658 research outputs found
Stabilized jellium model and structural relaxation effects on the fragmentation energies of ionized silver clusters
Using the stabilized jellium model in two schemes of `relaxed' and `rigid',
we have calculated the dissociation energies and the fission barrier heights
for the binary fragmentations of singly-ionized and doubly-ionized Ag clusters.
In the calculations, we have assumed spherical geometries for the clusters.
Comparison of the fragmentation energies in the two schemes show differences
which are significant in some cases. This result reveals the advantages of the
relaxed SJM over the rigid SJM in dynamical processes such as fragmentation.
Comparing the relaxed SJM results and axperimental data on fragmentation
energies, it is possible to predict the sizes of the clusters just before their
fragmentations.Comment: 9 pages, 12 JPG figure
Self-similar chain conformations in polymer gels
We use molecular dynamics simulations to study the swelling of randomly
end-cross-linked polymer networks in good solvent conditions. We find that the
equilibrium degree of swelling saturates at Q_eq = N_e**(3/5) for mean strand
lengths N_s exceeding the melt entanglement length N_e. The internal structure
of the network strands in the swollen state is characterized by a new exponent
nu=0.72. Our findings are in contradiction to de Gennes' c*-theorem, which
predicts Q_eq proportional N_s**(4/5) and nu=0.588. We present a simple Flory
argument for a self-similar structure of mutually interpenetrating network
strands, which yields nu=7/10 and otherwise recovers the classical Flory-Rehner
theory. In particular, Q_eq = N_e**(3/5), if N_e is used as effective strand
length.Comment: 4 pages, RevTex, 3 Figure
Observation of mesoscopic crystalline structures in a two-dimensional Rydberg gas
The ability to control and tune interactions in ultracold atomic gases has
paved the way towards the realization of new phases of matter. Whereas
experiments have so far achieved a high degree of control over short-ranged
interactions, the realization of long-range interactions would open up a whole
new realm of many-body physics and has become a central focus of research.
Rydberg atoms are very well-suited to achieve this goal, as the van der Waals
forces between them are many orders of magnitude larger than for ground state
atoms. Consequently, the mere laser excitation of ultracold gases can cause
strongly correlated many-body states to emerge directly when atoms are
transferred to Rydberg states. A key example are quantum crystals, composed of
coherent superpositions of different spatially ordered configurations of
collective excitations. Here we report on the direct measurement of strong
correlations in a laser excited two-dimensional atomic Mott insulator using
high-resolution, in-situ Rydberg atom imaging. The observations reveal the
emergence of spatially ordered excitation patterns in the high-density
components of the prepared many-body state. They have random orientation, but
well defined geometry, forming mesoscopic crystals of collective excitations
delocalised throughout the gas. Our experiment demonstrates the potential of
Rydberg gases to realise exotic phases of matter, thereby laying the basis for
quantum simulations of long-range interacting quantum magnets.Comment: 10 pages, 7 figure
Phenomenological template family for black-hole coalescence waveforms
Recent progress in numerical relativity has enabled us to model the
non-perturbative merger phase of the binary black-hole coalescence problem.
Based on these results, we propose a phenomenological family of waveforms which
can model the inspiral, merger, and ring-down stages of black hole coalescence.
We also construct a template bank using this family of waveforms and discuss
its implementation in the search for signatures of gravitational waves produced
by black-hole coalescences in the data of ground-based interferometers. This
template bank might enable us to extend the present inspiral searches to
higher-mass binary black-hole systems, i.e., systems with total mass greater
than about 80 solar masses, thereby increasing the reach of the current
generation of ground-based detectors.Comment: Minor changes, Submitted to Class. Quantum Grav. (Proc. GWDAW11
A review of rapid serial visual presentation-based brain-computer interfaces
International audienceRapid serial visual presentation (RSVP) combined with the detection of event related brain responses facilitates the selection of relevant information contained in a stream of images presented rapidly to a human. Event related potentials (ERPs) measured non-invasively with electroencephalography (EEG) can be associated with infrequent targets amongst a stream of images. Human-machine symbiosis may be augmented by enabling human interaction with a computer, without overt movement, and/or enable optimization of image/information sorting processes involving humans. Features of the human visual system impact on the success of the RSVP paradigm, but pre-attentive processing supports the identification of target information post presentation of the information by assessing the co-occurrence or time-locked EEG potentials. This paper presents a comprehensive review and evaluation of the limited but significant literature on research in RSVP-based brain-computer interfaces (BCIs). Applications that use RSVP-based BCIs are categorized based on display mode and protocol design, whilst a range of factors influencing ERP evocation and detection are analyzed. Guidelines for using the RSVP-based BCI paradigms are recommended, with a view to further standardizing methods and enhancing the inter-relatability of experimental design to support future research and the use of RSVP-based BCIs in practice
The status of GEO 600
The GEO 600 laser interferometer with 600m armlength is part of a worldwide network of gravitational wave detectors. GEO 600 is unique in having advanced multiple pendulum suspensions with a monolithic last stage and in employing a signal recycled optical design. This paper describes the recent commissioning of the interferometer and its operation in signal recycled mode
Search for Gravitational Waves from Primordial Black Hole Binary Coalescences in the Galactic Halo
We use data from the second science run of the LIGO gravitational-wave
detectors to search for the gravitational waves from primordial black hole
(PBH) binary coalescence with component masses in the range 0.2--.
The analysis requires a signal to be found in the data from both LIGO
observatories, according to a set of coincidence criteria. No inspiral signals
were found. Assuming a spherical halo with core radius 5 kpc extending to 50
kpc containing non-spinning black holes with masses in the range 0.2--, we place an observational upper limit on the rate of PBH coalescence
of 63 per year per Milky Way halo (MWH) with 90% confidence.Comment: 7 pages, 4 figures, to be submitted to Phys. Rev.
Searching for gravitational waves from known pulsars
We present upper limits on the amplitude of gravitational waves from 28
isolated pulsars using data from the second science run of LIGO. The results
are also expressed as a constraint on the pulsars' equatorial ellipticities. We
discuss a new way of presenting such ellipticity upper limits that takes
account of the uncertainties of the pulsar moment of inertia. We also extend
our previous method to search for known pulsars in binary systems, of which
there are about 80 in the sensitive frequency range of LIGO and GEO 600.Comment: Accepted by CQG for the proceeding of GWDAW9, 7 pages, 2 figure
- …