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

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

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