2,825 research outputs found
Matrix Product States: Symmetries and Two-Body Hamiltonians
We characterize the conditions under which a translationally invariant matrix
product state (MPS) is invariant under local transformations. This allows us to
relate the symmetry group of a given state to the symmetry group of a simple
tensor. We exploit this result in order to prove and extend a version of the
Lieb-Schultz-Mattis theorem, one of the basic results in many-body physics, in
the context of MPS. We illustrate the results with an exhaustive search of
SU(2)--invariant two-body Hamiltonians which have such MPS as exact ground
states or excitations.Comment: PDFLatex, 12 pages and 6 figure
Exact renormalization in quantum spin chains
We introduce a real-space exact renormalization group method to find exactly
solvable quantum spin chains and their ground states. This method allows us to
provide a complete list for exact solutions within SU(2) symmetric quantum spin
chains with and nearest-neighbor interactions, as well as examples
with S=5. We obtain two classes of solutions: One of them converges to the
fixed points of renormalization group and the ground states are matrix product
states. Another one does not have renormalization fixed points and the ground
states are partially ferromagnetic states.Comment: 8 pages, 5 figures, references added, published versio
String order and symmetries in quantum spin lattices
We show that the existence of string order in a given quantum state is
intimately related to the presence of a local symmetry by proving that both
concepts are equivalent within the framework of finitely correlated states.
Once this connection is established, we provide a complete characterization of
local symmetries in these states. The results allow to understand in a
straightforward way many of the properties of string order parameters, like
their robustness/fragility under perturbations and their typical disappearance
beyond strictly one-dimensional lattices. We propose and discuss an alternative
definition, ideally suited for detecting phase transitions, and generalizations
to two and more spatial dimensions.Comment: 5 pages, 1 figur
Variabilidad de la Respuesta de las CĂ©lulas DendrĂticas Estimuladas in vitro con Porphyromonas gingivalis y Aggregatibacter actinomycetemcomitans
ResumenLas cĂ©lulas dendrĂticas son cĂ©lulas presentadoras de antĂgeno capaces de inducir la activaciĂłn y maduraciĂłn de linfocitos T CD4+ vĂrgenes hacia un fenotipo efector especifico Th1 o Th2, dependiendo del tipo de antigeno presentado, las senales co-estimuladoras que expresan y el ambiente de citoquinas en el cual se produce la interaccion celula-celula. El objetivo del presente trabajo es analizar la respuesta de las celulas dendriticas estimuladas in vitro con distintas concentraciones de las bacterias periodontopatĂłgenas Pg y Aa. En celulas dendriticas derivadas de monocitos de sangre periferica estimuladas con 101 a 109 bacterias/mL de Pg y Aa se evaluo la expresion del marcador de maduracion CD80 mediante citometria de flujo y de las citoquinas IL1ÎČ, IL2, IL5, IL6, IL10, IL12, IL13, IFNÎł, TNFα y TNFÎČ mediante RT-PCR cuantitativa. Aa y Pg indujeron maduracion de las celulas dendriticas, detectandose significativamente mayor expresion de CD80 con la estimulacion de Aa, e indujeron predominantemente la expresion de citoquinas propias de una respuesta Th1. Dependiendo de la carga bacteriana, fueron detectados distintos umbrales de induccion de expresion de citoquinas. Aa indujo la sintesis de IL1ÎČ, IL12, IFNÎł, TNFα y TNFÎČ a menor carga bacteriana que Pg. Tomados en conjunto, estos datos nos permiten especular un mayor potencial antigenico y proyectar una mayor capacidad patogenica durante la infeccion periodontal de Aa en comparaciĂłn a Pg.AbstractDendritic cells are potent antigen-presenting cells able to prime naive T cells and polarize them towards a Th1 or Th2 response, depending on the type of the antigen presented to the TCR, the type of costimulatory signals, and the cytokine pattern in the environment. The aim of this work was to analyze the response of dendritic cells to in vitro stimulation with Pg and Aa. In monocyte-derived dendritic cells stimulated with 101 to 109 bacteria/mL of Pg or Aa were evaluated both the expression of the maturation marker CD80 by flow cytometry and the expression of the cytokines IL1ÎČ, IL2, IL5, IL6, IL10, IL12, IL13, IFNÎł, TNFα and TNFÎČ by quantitative RT-PCR. Both Pg and Aa led to dendritic cell maturation, detecting higher CD80 expression upon Aa-stimulation, and induced a Th1 pattern of cytokine expression. Aa-stimulated dendritic cells expressed IL1ÎČ, IL12, IFNÎł, TNFα and TNFÎČ mRNAs with lower bacterial charge than with Pg. Furthermore, our data indicated the existence of distinct thresholds for the induction of the different cytokines analyzed. Taken together, these data allow us to speculate a higher antigenic potential and higher pathogenic capacity of Aa than Pg during periodontal infections
Distribution of melanopsin positive neurons in pigmented and albino mice: evidence for melanopsin interneurons in the mouse retina.
Here we have studied the population of intrinsically photosensitive retinal ganglion cells (ipRGCs) in adult pigmented and albino mice. Our data show that although pigmented (C57Bl/6) and albino (Swiss) mice have a similar total number of ipRGCs, their distribution is slightly different: while in pigmented mice ipRGCs are more abundant in the temporal retina, in albinos the ipRGCs are more abundant in superior retina. In both strains, ipRGCs are located in the retinal periphery, in the areas of lower Brn3a(+)RGC density. Both strains also contain displaced ipRGCs (d-ipRGCs) in the inner nuclear layer (INL) that account for 14% of total ipRGCs in pigmented mice and 5% in albinos. Tracing from both superior colliculli shows that 98% (pigmented) and 97% (albino) of the total ipRGCs, become retrogradely labeled, while double immunodetection of melanopsin and Brn3a confirms that few ipRGCs express this transcription factor in mice. Rather surprisingly, application of a retrograde tracer to the optic nerve (ON) labels all ipRGCs, except for a sub-population of the d-ipRGCs (14% in pigmented and 28% in albino, respectively) and melanopsin positive cells residing in the ciliary marginal zone (CMZ) of the retina. In the CMZ, between 20% (pigmented) and 24% (albino) of the melanopsin positive cells are unlabeled by the tracer and we suggest that this may be because they fail to send an axon into the ON. As such, this study provides the first evidence for a population of melanopsin interneurons in the mammalian retina
A scenario of planet erosion by coronal radiation
Context: According to theory, high-energy emission from the coronae of cool
stars can severely erode the atmospheres of orbiting planets. No observational
tests of the long term effects of erosion have yet been made. Aims: To analyze
the current distribution of planetary mass with X-ray irradiation of the
atmospheres in order to make an observational assessment of the effects of
erosion by coronal radiation. Methods: We study a large sample of
planet-hosting stars with XMM-Newton, Chandra and ROSAT; make a careful
identification of X-ray counterparts; and fit their spectra to make accurately
measurements of the stellar X-ray flux. Results: The distribution of the
planetary masses with X-ray flux suggests that erosion has taken place: most
surviving massive planets, (M_p sin i >1.5 M_J), have been exposed to lower
accumulated irradiation. Heavy erosion during the initial stages of stellar
evolution is followed by a phase of much weaker erosion. A line dividing these
two phases could be present, showing a strong dependence on planet mass.
Although a larger sample will be required to establish a well-defined erosion
line, the distribution found is very suggestive. Conclusions: The distribution
of planetary mass with X-ray flux is consistent with a scenario in which planet
atmospheres have suffered the effects of erosion by coronal X-ray and EUV
emission. The erosion line is an observational constraint to models of
atmospheric erosion.Comment: A&A 511, L8 (2010). 4 pages, 3 figures, 1 online table (included).
Language edited; corrected a wrong unit conversion (g/s -> M_J/Gyr);
corrected values in column 12 of Table 1 (slightly underestimated in first
version), and Figure 2 updated accordingl
Relevance of d-D interactions on neutron and tritium production in IFMIF-EVEDA accelerator prototype
In the IFMIF-EVEDA accelerator prototype, deuterium is implanted in the components due to beam losses and in the beam dump, where the beam is stopped. The interaction of the deuterons with the deuterium previously implanted leads to the production of neutrons and tritium, which are important issues for radioprotection and safety analysis. A methodology to assess these production pathways in more realistic approach has been developed. The new tools and their main achievement are: (i) an âeffective diffusivity coefficientâ (deduced from available experimental data) that enables simulation of the diffusion phase, and (ii) the MCUNED code (able to handle deuteron transport libraries) allows to simulate the transport-slowdown of deuteron/tritium (to get the concentration profiles) and the neutron/tritium productions from d-Cu and d-D for up to 9 MeV incident deuteron. The results with/without theses tools are presented and their effect on the relevance of d-D sources versus d-Cu is evaluated
Estimation of the XUV radiation onto close planets and their evaporation
Context: The current distribution of planet mass vs. incident stellar X-ray
flux supports the idea that photoevaporation of the atmosphere may take place
in close-in planets. Integrated effects have to be accounted for. A proper
calculation of the mass loss rate due to photoevaporation requires to estimate
the total irradiation from the whole XUV range. Aims: The purpose of this paper
is to extend the analysis of the photoevaporation in planetary atmospheres from
the accessible X-rays to the mostly unobserved EUV range by using the coronal
models of stars to calculate the EUV contribution to the stellar spectra. The
mass evolution of planets can be traced assuming that thermal losses dominate
the mass loss of their atmospheres. Methods: We determine coronal models for 82
stars with exoplanets that have X-ray observations available. Then a synthetic
spectrum is produced for the whole XUV range (~1-912 {\AA}). The determination
of the EUV stellar flux, calibrated with real EUV data, allows us to calculate
the accumulated effects of the XUV irradiation on the planet atmosphere with
time, as well as the mass evolution for planets with known density. Results: We
calibrate for the first time a relation of the EUV luminosity with stellar age
valid for late-type stars. In a sample of 109 exoplanets, few planets with
masses larger than ~1.5 Mj receive high XUV flux, suggesting that intense
photoevaporation takes place in a short period of time, as previously found in
X-rays. The scenario is also consistent with the observed distribution of
planet masses with density. The accumulated effects of photoevaporation over
time indicate that HD 209458b may have lost 0.2 Mj since an age of 20 Myr.
Conclusions: Coronal radiation produces rapid photoevaporation of the
atmospheres of planets close to young late-type stars. More complex models are
needed to explain fully the observations.Comment: Accepted by A&A. 10 pages, 8 figures, 7 Tables (2 online). Additional
online material includes 7 pages, 6 figures and 6 tables, all include
Strongly hyperbolic Hamiltonian systems in numerical relativity: Formulation and symplectic integration
We consider two strongly hyperbolic Hamiltonian formulations of general
relativity and their numerical integration with a free and a partially
constrained symplectic integrator. In those formulations we use hyperbolic
drivers for the shift and in one case also for the densitized lapse. A system
where the densitized lapse is an external field allows to enforce the momentum
constraints in a holonomically constrained Hamiltonian system and to turn the
Hamilton constraint function from a weak to a strong invariant.
These schemes are tested in a perturbed Minkowski and the Schwarzschild
space-time. In those examples we find advantages of the strongly hyperbolic
formulations over the ADM system presented in [arXiv:0807.0734]. Furthermore we
observe stabilizing effects of the partially constrained evolution in
Schwarzschild space-time as long as the momentum constraints are enforced.Comment: This version clarifies some points concerning the interpretation of
the result
The IFMIF-EVEDA accelerator beam dump design
The IFMIF-EVEDA accelerator will be a 9 MeV, 125 mA cw deuteron accelerator prototype for verifying the validity of the 40 MeV accelerator design for IFMIF. A beam dump designed for maximum power of 1.12 MW will be used to stop the beam at the accelerator exit. The conceptual design for the IFMIF-EVEDA accelerator beam dump is based on a conical beam stop made of OFE copper. The cooling system uses an axial high velocity flow of water pressurized up to 3.4 Ă 105 Pa to avoid boiling. The design has been shown to be compliant with ASME mechanical design rules under nominal full power conditions. A sensitivity analysis has been performed to take into account the possible margin on the beam properties at the beam dump entrance. This analysis together with the study of the maintenance issues and the mounting and dismounting operations has led to the complete design definition
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