12,836 research outputs found
Orientation effects in reactions of allenyl cations with styrene
Allenyl cations, generated from allenyl or alkynyl halides and Ag+, attack styrene at the side chain or at the aromatic nucleus. The allenyl/alkynyl product ratio is dependent on the structure of the precursor halide except for highly substituted systems
Protonolysis of a Ruthenium–Carbene Bond and Applications in Olefin Metathesis
The synthesis of a ruthenium complex containing an N-heterocylic carbene (NHC) and a mesoionic carbene (MIC) is described wherein addition of a Brønsted acid results in protonolysis of the Ru–MIC bond to generate an extremely active metathesis catalyst. Mechanistic studies implicated a rate-determining protonation step in the generation of the metathesis-active species. The activity of the NHC/MIC catalyst was found to exceed those of current commercial ruthenium catalysts
Ring-apodized vortex coronagraphs for obscured telescopes. I. Transmissive ring apodizers
The vortex coronagraph (VC) is a new generation small inner working angle
(IWA) coronagraph currently offered on various 8-meter class ground-based
telescopes. On these observing platforms, the current level of performance is
not limited by the intrinsic properties of actual vortex devices, but by
wavefront control residuals and incoherent background (e.g. thermal emission of
the sky) or the light diffracted by the imprint of the secondary mirror and
support structures on the telescope pupil. In the particular case of unfriendly
apertures (mainly large central obscuration) when very high contrast is needed
(e.g. direct imaging of older exoplanets with extremely large telescopes or
space- based coronagraphs), a simple VC, as most coronagraphs, can not deliver
its nominal performance because of the contamination due to the diffraction
from the obscured part of the pupil. Here we propose a novel yet simple concept
that circumvents this problem. We combine a vortex phase mask in the image
plane of a high-contrast instrument with a single pupil-based amplitude ring
apodizer, tailor designed to exploit the unique convolution properties of the
VC at the Lyot-stop plane. We show that such a ring-apodized vortex coronagraph
(RAVC) restores the perfect attenuation property of the VC regardless of the
size of the central obscuration, and for any (even) topological charge of the
vortex. More importantly the RAVC maintains the IWA and conserves a fairly high
throughput, which are signature properties of the VC.Comment: 10 pages, 6 figure
Imaging and manipulating electrons in a 1D quantum dot with Coulomb blockade microscopy
Motivated by the recent experiments by the Westervelt group using a mobile
tip to probe the electronic state of quantum dots formed on a segmented
nanowire, we study the shifts in Coulomb blockade peak positions as a function
of the spatial variation of the tip potential, which can be termed "Coulomb
blockade microscopy". We show that if the tip can be brought sufficiently close
to the nanowire, one can distinguish a high density electronic liquid state
from a Wigner crystal state by microscopy with a weak tip potential. In the
opposite limit of a strongly negative tip potential, the potential depletes the
electronic density under it and divides the quantum wire into two partitions.
There the tip can push individual electrons from one partition to the other,
and the Coulomb blockade micrograph can clearly track such transitions. We show
that this phenomenon can be used to qualitatively estimate the relative
importance of the electron interaction compared to one particle potential and
kinetic energies. Finally, we propose that a weak tip Coulomb blockade
micrograph focusing on the transition between electron number N=0 and N=1
states may be used to experimentally map the one-particle potential landscape
produced by impurities and inhomogeneities.Comment: 4 pages 7 figure
Hybridization gap and anisotropic far-infrared optical conductivity of URu2Si2
We performed far-infrared optical spectroscopy measurements on the heavy
fermion compound URu 2 Si 2 as a function of temperature. The light's
electric-field was applied along the a-axis or the c-axis of the tetragonal
structure. We show that in addition to a pronounced anisotropy, the optical
conductivity exhibits for both axis a partial suppression of spectral weight
around 12 meV and below 30 K. We attribute these observations to a change in
the bandstructure below 30 K. However, since these changes have no noticeable
impact on the entropy nor on the DC transport properties, we suggest that this
is a crossover phenomenon rather than a thermodynamic phase transition.Comment: To be published in Physical Review
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