7,609 research outputs found
A new era of spectroscopy: SINFONI, NIR integral field spectroscopy at the diffraction limit of an 8m telescope
SINFONI, the SINgle Faint Object Near-infrared Investigation, is an
instrument for the Very Large Telescope (VLT), which will start its operation
mid 2002 and allow for the first time near infrared (NIR) integral field
spectroscopy at the diffraction limit of an 8-m telescope. SINFONI is the
combination of two state-of-the art instruments, the integral field
spectrometer SPIFFI, built by the Max-Planck-Institut fuer extraterrestrische
Physik (MPE), and the adaptive optics (AO) system MACAO, built by the European
Southern Observatory (ESO). It will allow a unique type of observations by
delivering simultaneously high spatial resolution (pixel sizes 0.025arcsec to
0.25arcsec) and a moderate spectral resolution (R~2000 to R~4500), where the
higher spectral resolution mode will allow for software OH suppression. This
opens new prospects for astronomy.Comment: 9 pages, 4 figures, to appear in SPIE proceedings "Astronomical
Telescopes and Instrumentation 2000". More recent sensitivity estimates are
available at http://www.mpe.mpg.de/www_ir/ir_instruments/sinfoni/spiffi.ht
Electronic properties of disordered corner-sharing tetrahedral lattices
We have examined the behaviour of noninteracting electrons moving on a
corner-sharing tetrahedral lattice into which we introduce a uniform (box)
distribution, of width W, of random on-site energies. We have used both the
relative localization length and the spectral rigidity to analyze the nature of
the eigenstates, and have determined both the mobility edge trajectories as a
function of W, and the critical disorder, Wc, beyond which all states are
localized. We find (i) that the mobility edge trajectories (energies Ec vs.
disorder W) are qualitatively different from those found for a simple cubic
lattice, and (ii) that the spectral rigidity is scale invariant at Wc and thus
provides a reliable method of estimating this quantity -- we find Wc/t=14.5. We
discuss our results in the context of the metal-to-insulator transition
undergone by LiAlyTi{2-y}O4 in a quantum site percolation model that also
includes the above-mentioned Anderson disorder, and show that the effects
produced by Anderson disorder are far less important than those produced by
quantum site percolation, at least in the determination of the doping
concentration at which the metal-to-insulator transition is predicted to occur
Multifractal analysis of the metal-insulator transition in anisotropic systems
We study the Anderson model of localization with anisotropic hopping in three
dimensions for weakly coupled chains and weakly coupled planes. The eigenstates
of the Hamiltonian, as computed by Lanczos diagonalization for systems of sizes
up to , show multifractal behavior at the metal-insulator transition even
for strong anisotropy. The critical disorder strength determined from the
system size dependence of the singularity spectra is in a reasonable agreement
with a recent study using transfer matrix methods. But the respective spectrum
at deviates from the ``characteristic spectrum'' determined for the
isotropic system. This indicates a quantitative difference of the multifractal
properties of states of the anisotropic as compared to the isotropic system.
Further, we calculate the Kubo conductivity for given anisotropies by exact
diagonalization. Already for small system sizes of only sites we observe
a rapidly decreasing conductivity in the directions with reduced hopping if the
coupling becomes weaker.Comment: 25 RevTeX pages with 10 PS-figures include
Mixing-induced anisotropic correlations in molecular crystalline systems
We investigate the structure of mixed thin films composed of pentacene (PEN)
and diindenoperylene (DIP) using X-ray reflectivity and grazing incidence X-ray
diffraction. For equimolar mixtures we observe vanishing in-plane order
coexisting with an excellent out-of-plane order, a yet unreported disordering
behavior in binary mixtures of organic semiconductors, which are crystalline in
their pure form. One approach to rationalize our findings is to introduce an
anisotropic interaction parameter in the framework of a mean field model. By
comparing the structural properties with those of other mixed systems, we
discuss the effects of sterical compatibility and chemical composition on the
mixing behavior, which adds to the general understanding of interactions in
molecular mixtures.Comment: 5 pages, 5 figures, accepted by Phys. Rev. Let
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