131 research outputs found
Crossing w=-1 in Gauss-Bonnet Brane World with Induced Gravity
Recent type Ia supernovas data seemingly favor a dark energy model whose
equation of state crosses -1 very recently, which is a much more amazing
problem than the acceleration of the universe. In this paper we show that it is
possible to realize such a crossing without introducing any phantom component
in a Gauss-Bonnet brane world with induced gravity, where a four dimensional
curvature scalar on the brane and a five dimensional Gauss-Bonnet term in the
bulk are present. In this realization, the Gauss-Bonnet term and the mass
parameter in the bulk play a crucial role.Comment: Revtex 16 pages including 10 eps files, references added, to appear
in Comm. Theor. Phy
Local moment versus Kondo behavior of the 4f-electrons in rare-earth iron oxypnictides
We consider the role played by the 4f states in the rare-earth oxyarsenides
REOFeAs (RE=Ce,Pr,Nd) and the oxyphosphate CeOFeP, using a first-principles
technique that combines the local density approximation and dynamical
mean-field theory (LDA+DMFT). In the Pr and Nd compounds, the 4f states are
located well below and above the Fermi level E_F, and essentially do not
interact with the iron 3d bands located near E_F, resulting in local moment
behavior. In the Ce compounds, our results reveal a qualitatively different
picture, with the 3d-4f hybridization being sufficiently strong to give rise to
an observable Kondo screening of the local 4f moment. Our LDA+DMFT electronic
structure calculations allow us to estimate the Kondo temperature T_K for both
CeOFeP and CeOFeAs. For the phosphate, the order of magnitude of our estimate
is consistent with the experimental observation of T_K around 10 K. At ambient
pressure, T_K is found to be negligibly small for CeOFeAs. Under applied
hydrostatic pressure, we predict an exponential increase of T_K which reaches
values comparable to the superconducting T_c around 40 K at pressures above 10
GPa. We conjecture that the competition between the Kondo effect and
superconductivity may be at the origin of the monotonous decrease of T_c
observed in CeOFeAs under pressure. We argue that the quantitative aspects of
this competition are inconsistent with a weak-coupling BCS description of the
superconductivity in the oxyarsenides.Comment: 6 pages, 3 figure
Electronic Structure of Fe-Based Superconductors
The electronic structure of the Fe-based superconductors is discussed, mainly
from the point of view of first principles calculations in relation to
experimental data. Comparisons and contrasts with cuprates are made. The
problem of reconciling experiments indicating an symmetry gap with
experiments indicating line nodes is discussed and a possible resolution is
given.Comment: Updated references. Additional discussio
Dynamical Mean-Field Theory
The dynamical mean-field theory (DMFT) is a widely applicable approximation
scheme for the investigation of correlated quantum many-particle systems on a
lattice, e.g., electrons in solids and cold atoms in optical lattices. In
particular, the combination of the DMFT with conventional methods for the
calculation of electronic band structures has led to a powerful numerical
approach which allows one to explore the properties of correlated materials. In
this introductory article we discuss the foundations of the DMFT, derive the
underlying self-consistency equations, and present several applications which
have provided important insights into the properties of correlated matter.Comment: Chapter in "Theoretical Methods for Strongly Correlated Systems",
edited by A. Avella and F. Mancini, Springer (2011), 31 pages, 5 figure
Pairing symmetry and properties of iron-based high temperature superconductors
Pairing symmetry is important to indentify the pairing mechanism. The
analysis becomes particularly timely and important for the newly discovered
iron-based multi-orbital superconductors. From group theory point of view we
classified all pairing matrices (in the orbital space) that carry irreducible
representations of the system. The quasiparticle gap falls into three
categories: full, nodal and gapless. The nodal-gap states show conventional
Volovik effect even for on-site pairing. The gapless states are odd in orbital
space, have a negative superfluid density and are therefore unstable. In
connection to experiments we proposed possible pairing states and implications
for the pairing mechanism.Comment: 4 pages, 1 table, 2 figures, polished versio
Improved high-temperature expansion and critical equation of state of three-dimensional Ising-like systems
High-temperature series are computed for a generalized Ising model with
arbitrary potential. Two specific ``improved'' potentials (suppressing leading
scaling corrections) are selected by Monte Carlo computation. Critical
exponents are extracted from high-temperature series specialized to improved
potentials, achieving high accuracy; our best estimates are:
, , , ,
. By the same technique, the coefficients of the small-field
expansion for the effective potential (Helmholtz free energy) are computed.
These results are applied to the construction of parametric representations of
the critical equation of state. A systematic approximation scheme, based on a
global stationarity condition, is introduced (the lowest-order approximation
reproduces the linear parametric model). This scheme is used for an accurate
determination of universal ratios of amplitudes. A comparison with other
theoretical and experimental determinations of universal quantities is
presented.Comment: 65 pages, 1 figure, revtex. New Monte Carlo data by Hasenbusch
enabled us to improve the determination of the critical exponents and of the
equation of state. The discussion of several topics was improved and the
bibliography was update
PgtE Enzyme of Salmonella enterica Shares the Similar Biological Roles to Plasminogen Activator (Pla) in Interacting With DEC-205 (CD205), and Enhancing Host Dissemination and Infectivity by Yersinia pestis
Yersinia pestis, the cause of plague, is a newly evolved Gram-negative bacterium. Through the acquisition of the plasminogen activator (Pla), Y. pestis gained the means to rapidly disseminate throughout its mammalian hosts. It was suggested that Y. pestis utilizes Pla to interact with the DEC-205 (CD205) receptor on antigen-presenting cells (APCs) to initiate host dissemination and infection. However, the evolutionary origin of Pla has not been fully elucidated. The PgtE enzyme of Salmonella enterica, involved in host dissemination, shows sequence similarity with the Y. pestis Pla. In this study, we demonstrated that both Escherichia coli K-12 and Y. pestis bacteria expressing the PgtE-protein were able to interact with primary alveolar macrophages and DEC-205-transfected CHO cells. The interaction between PgtE-expressing bacteria and DEC-205-expressing transfectants could be inhibited by the application of an anti-DEC-205 antibody. Moreover, PgtE-expressing Y. pestis partially re-gained the ability to promote host dissemination and infection. In conclusion, the DEC-205-PgtE interaction plays a role in promoting the dissemination and infection of Y. pestis, suggesting that Pla and the PgtE of S. enterica might share a common evolutionary origin.Peer reviewe
Consensus guidelines for the use and interpretation of angiogenesis assays
The formation of new blood vessels, or angiogenesis, is a complex process that plays important roles in growth and development, tissue and organ regeneration, as well as numerous pathological conditions. Angiogenesis undergoes multiple discrete steps that can be individually evaluated and quantified by a large number of bioassays. These independent assessments hold advantages but also have limitations. This article describes in vivo, ex vivo, and in vitro bioassays that are available for the evaluation of angiogenesis and highlights critical aspects that are relevant for their execution and proper interpretation. As such, this collaborative work is the first edition of consensus guidelines on angiogenesis bioassays to serve for current and future reference
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