6,027 research outputs found
Kepler super-flare stars: what are they?
The Kepler mission has led to the serendipitous discovery of a significant
number of `super flares' - white light flares with energies between 10^33 erg
and 10^36 erg - on solar-type stars. It has been speculated that these could be
`freak' events that might happen on the Sun, too. We have started a programme
to study the nature of the stars on which these super flares have been
observed. Here we present high-resolution spectroscopy of 11 of these stars and
discuss our results. We find that several of these stars are very young,
fast-rotating stars where high levels of stellar activity can be expected, but
for some other stars we do not find a straightforward explanation for the
occurrence of super flares.Comment: Accepted for publication in A&A, 9 pages, 4 figure
Conjunctive query inseparability of OWL 2 QL TBoxes
The OWL2 profile OWL 2 QL, based on the DL-Lite family of description logics, is emerging as a major language for developing new ontologies and approximating the existing ones. Its main application is ontology based data access, where ontologies are used to provide background knowledge for answering queries over data. We investigate the corresponding notion of query inseparability (or equivalence) for OWL 2 QL ontologies and show that deciding query inseparability is PSpace-hard and in ExpTime. We give polynomial-time (incomplete) algorithms and demonstrate by experiments that they can be used for practical module extraction
Amorphous ferromagnetism and re-entrant magnetic glassiness in SmMoO: new insights into the electronic phase diagram of pyrochlore molybdates
We discuss the magnetic properties of a SmMoO single
crystal as investigated by means of different experimental techniques. In the
literature, a conventional itinerant ferromagnetic state is reported for the
Mo sublattice below K. However, our results of dc
magnetometry, muon spin spectroscopy (SR) and high-harmonics magnetic
ac susceptibility unambiguously evidence highly disordered conditions in this
phase, in spite of the crystalline and chemical order. This disordered magnetic
state shares several common features with amorphous ferromagnetic alloys. This
scenario for SmMoO is supported by the anomalously high
values of the critical exponents, as mainly deduced by a scaling analysis of
our dc magnetization data and confirmed by the other techniques. Moreover,
SR detects a significant static magnetic disorder at the microscopic
scale. At the same time, the critical divergence of the third-harmonic
component of the ac magnetic susceptibility around K leads to
additional evidence towards the glassy nature of this magnetic phase. Finally,
the longitudinal relaxation of spin polarization (also supported by
results of ac susceptibility) evidences re-entrant glassy features similar to
amorphous ferromagnets.Comment: 15 pages, 13 figure
Module extraction via query inseparability in OWL 2 QL
We show that deciding conjunctive query inseparability for OWL 2 QL ontologies is PSpace-hard and in ExpTime. We give polynomial-time (incomplete) algorithms and demonstrate by experiments that they can be used for practical module extraction
Fishtail effect and vortex dynamics in LiFeAs single crystals
We investigate the fishtail effect, critical current density () and
vortex dynamics in LiFeAs single crystals. The sample exhibits a second peak
(SP) in the magnetization loop only with the field c-axis. We calculate a
reasonably high , however, values are lower than in 'Ba-122' and
'1111'-type FeAs-compounds. Magnetic relaxation data imply a strong pinning
which appears not to be due to conventional defects. Instead, its behavior is
similar to that of the triplet superconductor SrRuO. Our data suggest
that the origin of the SP may be related to a vortex lattice phase transition.
We have constructed the vortex phase diagram for LiFeAs on the
field-temperature plane.Comment: 5 pages, 5 figure
Exact learning description logic ontologies from data retrieval examples
We investigate the complexity of learning description logic ontologies in Angluin et al.'s framework of exact learning via queries posed to an oracle. We consider membership queries of the form "is individual a a certain answer to a data retrieval query q in a given ABox and the unkown target TBox?" and equivalence queries of the form "is a given TBox equivalent to the unknown target TBox?".We show that (i) DL-Lite TBoxes with role inclusions and ELI concept expressions on the right-hand side of inclusions and (ii) EL TBoxes without complex concept expressions on the right-hand side of inclusions can be learned in polynomial time. Both results are proved by a non-trivial reduction to learning from subsumption examples.We also show that arbitrary EL TBoxes cannot be learned in polynomial time
Disorder-induced Spin Gap in the Zigzag Spin-1/2 Chain Cuprate Sr_{0.9}Ca_{0.1}CuO_2
We report a comparative study of 63Cu Nuclear Magnetic Resonance spin lattice
relaxation rates, T_1^{-1}, on undoped SrCuO_2 and Ca doped
Sr_{0.9}Ca_{0.1}CuO_2 spin chain compounds. A temperature independent T_1^{-1}
is observed for SrCuO_2 as expected for an S=1/2 Heisenberg chain.
Surprisingly, we observe an exponential decrease of T_1^{-1} for T < 90,K in
the Ca-doped sample evidencing the opening of a spin gap. The data analysis
within the J_1-J_2 Heisenberg model employing density-matrix renormalization
group calculations suggests an impurity driven small alternation of the
J_2-exchange coupling as a possible cause of the spin gap.Comment: 4 pages, 4 figure
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