1,107 research outputs found
On the relation of quark confinement and chiral symmetry breaking
We study the phase diagram of QCD with the help of order parameters for
chiral symmetry breaking and quark confinement. We also introduce a new order
parameter for the confinement phase transition, which is related to the quark
density. It is easily accessible by different theoretical approaches, such as
functional approaches or lattice simulations. Its relation to the Polyakov loop
expectation value is discussed and the QCD phase diagram is analysed. Our
results suggest a close relation between the chiral and the confinement phase
transition.Comment: 5 pages, 3 figure
Drag and diffusion coefficients of mesons in hot hadronic matter
The drag and diffusion coefficients of a hot hadronic medium consisting of
pions, kaons and eta using open beauty mesons as a probe have been evaluated.
The interaction of the probe with the hadronic matter has been treated in the
framework of chiral perturbation theory. It is observed that the magnitude of
both the transport coefficients are significant, indicating substantial amount
of interaction of the heavy mesons with the thermal bath. The results may have
significant impact on the experimental observables like the suppression of
single electron spectra originating from the decays of heavy mesons produced in
nuclear collisions at RHIC and LHC energiesComment: 5 pages and 3 fig
Probing the QGP Phase Boundary with Thermal Properties of Mesons
A novel attempt has been made to probe the QCD phase boundary by using the
experimental data for transverse momenta of {\phi} mesons produced in nuclear
collisions at AGS, SPS and RHIC energies. The data are confronted with simple
thermodynamic expectations and lattice QCD results. The experimental data
indicate a first-order phase transition, with a mixed phase stretching the
energy density between \sim1 and 3.2 GeV/fm3 corresponding to SPS energies.Comment: 8-pages, 3-figs, Replaced with the published versio
Iron–molybdenum-oxo complexes as initiators for olefin autoxidation with O2
The reaction between [(TPA)Fe(MeCN)2](OTf)2 and [nBu4N](Cp*MoO3) yields the
novel tetranuclear complex [(TPA)Fe(μ-Cp*MoO3)]2(OTf)2, 1, with a rectangular
[Mo–O–Fe–O–]2 core containing high-spin iron(II) centres. 1 proved to be an
efficient initiator/(pre)catalyst for the autoxidation of cis-cyclooctene with
O2 to give cyclooctene epoxide. To test, which features of 1 are essential in
this regard, analogues with zinc(II) and cobalt(II) central atoms, namely
[(TPA)Zn(Cp*MoO3)](OTf), 3, and [(TPA)Co(Cp*MoO3)](OTf), 4, were prepared,
which proved to be inactive. The precursor compounds of 1,
[(TPA)Fe(MeCN)2](OTf)2 and [nBu4N](Cp*MoO3) as well as Cp2*Mo2O5, were found
to be inactive, too. Reactivity studies in the absence of cyclooctene revealed
that 1 reacts both with O2 and PhIO via loss of the Cp* ligands to give the
triflate salt 2 of the known cation [((TPA)Fe)2(μ-O)(μ-MoO4)]2+. The cobalt
analogue 4 reacts with O2 in a different way yielding
[((TPA)Co)2(μ-Mo2O8)](OTf)2, 5, featuring a Mo2O84− structural unit which is
novel in coordination chemistry. The compound [(TPA)Fe(μ-MoO4)]2, 6, being
related to 1, but lacking Cp* ligands failed to trigger autoxidation of
cyclooctene. However, initiation of autoxidation by Cp* radicals was excluded
via experiments including thermal dissociation of Cp2*
Fixed-N Superconductivity: The Crossover from the Bulk to the Few-Electron Limit
We present a truly canonical theory of superconductivity in ultrasmall
metallic grains by variationally optimizing fixed-N projected BCS
wave-functions, which yields the first full description of the entire crossover
from the bulk BCS regime (mean level spacing bulk gap )
to the ``fluctuation-dominated'' few-electron regime (). A
wave-function analysis shows in detail how the BCS limit is recovered for , and how for pairing correlations become
delocalized in energy space. An earlier grand-canonical prediction for an
observable parity effect in the spectral gaps is found to survive the fixed-N
projection.Comment: 4 pages, 3 figures, RevTeX, V2: minor charges to mach final printed
versio
Identification of intramembrane hydrogen bonding between 131 keto group of bacteriochlorophyll and serine residue α27 in the LH2 light-harvesting complex
AbstractIntramembrane hydrogen bonding and its effect on the structural integrity of purple bacterial light-harvesting complex 2, LH2, have been assessed in the native membrane environment. A novel hydrogen bond has been identified by Raman resonance spectroscopy between a serine residue of the membrane-spanning region of LH2 α-subunit, and the C-131 keto carbonyl of bacteriochlorophyll (BChl) B850 bound to the β-subunit. Replacement of the serine by alanine disrupts this strong hydrogen bond, but this neither alters the strongly red-shifted absorption nor the structural arrangement of the BChls, as judged from circular dichroism. It also decreases only slightly the thermal stability of the mutated LH2 in the native membrane environment. The possibility is discussed that weak H-bonding between the C-131 keto carbonyl and a methyl hydrogen of the alanine replacing serine(−4) or the imidazole group of the nearby histidine maintains structural integrity in this very stable bacterial light-harvesting complex. A more widespread occurrence of H-bonding to C-131 not only in BChl, but also in chlorophyll proteins, is indicated by a theoretical analysis of chlorophyll/polypeptide contacts at <3.5 Å in the high-resolution structure of Photosystem I. Nearly half of the 96 chlorophylls have aa residues suitable as hydrogen bond donors to their keto groups
a retrospective analysis
Background Several animal studies suggest beneficial effects on kidney
function upon administration of levosimendan. As recent data from clinical
studies are heterogeneous, we sought to investigate whether levosimendan is
associated with improved postoperative kidney function in cardiac surgery
patients with respect to timing of its administration. Methods Retrospective,
single centre, observational analysis at a university hospital in Berlin,
Germany. All adult patients without preoperative renal dysfunction that
underwent coronary artery bypass grafting and/or valve
reconstruction/replacement between 01/01/2007 and 31/12/2011 were considered
for analyses. Results Out of 1.095 included patients, 46 patients were treated
with levosimendan due to a severely reduced left ventricular systolic function
preoperatively (LVEF < 35%) and/or clinical signs of a low cardiac output
syndrome. Sixty-one percent received the drug whilst in the OR, 39% after
postoperative intensive care unit admission. When levosimendan was given
immediately after anaesthesia induction, creatinine plasma levels (p = 0.009
for nonparametric analysis of longitudinal data in a two-factorial design) and
incidence of postoperative renal dysfunction (67.9% vs. 94.4%; p = 0.033) were
significantly reduced in contrast to a later start of treatment. In addition,
duration of renal replacement therapy was significantly shorter (79 [35;332]
vs. 272 [132;703] minutes; p = 0.046) in that group. Conclusions Postoperative
kidney dysfunction is a common condition in patients under going cardiac
surgery. Patients with severely reduced left ventricular function and/or
clinical signs of a low cardiac output syndrome who preoperatively presented
with a normal kidney function may benefit from an early start of levosimendan
administration, i.e. immediately after anaesthesia
Superconductivity in Ultrasmall Metallic Grains
We develop a theory of superconductivity in ultrasmall (nm-scale) metallic
grains having a discrete electronic eigenspectrum with a mean level spacing of
order of the bulk gap. The theory is based on calculating the eigenspectrum
using a generalized BCS variational approach, whose applicability has been
extensively demonstrated in studies of pairing correlations in nuclear physics.
We discuss how conventional mean field theory breaks down with decreasing
sample size, how the so-called blocking effect weakens pairing correlations in
states with non-zero total spin, and how this affects the discrete
eigenspectrum's behavior in a magnetic field, which favors non-zero total spin.
In ultrasmall grains, spin magnetism dominates orbital magnetism, just as in
thin films in a parallel field; but whereas in the latter the magnetic-field
induced transition to a normal state is known to be first-order, we show that
in ultrasmall grains it is softened by finite size effects. Our calculations
qualitatively reproduce the magnetic-field dependent tunneling spectra for
individual aluminum grains measured recently by Ralph, Black and Tinkham. We
argue that previously-discussed parity effects for the odd-even ground state
energy difference are presently not observable for experimental reasons, and
propose an analogous parity effect for the pair-breaking energy that should be
observable provided that the grain size can be controlled sufficiently well.
Finally, experimental evidence is pointed out that the dominant role played by
time-reversed pairs of states, well-established in bulk and in dirty
superconductors, persists also in ultrasmall grains.Comment: 21 pages RevTeX, 12 EPS figures included, uses epsf.st
Electron correlations in CoMnFeSi Heusler compounds
This study presents the effect of local electronic correlations on the
Heusler compounds CoMnFeSi as a function of the concentration
. The analysis has been performed by means of first-principles
band-structure calculations based on the local approximation to spin-density
functional theory (LSDA). Correlation effects are treated in terms of the
Dynamical Mean-Field Theory (DMFT) and the LSDA+U approach. The formalism is
implemented within the Korringa-Kohn-Rostoker (KKR) Green's function method.
In good agreement with the available experimental data the magnetic and
spectroscopic properties of the compound are explained in terms of strong
electronic correlations. In addition the correlation effects have been analysed
separately with respect to their static or dynamical origin. To achieve a
quantitative description of the electronic structure of
CoMnFeSi both static and dynamic correlations must be treated
on equal footing.Comment: 12 pages, 5 figure
A very brief description of LOFAR - the Low Frequency Array
LOFAR (Low Frequency Array) is an innovative radio telescope optimized for
the frequency range 30-240 MHz. The telescope is realized as a phased aperture
array without any moving parts. Digital beam forming allows the telescope to
point to any part of the sky within a second. Transient buffering makes
retrospective imaging of explosive short-term events possible. The scientific
focus of LOFAR will initially be on four key science projects (KSPs): 1)
detection of the formation of the very first stars and galaxies in the universe
during the so-called epoch of reionization by measuring the power spectrum of
the neutral hydrogen 21-cm line (Shaver et al. 1999) on the ~5' scale; 2)
low-frequency surveys of the sky with of order expected new sources; 3)
all-sky monitoring and detection of transient radio sources such as gamma-ray
bursts, x-ray binaries, and exo-planets (Farrell et al. 2004); and 4) radio
detection of ultra-high energy cosmic rays and neutrinos (Falcke & Gorham 2003)
allowing for the first time access to particles beyond 10^21 eV (Scholten et
al. 2006). Apart from the KSPs open access for smaller projects is also
planned. Here we give a brief description of the telescope.Comment: 2 pages, IAU GA 2006, Highlights of Astronomy, Volume 14, K.A. van
der Hucht, e
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