3,627 research outputs found
Iron valence in double-perovskite (Ba,Sr,Ca)2FeMoO6: Isovalent substitution effect
In the Fe-Mo based B-site ordered double-perovskite, A2FeMoO6.0, with iron in
the mixed-valence II/III state, the valence value of Fe is not precisely fixed
at 2.5 but may be fine-tuned by means of applying chemical pressure at the
A-cation site. This is shown through a systematic 57Fe Mossbauer spectroscopy
study using a series of A2FeMoO6.0 [A = (Ba,Sr) or (Sr,Ca)] samples with high
degree of Fe/Mo order, the same stoichiometric oxygen content and also almost
the same grain size. The isomer shift values and other hyperfine parameters
obtained from the Mossbauer spectra confirm that Fe remains in the
mixed-valence state within the whole range of A constituents. However, upon
increasing the average cation size at the A site the precise valence of Fe is
found to decrease such that within the A = (Ba,Sr) regime the valence of Fe is
closer to II, while within the A = (Sr,Ca) regime it is closer to the actual
mixed-valence II/III state. As the valence of Fe approaches II, the difference
in charges between Fe and Mo increases, and parallel with this the degree of
Fe/Mo order increases. Additionally, for the less-ordered samples an increased
tendency of clustering of the anti-site Fe atoms is deduced from the Mossbauer
data.Comment: 19 pages, 6 figures, submitted to Phys. Rev.
Generalized Elliptic Integrals and the Legendre M-function
We study monotonicity and convexity properties of functions arising in the
theory of elliptic integrals, and in particular in the case of a
Schwarz-Christoffel conformal mapping from a half-plane to a trapezoid. We
obtain sharp monotonicity and convexity results for combinations of these
functions, as well as functional inequalities and a linearization property.Comment: 28 page
Decay times in turnover statistics of single enzymes
The first passage times for enzymatic turnovers in non-equilibrium steady
state display a statistical symmetry property related to non-equilibrium
fluctuation theorems, that makes it possible to extract the chemical driving
force from single molecule trajectories in non-equilibrium steady state. Below,
we show that this system violates the general expectation that the number of
decay constants needed to fit a first passage time distribution reflects the
number of states in the escape problem. In fact, the structure of the kinetic
mechanism makes half of the decay times vanish identically from the turnover
time distribution. The terms that cancel out correspond to the eigenvalues of a
certain sub-matrix of the master equation matrix for the first exit time
problem. We discuss how these results make modeling and data analysis easier
for such systems, and how the turnovers can be measured.Comment: 4 pages, 1 figure v2: Published version, minor corrections in
response to referee comment
Multiple Lac-mediated loops revealed by Bayesian statistics and tethered particle motion
The bacterial transcription factor LacI loops DNA by binding to two separate
locations on the DNA simultaneously. Despite being one of the best-studied
model systems for transcriptional regulation, the number and conformations of
loop structures accessible to LacI remain unclear, though the importance of
multiple co-existing loops has been implicated in interactions between LacI and
other cellular regulators of gene expression. To probe this issue, we have
developed a new analysis method for tethered particle motion, a versatile and
commonly-used in vitro single-molecule technique. Our method, vbTPM, performs
variational Bayesian inference in hidden Markov models. It learns the number of
distinct states (i.e., DNA-protein conformations) directly from tethered
particle motion data with better resolution than existing methods, while easily
correcting for common experimental artifacts. Studying short (roughly 100 bp)
LacI-mediated loops, we provide evidence for three distinct loop structures,
more than previously reported in single-molecule studies. Moreover, our results
confirm that changes in LacI conformation and DNA binding topology both
contribute to the repertoire of LacI-mediated loops formed in vitro, and
provide qualitatively new input for models of looping and transcriptional
regulation. We expect vbTPM to be broadly useful for probing complex
protein-nucleic acid interactions.Comment: 34 pages, 25 figures, including Supporting information. To appear in
Nucleic Acids Research. Accompanying open-source software:
http://sourceforge.net/projects/vbtpm
Precise determination of the hyperfine parameters of europium in multifluorite perovskites by 151Eu Mössbauer spectroscopy
The hyperfine interactions at the europium lattice sites in samples of the homologous (Fe,Cu)Sr2(Eu,Ce)nCu2O4+2n+z (n=2,3) series were studied by Eu151 Mössbauer spectroscopy. The work was motivated by the search for new superconducting phases. This homologous series is based on the YBa2Cu3O7âÎŽ (1:2:3) structure. The samples used in the Mössbauer measurements consisted of crystallites with random orientation and grain oriented crystallites. The texture of oriented samples was analyzed by x-ray diffraction. The complete quadrupole Hamiltonian of the 21.5-keV Îł-transition of Eu151 was successfully applied in the analyses of all the Mössbauer spectra. In samples having nâ€2 the europium atoms occupy a single lattice site, whereas the spectra of the n=3 samples exhibit hyperfine interactions of the two different europium sites. Analyzing the hyperfine parameters of the latter samples was made possible by simultaneous fitting of three spectra, corresponding to three different crystal orientations of the same specimen. This fitting scheme also enables more precise determination of the hyperfine parameters in the n=2 samples. In these samples an electric field gradient (EFG), with a large negative-valued main component (Vzz) parallel with the crystal c axis, was found. In the n=3 samples, the two EFGâs of the europium sites were found to have Vzz components of opposite signs. The negative Vzz value was attributed to the rare-earth site adjacent to the CuO layer. This site, found in all samples of the series, corresponds to the rare-earth site of the 1:2:3 system.Peer reviewe
Structural and Magnetic Properties of MrSrâYâ.â Ceâ.â CuâOz (M-1222) Compounds with M = Fe and Co
The MSr2Y1.5Ce0.5Cu2Oz (M-1222) compounds, with M = Fe and Co, have been synthesized through a solid-state reaction route. Both compounds crystallize in a tetragonal structure (space group 14/mmm). A Rietveld structural refinement of the room-temperature neutron diffraction data for Fe-1222 reveals that nearly half the Fe remains at the M site, while the other half goes to the Cu site in the CuO2 planes. Existence of Fe at two different lattice sites is also confirmed by 57Fe Mössbauer spectroscopy from which it is inferred that ~50% of the total Fe occupies the Cu site in the CuO2 planes as Fe3+, whereas the other ~50% is located at the M site with ~40% as Fe4+ and ~10% as Fe3+. For the M[Double Bond]Co compound, nearly 84% of Co remains at its designated M site, while the rest occupies the Cu site in the CuO2 planes
Antimatter and Matter Production in Heavy Ion Collisions at CERN (The NEWMASS Experiment NA52)
Besides the dedicated search for strangelets NA52 measures light
(anti)particle and (anti)nuclei production over a wide range of rapidity.
Compared to previous runs the statistics has been increased in the 1998 run by
more than one order of magnitude for negatively charged objects at different
spectrometer rigidities. Together with previous data taking at a rigidity of
-20 GeV/c we obtained 10^6 antiprotons 10^3 antideuterons and two antihelium3
without centrality requirements. We measured nuclei and antinuclei
(p,d,antiprotons, antideuterons) near midrapidity covering an impact parameter
range of b=2-12 fm. Our results strongly indicate that nuclei and antinuclei
are mainly produced via the coalescence mechanism. However the centrality
dependence of the antibaryon to baryon ratios show that antibaryons are
diminished due to annihilation and breakup reactions in the hadron dense
environment. The volume of the particle source extracted from coalescence
models agrees with results from pion interferometry for an expanding source.
The chemical and thermal freeze-out of nuclei and antinuclei appear to coincide
with each other and with the thermal freeze-out of hadrons.Comment: 12 pages, 8 figures, to appear in the proceedings of the conference
on 'Fundamental Issues in Elementary Matter' Bad Honnef, Germany, Sept.
25-29, 200
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