222 research outputs found
In-plane polarized collective modes in detwinned YBaCuO observed by spectral ellipsometry
The in-plane dielectric response of detwinned YBaCuO has
been studied by far-infared ellipsometry. A surprisingly lare number of
in-plane polarized modes are observed. Some of them correspond to pure phonon
modes. Others posses a large electronic contribution which strongly increases
in the superconducting state. The free carrier response and the collective
modes exhibit a pronounced a-b anisotropy. We discuss our results in terms of a
CDW state in the 1-d CuO chains and induced charge density fluctuations within
the 2-d CuO planes
Optical Conductivity in Mott-Hubbard Systems
We study the transfer of spectral weight in the optical spectra of a strongly
correlated electron system as a function of temperature and interaction
strength. Within a dynamical mean field theory of the Hubbard model that
becomes exact in the limit of large lattice coordination, we predict an
anomalous enhancement of spectral weight as a function of temperature in the
correlated metallic state and report on experimental measurements which agree
with this prediction in . We argue that the optical conductivity
anomalies in the metal are connected to the proximity to a crossover region in
the phase diagram of the model.Comment: 12 pages and 4 figures, to appear in Phys. Rev. Lett., v 75, p 105
(1995
Theory of Electronic Ferroelectricity
We present a theory of the linear and nonlinear optical characteristics of
the insulating phase of the Falicov-Kimball model within the self-consistent
mean-field approximation. The Coulomb attraction between the itinerant
d-electrons and the localized f-holes gives rise to a built-in coherence
between the d and f-states, which breaks the inversion symmetry of the
underlying crystal, leading to: (1) electronic ferroelectricity, (2)
ferroelectric resonance, and (3) a nonvanishing susceptibility for
second-harmonic generation. As experimental tests of such a built-in coherence
in mixed-valent compounds we propose measurements of the static dielectric
constant, the microwave absorption spectrum, and the dynamic second-order
susceptibility.Comment: 15 pages, 5 PostScript figures, submitted to Physical Review
Transfer of Spectral Weight in Spectroscopies of Correlated Electron Systems
We study the transfer of spectral weight in the photoemission and optical
spectra of strongly correlated electron systems. Within the LISA, that becomes
exact in the limit of large lattice coordination, we consider and compare two
models of correlated electrons, the Hubbard model and the periodic Anderson
model. The results are discussed in regard of recent experiments. In the
Hubbard model, we predict an anomalous enhancement optical spectral weight as a
function of temperature in the correlated metallic state which is in
qualitative agreement with optical measurements in . We argue that
anomalies observed in the spectroscopy of the metal are connected to the
proximity to a crossover region in the phase diagram of the model. In the
insulating phase, we obtain an excellent agreement with the experimental data
and present a detailed discussion on the role of magnetic frustration by
studying the resolved single particle spectra. The results for the periodic
Anderson model are discussed in connection to recent experimental data of the
Kondo insulators and . The model can successfully explain
the different energy scales that are associated to the thermal filling of the
optical gap, which we also relate to corresponding changes in the density of
states. The temperature dependence of the optical sum rule is obtained and its
relevance for the interpretation of the experimental data discussed. Finally,
we argue that the large scattering rate measured in Kondo insulators cannot be
described by the periodic Anderson model.Comment: 19 pages + 29 figures. Submitted to PR
Gauged Inflation
We propose a model for cosmic inflation which is based on an effective
description of strongly interacting, nonsupersymmetric matter within the
framework of dynamical Abelian projection and centerization. The underlying
gauge symmetry is assumed to be with . Appealing to a
thermodynamical treatment, the ground-state structure of the model is
classically determined by a potential for the inflaton field (dynamical
monopole condensate) which allows for nontrivially BPS saturated and thereby
stable solutions. For this leads to decoupling of gravity from the
inflaton dynamics. The ground state dynamics implies a heat capacity for the
vacuum leading to inflation for temperatures comparable to the mass scale
of the potential. The dynamics has an attractor property. In contrast to the
usual slow-roll paradigm we have during inflation. As a consequence,
density perturbations generated from the inflaton are irrelevant for the
formation of large-scale structure, and the model has to be supplemented with
an inflaton independent mechanism for the generation of spatial curvature
perturbations. Within a small fraction of the Hubble time inflation is
terminated by a transition of the theory to its center symmetric phase. The
spontaneously broken symmetry stabilizes relic vector bosons in the
epochs following inflation. These heavy relics contribute to the cold dark
matter of the universe and potentially originate the UHECRs beyond the GZK
bound.Comment: 23 pages, 4 figures, subsection added, revision of text, to app. in
PR
Fine mapping of genetic polymorphisms of pulmonary tuberculosis within chromosome 18q11.2 in the Chinese population: a case-control study
<p>Abstract</p> <p>Background</p> <p>Recently, one genome-wide association study identified a susceptibility locus of rs4331426 on chromosome 18q11.2 for tuberculosis in the African population. To validate the significance of this susceptibility locus in other areas, we conducted a case-control study in the Chinese population.</p> <p>Methods</p> <p>The present study consisted of 578 cases and 756 controls. The SNP rs4331426 and other six tag SNPs in the 100 Kbp up and down stream of rs4331426 on chromosome 18q11.2 were genotyped by using the Taqman-based allelic discrimination system.</p> <p>Results</p> <p>As compared with the findings from the African population, genetic variation of the SNP rs4331426 was rare among the Chinese. No significant differences were observed in genotypes or allele frequencies of the tag SNPs between cases and controls either before or after adjusting for age, sex, education, smoking, and drinking history. However, we observed strong linkage disequilibrium of SNPs. Constructed haplotypes within this block were linked the altered risks of tuberculosis. For example, in comparison with the common haplotype AA<sub>(rs8087945-rs12456774)</sub>, haplotypes AG<sub>(rs8087945-rs12456774) </sub>and GA<sub>(rs8087945-rs12456774) </sub>were associated with a decreased risk of tuberculosis, with the adjusted odds ratio(95% confidence interval) of 0.34(0.27-0.42) and 0.22(0.16-0.29), respectively.</p> <p>Conclusions</p> <p>Susceptibility locus of rs4331426 discovered in the African population could not be validated in the Chinese population. None of genetic polymorphisms we genotyped were related to tuberculosis in the single-point analysis. However, haplotypes on chromosome 18q11.2 might contribute to an individual's susceptibility. More work is necessary to identify the true causative variants of tuberculosis.</p
Single port/incision laparoscopic surgery compared with standard three-port laparoscopic surgery for appendicectomy : a randomised controlled trial
PMID: 23111090 [PubMed - indexed for MEDLINE] PMCID: PMC3543288 Free PMC ArticlePeer reviewedPublisher PD
The Effect of Chemical Doping and Hydrostatic Pressure on Tc of Y1-yCayBa2Cu3Ox Single Crystals
We performed susceptibility measurements on Y1-yCayBa2Cu3Ox single crystals
under high He pressure. For each Ca content various samples with different
oxygen contents have been prepared to probe the influence of Ca on Tc(x),
dTc/dp(x) and Tc,max. Starting from the parabolic Tc(nh) behavior we calculated
nh values from Tc and Tc,max for each sample. It is shown that in the overdoped
region dTc/dp can be described by a pressure induced charge transfer with
dnh/dp = 3.7E-3 [1/GPa] and a dTc,max/dp value of 0.8 K/GPa, irrespective of
the Ca content. In the underdoped region additional pressure effects lead to a
peak in dTc/dp at approximately 0.11 holes/CuO2 plane. However, with increasing
Ca content this peak is strongly depressed. This is explained in terms of an
increasing disorder in the CuO chain system due to doping. Deviations in dTc/dp
at very low nh values can be assigned to the ortho II ordering in the CuO chain
system.Comment: 13 pages with 6 figures, accepted for publication in Physica
TRY plant trait database - enhanced coverage and open access
Plant traits-the morphological, anatomical, physiological, biochemical and phenological characteristics of plants-determine how plants respond to environmental factors, affect other trophic levels, and influence ecosystem properties and their benefits and detriments to people. Plant trait data thus represent the basis for a vast area of research spanning from evolutionary biology, community and functional ecology, to biodiversity conservation, ecosystem and landscape management, restoration, biogeography and earth system modelling. Since its foundation in 2007, the TRY database of plant traits has grown continuously. It now provides unprecedented data coverage under an open access data policy and is the main plant trait database used by the research community worldwide. Increasingly, the TRY database also supports new frontiers of trait-based plant research, including the identification of data gaps and the subsequent mobilization or measurement of new data. To support this development, in this article we evaluate the extent of the trait data compiled in TRY and analyse emerging patterns of data coverage and representativeness. Best species coverage is achieved for categorical traits-almost complete coverage for 'plant growth form'. However, most traits relevant for ecology and vegetation modelling are characterized by continuous intraspecific variation and trait-environmental relationships. These traits have to be measured on individual plants in their respective environment. Despite unprecedented data coverage, we observe a humbling lack of completeness and representativeness of these continuous traits in many aspects. We, therefore, conclude that reducing data gaps and biases in the TRY database remains a key challenge and requires a coordinated approach to data mobilization and trait measurements. This can only be achieved in collaboration with other initiatives
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