27,020 research outputs found
Island formation without attractive interactions
We show that adsorbates on surfaces can form islands even if there are no
attractive interactions. Instead strong repulsion between adsorbates at short
distances can lead to islands, because such islands increase the entropy of the
adsorbates that are not part of the islands. We suggest that this mechanism
cause the observed island formation in O/Pt(111), but it may be important for
many other systems as well.Comment: 11 pages, 4 figure
Graph-Based Shape Analysis Beyond Context-Freeness
We develop a shape analysis for reasoning about relational properties of data
structures. Both the concrete and the abstract domain are represented by
hypergraphs. The analysis is parameterized by user-supplied indexed graph
grammars to guide concretization and abstraction. This novel extension of
context-free graph grammars is powerful enough to model complex data structures
such as balanced binary trees with parent pointers, while preserving most
desirable properties of context-free graph grammars. One strength of our
analysis is that no artifacts apart from grammars are required from the user;
it thus offers a high degree of automation. We implemented our analysis and
successfully applied it to various programs manipulating AVL trees,
(doubly-linked) lists, and combinations of both
Chiral gauge theories with domain wall fermions
We have investigated a proposal to construct chiral gauge theories on the
lattice using domain wall fermions. The model contains two opposite chirality
zeromodes, which live on two domain walls. We couple only one of them to a
gauge field, but find that mirror fermions which also couple to the gauge field
always seem to exist.Comment: 3 pages. ref. added, some rewording at the end. contribution to
Lattice'93. latex file, style file espcrc2.sty needed (appended), compressed
tar file with two figures appended at the end (look for FIGURES
Exciton-Exciton Annihilation Is Coherently Suppressed in H-Aggregates, but Not in J-Aggregates
We theoretically demonstrate a strong dependence of the annihilation rate
between (singlet) excitons on the sign of dipole-dipole couplings between
molecules. For molecular H-aggregates, where this sign is positive, the phase
relation of the delocalized two-exciton wavefunctions causes a destructive
interference in the annihilation probability. For J-aggregates, where this sign
is negative, the interference is constructive instead, as a result of which no
such coherent suppression of the annihilation rate occurs. As a consequence,
room temperature annihilation rates of typical H- and J-aggregates differ by a
factor of ~3, while an order of magnitude difference is found for
low-temperature aggregates with a low degree of disorder. These findings, which
explain experimental observations, reveal a fundamental principle underlying
exciton-exciton annihilation, with major implications for technological devices
and experimental studies involving high excitation densities
The eta' meson from lattice QCD
We study the flavour singlet pseudoscalar mesons from first principles using
lattice QCD. With N_f=2 flavours of light quark, this is the so-called eta_2
meson and we discuss the phenomenological status of this. Using maximally
twisted-mass lattice QCD, we extract the mass of the eta_2 meson at two values
of the lattice spacing for lighter quarks than previously discussed in the
literature. We are able to estimate the mass value in the limit of light quarks
with their physical masses.Comment: 16 pages: version accepted for publicatio
Sigma-terms and axial charges for hyperons and charmed baryons
We present results for the -terms and axial charges for various
hyperons and charmed baryons using twisted mass fermions. For the
computation of the three-point function we use the fixed current method. For
one of the ensembles with pion mass of 373 MeV we compare the
results of the fixed current method with those obtained with a stochastic
method for computing the all-to-all propagator involved in the evaluation of
the three point functions.Comment: Talk presented at 31st International Symposium on Lattice Field
Theory LATTICE 2013, July 29 - August 3, 2013, Mainz, Germany, PoS(LATTICE
2013)279. 7 pages and6 figure
Are white-beaked dolphins Lagenorhynchus albirostris food specialst? Their diet in the southern North Sea
The white-beaked dolphin Lagenorhynchus albirostris is the most numerous cetacean after the harbour porpoise Phocoena phocoena in the North Sea, including Dutch coastal waters. In this study, the diet of 45 white-beaked dolphins stranded on the Dutch coast between 1968 and 2005 was determined by analysis of stomach contents. Although 25 fish species were identified, the diet was dominated by Gadidae (98.0% by weight, 40.0% in numbers), found in all stomachs. All other prey species combined contributed little to the diet by weight (2.0%W). The two most important prey species were whiting Merlangius merlangus (91.1% frequency of occurrence (FO), 30.5%N, 37.6%W) and cod Gadus morhua (73.3%FO, 7.4%N, 55.9%W). In numbers, gobies were most common (54.6%N), but contributed little to the diet by weight (0.6%W). Three stomachs contained different prey compared to the others: one animal had taken 2250 gobies, accounting for 96.4% of all gobies found; one animal had fed on 29 small sepiolids; and one animal had solely taken haddock Melanogrammus aeglefinus. Squid and haddock were not found in any other stomach. The overall diet showed a lasting predominance of whiting and cod, without clear changes over time (35 years) or differences between sexes or size-classes of dolphins. This study adds to earlier published and unpublished data for Dutch coastal waters and agrees well with studies of white-beaked dolphins from other parts of the species’ range, in the North Sea and in Canadian waters, with Gadidae dominating the diet on both sides of the Atlantic
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