liquid crystals and supercooled molecular liquids.Comment: 16 pages and 4 figure
Truncated Harmonic Osillator and Parasupersymmetric Quantum Mechanics
We discuss in detail the parasupersymmetric quantum mechanics of arbitrary
order where the parasupersymmetry is between the normal bosons and those
corresponding to the truncated harmonic oscillator. We show that even though
the parasusy algebra is different from that of the usual parasusy quantum
mechanics, still the consequences of the two are identical. We further show
that the parasupersymmetric quantum mechanics of arbitrary order p can also be
rewritten in terms of p supercharges (i.e. all of which obey ).
However, the Hamiltonian cannot be expressed in a simple form in terms of the p
supercharges except in a special case. A model of conformal parasupersymmetry
is also discussed and it is shown that in this case, the p supercharges, the p
conformal supercharges along with Hamiltonian H, conformal generator K and
dilatation generator D form a closed algebra.Comment: 9 page
The Adaptation and Variability of Response of the Human Brain
Electrical potentials have been recorded from the brain of five normal human subjects by means of needle electrodes inserted about half a centimeter through the scalp, one near the external occipital protuberance and another about three inches forward and an inch to the side from the median line. The high time-constant of the amplifier, which was about a second, made it possible to obtain an almost distortionless recording of the low frequency waves. The amplifier was connected to the oscillograph element. The oscillation of the light beam projected from the element was photographed on sensitized paper
Providing Information Feedback to Bidders in Online Multi-unit Combinatorial Auctions
Bidders in online multi-unit combinatorial auctions face the acute problem of estimating the valuations of an immense number of packages. Can the seller guide the bidders to avoid placing bids that are too high or too low? In the single unit case, fast methods are now available for incrementally computing, for each package at each time instant, the recommended lower bound (Deadness Level) and upper bound (Winning Level) on the next bid. But when there are multiple units of items, it becomes difficult to compute the Deadness Level of a package accurately. An upper bound on this quantity can be derived however, and a bid that stays within this bound and the Winning Level is “safe”, in the sense that it is not wasted and has the potential to become a winning bid. What is now needed is an incremental procedure for speeding up the computation of this bound
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