733 research outputs found
Generalized seniority scheme in light Sn isotopes
The yrast generalized seniority states are compared with the corresponding
shell model states for the case of the Sn isotopes Sn. For most of
the cases the energies agree within 100 keV and the overlaps of the wave
functions are greater than 0.7.Comment: 8 pages, revtex. Submitted to Phys. Rev.
Application of realistic effective interactions to the structure of the Zr isotopes
We calculate the low-lying spectra of the zirconium isotopes Z=40 with
neutron numbers from N=52 to N=60 using the 1p1/20g9/2 proton and
2s1d0g7/20h11/2 neutron sub-shells to define the model space. Effective
proton-proton, neutron--neutron and proton-neutron interactions have been
derived using 88Sr as closed core and employing perturbative many-body
techniques. The starting point is the nucleon-nucleon potential derived from
modern meson exchange models. The comprehensive shell-model calculation
performed in this work provides a qualitative reproduction of essential
properties such as the sub-shell closures in 96Zr and 98Zr.Comment: To appear in Phys Rev C, june 2000, 8 figs, Revtex latex styl
Chiral three-nucleon forces and bound excited states in neutron-rich oxygen isotopes
We study the spectra of neutron-rich oxygen isotopes based on chiral two- and
three-nucleon interactions. First, we benchmark our many-body approach by
comparing ground-state energies to coupled-cluster results for the same
two-nucleon interaction, with overall good agreement. We then calculate bound
excited states in 21,22,23O, focusing on the role of three-nucleon forces, in
the standard sd shell and an extended sdf7/2p3/2 valence space. Chiral
three-nucleon forces provide important one- and two-body contributions between
valence neutrons. We find that both these contributions and an extended valence
space are necessary to reproduce key signatures of novel shell evolution, such
as the N = 14 magic number and the low-lying states in 21O and 23O, which are
too compressed with two-nucleon interactions only. For the extended space
calculations, this presents first work based on nuclear forces without
adjustments. Future work is needed and open questions are discussed.Comment: 6 pages, 4 figures, published versio
Living on the edge of stability, the limits of the nuclear landscape
A first-principles description of nuclear systems along the drip lines
presents a substantial theoretical and computational challenge. In this paper,
we discuss the nuclear theory roadmap, some of the key theoretical approaches,
and present selected results with a focus on long isotopic chains. An important
conclusion, which consistently emerges from these theoretical analyses, is that
three-nucleon forces are crucial for both global nuclear properties and
detailed nuclear structure, and that many-body correlations due to the coupling
to the particle continuum are essential as one approaches particle drip lines.
In the quest for a comprehensive nuclear theory, high performance computing
plays a key role.Comment: Contribution to proceedings of Nobel Symposium 152: Physics with
radioactive beams, June 2012, Gothenburg, Swede
Phase transition and selection in a four-species cyclic Lotka-Volterra model
We study a four species ecological system with cyclic dominance whose
individuals are distributed on a square lattice. Randomly chosen individuals
migrate to one of the neighboring sites if it is empty or invade this site if
occupied by their prey. The cyclic dominance maintains the coexistence of all
the four species if the concentration of vacant sites is lower than a threshold
value. Above the treshold, a symmetry breaking ordering occurs via growing
domains containing only two neutral species inside. These two neutral species
can protect each other from the external invaders (predators) and extend their
common territory. According to our Monte Carlo simulations the observed phase
transition is equivalent to those found in spreading models with two equivalent
absorbing states although the present model has continuous sets of absorbing
states with different portions of the two neutral species. The selection
mechanism yielding symmetric phases is related to the domain growth process
whith wide boundaries where the four species coexist.Comment: 4 pages, 5 figure
Sperm storage by males causes changes in sperm phenotype and influences the reproductive fitness of males and their sons
Recent studies suggest that environmentally induced effects on sperm phenotype can influence offspring phenotype beyond the classic Mendelian inheritance mechanism. However, establishing whether such effects are conveyed purely through ejaculates, independently of maternal environmental effects, remains a significant challenge. Here, we assess whether environmentally induced effects on sperm phenotype affects male reproductive success and offspring fitness. We experimentally manipulated the duration of sperm storage by males, and thus sperm age, in the internally fertilizing fish Poecilia reticulata. We first confirm that sperm ageing influences sperm quality and consequently males reproductive success. Specifically, we show that aged sperm exhibit impaired velocity and are competitively inferior to fresh sperm when ejaculates compete to fertilize eggs. We then used homospermic (noncompetitive) artificial insemination to inseminate females with old or fresh sperm and found that male offspring arising from fertilizations by experimentally aged sperm suffered consistently impaired sperm quality when just sexually mature (four months old) and subsequently as adults (13 months old). Although we have yet to determine whether these effects have a genetic or epigenetic basis, our analyses provide evidence that environmentally induced variation in sperm phenotype constitutes an important source of variation in male reproductive fitness that has far reaching implications for offspring fitness
A high-affinity, bivalent PDZ domain inhibitor complexes PICK1 to alleviate neuropathic pain
Maladaptive plasticity involving increased expression of AMPAâtype glutamate receptors is involved in several pathologies, including neuropathic pain, but direct inhibition of AMPARs is associated with side effects. As an alternative, we developed a cellâpermeable, highâaffinity (~2 nM) peptide inhibitor, TatâPâ(C5), of the PDZ domain protein PICK1 to interfere with increased AMPAR expression. The affinity is obtained partly from the Tat peptide and partly from the bivalency of the PDZ motif, engaging PDZ domains from two separate PICK1 dimers to form a tetrameric complex. Bivalent TatâPâ(C5) disrupts PICK1 interaction with membrane proteins on supported cell membrane sheets and reduce the interaction of AMPARs with PICK1 and AMPAâreceptor surface expression in vivo. Moreover, TatâPâ(C5) administration reduces spinal cord transmission and alleviates mechanical hyperalgesia in the spared nerve injury model of neuropathic pain. Taken together, our data reveal TatâPâ(C5) as a novel promising lead for neuropathic pain treatment and expand the therapeutic potential of bivalent inhibitors to nonâtandem proteinâprotein interaction domains
The nuclear energy density functional formalism
The present document focuses on the theoretical foundations of the nuclear
energy density functional (EDF) method. As such, it does not aim at reviewing
the status of the field, at covering all possible ramifications of the approach
or at presenting recent achievements and applications. The objective is to
provide a modern account of the nuclear EDF formalism that is at variance with
traditional presentations that rely, at one point or another, on a {\it
Hamiltonian-based} picture. The latter is not general enough to encompass what
the nuclear EDF method represents as of today. Specifically, the traditional
Hamiltonian-based picture does not allow one to grasp the difficulties
associated with the fact that currently available parametrizations of the
energy kernel at play in the method do not derive from a genuine
Hamilton operator, would the latter be effective. The method is formulated from
the outset through the most general multi-reference, i.e. beyond mean-field,
implementation such that the single-reference, i.e. "mean-field", derives as a
particular case. As such, a key point of the presentation provided here is to
demonstrate that the multi-reference EDF method can indeed be formulated in a
{\it mathematically} meaningful fashion even if does {\it not} derive
from a genuine Hamilton operator. In particular, the restoration of symmetries
can be entirely formulated without making {\it any} reference to a projected
state, i.e. within a genuine EDF framework. However, and as is illustrated in
the present document, a mathematically meaningful formulation does not
guarantee that the formalism is sound from a {\it physical} standpoint. The
price at which the latter can be enforced as well in the future is eventually
alluded to.Comment: 64 pages, 8 figures, submitted to Euroschool Lecture Notes in Physics
Vol.IV, Christoph Scheidenberger and Marek Pfutzner editor
Polyamines are essential for virulence in Salmonella enterica serovar Gallinarum despite evolutionary decay of polyamine biosynthesis genes
Modulation of crude glycerol fermentation byClostridium pasteurianum DSM 525 towards theproduction of butanol
High production yields and productivities are requisites for the development of an industrial
butanol production process based on biodiesel-derived crude glycerol. However,
impurities present in this substrate and/or the concentration of glycerol itself can affect
the microbial metabolism. In this work, the effect of crude glycerol concentration on the
production of butanol and 1,3-propanediol (1,3-PDO) by Clostridium pasteurianum DSM 525 is
studied. Also, the effect of acetate and butyrate supplementation to the culture medium
and the culture medium composition are evaluated. The results showed a marked effect of
crude glycerol concentration on the product yield. The competitive nature of butanol and
1,3-PDO pathways has been evident, and a shift to the butanol pathway once using higher
substrate concentrations (up to 35 g l 1) was clearly observed. Butyrate supplementation to
the culture medium resulted in a 45% higher butanol titre, a lower production of 1,3-PDO
and it decreased the fermentation time. Acetate supplementation also increased the
butanol titre but the fermentation was longer. Even though glycerol consumption could not
be increased over 32 g l 1, when the concentrations of NH4Cl and FeCl2 were simultaneously
increased, the results obtained were similar to those observed when butyrate was
supplemented to the culture medium; a 35% higher butanol yield at the expense of 1,3-PDO
and a shorter fermentation. The results herein gathered suggest that there are other factors
besides butanol inhibition and nutrient limitation that affect the glycerol
consumption.The authors acknowledge the financial support from the Strategic Project PEst-OE/EQB/LA0023/2013; the project ref. RECI/BBB-EBI/0179/2012 (project number FCOMP-01-0124-FEDER-027462); and the PhD grant given to R. Gallardo (ref SFRH/BD/42900/2008) funded by Fundacao para a Ciencia e a Tecnologia. The authors thank the MIT-Portugal Program for the support given to R. Gallardo
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