15,986 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
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
Phonon-modulated magnetic interactions and spin Tomonaga-Luttinger liquid in the p-orbital antiferromagnet CsO2
The magnetic response of antiferromagnetic CsO2, coming from the p-orbital
S=1/2 spins of anionic O2- molecules, is followed by 133Cs nuclear magnetic
resonance across the structural phase transition occuring at Ts1=61 K on
cooling. Above Ts1, where spins form a square magnetic lattice, we observe a
huge, nonmonotonic temperature dependence of the exchange coupling originating
from thermal librations of O2- molecules. Below Ts1, where antiferromagnetic
spin chains are formed as a result of p-orbital ordering, we observe a spin
Tomonaga-Luttinger-liquid behavior of spin dynamics. These two interesting
phenomena, which provide rare simple manifestations of the coupling between
spin, lattice and orbital degrees of freedom, establish CsO2 as a model system
for molecular solids.Comment: 9 pages, 5 figures (with Supplemental Material), to appear in
Physical Review Letter
Lifeview and perception of message in drama
The age-old controversy between church and the arts has not been resolved by a long shot. The ethically repellent in modern art is definitely over-emphasized at the cost of the ethically acceptable. This leads to the question about the ChriÂstian response to a play with a non-Christian lifeview, one in which the characÂters use crude language and in which vulgar actions are indulged in
Nucleon form factors and moments of parton distributions in twisted mass lattice QCD
We present results on the electroweak form factors and on the lower moments
of parton distributions of the nucleon, within lattice QCD using two dynamical
flavors of degenerate twisted mass fermions. Results are obtained on lattices
with three different values of the lattice spacings, namely a=0.089 fm, a=0.070
fm and a=0.056 fm, allowing the investigation of cut-off effects. The volume
dependence is examined by comparing results on two lattices of spatial length
L=2.1 fm and L=2.8 fm. The simulations span pion masses in the range of 260-470
MeV. Our results are renormalized non-perturbatively and the values are given
in the MS-scheme at a scale mu=2 GeV.Comment: Talk presented in the XXIst International Europhysics Conference on
High Energy Physics, 21-27 July 2011, Grenoble, Rhones Alpes Franc
One-dimensional quantum antiferromagnetism in the orbital CsO compound revealed by electron paramagnetic resonance
Recently it was proposed that the orbital ordering of molecular
orbitals in the superoxide CsO compound leads to the formation of spin-1/2
chains below the structural phase transition occuring at ~K on
cooling. Here we report a detailed X-band electron paramagnetic resonance (EPR)
study of this phase in CsO powder. The EPR signal appears as a broad line
below , which is replaced by the antiferromagnetic resonance below
the N\'{e}el temperature ~K. The temperature dependence of the
EPR linewidth between and agrees with the
predictions for the one-dimensional Heisenberg antiferromagnetic chain of
spins in the presence of symmetric anisotropic exchange interaction.
Complementary analysis of the EPR lineshape, linewidth and the signal intensity
within the Tomonaga-Luttinger liquid (TLL) framework allows for a determination
of the TLL exponent . Present EPR data thus fully comply with the
quantum antiferromagnetic state of spin-1/2 chains in the orbitally ordered
phase of CsO, which is, therefore, a unique orbital system where such a
state could be studied.Comment: 6 pages, 3 figure
Mass loss out of close binaries. II
Liberal evolution of interacting binaries has been proposed previously by
several authors in order to meet various observed binary characteristics better
than conservative evolution does. Since Algols are eclipsing binaries the
distribution of their orbital periods is precisely known. The distribution of
their mass ratios contains however more uncertainties. We try to reproduce
these two distributions theoretically using a liberal scenario in which the
gainer star can lose mass into interstellar space as a consequence of its rapid
rotation and the energy of a hot spot. In a recent paper (Van Rensbergen et al.
2010, A&A) we calculated the liberal evolution of binaries with a B-type
primary at birth where mass transfer starts during core hydrogen burning of the
donor. In this paper we include the cases where mass transfer starts during
hydrogen shell burning and it is our aim to reproduce the observed
distributions of the system parameters of Algol-type semi-detached systems. Our
calculations reveal the amount of time that an Algol binary lives with a well
defined value of mass ratio and orbital period. We use these data to simulate
the distribution of mass ratios and orbital periods of Algols. Binaries with a
late B-type initial primary hardly lose any mass whereas those with an early B
primary evolve in a non-conservative way. Conservative binary evolution
predicts only ~ 12 % of Algols with a mass ratio q above 0.4. This value is
raised up to ~ 17 % using our scenario of liberal evolution, which is still far
below the ~ 45 % that is observed. Observed orbital periods of Algol binaries
larger than one day are faithfully reproduced by our liberal scenario. Mass
ratios are reproduced better than with conservative evolution, but the
resemblance is still poor.Comment: 11 pages, 6 figures, accepted for publication in A&A; accepted
versio
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