121 research outputs found
On the Nagaoka polaron in the t-J model
It is widely believed that a single hole in the two (or three) dimensional
t-J model, for sufficiently small exchange coupling J, creates a ferromagnetic
bubble around itself, a finite J remnant of the ferromagnetic groundstate at
J=0 (the infinite U Hubbard model), first established by Nagaoka. We
investigate this phenomenon in two dimensions using the density matrix
renormalization group, for system sizes up to 9x9. We find that the polaron
forms for J/t<0.02-0.03 (a somewhat larger value than estimated previously).
Although finite-size effects appear large, our data seems consistent with the
expected 1.1(J/t)^{-1/4} variation of polarion radius. We also test the
Brinkman-Rice model of non-retracing paths in a Neel background, showing that
it is quite accurate, at larger J. Results are also presented in the case where
the
Heisenberg interaction is dropped (the t-J^z model). Finally we discuss a
"dressed polaron" picture in which the hole propagates freely inside a finite
region but makes only self-retracing excursions outside this region.Comment: 7 pages, 9 encapsulated figure
Genotype versus phenotype in families with androgen insensitivity syndrome
Androgen insensitivity syndrome encompasses a wide range of phenotypes,
which are caused by numerous different mutations in the AR gene. Detailed
information on the genotype/phenotype relationship in androgen
insensitivity syndrome is important for sex assignment, treatment of
androgen insensitivity syndrome patients, genetic counseling of their
families, and insight into the functional domains of the AR. The commonly
accepted concept of dependence on fetal androgens of the development of
Wolffian ducts was studied in complete androgen insensitivity syndrome
(CAIS) patients. In a nationwide survey in The Netherlands, all cases (n =
49) with the presumptive diagnosis androgen insensitivity syndrome known
to pediatric endocrinologists and clinical geneticists were studied. After
studying the clinical phenotype, mutation analysis and functional analysis
of mutant receptors were performed using genital skin fibroblasts and in
vitro expression studies. Here we report the findings in families with
multiple affected cases. Fifty-nine percent of androgen insensitivity
syndrome patients had other affected relatives. A total of 17 families
were studied, seven families with CAIS (18 patients), nine families with
partial androgen insensitivity (24 patients), and one family with female
prepubertal phenotypes (two patients). No phenotypic variation was
observed in families with CAIS. However, phenotypic variation was observed
in one-third of families with partial androgen insensitivity resulting in
different sex of rearing and differences in requirement of reconstructive
surgery. Intrafamilial phenotypic variation was observed for mutations
R846H, M771I, and deletion of amino acid N682. Four newly identified
mutations were found. Follow-up in families with different AR gene
mutations provided information on residual androgen action in vivo and the
development of the prepubertal and adult phenotype. Patients with a
functional complete defective AR had some pubic hair, Tanner stage P2, and
vestigial Wolffian duct derivatives despite absence of AR expression.
Vaginal length was functional in most but not all CAIS patients. The
minimal incidence of androgen insensitivity syndrome in The Netherlands,
based on patients with molecular proof of the diagnosis is 1:99,000.
Phenotypic variation was absent in families with CAIS, but distinct
phenotypic variation was observed relatively frequent in families with
partial androgen insensitivity. Molecular observations suggest that
phenotypic variation had different etiologies among these families. Sex
assignment of patients with partial androgen insensitivity cannot be based
on a specific identified AR gene mutation because distinct phenotypic
variation in partial androgen insensitivity families is relatively
frequent. In genetic counseling of partial androgen insensitivity
families, this frequent occurrence of variable expression resulting in
differences in sex of rearing and/or requirement of reconstructive surgery
is important information. During puberty or normal dose androgen therapy,
no or only minimal virilization may occur even in patients with
significant (but still deficient) prenatal virilization. Wolffian duct
remnants remain detectable but differentiation does not occur in the
absence of a functional AR. In many CAIS patients, surgical elongation of
the vagina is not indicated
Intergranular flux pinning in underdoped and overdoped R(1-x)CaxBa2Cu3Oz (R=Y,Gd; x=0,0.2) samples
The temperature dependences of AC magnetic susceptibility at different
magnetic field amplitudes and frequencies are investigated for underdoped and
overdoped R(1-x)Ca(x)Ba(2)Cu(3)O(7-delta)(R=Y; Gd and x=0; 0.2) polycrystalline
samples. The activation energy, Ea, for thermally assisted flux flow (TAFF) in
intergranular region is determined. It was established that a correlation
exists between the intergranular critical current and flux pinning activation
energy. In underdoped samples the intergranular current shows S-I-S behaviour
and the activation energy is small, while in overdoped samples the
intergranular current is changed to S-N-S type and the activation energy
increases. 2D pancake vortices are characteristic of underdoped samples, while
3D vortex system exists in overdoped samples. In fact we demonstrate that Ca
substitution not only increases carrier concentration, but improves
intergranular activation energy for TAFF.Comment: 20 pages including 7 figures and one Table. accepted in Physica
Kondo effect in coupled quantum dots: a Non-crossing approximation study
The out-of-equilibrium transport properties of a double quantum dot system in
the Kondo regime are studied theoretically by means of a two-impurity Anderson
Hamiltonian with inter-impurity hopping. The Hamiltonian, formulated in
slave-boson language, is solved by means of a generalization of the
non-crossing approximation (NCA) to the present problem. We provide benchmark
calculations of the predictions of the NCA for the linear and nonlinear
transport properties of coupled quantum dots in the Kondo regime. We give a
series of predictions that can be observed experimentally in linear and
nonlinear transport measurements through coupled quantum dots. Importantly, it
is demonstrated that measurements of the differential conductance , for the appropriate values of voltages and inter-dot tunneling
couplings, can give a direct observation of the coherent superposition between
the many-body Kondo states of each dot. This coherence can be also detected in
the linear transport through the system: the curve linear conductance vs
temperature is non-monotonic, with a maximum at a temperature
characterizing quantum coherence between both Kondo states.Comment: 20 pages, 17 figure
Tight-binding parameters for charge transfer along DNA
We systematically examine all the tight-binding parameters pertinent to
charge transfer along DNA. The molecular structure of the four DNA bases
(adenine, thymine, cytosine, and guanine) is investigated by using the linear
combination of atomic orbitals method with a recently introduced
parametrization. The HOMO and LUMO wavefunctions and energies of DNA bases are
discussed and then used for calculating the corresponding wavefunctions of the
two B-DNA base-pairs (adenine-thymine and guanine-cytosine). The obtained HOMO
and LUMO energies of the bases are in good agreement with available
experimental values. Our results are then used for estimating the complete set
of charge transfer parameters between neighboring bases and also between
successive base-pairs, considering all possible combinations between them, for
both electrons and holes. The calculated microscopic quantities can be used in
mesoscopic theoretical models of electron or hole transfer along the DNA double
helix, as they provide the necessary parameters for a tight-binding
phenomenological description based on the molecular overlap. We find that
usually the hopping parameters for holes are higher in magnitude compared to
the ones for electrons, which probably indicates that hole transport along DNA
is more favorable than electron transport. Our findings are also compared with
existing calculations from first principles.Comment: 15 pages, 3 figures, 7 table
Fitting the integrated Spectral Energy Distributions of Galaxies
Fitting the spectral energy distributions (SEDs) of galaxies is an almost
universally used technique that has matured significantly in the last decade.
Model predictions and fitting procedures have improved significantly over this
time, attempting to keep up with the vastly increased volume and quality of
available data. We review here the field of SED fitting, describing the
modelling of ultraviolet to infrared galaxy SEDs, the creation of
multiwavelength data sets, and the methods used to fit model SEDs to observed
galaxy data sets. We touch upon the achievements and challenges in the major
ingredients of SED fitting, with a special emphasis on describing the interplay
between the quality of the available data, the quality of the available models,
and the best fitting technique to use in order to obtain a realistic
measurement as well as realistic uncertainties. We conclude that SED fitting
can be used effectively to derive a range of physical properties of galaxies,
such as redshift, stellar masses, star formation rates, dust masses, and
metallicities, with care taken not to over-interpret the available data. Yet
there still exist many issues such as estimating the age of the oldest stars in
a galaxy, finer details ofdust properties and dust-star geometry, and the
influences of poorly understood, luminous stellar types and phases. The
challenge for the coming years will be to improve both the models and the
observational data sets to resolve these uncertainties. The present review will
be made available on an interactive, moderated web page (sedfitting.org), where
the community can access and change the text. The intention is to expand the
text and keep it up to date over the coming years.Comment: 54 pages, 26 figures, Accepted for publication in Astrophysics &
Space Scienc
A First Search for coincident Gravitational Waves and High Energy Neutrinos using LIGO, Virgo and ANTARES data from 2007
We present the results of the first search for gravitational wave bursts
associated with high energy neutrinos. Together, these messengers could reveal
new, hidden sources that are not observed by conventional photon astronomy,
particularly at high energy. Our search uses neutrinos detected by the
underwater neutrino telescope ANTARES in its 5 line configuration during the
period January - September 2007, which coincided with the fifth and first
science runs of LIGO and Virgo, respectively. The LIGO-Virgo data were analysed
for candidate gravitational-wave signals coincident in time and direction with
the neutrino events. No significant coincident events were observed. We place
limits on the density of joint high energy neutrino - gravitational wave
emission events in the local universe, and compare them with densities of
merger and core-collapse events.Comment: 19 pages, 8 figures, science summary page at
http://www.ligo.org/science/Publication-S5LV_ANTARES/index.php. Public access
area to figures, tables at
https://dcc.ligo.org/cgi-bin/DocDB/ShowDocument?docid=p120000
Um balanço das políticas do governo Lula para a educação superior: continuidade e ruptura
Search for Neutrinoless Double- β Decay with the Complete EXO-200 Dataset
A search for neutrinoless double-β decay (0νββ) in Xe136 is performed with the full EXO-200 dataset using a deep neural network to discriminate between 0νββ and background events. Relative to previous analyses, the signal detection efficiency has been raised from 80.8% to 96.4±3.0%, and the energy resolution of the detector at the Q value of Xe136 0νββ has been improved from σ/E=1.23% to 1.15±0.02% with the upgraded detector. Accounting for the new data, the median 90% confidence level 0νββ half-life sensitivity for this analysis is 5.0×1025 yr with a total Xe136 exposure of 234.1 kg yr. No statistically significant evidence for 0νββ is observed, leading to a lower limit on the 0νββ half-life of 3.5×1025 yr at the 90% confidence level
Novel genetic loci associated with hippocampal volume
The hippocampal formation is a brain structure integrally involved in episodic memory, spatial navigation, cognition and stress responsiveness. Structural abnormalities in hippocampal volume and shape are found in several common neuropsychiatric disorders. To identify the genetic underpinnings of hippocampal structure here we perform a genome-wide association study (GWAS) of 33,536 individuals and discover six independent loci significantly associated with hippocampal volume, four of them novel. Of the novel loci, three lie within genes (ASTN2, DPP4 and MAST4) and one is found 200 kb upstream of SHH. A hippocampal subfield analysis shows that a locus within the MSRB3 gene shows evidence of a localized effect along the dentate gyrus, subiculum, CA1 and fissure. Further, we show that genetic variants associated with decreased hippocampal volume are also associated with increased risk for Alzheimer's disease (rg =-0.155). Our findings suggest novel biological pathways through which human genetic variation influences hippocampal volume and risk for neuropsychiatric illness
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