2,910 research outputs found
Ground-based adaptive optics coronagraphic performance under closed-loop predictive control
The discovery of the exoplanet Proxima b highlights the potential for the
coming generation of giant segmented mirror telescopes (GSMTs) to characterize
terrestrial --- potentially habitable --- planets orbiting nearby stars with
direct imaging. This will require continued development and implementation of
optimized adaptive optics systems feeding coronagraphs on the GSMTs. Such
development should proceed with an understanding of the fundamental limits
imposed by atmospheric turbulence. Here we seek to address this question with a
semi-analytic framework for calculating the post-coronagraph contrast in a
closed-loop AO system. We do this starting with the temporal power spectra of
the Fourier basis calculated assuming frozen flow turbulence, and then apply
closed-loop transfer functions. We include the benefits of a simple predictive
controller, which we show could provide over a factor of 1400 gain in raw PSF
contrast at 1 on bright stars, and more than a factor of 30 gain on
an I = 7.5 mag star such as Proxima. More sophisticated predictive control can
be expected to improve this even further. Assuming a photon noise limited
observing technique such as High Dispersion Coronagraphy, these gains in raw
contrast will decrease integration times by the same large factors. Predictive
control of atmospheric turbulence should therefore be seen as one of the key
technologies which will enable ground-based telescopes to characterize
terrrestrial planets.Comment: Accepted to JATI
Maximally-localized Wannier functions for entangled energy bands
We present a method for obtaining well-localized Wannier-like functions (WFs)
for energy bands that are attached to or mixed with other bands. The present
scheme removes the limitation of the usual maximally-localized WFs method (N.
Marzari and D. Vanderbilt, Phys. Rev. B 56, 12847 (1997)) that the bands of
interest should form an isolated group, separated by gaps from higher and lower
bands everywhere in the Brillouin zone. An energy window encompassing N bands
of interest is specified by the user, and the algorithm then proceeds to
disentangle these from the remaining bands inside the window by filtering out
an optimally connected N-dimensional subspace. This is achieved by minimizing a
functional that measures the subspace dispersion across the Brillouin zone. The
maximally-localized WFs for the optimal subspace are then obtained via the
algorithm of Marzari and Vanderbilt. The method, which functions as a
postprocessing step using the output of conventional electronic-structure
codes, is applied to the s and d bands of copper, and to the valence and
low-lying conduction bands of silicon. For the low-lying nearly-free-electron
bands of copper we find WFs which are centered at the tetrahedral interstitial
sites, suggesting an alternative tight-binding parametrization.Comment: 13 pages, with 9 postscript figures embedded. Uses REVTEX and epsf
macro
Conductance fingerprints of non-collinear magnetic states in single atom contacts: a first-principles Wannier functions study
We present a first-principles computational scheme for investigating the
ballistic transport properties of one-dimensional nanostructures with
non-collinear magnetic order. The electronic structure is obtained within
density functional theory as implemented in the full-potential linearized
augmented plane-wave (FLAPW) method and mapped to a tight-binding like
transport Hamiltonian via non-collinear Wannier functions. The conductance is
then computed based on the Landauer formula using the Green's function method.
As a first application we study the conductance between two ferromagnetic Co
monowires terminated by single Mn apex atoms as a function of Mn-Mn separation.
We vary the Mn-Mn separation from the contact (about 2.5 to 5 {\AA}) to the far
tunneling regime (5 to 10 {\AA}). The magnetization direction of the Co
electrodes is chosen either in parallel or antiparallel alignment and we allow
for different spin configurations of the two Mn spins. In the tunneling and
into the contact regime the conductance is dominated by --states.
In the close contact regime (below 3.5 {\AA}) there is an additional
contribution for a parallel magnetization alignment from the - and
-states which give rise to an increase of the magnetoresistance as it
is absent for antiparallel magnetization. If we allow the Mn spins to relax a
non-collinear spin state is formed close to contact. We demonstrate that the
transition from a collinear to such a non-collinear spin structure as the two
Mn atoms approach leaves a characteristic fingerprint in the distance-dependent
conductance and magnetoresistance of the junction. We explain the effect of the
non-collinear spin state on the conductance based on the spin-dependent
hybridization between the -states of the Mn spins and their coupling
to the Co electrodes.Comment: 13 pages, 5 figure
Equality-friendly well-founded semantics and applications to description logics
We tackle the problem of defining a well-founded semantics (WFS) for Datalog rules with existentially quantified variables in their heads and nega- tions in their bodies. In particular, we provide a WFS for the recent Datalog± family of ontology languages, which covers several important description logics (DLs). To do so, we generalize Datalog± by non-stratified nonmonotonic nega- tion in rule bodies, and we define a WFS for this generalization via guarded fixed point logic. We refer to this approach as equality-friendly WFS, since it has the advantage that it does not make the unique name assumption (UNA); this brings it close to OWL and its profiles as well as typical DLs, which also do not make the UNA. We prove that for guarded Datalog± with negation under the equality- friendly WFS, conjunctive query answering is decidable, and we provide precise complexity results for this problem. From these results, we obtain precise defi- nitions of the standard WFS extensions of EL and of members of the DL-Lite family, as well as corresponding complexity results for query answering
- …