Biomedical subjects

V Chernyak

Publications and source records attributed to V Chernyak.

15 recordsLinked to original sources

PubMed2005

Diagnosis of weaknesses in modern error correction codes: a physics approach.

One of the main obstacles to the wider use of the modern error-correction codes is that, due to the complex behavior of their decoding algorithms, no systematic method which would allow characterization of the bit-error-rate (BER) is known. This is especially true at the weak noise where many systems operate and where coding performance is difficult to estimate because of the diminishingly small number of errors. We show how the instanton method of physics allows one to solve the problem of BER analysis in the weak noise range by recasting it as a computationally tractable minimization problem.

Algorithms↗

PubMed2005

Dynamical generalization of nonequilibrium work relation.

The evolution of systems in contact with thermal, chaotic, or turbulent surroundings-often modeled with stochastic equations of motion-can be particularly complex when these equations of motion are nonautonomous, that is, when external parameters of the surroundings are varied with time. In this paper we establish a rigorous equality relating the nonautonomous behavior of such a system, to solutions of the corresponding autonomous equations of motion, for arbitrary initial conditions. If the system is initially in thermal equilibrium, we recover previously known results relating nonequilibrium work values to equilibrium probability distributions. We discuss specific examples of our result, and suggest an experimental setting in which it might be verified.

Journal Article↗

PubMed2004

Error correction on a tree: an instanton approach.

We introduce a method that allows analytical or semianalytical estimating of the post-error correction bit error rate (BER) when a forward-error correction is utilized for transmitting information through a noisy channel. The generic method that applies to a variety of error-correction schemes in the regimes where the BER is low is illustrated using the example of a finite-size code approximated by a treelike structure. Exploring the statistical physics formulation of the problem we find that the BER decreases with the signal-to-noise ratio nonuniformly, i.e., crossing over through a sequence of phases. The higher the signal-to-noise ratio the lower the symmetry of the phase dominating BER.

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PubMed2003

Probability of anomalously large bit-error rate in long haul optical transmission.

We consider a linear model of optical transmission through a fiber with birefringent disorder in the presence of amplifier noise. Both disorder and noise are assumed to be weak, i.e., the average bit-error rate (BER) is small. The probability distribution function (PDF) of rare violent events leading to the values of BER much larger than its typical value is estimated. We show that the PDF has a long algebraic-like tail.

Journal Article↗

PubMed2001

Exciton coherence and electron energy loss spectroscopy of conjugated molecules.

Signatures of the exciton coherence size, which controls the nonlinear optical response and luminescence of conjugated systems, in the electronic dynamic structure factor S(q,omega) are calculated. We find that for small molecules the momentum dependence of the lowest exciton resonance is purely geometric, reflecting the molecular size rather than a universal exciton size, as suggested recently. For long chains the q dependence is determined by the interplay of the exciton size and the bond-alternation length scales.

Journal Article↗

PubMed2000

Excitonic funneling in extended dendrimers with nonlinear and random potentials

The mean first passage time (MFPT) for photoexcitations diffusion in a funneling potential of artificial treelike light-harvesting antennas (phenylacetylene dendrimers with generation-dependent segment lengths) is computed. Effects of the nonlinearity of the realistic funneling potential and slow random solvent fluctuations considerably slow down the center-bound diffusion beyond a temperature-dependent optimal size. Diffusion on a disordered Cayley tree with a linear potential is investigated analytically. At low temperatures we predict a phase in which the MFPT is dominated by a few paths.

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PubMed1999

Excitonic couplings and electronic coherence in bridged naphthalene dimers.

The electronic excitations of naphthalene and a family of bridged naphthalene dimers are calculated and analyzed by using the Collective Electronic Oscillator method combined with the oblique Lanczos algorithm. All experimentally observed trends in absorption profiles and radiative lifetimes are reproduced. Each electronic excitation is linked to the corresponding real-space transition density matrix, which represents the motions of electrons and holes created in the molecule by photon absorption. Two-dimensional plots of these matrices help visualize the degree of exciton localization and explain the dependence of the electronic interaction between chromophores on their separation.

Journal Article↗

PubMed1996