791 research outputs found
Synthesis of 3,3-disubstituted heterocycles by Pd-catalyzed arylallylation of unactivated alkenes
Finding new methods of carbon–carbon bond formation is a key goal in expanding current
methodology for heterocycle formation. Because of their inherently non-planar shape, new methods of
forming sp3-rich scaffolds are of particular importance. While there are methods for combining
heterocyclisation and formation of new sp3–sp3 carbon–carbon bonds, these form the carbon–
heteroatom bond rather than a carbon–carbon bond of the heterocycle. Here we show a new alkene
arylallylation reaction that generates a heterocycle with concomitant formation of two new carbon–
carbon bonds. Furthermore, we demonstrate that this process occurs through an isohypsic (redox
neutral) mechanism. Overall, this carboallylation reaction gives a new route to the synthesis of 3,3-
disubstituted heterocycles
One-dimensional tight-binding models with correlated diagonal and off-diagonal disorder
We study localization properties of electronic states in one-dimensional
lattices with nearest-neighbour interaction. Both the site energies and the
hopping amplitudes are supposed to be of arbitrary form. A few cases are
considered in details. We discuss first the case in which both the diagonal
potential and the fluctuating part of the hopping amplitudes are small. In this
case we derive a general analytical expression for the localization length,
which depends on the pair correlators of the diagonal and off-diagonal matrix
elements. The second case we investigate is that of strong uncorrelated
disorder, for which approximate analytical estimates are given and compared
with numerical data. Finally, we study the model with short-range correlations
which constitutes an extension with off-diagonal disorder of the random dimer
model.Comment: 11 pages, 7 EPS figures; submitted to "Physica E
Delocalization and spin-wave dynamics in ferromagnetic chains with long-range correlated random exchange
We study the one-dimensional quantum Heisenberg ferromagnet with exchange
couplings exhibiting long-range correlated disorder with power spectrum
proportional to , where is the wave-vector of the modulations
on the random coupling landscape. By using renormalization group, integration
of the equations of motion and exact diagonalization, we compute the spin-wave
localization length and the mean-square displacement of the wave-packet. We
find that, associated with the emergence of extended spin-waves in the
low-energy region for , the wave-packet mean-square displacement
changes from a long-time super-diffusive behavior for to a
long-time ballistic behavior for . At the vicinity of ,
the mobility edge separating the extended and localized phases is shown to
scale with the degree of correlation as .Comment: PRB to appea
Delocalization in harmonic chains with long-range correlated random masses
We study the nature of collective excitations in harmonic chains with masses
exhibiting long-range correlated disorder with power spectrum proportional to
, where is the wave-vector of the modulations on the random
masses landscape. Using a transfer matrix method and exact diagonalization, we
compute the localization length and participation ratio of eigenmodes within
the band of allowed energies. We find extended vibrational modes in the
low-energy region for . In order to study the time evolution of an
initially localized energy input, we calculate the second moment of
the energy spatial distribution. We show that , besides being dependent
of the specific initial excitation and exhibiting an anomalous diffusion for
weakly correlated disorder, assumes a ballistic spread in the regime
due to the presence of extended vibrational modes.Comment: 6 pages, 9 figure
Large atom number dual-species magneto-optical trap for fermionic 6Li and 40K atoms
We present the design, implementation and characterization of a dual-species
magneto-optical trap (MOT) for fermionic 6Li and 40K atoms with large atom
numbers. The MOT simultaneously contains 5.2x10^9 6Li-atoms and 8.0x10^9
40K-atoms, which are continuously loaded by a Zeeman slower for 6Li and a
2D-MOT for 40K. The atom sources induce capture rates of 1.2x10^9 6Li-atoms/s
and 1.4x10^9 40K-atoms/s. Trap losses due to light-induced interspecies
collisions of ~65% were observed and could be minimized to ~10% by using low
magnetic field gradients and low light powers in the repumping light of both
atomic species. The described system represents the starting point for the
production of a large-atom number quantum degenerate Fermi-Fermi mixture
Testing isotropy of the universe using the Ramsey resonance technique on ultracold neutron spins
Physics at the Planck scale could be revealed by looking for tiny violations
of fundamental symmetries in low energy experiments. In 2008, a sensitive test
of the isotropy of the Universe using has been performed with stored ultracold
neutrons (UCN), this is the first clock-comparison experiment performed with
free neutrons. During several days we monitored the Larmor frequency of neutron
spins in a weak magnetic field using the Ramsey resonance technique. An
non-zero cosmic axial field, violating rotational symmetry, would induce a
daily variation of the precession frequency. Our null result constitutes one of
the most stringent tests of Lorentz invariance to date.Comment: proceedings of the PNCMI2010 conferenc
The ^4He trimer as an Efimov system
We review the results obtained in the last four decades which demonstrate the
Efimov nature of the He three-atomic system.Comment: Review article for a special issue of the Few-Body Systems journal
devoted to Efimov physic
Green functions for generalized point interactions in 1D: A scattering approach
Recently, general point interactions in one dimension has been used to model
a large number of different phenomena in quantum mechanics. Such potentials,
however, requires some sort of regularization to lead to meaningful results.
The usual ways to do so rely on technicalities which may hide important
physical aspects of the problem. In this work we present a new method to
calculate the exact Green functions for general point interactions in 1D. Our
approach differs from previous ones because it is based only on physical
quantities, namely, the scattering coefficients, and , to construct .
Renormalization or particular mathematical prescriptions are not invoked. The
simple formulation of the method makes it easy to extend to more general
contexts, such as for lattices of general point interactions; on a line; on
a half-line; under periodic boundary conditions; and confined in a box.Comment: Revtex, 9 pages, 3 EPS figures. To be published in PR
First-principles calculation of the effect of strain on the diffusion of Ge adatoms on Si and Ge (001) surfaces
First-principles calculations are used to calculate the strain dependencies
of the binding and diffusion-activation energies for Ge adatoms on both Si(001)
and Ge(001) surfaces. Our calculations reveal that the binding and activation
energies on a strained Ge(001) surface increase and decrease, respectively, by
0.21 eV and 0.12 eV per percent compressive strain. For a growth temperature of
600 degrees C, these strain-dependencies give rise to a 16-fold increase in
adatom density and a 5-fold decrease in adatom diffusivity in the region of
compressive strain surrounding a Ge island with a characteristic size of 10 nm.Comment: 4 pages, 4 figure
Deriving the dietary approaches to stop hypertension (DASH) score in women from seven pregnancy cohorts from the European alphabet consortium
The ALPHABET consortium aims to examine the interplays between maternal diet quality, epigenetics and offspring health in seven pregnancy/birth cohorts from five European countries. We aimed to use the Dietary Approaches to Stop Hypertension (DASH) score to assess diet quality, but different versions have been published. To derive a single DASH score allowing cross-country, cross-cohort and cross-period comparison and limiting data heterogeneity within the ALPHABET consortium, we harmonised food frequency questionnaire (FFQ) data collected before and during pregnancy in ≥26,500 women. Although FFQs differed strongly in length and content, we derived a consortium DASH score composed of eight food components by combining the prescriptive original DASH and the DASH described by Fung et al. Statistical issues tied to the nature of the FFQs led us to re-classify two food groups (grains and dairy products). Most DASH food components exhibited pronounced between-cohort variability, including non-full-fat dairy products (median intake ranging from 0.1 to 2.2 servings/day), sugar-sweetened beverages/sweets/added sugars (0.3–1.7 servings/day), fruits (1.1–3.1 servings/day), and vegetables (1.5–3.6 servings/day). We successfully developed a harmonized DASH score adapted to all cohorts being part of the ALPHABET consortium. This methodological work may benefit other research teams in adapting the DASH to their study’s specificities
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