6,156 research outputs found
The reason why doping causes superconductivity in LaFeAsO
The experimental observation of superconductivity in LaFeAsO appearing on
doping is analyzed with the group-theoretical approach that evidently led in a
foregoing paper (J. Supercond 24:2103, 2011) to an understanding of the cause
of both the antiferromagnetic state and the accompanying structural distortion
in this material. Doping, like the structural distortions, means also a
reduction of the symmetry of the pure perfect crystal. In the present paper we
show that this reduction modifies the correlated motion of the electrons in a
special narrow half-filled band of LaFeAsO in such a way that these electrons
produce a stable superconducting state
The structural distortion in antiferromagnetic LaFeAsO investigated by a group-theoretical approach
As experimentally well established, undoped LaFeAsO is antiferromagnetic
below 137K with the magnetic moments lying on the Fe sites. We determine the
orthorhombic body-centered group Imma (74) as the space group of the
experimentally observed magnetic structure in the undistorted lattice, i.e., in
a lattice possessing no structural distortions in addition to the
magnetostriction. We show that LaFeAsO possesses a partly filled "magnetic
band" with Bloch functions that can be unitarily transformed into optimally
localized Wannier functions adapted to the space group Imma. This finding is
interpreted in the framework of a nonadiabatic extension of the Heisenberg
model of magnetism, the nonadiabatic Heisenberg model. Within this model,
however, the magnetic structure with the space group Imma is not stable but can
be stabilized by a (slight) distortion of the crystal turning the space group
Imma into the space group Pnn2 (34). This group-theoretical result is in
accordance with the experimentally observed displacements of the Fe and O atoms
in LaFeAsO as reported by Clarina de la Cruz et al. [nature 453, 899 (2008)]
Impact-Generated Dust Clouds Surrounding the Galilean Moons
Tenuous dust clouds of Jupiter's Galilean moons Io, Europa, Ganymede and
Callisto have been detected with the in-situ dust detector on board the Galileo
spacecraft. The majority of the dust particles have been sensed at altitudes
below five radii of these lunar-sized satellites. We identify the particles in
the dust clouds surrounding the moons by their impact direction, impact
velocity, and mass distribution. Average particle sizes are 0.5 to , just above the detector threshold, indicating a size distribution with
decreasing numbers towards bigger particles. Our results imply that the
particles have been kicked up by hypervelocity impacts of micrometeoroids onto
the satellites' surfaces. The measured radial dust density profiles are
consistent with predictions by dynamical modeling for satellite ejecta produced
by interplanetary impactors (Krivov et al., PSS, 2003, 51, 251--269), assuming
yield, mass and velocity distributions of the ejecta from laboratory
measurements. The dust clouds of the three outer Galilean moons have very
similar properties and are in good agreement with the model predictions for
solid ice-silicate surfaces. The dust density in the vicinity of Io, however,
is more than an order of magnitude lower than expected from theory. This may be
due to a softer, fluffier surface of Io (volcanic deposits) as compared to the
other moons. The log-log slope of the dust number density in the clouds vs.
distance from the satellite center ranges between --1.6 and --2.8. Appreciable
variations of number densities obtained from individual flybys with varying
geometry, especially at Callisto, might be indicative of leading-trailing
asymmetries of the clouds due to the motion of the moons with respect to the
field of impactors.Comment: Icarus, in press, 46 pages, 16 figures, 5 table
Critical Point of an Interacting Two-Dimensional Atomic Bose Gas
We have measured the critical atom number in an array of harmonically trapped
two-dimensional (2D) Bose gases of rubidium atoms at different temperatures. We
found this number to be about five times higher than predicted by the
semi-classical theory of Bose-Einstein condensation (BEC) in the ideal gas.
This demonstrates that the conventional BEC picture is inapplicable in an
interacting 2D atomic gas, in sharp contrast to the three-dimensional case. A
simple heuristic model based on the Berezinskii-Kosterlitz-Thouless theory of
2D superfluidity and the local density approximation accounts well for our
experimental results
Dynamically controlled toroidal and ring-shaped magnetic traps
We present traps with toroidal and ring-shaped topologies, based on
adiabatic potentials for radio-frequency dressed Zeeman states in a ring-shaped
magnetic quadrupole field. Simple adjustment of the radio-frequency fields
provides versatile possibilities for dynamical parameter tuning, topology
change, and controlled potential perturbation. We show how to induce toroidal
and poloidal rotations, and demonstrate the feasibility of preparing degenerate
quantum gases with reduced dimensionality and periodic boundary conditions. The
great level of dynamical and even state dependent control is useful for atom
interferometry.Comment: 6 pages, 4 figures. Paragraphs on gravity compensation and expected
trap lifetimes adde
Theoretical study of the local atomic and electronic structure of dimetacyano azobenzene molecules on Bi (111) substrate
The present work is devoted to the local atomic and electronic structure of
dimetacyano (DMC) molecules deposited on a Bi (111) substrate before and after
irradiation by X-rays and UV light using density functional theory (DFT) and a
theoretical analysis of the X-ray absorption near edge tructure (XANES)
spectroscopy. As a result of the calculations the low-energy structure for DMC
zobenzene molecules on a Bi (111) substrate was obtained.1\. Auflag
Experimental Investigation of Pervaporation Membranes for Biobutanol Separation
Biotechnological production of chemical building blocks is one important step towards a more sustainable production. Unfortunately, the products to be separated are often highly diluted. Pervaporation has received increasing attention for the separation of small amounts of organic compounds from aqueous solutions, especially in the separation of butanol from water or from fermentation broth. To evaluate the potential of pervaporation for biobutanol recovery a consistent database is required, describing the dependency of permeate fluxes and selectivities on process variables like temperature, permeate pressure as well as feed concentrations and compositions. Therefore, within this work we investigated the separation behaviour of a commercially available polydimethylsiloxane (PDMS) membrane and membranes based on poly(ether block amide) (PEBA) fabricated in our own laboratory. The membranes were tested under varying operating conditions. Fermentation by-products or impurities may affect the pervaporation separation performance. Therefore, in addition, the permeate fluxes and the influence of acetone, ethanol, acetic and butyric acid and 1,3-propanediol have been investigated in detail as well. Several differences in the permeability and selectivity of PDMS and PEBA were observed during the experimental study. Swelling experiments were applied to further analyse the separation behaviour of PDMS and PEBA more in detail. Finally the influence of the observed separation performances on the overall butanol pervaporation process is discussed. It was found that especially well permeating by-products like acetone can drastically influence the subsequent downstreaming process
Long-Time Asymptotics of the Toda Lattice for Decaying Initial Data Revisited
The purpose of this article is to give a streamlined and self-contained
treatment of the long-time asymptotics of the Toda lattice for decaying initial
data in the soliton and in the similarity region via the method of nonlinear
steepest descent.Comment: 41 page
Double-lepton polarization asymmetries in the (B -> K l^+ l^-) decay beyond the Standard Model
General expressions for the double-lepton polarizations in the (B -> K l^+
l^-) decay are obtained, using model independent effective Hamiltonian,
including all possible interactions. Correlations between the averaged
double-lepton polarization asymmetries and the branching ratio, as well as, the
averaged single-lepton polarization asymmetry are studied. It is observed that,
study of the double-lepton polarization asymmetries can serve as a good test
for establishing new physics beyond the Standard Model.Comment: 21 pages, 18 figures, LaTeX formatte
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