266 research outputs found
An effective thermodynamic potential from the instanton with Polyakov-loop contributions
We derive an effective thermodynamic potential (Omega_eff) at finite
temperature (T>0) and zero quark-chemical potential (mu_R=0), using the
singular-gauge instanton solution and Matsubara formula for N_c=3 and N_f=2 in
the chiral limit. The momentum-dependent constituent-quark mass is also
obtained as a function of T, employing the Harrington-Shepard caloron solution
in the large-N_c limit. In addition, we take into account the imaginary quark
chemical potential mu_I = A_4, translated as the traced Polayakov-loop (Phi) as
an order parameter for the Z(N_c) symmsetry, characterizing the confinement
(intact) and deconfinement (spontaneously broken) phases. As a result, we
observe the crossover of the chiral (chi) order parameter sigma^2 and Phi. It
also turns out that the critical temperature for the deconfinment phase
transition, T^Z_c is lowered by about (5-10)% in comparison to the case with a
constant constituent-quark mass. This behavior can be understood by
considerable effects from the partial chiral restoration and nontrivial QCD
vacuum on Phi. Numerical calculations show that the crossover transitions occur
at (T^chi_c,T^Z_c) ~ (216,227) MeV.Comment: 15 pages, 7 figure
Self-similar solution of fast magnetic reconnection: Semi-analytic study of inflow region
An evolutionary process of the fast magnetic reconnection in ``free space''
which is free from any influence of outer circumstance has been studied
semi-analytically, and a self-similarly expanding solution has been obtained.
The semi-analytic solution is consistent with the results of our numerical
simulations performed in our previous paper (see Nitta et al. 2001). This
semi-analytic study confirms the existence of self-similar growth. On the other
hand, the numerical study by time dependent computer simulation clarifies the
stability of the self-similar growth with respect to any MHD mode. These
results confirm the stable self-similar evolution of the fast magnetic
reconnection system.Comment: 15 pages, 7 figure
Fast magnetic reconnection in free space: self-similar evolution process
We present a new model for time evolution of fast magnetic reconnection in
free space, which is characterized by self-similarity. Reconnection triggered
by locally enhanced resistivity assumed at the center of the current sheet can
self-similarly and unlimitedly evolve until external factors affect the
evolution. The possibility and stability of this type of evolution are verified
by numerical simulations in a very wide spatial dynamic range. Actual
astrophysical reconnection in solar flares and geomagnetospheric substorms can
be treated as an evolutionary process in free space, because the resultant
scale is much larger than the initial scale. In spite of this fact, most of the
previous numerical works focused on the evolutionary characters strongly
affected by artificial boundary conditions on the simulation boundary. Our new
model clarifies a realistic evolution for such cases. The characteristic
structure around the diffusion region is quite similar to the Petschek model
which is characterized by a pair of slow-mode shocks and the fast-mode
rarefaction-dominated inflow. However, in the outer region, a vortex-like
return flow driven by the fast-mode compression caused by the piston effect of
the plasmoid takes place. The entire reconnection system expands
self-similarly.Comment: 17 Pages, 17 Figure
Memory deficits in APP23/Abca1+/− mice correlate with the level of Aβ oligomers
ABCA1, a member of the ATP-binding cassette family of transporters, lipidates ApoE (apolipoprotein A) and is essential for the generation of HDL (high-density lipoprotein)-like particles in the CNS (central nervous system). Lack of Abca1 increases amyloid deposition in several AD (Alzheimer's disease) mouse models. We hypothesized that deletion of only one copy of Abca1 in APP23 (where APP is amyloid precursor protein) AD model mice will aggravate memory deficits in these mice. Using the Morris Water Maze, we demonstrate that 2-year-old Abca1 heterozygous APP23 mice (referred to as APP23/het) have impaired learning during acquisition, and impaired memory retention during the probe trial when compared with age-matched wild-type mice (referred to as APP23/wt). As in our previous studies, the levels of ApoE in APP23/het mice were decreased, but the differences in the levels of Aβ and thioflavin-S-positive plaques between both groups were insignificant. Importantly, dot blot analysis demonstrated that APP23/het mice have a significantly higher level of soluble A11-positive Aβ (amyloid β protein) oligomers compared with APP23/wt which correlated negatively with cognitive performance. To confirm this finding, we performed immunohistochemistry with the A11 antibody, which revealed a significant increase of A11-positive oligomer structures in the CA1 region of hippocampi of APP23/het. This characteristic region-specific pattern of A11 staining was age-dependent and was missing in younger APP23 mice lacking Abca1. In contrast, the levels of Aβ*56, as well as other low-molecular-mass Aβ oligomers, were unchanged among the groups. Overall, the results of the present study demonstrate that in aged APP23 mice memory deficits depend on Abca1 and are likely to be mediated by the amount of Aβ oligomers deposited in the hippocampus
Competition between phonon superconductivity and Kondo screening in mixed valence and heavy fermion compounds
We consider competition of Kondo effect and s-wave superconductivity in heavy
fermion and mixed valence superconductors, using the phenomenological approach
for the periodic Anderson model. Similar to the well known results for
single-impurity Kondo effect in superconductors, we have found principal
possibility of a re-entrant regime of the superconducting transition
temperature, , in heavy fermion superconductors in a narrow range of model
parameters and concentration of f-electrons. Suppression of in mixed
valence superconductors is much weaker. Our theory has most validity in the
low-temperature Fermi liquid regime, without re-entrant behavior of . To
check its applicability, we performed the fit for the -dependence of
in CeLaRuSi and obtained an excellent agreement with the
experimental data, although no re-entrance was found in this case. Other
experimental data are discussed in the light of our theoretical analysis. In
particular, we compare temperatures of the superconducting transition for some
known homologs, i.e., the analog periodic lattice compounds with and without
f-elements. For a few pairs of homologs superconductivity exists only in the
heavy fermion materials, thus confirming uniqueness of superconductivity
mechanisms for the latter. We suggest that for some other compounds the value
of may remain of the same order in the two homologs, if superconductivity
originates mainly on some light Fermi surface, but induces sizable
superconducting gap on another Fermi surface,for which hybridization or other
heavy fermion effects are more significant.Comment: 11 pages, 4 figures, pd
The Influence of Spin-Labeled Fluorene Compounds on the Assembly and Toxicity of the Aβ Peptide
The deposition and oligomerization of amyloid β (Aβ) peptide plays a key role in the pathogenesis of Alzheimer's disease (AD). Aβ peptide arises from cleavage of the membrane-associated domain of the amyloid precursor protein (APP) by β and γ secretases. Several lines of evidence point to the soluble Aβ oligomer (AβO) as the primary neurotoxic species in the etiology of AD. Recently, we have demonstrated that a class of fluorene molecules specifically disrupts the AβO species.
Methodology/Principal Findings
To achieve a better understanding of the mechanism of action of this disruptive ability, we extend the application of electron paramagnetic resonance (EPR) spectroscopy of site-directed spin labels in the Aβ peptide to investigate the binding and influence of fluorene compounds on AβO structure and dynamics. In addition, we have synthesized a spin-labeled fluorene (SLF) containing a pyrroline nitroxide group that provides both increased cell protection against AβO toxicity and a route to directly observe the binding of the fluorene to the AβO assembly. We also evaluate the ability of fluorenes to target multiple pathological processes involved in the neurodegenerative cascade, such as their ability to block AβO toxicity, scavenge free radicals and diminish the formation of intracellular AβO species.
Conclusions
Fluorene modified with pyrroline nitroxide may be especially useful in counteracting Aβ peptide toxicity, because they posses both antioxidant properties and the ability to disrupt AβO species
Subgap conductivity in SIN-junctions of high barrier transparency
We investigate the current-voltage characteristics of high-transparency
superconductor-insulator-normal metal (SIN) junctions with the specific tunnel
resistance below 30 kOhm per square micron. The junctions were fabricated from
different superconducting and normal conducting materials, including Nb, Al,
AuPd and Cu. The subgap leakage currents were found to be appreciably larger
than those given by the standard tunnelling model. We explain our results using
the model of two-electron tunnelling in the coherent diffusive transport
regime. We demonstrate that even in the high-transparency SIN-junctions, a
noticeable reduction of the subgap current can be achieved by splitting a
junction into several submicron sub-junctions. These structures can be used as
nonlinear low-noise shunts in Rapid-Single-Flux-Quantum (RSFQ) circuitry for
controlling Josephson qubits.Comment: 6 pages, 5 figures, 1 tabl
A limiting velocity for quarkonium propagation in a strongly coupled plasma via AdS/CFT
We study the dispersion relations of mesons in a particular hot strongly
coupled supersymmetric gauge theory plasma. We find that at large momentum k
the dispersion relations become omega = v_0 k + a + b/k + ..., where the
limiting velocity v_0 is the same for mesons with any quantum numbers and
depends only on the ratio of the temperature to the quark mass T/m_q. We
compute a and b in terms of the meson quantum numbers and T/m_q. The limiting
meson velocity v_0 becomes much smaller than the speed of light at temperatures
below but close to T_diss, the temperature above which no meson bound states at
rest in the plasma are found. From our result for v_0, we find that the
temperature above which no meson bound states with velocity v exist is
T_diss(v) \simeq (1-v^2)^(1/4) T_diss, up to few percent corrections.We thus
confirm by direct calculation of meson dispersion relations a result inferred
indirectly in previous work via analysis of the screening length between a
static quark and antiquark in a moving plasma. Although we do not do our
calculations in QCD, we argue that the qualitative features of the dispersion
relation we compute, including in particular the relation between dissociation
temperature and meson velocity, may apply to bottomonium and charmonium mesons
propagating in the strongly coupled plasma of QCD. We discuss how our results
can contribute to understanding quarkonium physics in heavy ion collisions.Comment: 57 pages, 12 figures; references adde
Atlas of Signaling for Interpretation of Microarray Experiments
Microarray-based expression profiling of living systems is a quick and inexpensive method to obtain insights into the nature of various diseases and phenotypes. A typical microarray profile can yield hundreds or even thousands of differentially expressed genes and finding biologically plausible themes or regulatory mechanisms underlying these changes is a non-trivial and daunting task. We describe a novel approach for systems-level interpretation of microarray expression data using a manually constructed “overview” pathway depicting the main cellular signaling channels (Atlas of Signaling). Currently, the developed pathway focuses on signal transduction from surface receptors to transcription factors and further transcriptional regulation of cellular “workhorse” proteins. We show how the constructed Atlas of Signaling in combination with an enrichment analysis algorithm allows quick identification and visualization of the main signaling cascades and cellular processes affected in a gene expression profiling experiment. We validate our approach using several publicly available gene expression datasets
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