4,527 research outputs found
Origins of the Quantum Efficiency Duality In the Primary Photochemical Event of Bacteriorhodopsin
Experimental and theoretical evidence is presented which suggests that two distinct forms of light-adapted bacteriorhodopsin may exist. We propose that these two forms have characteristic photocycles with significantly different primary quantum yields. INDO-PSDCI molecular orbital procedures and semiempirical molecular dynamics simulations predict that one ground state geometry of bR undergoes photochemistry with a primary quantum yield, Φ1, of ~ 0.27, and that a second ground state geometry, with a slightly displaced counterion, yields Φ1 ~ 0.74. This theoretical model is supported by the observation that literature measurements of Φ1 tend to fall into one of two categories- those that observe Φ1 ~ 0.33 or below, and those that observe Φ1 ~ 0.6 or above. The observation that all photostationary state measurements of the primary quantum yield give values near 0.3, and all direct measurements of the quantum yield result in values near 0.6, suggests that photochemical back reactions may select the bacteriorhodopsin conformation with the lower quantum yield. The two photocycles may have developed as a natural biological requirement that the bacterium have the capacity to adjust the efficiency of the photocycle in relation to the intensity of light and/or membrane electrochemical gradient
Hierarchical Organization in Complex Networks
Many real networks in nature and society share two generic properties: they
are scale-free and they display a high degree of clustering. We show that these
two features are the consequence of a hierarchical organization, implying that
small groups of nodes organize in a hierarchical manner into increasingly large
groups, while maintaining a scale-free topology. In hierarchical networks the
degree of clustering characterizing the different groups follows a strict
scaling law, which can be used to identify the presence of a hierarchical
organization in real networks. We find that several real networks, such as the
World Wide Web, actor network, the Internet at the domain level and the
semantic web obey this scaling law, indicating that hierarchy is a fundamental
characteristic of many complex systems
Extending the Canada-France brown Dwarfs Survey to the near-infrared: first ultracool brown dwarfs from CFBDSIR
We present the first results of the ongoing Canada-France Brown Dwarfs
Survey-InfraRed, hereafter CFBDSIR, a Near InfraRed extension to the optical
wide-field survey CFBDS. Our final objectives are to constrain ultracool
atmosphere physics by finding a statistically significant sample of objects
cooler than 650K and to explore the ultracool brown dwarf mass function
building on a well defined sample of such objects. Candidates are identified in
CFHT/WIRCam J and CFHT/MegaCam z' images using optimised psf-fitting, and we
follow them up with pointed near infrared imaging with SOFI at NTT. We finally
obtain low resolution spectroscopy of the coolest candidates to characterise
their atmospheric physics. We have so far analysed and followed up all
candidates on the first 66 square degrees of the 335 square degrees survey. We
identified 55 T-dwarfs candidates with z'-J > 3:5 and have confirmed six of
them as T-dwarfs, including 3 that are strong later-than-T8 candidates, based
on their far-red and NIR colours. We also present here the NIR spectra of one
of these ultracool dwarfs, CFBDSIR1458+1013 which confirms it as one of the
coolest brown dwarf known, possibly in the 550-600K temperature range. From the
completed survey we expect to discover 10 to 15 dwarfs later than T8, more than
doubling the known number of such objects. This will enable detailed studies of
their extreme atmospheric properties and provide a stronger statistical base
for studies of their luminosity function.Comment: A&A, Accepte
From white elephant to Nobel Prize: Dennis Gabor’s wavefront reconstruction
Dennis Gabor devised a new concept for optical imaging in 1947 that went by a variety of names over the following decade: holoscopy, wavefront reconstruction, interference microscopy, diffraction microscopy and Gaboroscopy. A well-connected and creative research engineer, Gabor worked actively to publicize and exploit his concept, but the scheme failed to capture the interest of many researchers. Gabor’s theory was repeatedly deemed unintuitive and baffling; the technique was appraised by his contemporaries to be of dubious practicality and, at best, constrained to a narrow branch of science. By the late 1950s, Gabor’s subject had been assessed by its handful of practitioners to be a white elephant. Nevertheless, the concept was later rehabilitated by the research of Emmett Leith and Juris Upatnieks at the University of Michigan, and Yury Denisyuk at the Vavilov Institute in Leningrad. What had been judged a failure was recast as a success: evaluations of Gabor’s work were transformed during the 1960s, when it was represented as the foundation on which to construct the new and distinctly different subject of holography, a re-evaluation that gained the Nobel Prize for Physics for Gabor alone in 1971. This paper focuses on the difficulties experienced in constructing a meaningful subject, a practical application and a viable technical community from Gabor’s ideas during the decade 1947-1957
Good practices for a literature survey are not followed by authors while preparing scientific manuscripts
The number of citations received by authors in scientific journals has become
a major parameter to assess individual researchers and the journals themselves
through the impact factor. A fair assessment therefore requires that the
criteria for selecting references in a given manuscript should be unbiased with
respect to the authors or the journals cited. In this paper, we advocate that
authors should follow two mandatory principles to select papers (later
reflected in the list of references) while studying the literature for a given
research: i) consider similarity of content with the topics investigated, lest
very related work should be reproduced or ignored; ii) perform a systematic
search over the network of citations including seminal or very related papers.
We use formalisms of complex networks for two datasets of papers from the arXiv
repository to show that neither of these two criteria is fulfilled in practice
Statistical mechanics of complex networks
Complex networks describe a wide range of systems in nature and society, much
quoted examples including the cell, a network of chemicals linked by chemical
reactions, or the Internet, a network of routers and computers connected by
physical links. While traditionally these systems were modeled as random
graphs, it is increasingly recognized that the topology and evolution of real
networks is governed by robust organizing principles. Here we review the recent
advances in the field of complex networks, focusing on the statistical
mechanics of network topology and dynamics. After reviewing the empirical data
that motivated the recent interest in networks, we discuss the main models and
analytical tools, covering random graphs, small-world and scale-free networks,
as well as the interplay between topology and the network's robustness against
failures and attacks.Comment: 54 pages, submitted to Reviews of Modern Physic
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