25,939 research outputs found
Fitness-based network growth with dynamic feedback
This article is a preprint of a paper that is currently under review with Physical Review E.We study a class of network growth models in which the choice of attachment by new nodes is governed by intrinsic attractiveness, or tness, of the existing nodes. The key feature of the models is a feedback mechanism whereby the distribution from which fitnesses of new nodes are drawn is dynamically updated to account for the evolving degree distribution. It is shown that in the case of linear mapping between fitnesses and degrees, the models lead to tunable stationary powerlaw degree distribution, while in the non-linear case the distributions converge to the stretched exponential form
Energy-dependent quenching adjusts the excitation diffusion length to regulate photosynthetic light harvesting
An important determinant of crop yields is the regulation of photosystem II
(PSII) light harvesting by energy-dependent quenching (qE). However, the
molecular details of excitation quenching have not been quantitatively
connected to the PSII yield, which only emerges on the 100 nm scale of the
grana membrane and determines flux to downstream metabolism. Here, we
incorporate excitation dissipation by qE into a pigment-scale model of
excitation transfer and trapping for a 200 nm x 200 nm patch of the grana
membrane. We demonstrate that single molecule measurements of qE are consistent
with a weak-quenching regime. Consequently, excitation transport can be
rigorously coarse-grained to a 2D random walk with an excitation diffusion
length determined by the extent of quenching. A diffusion-corrected lake model
substantially improves the PSII yield determined from variable chlorophyll
fluorescence measurements and offers an improved model of PSII for
photosynthetic metabolism.Comment: 19 pages, 4 figures, 3 supplementary figure
Dynamics of a two-level system strongly coupled to a high-frequency quantum oscillator
Recent experiments on quantum behavior in microfabricated solid-state systems
suggest tantalizing connections to quantum optics. Several of these experiments
address the prototypical problem of cavity quantum electrodynamics: a two-level
system coupled to a quantum harmonic oscillator. Such devices may allow the
exploration of parameter regimes outside the near-resonance and weak-coupling
assumptions of the ubiquitous rotating-wave approximation (RWA), necessitating
other theoretical approaches. One such approach is an adiabatic approximation
in the limit that the oscillator frequency is much larger than the
characteristic frequency of the two-level system. A derivation of the
approximation is presented and the time evolution of the two-level-system
occupation probability is calculated using both thermal- and coherent-state
initial conditions for the oscillator. Closed-form evaluation of the time
evolution in the weak-coupling limit provides insight into the differences
between the thermal- and coherent-state models. Finally, potential experimental
observations in solid-state systems, particularly the Cooper-pair
box--nanomechanical resonator system, are discussed and found to be promising.Comment: 16 pages, 11 figures; revised abstract; some text revisions; added
two figures and combined others; added references. Submitted to Phys. Rev.
The possibility of a metal insulator transition in antidot arrays induced by an external driving
It is shown that a family of models associated with the kicked Harper model
is relevant for cyclotron resonance experiments in an antidot array. For this
purpose a simplified model for electronic motion in a related model system in
presence of a magnetic field and an AC electric field is developed. In the
limit of strong magnetic field it reduces to a model similar to the kicked
Harper model. This model is studied numerically and is found to be extremely
sensitive to the strength of the electric field. In particular, as the strength
of the electric field is varied a metal -- insulator transition may be found.
The experimental conditions required for this transition are discussed.Comment: 6 files: kharp.tex, fig1.ps fig2.ps fi3.ps fig4.ps fig5.p
Extragalactic infrared spectroscopy
The spectra of galaxies in the near infrared atmospheric transmission windows are explored. Emission lines were detected due to molecular hydrogen, atomic hydrogen recombination lines, a line attributed to FEII, and a broad CO absorption feature. Lines due to H2 and FEII are especially strong in interacting and merging galaxies, but they were also detected in Seyferts and normal spirals. These lines appear to be shock excited. Multi-aperture measurements show that they emanate from regions as large as 15 kpc. It is argued that starbursts provide the most plausible and consistent model for the excitation of these lines, but the changes of relative line intensity of various species with aperture suggest that other excitation mechanisms are also operating in the outer regions of these galaxies
A Correlation between Galaxy Light Concentration and Supermassive Black Hole Mass
We present evidence for a strong correlation between the concentration of
bulges and the mass of their central supermassive black hole (M_bh) -- more
concentrated bulges have more massive black holes. Using C_{r_e}(1/3) from
Trujillo, Graham & Caon (2001b) as a measure of bulge concentration, we find
that log (M_bh/M_sun) = 6.81(+/-0.95)C_{r_e}(1/3) + 5.03(+/-0.41). This
correlation is shown to be marginally stronger (Spearman's r_s=0.91) than the
relationship between the logarithm of the stellar velocity dispersion and log
M_bh (Spearman's r_s=0.86), and has comparable, or less, scatter (0.31 dex in
log M_bh), which decreases to 0.19 dex when we use only those galaxies whose
supermassive black hole's radius of influence is resolved and remove one well
understood outlying data point).Comment: 7 pages, 1 table, 2 figures. ApJ Letters, accepte
Large rare fluctuations in systems with delayed dissipation
We study the probability distribution and the escape rate in systems with
delayed dissipation that comes from the coupling to a thermal bath. To
logarithmic accuracy in the fluctuation intensity, the problem is reduced to a
variational problem. It describes the most probable fluctuational paths, which
are given by acausal equations due to the delay. In thermal equilibrium, the
most probable path passing through a remote state has time reversal symmetry,
even though one cannot uniquely define a path that starts from a state with
given system coordinate and momentum. The corrections to the distribution and
the escape activation energy for small delay and small noise correlation time
are obtained in the explicit form.Comment: 9 page
Discovery of an extended debris disk around the F2V star HD 15745
Using the Advanced Camera for Surveys aboard the Hubble Space Telescope, we
have discovered dust-scattered light from the debris disk surrounding the F2V
star HD 15745. The circumstellar disk is detected between 2.0" and 7.5" radius,
corresponding to 128 - 480 AU radius. The circumstellar disk morphology is
asymmetric about the star, resembling a fan, and consistent with forward
scattering grains in an optically thin disk with an inclination of ~67 degrees
to our line of sight. The spectral energy distribution and scattered light
morphology can be approximated with a model disk composed of silicate grains
between 60 and 450 AU radius, with a total dust mass of 10E-7 M_sun (0.03
M_earth) representing a narrow grain size distribution (1 - 10 micron).
Galactic space motions are similar to the Castor Moving Group with an age of
~10E+8 yr, although future work is required to determine the age of HD 15745
using other indicators.Comment: 7 pages, 4 figures, ApJ Letters, in pres
Scaling function for the noisy Burgers equation in the soliton approximation
We derive the scaling function for the one dimensional noisy Burgers equation
in the two-soliton approximation within the weak noise canonical phase space
approach. The result is in agreement with an earlier heuristic expression and
exhibits the correct scaling properties. The calculation presents the first
step in a many body treatment of the correlations in the Burgers equation.Comment: Replacement: Several corrections, 4 pages, Revtex file, 3 figures. To
appear in Europhysics Letter
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