3,039 research outputs found
Ferromagnetic coupling and magnetic anisotropy in molecular Ni(II) squares
We investigated the magnetic properties of two isostructural Ni(II) metal
complexes [Ni4Lb8] and [Ni4Lc8]. In each molecule the four Ni(II) centers form
almost perfect regular squares. Magnetic coupling and anisotropy of single
crystals were examined by magnetization measurements and in particular by
high-field torque magnetometry at low temperatures. The data were analyzed in
terms of an effective spin Hamiltonian appropriate for Ni(II) centers. For both
compounds, we found a weak intramolecular ferromagnetic coupling of the four
Ni(II) spins and sizable single-ion anisotropies of the easy-axis type. The
coupling strengths are roughly identical for both compounds, whereas the
zero-field-splitting parameters are significantly different. Possible reasons
for this observation are discussed.Comment: 7 pages, 7 figure
Q-dependence of the inelastic neutron scattering cross section for molecular spin clusters with high molecular symmetry
For powder samples of polynuclear metal complexes the dependence of the
inelastic neutron scattering intensity on the momentum transfer Q is known to
be described by a combination of so called interference terms. They reflect the
interplay between the geometrical structure of the compound and the spatial
properties of the wave functions involved in the transition. In this work, it
is shown that the Q-dependence is strongly interrelated with the molecular
symmetry of molecular nanomagnets, and, if the molecular symmetry is high
enough, is actually completely determined by it. A general formalism connecting
spatial symmetry and interference terms is developed. The arguments are
detailed for cyclic spin clusters, as experimentally realized by e.g. the
octanuclear molecular wheel Cr8, and the star like tetranuclear cluster Fe4.Comment: 8 pages, 1 figures, REVTEX
Classification of Invariant Star Products up to Equivariant Morita Equivalence on Symplectic Manifolds
In this paper we investigate equivariant Morita theory for algebras with
momentum maps and compute the equivariant Picard groupoid in terms of the
Picard groupoid explicitly. We consider three types of Morita theory:
ring-theoretic equivalence, *-equivalence and strong equivalence. Then we apply
these general considerations to star product algebras over symplectic manifolds
with a Lie algebra symmetry. We obtain the full classification up to
equivariant Morita equivalence.Comment: 28 pages. Minor update, fixed typos
A new approach to analysing HST spatial scans: the transmission spectrum of HD 209458 b
The Wide Field Camera 3 (WFC3) on Hubble Space Telescope (HST) is currently
one of the most widely used instruments for observing exoplanetary atmospheres,
especially with the use of the spatial scanning technique. An increasing number
of exoplanets have been studied using this technique as it enables the
observation of bright targets without saturating the sensitive detectors. In
this work we present a new pipeline for analyzing the data obtained with the
spatial scanning technique, starting from the raw data provided by the
instrument. In addition to commonly used correction techniques, we take into
account the geometric distortions of the instrument, whose impact may become
important when combined to the scanning process. Our approach can improve the
photometric precision for existing data and also push further the limits of the
spatial scanning technique, as it allows the analysis of even longer spatial
scans. As an application of our method and pipeline, we present the results
from a reanalysis of the spatially scanned transit spectrum of HD 209458 b. We
calculate the transit depth per wavelength channel with an average relative
uncertainty of 40 ppm. We interpret the final spectrum with T-Rex, our fully
Bayesian spectral retrieval code, which confirms the presence of water vapor
and clouds in the atmosphere of HD 209458 b. The narrow wavelength range limits
our ability to disentangle the degeneracies between the fitted atmospheric
parameters. Additional data over a broader spectral range are needed to address
this issue.Comment: 13 pages, 15 figures, 7 tables, Accepted for publication in Ap
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Automation of a Positron-emission Tomography (PET) Radiotracer Synthesis Protocol for Clinical Production.
The development of new positron-emission tomography (PET) tracers is enabling researchers and clinicians to image an increasingly wide array of biological targets and processes. However, the increasing number of different tracers creates challenges for their production at radiopharmacies. While historically it has been practical to dedicate a custom-configured radiosynthesizer and hot cell for the repeated production of each individual tracer, it is becoming necessary to change this workflow. Recent commercial radiosynthesizers based on disposable cassettes/kits for each tracer simplify the production of multiple tracers with one set of equipment by eliminating the need for custom tracer-specific modifications. Furthermore, some of these radiosynthesizers enable the operator to develop and optimize their own synthesis protocols in addition to purchasing commercially-available kits. In this protocol, we describe the general procedure for how the manual synthesis of a new PET tracer can be automated on one of these radiosynthesizers and validated for the production of clinical-grade tracers. As an example, we use the ELIXYS radiosynthesizer, a flexible cassette-based radiochemistry tool that can support both PET tracer development efforts, as well as routine clinical probe manufacturing on the same system, to produce [18F]Clofarabine ([18F]CFA), a PET tracer to measure in vivo deoxycytidine kinase (dCK) enzyme activity. Translating a manual synthesis involves breaking down the synthetic protocol into basic radiochemistry processes that are then translated into intuitive chemistry "unit operations" supported by the synthesizer software. These operations can then rapidly be converted into an automated synthesis program by assembling them using the drag-and-drop interface. After basic testing, the synthesis and purification procedure may require optimization to achieve the desired yield and purity. Once the desired performance is achieved, a validation of the synthesis is carried out to determine its suitability for the production of the radiotracer for clinical use
Subalgebras with Converging Star Products in Deformation Quantization: An Algebraic Construction for \complex \mbox{\LARGE P}^n
Based on a closed formula for a star product of Wick type on \CP^n, which
has been discovered in an earlier article of the authors, we explicitly
construct a subalgebra of the formal star-algebra (with coefficients contained
in the uniformly dense subspace of representative functions with respect to the
canonical action of the unitary group) that consists of {\em converging} power
series in the formal parameter, thereby giving an elementary algebraic proof of
a convergence result already obtained by Cahen, Gutt, and Rawnsley. In this
subalgebra the formal parameter can be substituted by a real number :
the resulting associative algebras are infinite-dimensional except for the case
, a positive integer, where they turn out to be isomorphic to
the finite-dimensional algebra of linear operators in the th energy
eigenspace of an isotropic harmonic oscillator with degrees of freedom.
Other examples like the -torus and the Poincar\'e disk are discussed.Comment: 16 pages, LaTeX with AMS Font
Collective resonance modes of Josephson vortices in sandwiched stack of BiSrCaCuO intrinsic Josephson junctions
We observed splitting of the low-bias vortex-flow branch in a
dense-Josephson-vortex state into multiple sub-branches in current-voltage
characteristics of intrinsic Josephson junctions (IJJs) of
BiSrCaCuO single crystals in the long-junction limit.
Each sub-branch corresponds to a plasma mode in serially coupled Josephson
junctions. Splitting into low-bias linear sub-branches with a spread in the
slopes and the inter-sub-branch mode-switching character are in good
quantitative agreement with the prediction of the weak but finite
inter-junction capacitive-coupling model incorporated with the inductive
coupling. This suggests the importance of the role of the capacitive coupling
in accurately describing the vortex dynamics in serially stacked IJJs.Comment: 4 pages, 3 figures, 1 tabl
The tight coupling between category and causal learning
The main goal of the present research was to demonstrate the interaction between category and causal induction in causal model learning. We used a two-phase learning procedure in which learners were presented with learning input referring to two interconnected causal relations forming a causal chain (Experiment 1) or a common-cause model (Experiments 2a, b). One of the three events (i.e., the intermediate event of the chain, or the common cause) was presented as a set of uncategorized exemplars. Although participants were not provided with any feedback about category labels, they tended to induce categories in the first phase that maximized the predictability of their causes or effects. In the second causal learning phase, participants had the choice between transferring the newly learned categories from the first phase at the cost of suboptimal predictions, or they could induce a new set of optimally predictive categories for the second causal relation, but at the cost of proliferating different category schemes for the same set of events. It turned out that in all three experiments learners tended to transfer the categories entailed by the first causal relation to the second causal relation
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