43,215 research outputs found
Instability of three dimensional conformally dressed black hole
The three dimensional black hole solution of Einstein equations with negative
cosmological constant coupled to a conformal scalar field is proved to be
unstable against linear circularly symmetric perturbations.Comment: 5 pages, REVTe
Exact States in Waveguides With Periodically Modulated Nonlinearity
We introduce a one-dimensional model based on the nonlinear
Schrodinger/Gross-Pitaevskii equation where the local nonlinearity is subject
to spatially periodic modulation in terms of the Jacobi dn function, with three
free parameters including the period, amplitude, and internal form-factor. An
exact periodic solution is found for each set of parameters and, which is more
important for physical realizations, we solve the inverse problem and predict
the period and amplitude of the modulation that yields a particular exact
spatially periodic state. Numerical stability analysis demonstrates that the
periodic states become modulationally unstable for large periods, and regain
stability in the limit of an infinite period, which corresponds to a bright
soliton pinned to a localized nonlinearity-modulation pattern. Exact
dark-bright soliton complex in a coupled system with a localized modulation
structure is also briefly considered . The system can be realized in planar
optical waveguides and cigar-shaped atomic Bose-Einstein condensates.Comment: EPL, in pres
Synchrotron brightness distribution of turbulent radio jets
Radio jets are considered as turbulent mixing regions and it is proposed that the essential small scale viscous dissipation in these jets is by emission of MHD waves and by their subsequent strong damping due, at least partly, to gyro-resonant acceleration of supra-thermal particles. A formula relating the synchrotron surface brightness of a radio jet to the turbulent power input is deduced from physical postulates, and is tested against the data for NGC315 and 3C31 (NGC383). The predicted brightness depends essentially on the collimation behavior of the jet, and, to a lesser extent, on the CH picture of a 'high' nozzle with accelerating flow. The conditions for forming a large scale jet at a high nozzle from a much smaller scale jet are discussed. The effect of entrainment on the prediction is discussed with the use of similarity solutions. Although entrainment is inevitably associated with the turbulent jet, it may or may not be a dominant factor depending on the ambient density profile
Theory of enhanced performance emerging in a sparsely-connected competitive population
We provide an analytic theory to explain Anghel et al.'s recent numerical
finding whereby a maximum in the global performance emerges for a
sparsely-connected competitive population [Phys. Rev. Lett. 92, 058701 (2004)].
We show that the effect originates in the highly-correlated dynamics of
strategy choice, and can be significantly enhanced using a simple modification
to the model.Comment: This revised version will appear in PRE as a Rapid Com
Magic gap ratio for optimally robust fermionic condensation and its implications for high-Tc superconductivity
Bardeen-Schrieffer-Cooper (BCS) and Bose-Einstein condensation (BEC) occur at
opposite limits of a continuum of pairing interaction strength between
fermions. A crossover between these limits is readily observed in a cold atomic
Fermi gas. Whether it occurs in other systems such as the high temperature
superconducting cuprates has remained an open question. We uncover here
unambiguous evidence for a BCS-BEC crossover in the cuprates by identifying a
universal magic gap ratio 2\Delta/k_BT_c ~ 6.5 (where \Delta is the pairing gap
and Tc is the transition temperature) at which paired fermion condensates
become optimally robust. At this gap ratio, corresponding to the unitary point
in a cold atomic Fermi gas, the condensate fraction N_0 and the height of the
jump \delta\gamma(Tc) in the coefficient \gamma of the fermionic specific heat
at Tc are strongly peaked. In the cuprates, \delta\gamma(Tc) is peaked at this
gap ratio when \Delta corresponds to the antinodal spectroscopic gap, thus
reinforcing its interpretation as the pairing gap. We find the peak in
\delta\gamma(Tc) also to coincide with a normal state maximum in \gamma, which
is indicative of a pairing fluctuation pseudogap above Tc.Comment: 15 pages including 8 figure
A study of inner zone electron data and their comparison with trapped radiation models
A summary and intercomparison of recent inner radiation zone electron data are presented. The morphology of the inner radiation zone is described and the data compared with the current generation of inner zone trapped electron models. An analytic representation of the inner zone equatorial pitch angle distribution is presented. This model was based upon data from eight satellites and was used to reduce all data to the form of equatorial flux. Although no Starfish-free high energy electron measurements were available from the inner portion of the inner radiation zone, it was found that the AE-6 model provided a good description of the present solar maximum environment
Bringing Order to Special Cases of Klee's Measure Problem
Klee's Measure Problem (KMP) asks for the volume of the union of n
axis-aligned boxes in d-space. Omitting logarithmic factors, the best algorithm
has runtime O*(n^{d/2}) [Overmars,Yap'91]. There are faster algorithms known
for several special cases: Cube-KMP (where all boxes are cubes), Unitcube-KMP
(where all boxes are cubes of equal side length), Hypervolume (where all boxes
share a vertex), and k-Grounded (where the projection onto the first k
dimensions is a Hypervolume instance).
In this paper we bring some order to these special cases by providing
reductions among them. In addition to the trivial inclusions, we establish
Hypervolume as the easiest of these special cases, and show that the runtimes
of Unitcube-KMP and Cube-KMP are polynomially related. More importantly, we
show that any algorithm for one of the special cases with runtime T(n,d)
implies an algorithm for the general case with runtime T(n,2d), yielding the
first non-trivial relation between KMP and its special cases. This allows to
transfer W[1]-hardness of KMP to all special cases, proving that no n^{o(d)}
algorithm exists for any of the special cases under reasonable complexity
theoretic assumptions. Furthermore, assuming that there is no improved
algorithm for the general case of KMP (no algorithm with runtime O(n^{d/2 -
eps})) this reduction shows that there is no algorithm with runtime
O(n^{floor(d/2)/2 - eps}) for any of the special cases. Under the same
assumption we show a tight lower bound for a recent algorithm for 2-Grounded
[Yildiz,Suri'12].Comment: 17 page
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