15,793 research outputs found
Stokes trapping and planet formation
It is believed that planets are formed by aggregation of dust particles
suspended in the turbulent gas forming accretion disks around developing stars.
We describe a mechanism, termed 'Stokes trapping', by which turbulence limits
the growth of aggregates of dust particles, so that their Stokes number
(defined as the ratio of the damping time of the particles to the Kolmogorov
dissipation timescale) remains close to unity. We discuss possible mechanisms
for avoiding this barrier to further growth. None of these is found to be
satisfactory and we introduce a new theory which does not involve the growth of
small clusters of dust grains.Comment: 30 pages, 4 figures. Revised version has improved concluding remarks,
extended discussion of sticking velocit
The Quantum-Classical Crossover in the Adiabatic Response of Chaotic Systems
The autocorrelation function of the force acting on a slow classical system,
resulting from interaction with a fast quantum system is calculated following
Berry-Robbins and Jarzynski within the leading order correction to the
adiabatic approximation. The time integral of the autocorrelation function is
proportional to the rate of dissipation. The fast quantum system is assumed to
be chaotic in the classical limit for each configuration of the slow system. An
analytic formula is obtained for the finite time integral of the correlation
function, in the framework of random matrix theory (RMT), for a specific
dependence on the adiabatically varying parameter. Extension to a wider class
of RMT models is discussed. For the Gaussian unitary and symplectic ensembles
for long times the time integral of the correlation function vanishes or falls
off as a Gaussian with a characteristic time that is proportional to the
Heisenberg time, depending on the details of the model. The fall off is
inversely proportional to time for the Gaussian orthogonal ensemble. The
correlation function is found to be dominated by the nearest neighbor level
spacings. It was calculated for a variety of nearest neighbor level spacing
distributions, including ones that do not originate from RMT ensembles. The
various approximate formulas obtained are tested numerically in RMT. The
results shed light on the quantum to classical crossover for chaotic systems.
The implications on the possibility to experimentally observe deterministic
friction are discussed.Comment: 26 pages, including 6 figure
Structure of purine nucleoside phosphorylase (DeoD) from Bacillus anthracis
Protein structures from the causative agent of anthrax (Bacillus anthracis) are being determined as part of a structural genomics programme. Amongst initial candidates for crystallographic analysis are enzymes involved in nucleotide biosynthesis, since these are recognized as potential targets in antibacterial therapy. Purine nucleoside phosphorylase is a key enzyme in the purine-salvage pathway. The crystal structure of purine nucleoside phosphorylase (DeoD) from B. anthracis has been solved by molecular replacement at 2.24 Å resolution and refined to an R factor of 18.4%. This is the first report of a DeoD structure from a Gram-positive bacterium
Bacillus subtilis regulatory protein GerE
GerE is the latest-acting of a series of factors which regulate gene expression in the mother cell during sporulation in Bacillus. The gene encoding GerE has been cloned from B. subtilis and overexpressed in Escherichia coli. Purified GerE has been crystallized by the hanging-drop vapour-diffusion method using polyethylene glycol as a precipitant. The small plate-like crystals belong to the monoclinic space group C2 and diffract beyond 2.2 Angstrom resolution with a synchrotron radiation X-ray source
Anomalous flux-flow dynamics in layered type-II superconductors at low temperatures
Low-temperature dissipation due to vortex motion in strongly anisotropic
type-II superconductors with a moderate disorder () is shown to be determined by the Zener-type transitions between
the localized electronic states in the vortex core. Statistics of these levels
is described by the random matrix ensemble of the class C defined recently by
Atland and Zirnbauer [cond-mat/9602137], so the vortex motion leads naturally
to the new example of a parametric statistics of energy levels. The flux-flow
conductivity is a bit lower than the quasiclassical one and {\it
grows} slowly with the increase of the electric field.Comment: 4 pages, Revte
Attempted Bethe ansatz solution for one-dimensional directed polymers in random media
We study the statistical properties of one-dimensional directed polymers in a
short-range random potential by mapping the replicated problem to a many body
quantum boson system with attractive interactions. We find the full set of
eigenvalues and eigenfunctions of the many-body system and perform the
summation over the entire spectrum of excited states. The analytic continuation
of the obtained exact expression for the replica partition function from
integer to non-integer replica parameter N turns out to be ambiguous.
Performing the analytic continuation simply by assuming that the parameter N
can take arbitrary complex values, and going to the thermodynamic limit of the
original directed polymer problem, we obtain the explicit universal expression
for the probability distribution function of free energy fluctuations.Comment: 32 pages, 1 figur
Higher Order Correlations in Quantum Chaotic Spectra
The statistical properties of the quantum chaotic spectra have been studied,
so far, only up to the second order correlation effects. The numerical as well
as the analytical evidence that random matrix theory can successfully model the
spectral fluctuatations of these systems is available only up to this order.
For a complete understanding of spectral properties it is highly desirable to
study the higher order spectral correlations. This will also inform us about
the limitations of random matrix theory in modelling the properties of quantum
chaotic systems. Our main purpose in this paper is to carry out this study by a
semiclassical calculation for the quantum maps; however results are also valid
for time-independent systems.Comment: Revtex, Four figures (Postscript files), Phys. Rev E (in press
Super-diffusion in optical realizations of Anderson localization
We discuss the dynamics of particles in one dimension in potentials that are
random both in space and in time. The results are applied to recent optics
experiments on Anderson localization, in which the transverse spreading of a
beam is suppressed by random fluctuations in the refractive index. If the
refractive index fluctuates along the direction of the paraxial propagation of
the beam, the localization is destroyed. We analyze this broken localization,
in terms of the spectral decomposition of the potential. When the potential has
a discrete spectrum, the spread is controlled by the overlap of Chirikov
resonances in phase space. As the number of Fourier components is increased,
the resonances merge into a continuum, which is described by a Fokker-Planck
equation. We express the diffusion coefficient in terms of the spectral
intensity of the potential. For a general class of potentials that are commonly
used in optics, the solutions of the Fokker-Planck equation exhibit anomalous
diffusion in phase space, implying that when Anderson localization is broken by
temporal fluctuations of the potential, the result is transport at a rate
similar to a ballistic one or even faster. For a class of potentials which
arise in some existing realizations of Anderson localization atypical behavior
is found.Comment: 11 pages, 2 figure
Caustics in turbulent aerosols
Networks of caustics can occur in the distribution of particles suspended in
a randomly moving gas. These can facilitate coagulation of particles by
bringing them into close proximity, even in cases where the trajectories do not
coalesce. We show that the long-time morphology of these caustic patterns is
determined by the Lyapunov exponents lambda_1, lambda_2 of the suspended
particles, as well as the rate J at which particles encounter caustics. We
develop a theory determining the quantities J, lambda_1, lambda_2 from the
statistical properties of the gas flow, in the limit of short correlation
times.Comment: 4 pages, 3 figure
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