62,105 research outputs found
Quantum phase transition in an atomic Bose gas near a Feshbach resonance
We study the quantum phase transition in an atomic Bose gas near a Feshbach
resonance in terms of the renormalization group. This quantum phase transition
is characterized by an Ising order parameter. We show that in the low
temperature regime where the quantum fluctuations dominate the low-energy
physics this phase transition is of first order because of the coupling between
the Ising order parameter and the Goldstone mode existing in the bosonic
superfluid. However, when the thermal fluctuations become important, the phase
transition turns into the second order one, which belongs to the
three-dimensional Ising universality class. We also calculate the damping rate
of the collective mode in the phase with only a molecular Bose-Einstein
condensate near the second-order transition line, which can serve as an
experimental signature of the second-order transition.Comment: 8 pages, 2 figures, published version in Phys. Rev.
Processing SPARQL queries with regular expressions in RDF databases
Background: As the Resource Description Framework (RDF) data model is widely used for modeling and sharing a lot of online bioinformatics resources such as Uniprot (dev.isb-sib.ch/projects/uniprot-rdf) or Bio2RDF (bio2rdf.org), SPARQL - a W3C recommendation query for RDF databases - has become an important query language for querying the bioinformatics knowledge bases. Moreover, due to the diversity of users' requests for extracting information from the RDF data as well as the lack of users' knowledge about the exact value of each fact in the RDF databases, it is desirable to use the SPARQL query with regular expression patterns for querying the RDF data. To the best of our knowledge, there is currently no work that efficiently supports regular expression processing in SPARQL over RDF databases. Most of the existing techniques for processing regular expressions are designed for querying a text corpus, or only for supporting the matching over the paths in an RDF graph.
Results: In this paper, we propose a novel framework for supporting regular expression processing in SPARQL query. Our contributions can be summarized as follows. 1) We propose an efficient framework for processing SPARQL queries with regular expression patterns in RDF databases. 2) We propose a cost model in order to adapt the proposed framework in the existing query optimizers. 3) We build a prototype for the proposed framework in C++ and conduct extensive experiments demonstrating the efficiency and effectiveness of our technique.
Conclusions: Experiments with a full-blown RDF engine show that our framework outperforms the existing ones by up to two orders of magnitude in processing SPARQL queries with regular expression patterns.X113sciescopu
Momentum Flow Correlations from Event Shapes: Factorized Soft Gluons and Soft-Collinear Effective Theory
The distributions of two-jet event shapes contain information on
hadronization in QCD. Near the two-jet limit, these distributions can be
described by convolutions of nonperturbative event shape functions with the
same distributions calculated in resummed perturbation theory. The shape
functions, in turn, are determined by correlations of momentum flow operators
with each other and with light-like Wilson lines, which describe the coupling
of soft, wide-angle radiation to jets. We observe that leading power
corrections to the mean values of event shapes are determined by the
correlation of a single momentum flow operator with the relevant Wilson lines.
This generalizes arguments for the universality of leading power corrections
based on the low-scale behavior of the running coupling or resummation. We also
show how a study of the angularity event shapes can provide information on
correlations involving multiple momentum flow operators, giving a window to the
system of QCD dynamics that underlies the variety of event shape functions. In
deriving these results, we review, develop and compare factorization techniques
in conventional perturbative QCD and soft-collinear effective theory (SCET). We
give special emphasis to the elimination of double counting of momentum regions
in these two formalisms.Comment: 25 pages revtex
Ballistic spin field-effect transistors: Multichannel effects
We study a ballistic spin field-effect transistor (SFET) with special
attention to the issue of multi-channel effects. The conductance modulation of
the SFET as a function of the Rashba spin-orbit coupling strength is
numerically examined for the number of channels ranging from a few to close to
100. Even with the ideal spin injector and collector, the conductance
modulation ratio, defined as the ratio between the maximum and minimum
conductances, decays rapidly and approaches one with the increase of the
channel number. It turns out that the decay is considerably faster when the
Rashba spin-orbit coupling is larger. Effects of the electronic coherence are
also examined in the multi-channel regime and it is found that the coherent
Fabry-Perot-like interference in the multi-channel regime gives rise to a
nested peak structure. For a nonideal spin injector/collector structure, which
consists of a conventional metallic ferromagnet-thin insulator-2DEG
heterostructure, the Rashba-coupling-induced conductance modulation is strongly
affected by large resonance peaks that arise from the electron confinement
effect of the insulators. Finally scattering effects are briefly addressed and
it is found that in the weakly diffusive regime, the positions of the resonance
peaks fluctuate, making the conductance modulation signal sample-dependent.Comment: 18 pages, 15 figure
Carbon-substitution effect on the electronic properties of MgB single crystals
The electronic properties of the carbon substituted MgB single crystals
are reported. The carbon substitution drops T below 2 K. In-plane
resistivity shows a remarkable increase in residual resistivity by
C-substitution, while the change of in-plane/out-of-plane Hall coefficients is
rather small. Raman scattering spectra indicate that the E-phonon
frequency radically hardens with increasing the carbon-content, suggesting the
weakening of electron-phonon coupling. Another striking C-effect is the
increases of the second critical fields in both in-plane and out-of-plane
directions, accompanied by a reduction in the anisotropy ratio. The possible
changes in the electronic state and the origin of T-suppression by
C-substitution are discussed.Comment: 6 pages, 8 figure
Dynamical renormalization group approach to the Altarelli-Parisi-Lipatov equations
The Altarelli-Parisi-Lipatov equations for the parton distribution functions
are rederived using the dynamical renormalization group approach to quantum
kinetics. This method systematically treats the ln Q^2 corrections that arises
in perturbation theory as a renormalization of the parton distribution function
and unambiguously indicates that the strong coupling must be allowed to run
with the scale in the evolution kernel. To leading logarithmic accuracy the
evolution equation is Markovian and the logarithmic divergences in the
perturbative expansion are identified with the secular divergences (terms that
grow in time) that emerge in a perturbative treatment of the kinetic equations
in nonequilibrium systems. The resummation of the leading logarithms by the
Altarelli-Parisi-Lipatov equation is thus similar to the resummation of the
leading secular terms by the Boltzmann kinetic equation.Comment: 8 pages, version to appear in Phys. Rev.
Glueball Masses in Relativistic Potential Model
The problem of glueball mass spectra using the relativistic Dirac equation is
studied. Also the Breit-Fermi approach used to obtaining hyperfine splitting in
glueballs. Our approach is based on the assumption, that the nature and the
forces between two gluons are the short-range. We were to calculate the
glueball masses with used screened potential.Comment: 7 pages, LaTe
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