1,681 research outputs found
Contribution of pulsars to the gamma-ray background and their observation with the space telescopes GLAST and AGILE
Luminosities and uxes of the expected population of galactic gamma-ray
pulsars become foreseeable if physical distributions at birth and evolutive
history are assigned. In this work we estimate the contribution of pulsar uxes
to the gamma-ray background, which has been measured by the EGRET experiment on
board of the CGRO. For pulsar luminosities we select some of the most important
gamma-ray emission models, taking into account both polar cap and outer gap
scenarios. We nd that this contribution strongly depends upon controversial
neutron star birth properties. A comparison between our simulation results and
EGRET data is presented for each model, nding an average contribution of about
10%. In addition, we perform the calculation of the number of new gamma-ray
pulsars detectable by GLAST and AGILE, showing a remarkable di erence between
the two classes of models. Finally, we suggest some improvements in the
numerical code, including more sophisticated galactic m odels and di erent
populations of pulsars like binaries, milliseconds, anomalous pulsars and
magnetars.Comment: 6 pages, 6 figures, to be published in the Proceedings of the 6th
International Symposium ''Frontiers of Fundamental and Computational
Physics'' (FFP6), Udine (Italy), Sep. 26-29, 200
Pairing, Charge, and Spin Correlations in the Three-Band Hubbard Model
Using the Constrained Path Monte Carlo (CPMC) method, we simulated the
two-dimensional, three-band Hubbard model to study pairing, charge, and spin
correlations as a function of electron and hole doping and the Coulomb
repulsion between charges on neighboring Cu and O lattice sites. As a
function of distance, both the -wave and extended s-wave pairing
correlations decayed quickly. In the charge-transfer regime, increasing
decreased the long-range part of the correlation functions in both
channels, while in the mixed-valent regime, it increased the long-range part of
the s-wave behavior but decreased that of the d-wave behavior. Still the d-wave
behavior dominated. At a given doping, increasing increased the
spin-spin correlations in the charge-transfer regime but decreased them in the
mixed-valent regime. Also increasing suppressed the charge-charge
correlations between neighboring Cu and O sites. Electron and hole doping away
from half-filling was accompanied by a rapid suppression of anti-ferromagnetic
correlations.Comment: Revtex, 8 pages with 15 figure
A Gaussian Theory of Superfluid--Bose-Glass Phase Transition
We show that gaussian quantum fluctuations, even if infinitesimal, are
sufficient to destroy the superfluidity of a disordered boson system in 1D and
2D. The critical disorder is thus finite no matter how small the repulsion is
between particles. Within the gaussian approximation, we study the nature of
the elementary excitations, including their density of states and mobility edge
transition. We give the gaussian exponent at criticality in 1D and show
that its ratio to of the pure system is universal.Comment: Revtex 3.0, 11 pages (4 figures will be sent through airmail upon
request
Entanglement detection via condition of quantum correlation
We develop a novel necessary condition of quantum correlation. It is utilized
to construct -level bipartite Bell-type inequality which is strongly
resistant to noise and requires only analyses of measurement outcomes
compared to the previous result . Remarkably, a connection between
the arbitrary high-dimensional bipartite Bell-type inequality and entanglement
witnesses is found. Through the necessary condition of quantum correlation, we
propose that the witness operators to detect truly multipartite entanglement
for a generalized Greenberger-Horne-Zeilinger (GHZ) state with two local
measurement settings and a four-qubit singlet state with three settings.
Moreover, we also propose the first robust entanglement witness to detect
four-level tripartite GHZ state with only two local measurement settings
Critical Exponents for Three-Dimensional Superfluid--Bose-Glass Phase Transition
The critical phenomenon of the zero temperature superfluid--Bose-glass phase
transition for hard-core bosons on a three-dimensional disordered lattice is
studied using a quantum real-space renormalization-group method. The
correlation-length exponent and the dynamic exponent z are computed. The
critical exponent z is found to be 2.5 for compressible states and 1.3 for
incompressible states. The exponent is shown to be insensitive to z as
that in the two-dimensional case, and has value roughly equal to 1.Comment: 11 pages, REVTE
Preparation of polarization entangled mixed states of two photons
We propose a scheme for preparing arbitrary two photons polarization
entangled mixed states via controlled location decoherence. The scheme uses
only linear optical devices and single-mode optical fibers, and may be feasible
in experiment within current optical technology.Comment: 3 pages, 5 figs. The article has been rewritten. Discussion about
experiment are added. To appear in Phys. Rev.
Disordered Boson Systems: A Perturbative Study
A hard-core disordered boson system is mapped onto a quantum spin 1/2
XY-model with transverse random fields. It is then generalized to a system of
spins with an arbitrary magnitude S and studied through a 1/S expansion. The
first order 1/S expansion corresponds to a spin-wave theory. The effect of weak
disorder is studied perturbatively within such a first order 1/S scheme. We
compute the reduction of the speed of sound and the life time of the Bloch
phonons in the regime of weak disorder. Generalizations of the present study to
the strong disordered regime are discussed.Comment: 27 pages, revte
Two dimensional scaling of resistance in flux flow region in thin films
The resistance of thin films has been measured when the
angle between the applied fields and -plane of the film is changed
continuously at various temperatures. Under various magnetic fields, the
resistance can be well scaled in terms of the c-axis component of the applied
fields at the same temperature in the whole angle range. Meanwhile, we show
that the measurement of resistance in this way is a complementary method to
determine the growth orientation of the anisotropic high- superconductors.Comment: 11 pages, 8 figures. Have been published in Physica
Superfluidity vs Bose-Einstein condensation in a Bose gas with disorder
We investigate the phenomenon of Bose-Einstein condensation and superfluidity
in a Bose gas at zero temperature with disorder. By using the Diffusion
Monte-Carlo method we calculate the superfluid and the condensate fraction of
the system as a function of density and strength of disorder. In the regime of
weak disorder we find agreement with the analytical results obtained within the
Bogoliubov model. For strong disorder the system enters an unusual regime where
the superfluid fraction is smaller than the condensate fraction.Comment: 4 pages, 4 Postscript figure
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