56 research outputs found
A Chiral Macrocyclic Oligothiophene with Broken Conjugation âEuro" Rapid Racemization through Internal Rotation
Mechanical Stabilization of Helical Chirality in a Macrocyclic Oligothiophene
We introduce a design principle to stabilize helically chiral structures from an achiral tetrasubstituted [2.2]paracyclophane by integrating it into a macrocycle. The [2.2]paracyclophane introduces a three-dimensional perturbation into a nearly planar macrocyclic oligothiophene. The resulting helical structure is stabilized by two bulky substituents installed on the [2.2]paracyclophane unit. The increased enantiomerization barrier enabled the separation of both enantiomers. The synthesis of the target helical macrocycle 1 involves a sequence of halogenation and cross-coupling steps and a high-dilution strategy to close the macrocycle. Substituents tuning the energy of the enantiomerization process can be introduced in the last steps of the synthesis. The chiral target compound 1 was fully characterized by NMR spectroscopy and mass spectrometry. The absolute configurations of the isolated enantiomers were assigned by comparing the data of circular dichroism spectroscopy with TD-DFT calculations. The enantiomerization dynamics was studied by dynamic HPLC and variable-temperature 2D exchange spectroscopy and supported by quantum-chemical calculations
Large Conductance Variations in a Mechanosensitive Single-Molecule Junction
The appealing feature of molecular electronics is the possibility of
exploiting functionality built within a single molecule. This functionality can
be employed, for example, for sensing or switching purposes. Thus, ideally, the
associated conductance changes should be sizable upon application of external
stimuli. Here, we show that a molecular spring can be mechanically compressed
or elongated to tune its conductance by up to an order of magnitude by
controlling the quantum interference between electronic pathways. Oscillations
in the conductance occur when the stress built up in the molecule is high
enough to allow the anchoring groups to move along the surface in a
stick-slip-like fashion. The mechanical control of quantum interference effects
and the resulting large change in molecular conductance open the door for
applications in, e.g., a minute mechanosensitive sensing device functional at
room temperature.Comment: 23 pages, 6 figure
The Atacama Cosmology Telescope: Physical Properties and Purity of a Galaxy Cluster Sample Selected via the Sunyaev-Zel'dovich Effect
We present optical and X-ray properties for the first confirmed galaxy
cluster sample selected by the Sunyaev-Zel'dovich Effect from 148 GHz maps over
455 square degrees of sky made with the Atacama Cosmology Telescope. These
maps, coupled with multi-band imaging on 4-meter-class optical telescopes, have
yielded a sample of 23 galaxy clusters with redshifts between 0.118 and 1.066.
Of these 23 clusters, 10 are newly discovered. The selection of this sample is
approximately mass limited and essentially independent of redshift. We provide
optical positions, images, redshifts and X-ray fluxes and luminosities for the
full sample, and X-ray temperatures of an important subset. The mass limit of
the full sample is around 8e14 Msun, with a number distribution that peaks
around a redshift of 0.4. For the 10 highest significance SZE-selected cluster
candidates, all of which are optically confirmed, the mass threshold is 1e15
Msun and the redshift range is 0.167 to 1.066. Archival observations from
Chandra, XMM-Newton, and ROSAT provide X-ray luminosities and temperatures that
are broadly consistent with this mass threshold. Our optical follow-up
procedure also allowed us to assess the purity of the ACT cluster sample.
Eighty (one hundred) percent of the 148 GHz candidates with signal-to-noise
ratios greater than 5.1 (5.7) are confirmed as massive clusters. The reported
sample represents one of the largest SZE-selected sample of massive clusters
over all redshifts within a cosmologically-significant survey volume, which
will enable cosmological studies as well as future studies on the evolution,
morphology, and stellar populations in the most massive clusters in the
Universe.Comment: 20 pages, 15 figures, 6 tables. Accepted for publication in ApJ.
Higher resolution figures available at:
http://peumo.rutgers.edu/~felipe/e-prints
The Atacama Cosmology Telescope: Extragalactic Sources at 148 GHz in the 2008 Survey
We report on extragalactic sources detected in a 455 square-degree map of the
southern sky made with data at a frequency of 148 GHz from the Atacama
Cosmology Telescope 2008 observing season. We provide a catalog of 157 sources
with flux densities spanning two orders of magnitude: from 15 to 1500 mJy.
Comparison to other catalogs shows that 98% of the ACT detections correspond to
sources detected at lower radio frequencies. Three of the sources appear to be
associated with the brightest cluster galaxies of low redshift X-ray selected
galaxy clusters. Estimates of the radio to mm-wave spectral indices and
differential counts of the sources further bolster the hypothesis that they are
nearly all radio sources, and that their emission is not dominated by
re-emission from warm dust. In a bright (>50 mJy) 148 GHz-selected sample with
complete cross-identifications from the Australia Telescope 20 GHz survey, we
observe an average steepening of the spectra between 5, 20, and 148 GHz with
median spectral indices of , , and . When the
measured spectral indices are taken into account, the 148 GHz differential
source counts are consistent with previous measurements at 30 GHz in the
context of a source count model dominated by radio sources. Extrapolating with
an appropriately rescaled model for the radio source counts, the Poisson
contribution to the spatial power spectrum from synchrotron-dominated sources
with flux density less than 20 mJy is C^{\rm Sync} = (2.8 \pm 0.3) \times
10^{-6} \micro\kelvin^2.Comment: Accepted to Ap
The Atacama Cosmology Telescope: A Measurement of the Cosmic Microwave Background Power Spectrum at 148 and 218 GHz from the 2008 Southern Survey
We present measurements of the cosmic microwave background (CMB) power
spectrum made by the Atacama Cosmology Telescope at 148 GHz and 218 GHz, as
well as the cross-frequency spectrum between the two channels. Our results
clearly show the second through the seventh acoustic peaks in the CMB power
spectrum. The measurements of these higher-order peaks provide an additional
test of the {\Lambda}CDM cosmological model. At l > 3000, we detect power in
excess of the primary anisotropy spectrum of the CMB. At lower multipoles 500 <
l < 3000, we find evidence for gravitational lensing of the CMB in the power
spectrum at the 2.8{\sigma} level. We also detect a low level of Galactic dust
in our maps, which demonstrates that we can recover known faint, diffuse
signals.Comment: 19 pages, 13 figures. Submitted to ApJ. This paper is a companion to
Hajian et al. (2010) and Dunkley et al. (2010
Chemical profiles of the oxides on tantalum in state of the art superconducting circuits
Over the past decades, superconducting qubits have emerged as one of the
leading hardware platforms for realizing a quantum processor. Consequently,
researchers have made significant effort to understand the loss channels that
limit the coherence times of superconducting qubits. A major source of loss has
been attributed to two level systems that are present at the material
interfaces. We recently showed that replacing the metal in the capacitor of a
transmon with tantalum yields record relaxation and coherence times for
superconducting qubits, motivating a detailed study of the tantalum surface. In
this work, we study the chemical profile of the surface of tantalum films grown
on c-plane sapphire using variable energy X-ray photoelectron spectroscopy
(VEXPS). We identify the different oxidation states of tantalum that are
present in the native oxide resulting from exposure to air, and we measure
their distribution through the depth of the film. Furthermore, we show how the
volume and depth distribution of these tantalum oxidation states can be altered
by various chemical treatments. By correlating these measurements with detailed
measurements of quantum devices, we can improve our understanding of the
microscopic device losses
CLASS Angular Power Spectra and Map-Component Analysis for 40 GHz Observations through 2022
Measurement of the largest angular scale () features of the cosmic
microwave background (CMB) polarization is a powerful way to constrain the
optical depth to reionization, , and search for the signature of
inflation through the detection of primordial -modes. We present an analysis
of maps covering nearly 75% of the sky made from the ground-based
channel of the Cosmology Large Angular Scale Surveyor
(CLASS) from August 2016 to May 2022. Using fast front-end polarization
modulation from the Atacama Desert in Chile, we show this channel achieves
higher sensitivity than the analogous frequencies from satellite measurements
in the range . After a final calibration adjustment, noise
simulations show the CLASS linear (circular) polarization maps have a white
noise level of . We measure the
Galaxy-masked and spectra of diffuse synchrotron radiation and
compare to space-based measurements at similar frequencies. In combination with
external data, we expand measurements of the spatial variations of the
synchrotron spectral energy density (SED) to include new regions of the sky and
measure the faint diffuse SED in the harmonic domain. We place a new upper
limit on a background of circular polarization in the range
with the first bin showing at 95%
confidence. These results establish a new standard for recovery of the
largest-scale CMB polarization from the ground and signal exciting
possibilities when the higher sensitivity and higher frequency CLASS channels
are included in the analysis.Comment: 36 pages, 24 figures, 6 tables. Submitted to The Astrophysical
Journa
Venus Observations at 40 and 90 GHz with CLASS
Using the Cosmology Large Angular Scale Surveyor, we measure the
disk-averaged absolute Venus brightness temperature to be 432.3 2.8 K and
355.6 1.3 K in the Q and W frequency bands centered at 38.8 and 93.7 GHz,
respectively. At both frequency bands, these are the most precise measurements
to date. Furthermore, we observe no phase dependence of the measured
temperature in either band. Our measurements are consistent with a
CO-dominant atmospheric model that includes trace amounts of additional
absorbers like SO and HSO.Comment: 7 pages, 3 figures, published in PS
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