14,005 research outputs found
Comment on "Chain Length Scaling of Protein Folding Time", PRL 77, 5433 (1996)
In a recent Letter, Gutin, Abkevich, and Shakhnovich (GAS) reported on a
series of dynamical Monte Carlo simulations on lattice models of proteins.
Based on these highly simplified models, they found that four different
potential energies lead to four different folding time scales tau_f, where
tau_f scales with chain length as N^lambda (see, also, Refs. [2-4]), with
lambda varying from 2.7 to 6.0. However, due to the lack of microscopic models
of protein folding dynamics, the interpretation and origin of the data have
remained somewhat speculative. It is the purpose of this Comment to point out
that the application of a simple "mesoscopic" model (cond-mat/9512019, PRL 77,
2324, 1996) of protein folding provides a full account of the data presented in
their paper. Moreover, we find a major qualitative disagreement with the
argumentative interpretation of GAS. Including, the origin of the dynamics, and
size of the critical folding nucleus.Comment: 1 page Revtex, 1 fig. upon request. Submitted to PR
A Criterion That Determines Fast Folding of Proteins: A Model Study
We consider the statistical mechanics of a full set of two-dimensional
protein-like heteropolymers, whose thermodynamics is characterized by the
coil-to-globular () and the folding () transition temperatures.
For our model, the typical time scale for reaching the unique native
conformation is shown to scale as , where
, is the number of residues, and scales
algebraically with . We argue that scales linearly with the inverse of
entropy of low energy non-native states, whereas is almost
independent of it. As , non-productive intermediates
decrease, and the initial rapid collapse of the protein leads to structures
resembling the native state. Based solely on {\it accessible} information,
can be used to predict sequences that fold rapidly.Comment: 10 pages, latex, figures upon reques
Spontaneous patterns in coherently driven polariton microcavities
We consider a polariton microcavity resonantly driven by two external lasers
which simultaneously pump both lower and upper polariton branches at normal
incidence. In this setup, we study the occurrence of instabilities of the
pump-only solutions towards the spontaneous formation of patterns. Their
appearance is a consequence of the spontaneous symmetry breaking of
translational and rotational invariance due to interaction induced parametric
scattering. We observe the evolution between diverse patterns which can be
classified as single-pump, where parametric scattering occurs at the same
energy as one of the pumps, and as two-pump, where scattering occurs at a
different energy. For two-pump instabilities, stripe and chequerboard patterns
become the dominant steady-state solutions because cubic parametric scattering
processes are forbidden. This contrasts with the single-pump case, where
hexagonal patterns are the most common arrangements. We study the possibility
of controlling the evolution between different patterns. Our results are
obtained within a linear stability analysis and are confirmed by finite size
full numerical calculations.Comment: 15 pages, 9 figure
Quantum nondemolition measurements of a particle in electric and gravitational fields
In this work we obtain a nondemolition variable for the case in which a
charged particle moves in the electric and gravitational fields of a spherical
body. Afterwards we consider the continuous monitoring of this nondemolition
parameter, and calculate along the ideas of the so called restricted path
integral formalism, the corresponding propagator. Using these results the
probabilities associated with the possible measurement outputs are evaluated.
The limit of our results, as the resolution of the measuring device goes to
zero, is analyzed, and the dependence of the corresponding propagator upon the
strength of the electric and gravitational fields are commented. The role that
mass plays in the corresponding results, and its possible connection with the
equivalence principle at quantum level, are studied.Comment: Accepted in International Journal of Modern Physics D, 14 page
Ultra--cold gases and the detection of the Earth's rotation: Bogoliubov space and gravitomagnetism
The present work analyzes the consequences of the gravitomagnetic effect of
the Earth upon a bosonic gas in which the corresponding atoms have a
non--vanishing orbital angular momentum. Concerning the ground state of the
Bogoliubov space of this system we deduce the consequences, on the pressure and
on the speed of sound, of the gravitomagnetic effect. We prove that the effect
on a single atom is very small, but we also show that for some thermodynamical
properties the consequences scale as a non--trivial function of the number of
particles.Comment: 4 page
Low-distortion slow light using two absorption resonances
We consider group delay and broadening using two strongly absorbing and
widely spaced resonances. We derive relations which show that very large pulse
bandwidths coupled with large group delays and small broadening can be
achieved. Unlike single resonance systems, the dispersive broadening dominates
the absorptive broadening which leads to a dramatic increase in the possible
group delay. We show that the double resonance systems are excellent candidates
for realizing all-optical delay lines. We report on an experiment which
achieved up to 50 pulse delays with 40% broadening.Comment: 4 pages 4 figure
Rapidly reconfigurable slow-light system based on off-resonant Raman absorption
We present a slow-light system based on dual Raman absorption resonances in warm rubidium vapor. Each
Raman absorption resonance is produced by a control beam in an off-resonant Î system. This system combines
all optical control of the Raman absorption and the low-dispersion broadening properties of the double Lorentzian absorption slow light. The bandwidth, group delay, and central frequency of the slow-light system can all be tuned dynamically by changing the properties of the control beam. We demonstrate multiple pulse delays with
low distortion and show that such a system has fast switching dynamics and thus fast reconfiguration rates
Deep exclusive reactions at Jefferson Lab Hall A
International audienceRecent results on the Generalized Parton Distribution (GPD) programat Jefferson Lab (JLab) will be presented. The emphasis will be in the Hall A program aiming at measuring Q2âdependences of different terms of the Deeply Virtual Compton Scattering (DVCS) cross section. This is a fundamental step before one can extract GPD information from JLab DVCS data. The upcoming program in Hall A, using both a 6 GeV beam (2010) and a 11 GeV beam (â 2015) will also be described
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