1,176 research outputs found
Phase separation and valence instabilities in cuprate superconductors. Effective one-band model approach
We study the Cu-O valence instability (VI) and the related phase separation
(PS) driven by Cu-O nearest-neighbor repulsion , using an effective
extended one-band Hubbard model () obtained from the extended
three-bandHubbard model, through an appropriate low-energy reduction.
is solved by exact diagonalization of a square cluster with 10 unit cells and
also within a slave-boson mean-field theory. Its parameters depend on doping
for or on-site O repulsion . The results using both
techniques coincide in that there is neither VI nor PS for doping levels
if eV. The PS region begins for eV
at large doping and increases with increasing . The PS also
increases with increasing on-site Cu repulsion .Comment: 16 pages and 10 figures in postscript format, compressed with uufile
Optical properties of an effective one-band Hubbard model for the cuprates
We study the Cu and O spectral density of states and the optical conductivity
of CuO_2 planes using an effective generalized one-band Hubbard model derived
from the extended three-band Hubbard model. We solve exactly a square cluster
of 10 unit cells and average the results over all possible boundary conditions,
what leads to smooth functions of frequency. Upon doping, the Fermi energy
jumps to Zhang-Rice states which are connected to the rest of the valence band
(in contrast to an isolated new band in the middle of the gap). The transfer of
spectral weight depends on the parameters of the original three-band model not
only through the one-band effective parameters but also through the relevant
matrix elements. We discuss the evolution of the gap upon doping. The optical
conductivity of the doped system shows a mid-infrared peak due to intraband
transitions, a pseudogap and a high frequency part related to interband
transitions. Its shape and integrated weight up to a given frequency (including
the Drude weight) agree qualitatively with experiments in the cuprates for low
to moderate doping levels, but significant deviations exist for doping .Comment: 11 pages (tex), 14 figures (ps
Role of Heme and Heme-Proteins in Trypanosomatid Essential Metabolic Pathways
Around the world, trypanosomatids are known for being etiological agents of several highly disabling and often fatal diseases like Chagas disease (Trypanosoma cruzi), leishmaniasis (Leishmania spp.), and African trypanosomiasis (Trypanosoma brucei). Throughout their life cycle, they must cope with diverse environmental conditions, and the mechanisms involved in these processes are crucial for their survival. In this review, we describe the role of heme in several essential metabolic pathways of these protozoans. Notwithstanding trypanosomatids lack of the complete heme biosynthetic pathway, we focus our discussion in the metabolic role played for important heme-proteins, like cytochromes. Although several genes for different types of cytochromes, involved in mitochondrial respiration, polyunsaturated fatty acid metabolism, and sterol biosynthesis, are annotated at the Tritryp Genome Project, the encoded proteins have not yet been deeply studied. We pointed our attention into relevant aspects of these protein functions that are amenable to be considered for rational design of trypanocidal agents
Charge and spin excitations of insulating lamellar copper oxides
A consistent description of low-energy charge and spin responses of the
insulating Sr_2CuO_2Cl_2 lamellar system is found in the framework of a
one-band Hubbard model which besides includes hoppings up to 3^{rd}
nearest-neighbors. By combining mean-field calculations, exact diagonalization
(ED) results, and Quantum Monte Carlo simulations (QMC), we analyze both charge
and spin degrees of freedom responses as observed by optical conductivity,
ARPES, Raman and inelastic neutron scattering experiments. Within this
effective model, long-range hopping processes flatten the quasiparticle band
around . We calculate also the non-resonant A_{1g} and B_{1g} Raman
profiles and show that the latter is composed by two main features, which are
attributed to 2- and 4-magnon scattering.Comment: 6 pages, 3 figures, To be published in PRB (july
A chiral model for bar{q}q and bar{q}bar{q}qq$ mesons
We point out that the spectrum of pseudoscalar and scalar mesons exhibits a
cuasi-degenerate chiral nonet in the energy region around 1.4 GeV whose scalar
component has a slightly inverted spectrum. Based on the empirical linear
rising of the mass of a hadron with the number of constituent quarks which
yields a mass around GeV for tetraquarks, we conjecture that this
cuasi-chiral nonet arises from the mixing of a chiral nonet composed of
tetraquarks with conventional bar{q}q states. We explore this possibility in
the framework of a chiral model assuming a tetraquark chiral nonet around 1.4
GeV with chiral symmetry realized directly. We stress that U_{A}(1)
transformations can distinguish bar{q}q from tetraquark states, although it
cannot distinguish specific dynamics in the later case. We find that the
measured spectrum is consistent with this picture. In general, pseudoscalar
states arise as mainly bar{q}q states but scalar states turn out to be strong
admixtures of bar{q}q and tetraquark states. We work out also the model
predictions for the most relevant couplings and calculate explicitly the strong
decays of the a_{0}(1450) and K_{0}^*(1430) mesons. From the comparison of some
of the predicted couplings with the experimental ones we conclude that
observable for the isovector and isospinor sectors are consistently described
within the model. The proper description of couplings in the isoscalar sectors
would require the introduction of glueball fields which is an important missing
piece in the present model.Comment: 20 pages, 3 figure
Two photon decay of neutral scalars below 1.5 GeV in a chiral model for bar{q}q and bar{q}bar{q}qq states
We study the two photon decay of neutral scalars below 1.5 GeV in the context
of a recently proposed chiral model for bar{q}q and bar{q}bar{q}qq states. We
find good agreement with experimental results for the a_{0}(980)->gamma gamma.
Our calculations for f_{0}(980)->gamma gamma shows that further work is
necessary in order to understand the structure of this meson. The model
predicts Gamma(a_{0}(1450)->gamma gamma)=0.16+/-0.10KeV, Gamma(sigma->gamma
gamma)=0.47+/-0.66 KeV, Gamma(f(1370)->gamma gamma)=0.07+/-0.15 KeV,
Gamma(f(1500)->gamma gamma)=0.74+/-0.78 KeV.Comment: 6 pages, 1 figur
Variación morfológica y distribución geográfica de Schizopetalon arcuatum Al-Shehbaz (Brassicaceae), una especie endémica y críptica de la Región de Atacama, Chile
La presente nota ofrece una revisión de la distribución geográfica y variación morfológica de Schizopetalon arcuatum, dado que ambas variables han generado confusión en la identificación de esta especie críptica. Se espera que la información presentada ayude a resolver este problema en relación a sus especies vecinas o cercanamente relacionadas
Bienes no tradicionales exportables en Colombia: evolución y perspectivas de expansión en los mercados internacionales entre el periodo 1990-2001
El comercio exterior colombiano ha tenido variaciones constantes en cuanto a
exportaciones se refiere en el transcurso de los años. Dentro de las más
significantes podemos destacar lo ocurrido en el periodo comprendido entre 1975-
1983, donde se nota un estancamiento de las exportaciones no tradicionales,
mientras que las tradicionales presentaron un dinamismo impulsadas por el café.
A diferencia del periodo anterior, entre 1984 y 1999 tanto las exportaciones
tradicionales como las no tradicionales incidieron positivamente en un crecimiento,
pero ya no fue el café el rubro dinámico sino el petróleo crudo. Cabe resaltar que
en los años 92, 96 y 99 existió una leve disminución.
Dentro de los productos que presentan una importante participación dentro de las
exportaciones no tradicionales, y que a su vez tienen participaciones crecientes
entre el periodo de 1970 y 1999, están las flores, azúcar, confecciones, artes
graficas y editoriales y químicos.
Para el año 2000 y 2001 las exportaciones no tradicionales crecieron debido al
comportamiento positivo de bienes industriales.
En cuanto a los sectores se refiere, en el periodo analizado, la mayor participación
se reflejo en el año 2001, representando el 55% del total exportado (Tabla 3). La
mayor contribución la determinó el sector industrial.
Por ultimo, los principales destinos de las exportaciones no tradicionales son
Estados Unidos y Venezuela, este último gracias a la apertura, aumentó año tras
año el valor de las ventas.Incluye conclusiones y bibliografí
A Fermi Surface Model for Large Supersymmetric AdS_5 Black Holes
We identify a large family of 1/16 BPS operators in N=4 SYM that
qualitatively reproduce the relations between charge, angular momentum and
entropy in regular supersymmetric AdS_5 black holes when the main contribution
to their masses is given by their angular momentum.Comment: 32 pages, 6 figures, LaTeX uses JHEP3 class; ver 2- added
acknowledgment, minor change
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