460 research outputs found
Phase Transitions in Finite Nuclei and the Integer Nucleon Number Problem
The study of spherical-deformed ground--state phase transitions in finite
nuclei as a function of N and Z is hindered by the discrete values of the
nucleon number. A resolution of the integer nucleon number problem, and
evidence relating to phase transitions in finite nuclei, are discussed from the
experimental point of view and interpreted within the framework of the
interacting boson model.Comment: 8 pages Latex + 8 figs (postscript). In Phys Rev Lett, June 199
Contact mechanics: contact area and interfacial separation from small contact to full contact
We present a molecular dynamics study of the contact between a rigid solid
with a randomly rough surface and an elastic block with a flat surface. The
numerical calculations mainly focus on the contact area and the interfacial
separation from small contact (low load) to full contact (high load). For small
load the contact area varies linearly with the load and the interfacial
separation depends logarithmically on the load. For high load the contact area
approaches the nominal contact area (i.e., complete contact), and the
interfacial separation approaches zero. The numerical results have been
compared with analytical theory and experimental results. They are in good
agreement with each other. The present findings may be very important for soft
solids, e.g., rubber, or for very smooth surfaces, where complete contact can
be reached at moderate high loads without plastic deformation of the solids.Comment: 15 pages, 23 figure
Fine sediment reduces vertical migrations of Gammarus pulex (Crustacea: Amphipoda) in response to surface water loss
Surface and subsurface sediments in river ecosystems are recognized as refuges that may promote invertebrate survival during disturbances such as floods and streambed drying. Refuge use is spatiotemporally variable, with environmental factors including substrate composition, in particular the proportion of fine sediment (FS), affecting the ability of organisms to move through interstitial spaces. We conducted a laboratory experiment to examine the effects of FS on the movement of Gammarus pulex Linnaeus (Crustacea: Amphipoda) into subsurface sediments in response to surface water loss. We hypothesized that increasing volumes of FS would impede and ultimately prevent individuals from migrating into the sediments. To test this hypothesis, the proportion of FS (1–2 mm diameter) present within an open gravel matrix (4–16 mm diameter) was varied from 10 to 20% by volume in 2.5% increments. Under control conditions (0% FS), 93% of individuals moved into subsurface sediments as the water level was reduced. The proportion of individuals moving into the subsurface decreased to 74% at 10% FS, and at 20% FS no individuals entered the sediments, supporting our hypothesis. These results demonstrate the importance of reducing FS inputs into river ecosystems and restoring FS-clogged riverbeds, to promote refuge use during increasingly common instream disturbances
Some New Results on Complex-Temperature Singularities in Potts Models on the Square Lattice
We report some new results on the complex-temperature (CT) singularities of
-state Potts models on the square lattice. We concentrate on the problematic
region (where ) in which CT zeros of the partition function
are sensitive to finite lattice artifacts. From analyses of low-temperature
series expansions for , we establish the existence, in this
region, of complex-conjugate CT singularities at which the magnetization and
susceptibility diverge. From calculations of zeros of the partition function,
we obtain evidence consistent with the inference that these singularities occur
at endpoints of arcs protruding into the (complex-temperature
extension of the) FM phase. Exponents for these singularities are determined;
e.g., for , we find , consistent with .
By duality, these results also imply associated arcs extending to the (CT
extension of the) symmetric PM phase. Analytic expressions are suggested for
the positions of some of these singularities; e.g., for , our finding is
consistent with the exact value . Further discussions of
complex-temperature phase diagrams are given.Comment: 26 pages, latex, with eight epsf figure
Measurements of single top quark production cross sections and |Vtb| in ppbar collisions at sqrt{s}=1.96 TeV
We present measurements of production cross sections of single top quarks in
\ppbar collisions at in a data sample corresponding
to an integrated luminosity of collected by the D0 detector
at the Fermilab Tevatron Collider. We select events with an isolated electron
or muon, an imbalance in transverse energy, and two, three, or four jets, with
one or two of them containing a bottom hadron. We obtain an inclusive cross
section of \sigma({\ppbar}{\rargap}tb+X, tqb+X) = 3.43\pm^{0.73}_{0.74}\;\rm
pb and use it to extract the CKM matrix element at
the 95% C.L. We also measure \sigma({\ppbar}{\rargap}tb+X) =
0.68\pm^{0.38}_{0.35}\;\rm pb and \sigma({\ppbar}{\rargap}tqb+X) =
2.86\pm^{0.69}_{0.63}\;\rm pb when assuming, respectively, and
production rates as predicted by the standard model.Comment: 11 pages, 8 figures, submitted to Phys. Rev.
Multifactor Analysis of Multiscaling in Volatility Return Intervals
We study the volatility time series of 1137 most traded stocks in the US
stock markets for the two-year period 2001-02 and analyze their return
intervals , which are time intervals between volatilities above a given
threshold . We explore the probability density function of ,
, assuming a stretched exponential function, . We find that the exponent depends on the threshold
in the range between and 6 standard deviations of the volatility. This
finding supports the multiscaling nature of the return interval distribution.
To better understand the multiscaling origin, we study how depends on
four essential factors, capitalization, risk, number of trades and return. We
show that depends on the capitalization, risk and return but almost
does not depend on the number of trades. This suggests that relates to
the portfolio selection but not on the market activity. To further characterize
the multiscaling of individual stocks, we fit the moments of , , in the range of by a
power-law, . The exponent is found also to
depend on the capitalization, risk and return but not on the number of trades,
and its tendency is opposite to that of . Moreover, we show that
decreases with approximately by a linear relation. The return
intervals demonstrate the temporal structure of volatilities and our findings
suggest that their multiscaling features may be helpful for portfolio
optimization.Comment: 16 pages, 6 figure
Responses of marine benthic microalgae to elevated CO<inf>2</inf>
Increasing anthropogenic CO2 emissions to the atmosphere are causing a rise in pCO2 concentrations in the ocean surface and lowering pH. To predict the effects of these changes, we need to improve our understanding of the responses of marine primary producers since these drive biogeochemical cycles and profoundly affect the structure and function of benthic habitats. The effects of increasing CO2 levels on the colonisation of artificial substrata by microalgal assemblages (periphyton) were examined across a CO2 gradient off the volcanic island of Vulcano (NE Sicily). We show that periphyton communities altered significantly as CO2 concentrations increased. CO2 enrichment caused significant increases in chlorophyll a concentrations and in diatom abundance although we did not detect any changes in cyanobacteria. SEM analysis revealed major shifts in diatom assemblage composition as CO2 levels increased. The responses of benthic microalgae to rising anthropogenic CO2 emissions are likely to have significant ecological ramifications for coastal systems. © 2011 Springer-Verlag
Activity-based E3 ligase profiling uncovers an E3 ligase with esterification activity
Ubiquitination is initiated by transfer of ubiquitin (Ub) from a ubiquitin-activating enzyme (E1) to a ubiquitin-conjugating enzyme (E2), producing a covalently linked intermediate (E2-Ub)(1). Ubiquitin ligases (E3s) of the 'really interesting new gene' (RING) class recruit E2-Ub via their RING domain and then mediate direct transfer of ubiquitin to substrates(2). By contrast, 'homologous to E6-AP carboxy terminus' (HECT) E3 ligases undergo a catalytic cysteine-dependent transthiolation reaction with E2-Ub, forming a covalent E3-Ub intermediate(3,4). Additionally, RING-between-RING (RBR) E3 ligases have a canonical RING domain that is linked to an ancillary domain. This ancillary domain contains a catalytic cysteine that enables a hybrid RING-HECT mechanism(5). Ubiquitination is typically considered a post-translational modification of lysine residues, as there are no known human E3 ligases with non-lysine activity. Here we perform activity-based protein profiling of HECT or RBR-like E3 ligases and identify the neuron-associated E3 ligase MYCBP2 (also known as PHR1) as the apparent single member of a class of RING-linked E3 ligase with esterification activity and intrinsic selectivity for threonine over serine. MYCBP2 contains two essential catalytic cysteine residues that relay ubiquitin to its substrate via thioester intermediates. Crystallographic characterization of this class of E3 ligase, which we designate RING-Cys-relay (RCR), provides insights into its mechanism and threonine selectivity. These findings implicate non-lysine ubiquitination in cellular regulation of higher eukaryotes and suggest that E3 enzymes have an unappreciated mechanistic diversity
The Development of Cephalic Armor in The Tokay Gecko (Squamata: Gekkonidae: \u3cem\u3eGekko gecko\u3c/em\u3e)
Armored skin resulting from the presence of bony dermal structures, osteoderms, is an exceptional phenotype in gekkotans (geckos and flap-footed lizards) only known to occur in three genera: Geckolepis, Gekko, and Tarentola. The Tokay gecko (Gekko gecko LINNAEUS 1758) is among the best-studied geckos due to its large size and wide range of occurrence, and although cranial dermal bone development has previously been investigated, details of osteoderm development along a size gradient remain less well-known. Likewise, a comparative survey of additional species within the broader Gekko clade to determine the uniqueness of this trait has not yet been completed. Here, we studied a large sample of gekkotans (38 spp.), including 18 specimens of G. gecko, using X-rays and high-resolution computed tomography for visualizing and quantifying the dermal armor in situ. Results from this survey confirm the presence of osteoderms in a second species within this genus, Gekko reevesii GRAY 1831, which exhibits discordance in timing and pattern of osteoderm development when compared with its sister taxon, G. gecko. We discuss the developmental sequence of osteoderms in these two species and explore in detail the formation and functionality of these enigmatic dermal ossifications. Finally, we conducted a comparative analysis of endolymphatic sacs in a wide array of gekkotans to explore previous ideas regarding the role of osteoderms as calcium reservoirs. We found that G. gecko and other gecko species with osteoderms have highly enlarged endolymphatic sacs relative to their body size, when compared to species without osteoderms, which implies that these membranous structures might fulfill a major role of calcium storage even in species with osteoderms
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