4,746 research outputs found
A generative model for feedback networks
We investigate a simple generative model for network formation. The model is
designed to describe the growth of networks of kinship, trading, corporate
alliances, or autocatalytic chemical reactions, where feedback is an essential
element of network growth. The underlying graphs in these situations grow via a
competition between cycle formation and node addition. After choosing a given
node, a search is made for another node at a suitable distance. If such a node
is found, a link is added connecting this to the original node, and increasing
the number of cycles in the graph; if such a node cannot be found, a new node
is added, which is linked to the original node. We simulate this algorithm and
find that we cannot reject the hypothesis that the empirical degree
distribution is a q-exponential function, which has been used to model
long-range processes in nonequilibrium statistical mechanics.Comment: 11 pages, 6 figure
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The effect of elevated hydrostatic pressure on the spectral absorption of deep-sea fish visual pigments
The effect of hydrostatic pressure (0.1-54 MPa, equivalent to pressures experienced by fish from the ocean's surface to depths of ca. 5400 m) on visual pigment absorption spectra was investigated for rod visual pigments extracted from the retinae of 12 species of deep-sea fish of diverse phylogeny and habitat. The wavelength of peak absorption (λmax) was shifted to longer wavelengths by an average of 1.35 nm at 40 MPa (a pressure approximately equivalent to average ocean depth) relative to measurements made at one atmosphere (ca. 0.1 MPa), but with little evidence of a change in absorbance at the λmax. We conclude that previousλ max measurements of deep-sea fish visual pigments, made at a pressure close to 0.1 MPa, provide a good indication ofλ max values at higher pressures when considering the ecology of vision in the deep-sea. Although not affecting the spectral sensitivity of the animal to any important degree, the observed shift inλ max may be of interest in the context of understanding opsin-chromophore interaction and spectral tuning of visual pigments
Cold dark matter models with high baryon content
Recent results have suggested that the density of baryons in the Universe,
OmegaB, is much more uncertain than previously thought, and may be
significantly higher. We demonstrate that a higher OmegaB increases the
viability of critical-density cold dark matter (CDM) models. High baryon
fraction offers the twin benefits of boosting the first peak in the microwave
anisotropy power spectrum and of suppressing short-scale power in the matter
power spectrum. These enable viable CDM models to have a larger Hubble constant
than otherwise possible. We carry out a general exploration of high OmegaB CDM
models, varying the Hubble constant h and the spectral index n. We confront a
variety of observational constraints and discuss specific predictions. Although
some observational evidence may favour baryon fractions as high as 20 per cent,
we find that values around 10 to 15 per cent provide a reasonable fit to a wide
range of data. We suggest that models with OmegaB in this range, with h about
0.5 and n about 0.8, are currently the best critical-density CDM models.Comment: 14 pages, LaTeX, with 9 included figures, to appear in MNRAS. Revised
version includes updated references, some changes to section 4. Conclusions
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The Navigability of Strong Ties: Small Worlds, Tie Strength and Network Topology
International audienceWe examine data on and models of small world properties and parameters of social networks. Our focus, on tie-strength, multilevel networks and searchability in strong-tie social networks, allows us to extend some of the questions and findings of recent research and the fit of small world models to sociological and anthropological data on human communities. We offer a 'navigability of strong ties' hypothesis about network topologies tested with data from kinship systems, but potentially applicable to corporate cultures and business networks
Stripe orientation in an anisotropic t-J model
The tilt pattern of the CuO_6 octahedra in the LTT phase of the cuprate
superconductors leads to planar anisotropies for the exchange coupling and
hopping integrals. Here, we show that these anisotropies provide a possible
structural mechanism for the orientation of stripes. A t_x-t_y-J_x-J_y model
thus serves as an effective Hamiltonian to describe stripe formation and
orientation in LTT-phase cuprates.Comment: 3 pages, 3 figure
Density-matrix-renormalization-group-based downfolding of the three-band Hubbard model: the importance of density-assisted hopping
Typical Wannier-function downfolding starts with a mean-field or density
functional set of bands to construct the Wannier functions. Here we carry out a
controlled approach, using DMRG-computed natural orbital bands, to downfold the
three-band Hubbard model to an effective single band model. A sharp drop-off in
the natural orbital occupancy at the edge of the first band provides a clear
justification for a single-band model. Constructing Wannier functions from the
first band, we compute all possible two-particle terms and retain those with
significant magnitude. The resulting single-band model includes two-site
density-assisted hopping terms with . These terms lead to a
reduction of the ratio , and are important in capturing the
doping-asymmetric carrier mobility, as well as in enhancing the pairing in a
single-band model for the hole-doped cuprates.Comment: 7+4 pages, 4+5 figures; revised some writing
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