16,364 research outputs found
Optical pumping of charged excitons in unintentionally doped InAs quantum dots
As an alternative to commonly used electrical methods, we have investigated
the optical pumping of charged exciton complexes addressing impurity related
transitions with photons of the appropriate energy. Under these conditions, we
demonstrate that the pumping fidelity can be very high while still maintaining
a switching behavior between the different excitonic species. This mechanism
has been investigated for single quantum dots of different size present in the
same sample and compared with the direct injection of spectator electrons from
nearby donors.Comment: 4 pages and 3 figures submitted to AP
A connection between accretion state and in an accreting neutron star: black hole-like soft state winds?
High resolution X-ray spectra of accreting stellar mass Black Holes reveal
the presence of accretion disc winds, traced by high ionisation Fe K lines.
These winds appear to have an equatorial geometry and to be observed only
during disc dominated states in which the radio jet is absent. Accreting
neutron star systems also show equatorial high ionisation absorbers. However,
the presence of any correlation with the accretion state has not been
previously tested. We have studied EXO 0748-676, a transient neutron star
system, for which we can reliably determine the accretion state, in order to
investigate the Fe K absorption/accretion state/jet connection. Not one of
twenty X-ray spectra obtained in the hard state revealed any significant Fe K
absorption line. However, intense Fe and Fe
(as well as a rarely observed Fe line plus S ; a blend of S and Ar ; Ca and Ca
, possibly produced by the same high ionisation
material) absorption lines ( eV, eV) are clearly detected during the only soft state
observation. This suggests that the connection between Fe K absorption and
states (and anticorrelation between the presence of Fe K absorption and jets)
is also valid for EXO 0748-676 and therefore it is not a unique property of
black hole systems but a more general characteristic of accreting sources.Comment: Accepted for publication in MNRAS Letter
Time-Delayed transfer functions simulations for LMXBs
Recent works (Steeghs & Casares 2002, Casares et al. 2003, Hynes et al. 2003)
have demonstrated that Bowen flourescence is a very efficient tracer of the
companion star in LMXBs. We present a numerical code to simulate time-delayed
transfer functions in LMXBs, specific to the case of reprocessing in emission
lines. The code is also able to obtain geometrical and binary parameters by
fitting observed (X-ray + optical) light curves using simulated annealing
methods. In this work we present the geometrical model for the companion star
and the analytical model for the disc and show synthetic time-delay transfer
functions for different orbital phases and system parameters.Comment: Contribution presented at the conference "Interacting Binaries:
Accretion, Evolution and Outcomes", held in Cefalu, Sicily (Italy) in July
2004. To be published by AIP (American Institute of Physics), eds. L. A.
Antonelli, L. Burderi, F. D'Antona, T. Di Salvo, G.L. Israel, L. Piersanti,
O. Straniero, A. Tornambe. 4 pages, 4 figure
Explanation and observability of diffraction in time
Diffraction in time (DIT) is a fundamental phenomenon in quantum dynamics due
to time-dependent obstacles and slits. It is formally analogous to diffraction
of light, and is expected to play an increasing role to design coherent matter
wave sources, as in the atom laser, to analyze time-of-flight information and
emission from ultrafast pulsed excitations, and in applications of coherent
matter waves in integrated atom-optical circuits. We demonstrate that DIT
emerges robustly in quantum waves emitted by an exponentially decaying source
and provide a simple explanation of the phenomenon, as an interference of two
characteristic velocities. This allows for its controllability and
optimization.Comment: 4 pages, 6 figure
Differential susceptibility to noise of mixed Turing and Hopf modes in a photosensitive chemical medium
We report on experiments with the photosensitive chlorine dioxide-iodine-malonic acid reaction (CDIMA) when forced with a random (spatiotemporally) distributed illumination. Acting on a mixed mode consisting of oscillating spots, close enough to the Hopf and Turing codimension-two bifurcation, we observe attenuation of oscillations while the spatial pattern is preserved. Numerical simulations confirm and extend these results. All together these observations point out to a larger vulnerability of the Hopf with respect to the Turing mode when facing noise of intermediate intensity and small correlation parameters.Peer ReviewedPostprint (published version
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