617 research outputs found
Basis set effects on the hyperpolarizability of CHCl_3: Gaussian-type orbitals, numerical basis sets and real-space grids
Calculations of the hyperpolarizability are typically much more difficult to
converge with basis set size than the linear polarizability. In order to
understand these convergence issues and hence obtain accurate ab initio values,
we compare calculations of the static hyperpolarizability of the gas-phase
chloroform molecule (CHCl_3) using three different kinds of basis sets:
Gaussian-type orbitals, numerical basis sets, and real-space grids. Although
all of these methods can yield similar results, surprisingly large, diffuse
basis sets are needed to achieve convergence to comparable values. These
results are interpreted in terms of local polarizability and
hyperpolarizability densities. We find that the hyperpolarizability is very
sensitive to the molecular structure, and we also assess the significance of
vibrational contributions and frequency dispersion
Stripes Disorder and Correlation lengths in doped antiferromagnets
For stripes in doped antiferromagnets, we find that the ratio of spin and
charge correlation lenghts, , provide a sharp criterion for
determining the dominant form of disorder in the system. If stripes disorder is
controlled by topological defects then . In contast,
if stripes correlations are disordered primarily by non-topological elastic
deformations (i.e., a Bragg-Glass type of disorder) then is expected. Therefore, the observation of in and in invariably implies that the stripes
are in a Bragg glass type state, and topological defects are much less relevant
than commonly assumed. Expected spectral properties are discussed. Thus, we
establish the basis for any theoretical analysis of the experimentally
obsereved glassy state in these material.Comment: 4 pages, 2 figure
Description of Lyme disease-like syndrome in Brazil: is it a new tick borne disease or Lyme disease variation?
An emerging clinical entity that reproduces clinical manifestations similar to those observed in Lyme disease (LD) has been recently under discussion in Brazil. Due to etiological and laboratory particularities it is named LD-like syndrome or LD imitator syndrome. The condition is considered to be a zoonosis transmitted by ticks of the genus Amblyomma, possibly caused by interaction of multiple fastidious microorganisms originating a protean clinical picture, including neurological, osteoarticular and erythema migrans-like lesions. When peripheral blood of patients with LD-like syndrome is viewed under a dark-field microscope, mobile uncultivable spirochete-like bacteria are observed. PCR carried out with specific or conservative primers to recognize Borrelia burgdorferi sensu stricto or the genus Borrelia has been negative in ticks and in biological samples. Two different procedures, respectively involving hematoxylin and eosin staining of cerebrospinal fluid and electron microscopy analysis of blood, have revealed spirochetes not belonging to the genera Borrelia, Leptospira or Treponema. Surprisingly, co-infection with microorganisms resembling Mycoplasma and Chlamydia was observed on one occasion by electron microscopy analysis. We discuss here the possible existence of a new tick-borne disease in Brazil imitating LD, except for a higher frequency of recurrence episodes observed along prolonged clinical follow-up
Amplificação do gene flgE fornece evidência para a existência de uma borreliose brasileira
INTRODUÇÃO: Os sintomas da borreliose brasileira se assemelham às manifestações clínicas da Doença de Lyme (DL), porém, existem diferenças epidemiológicas e laboratoriais entre essas enfermidades. Primers normalmente utilizados para diagnosticar a DL não conseguiram detectar cepas de borrelia no Brasil. OBJETIVO: O objetivo desse trabalho foi identificar a borrelia brasileira usando um gene conservado que sintetiza o gancho flagelar (flgE) da Borrelia burgdorferi sensu lato. MÉTODO: Três pacientes com eritema migratório e epidemiologia positiva foram recrutados para o estudo. Amostras de sangue foram coletadas, e o DNA foi extraído por kits comerciais. RESULTADOS: O gene flgE foi amplificado a partir do DNA de todos os pacientes selecionados. Após o sequenciamento, essas amostras positivas revelaram homologia de 99% para B. burgdorferi. CONCLUSÃO: Estes resultados reforçam a existência de borreliose no Brasil. No entanto, não está claro se esta borreliose é causada por uma variante geneticamente modificada da B. burgdorferi sensu stricto ou por uma nova espécie de Borrelia spp.INTRODUCTION: The symptoms of Brazilian borreliosis resemble the clinical manifestations of Lyme disease (LD). However, there are differences between the two in terms of epidemiological and laboratory findings. Primers usually employed to diagnose LD have failed to detect Borrelia strains in Brazil. OBJECTIVE: We aimed to identify the Brazilian Borrelia using a conserved gene that synthesizes the flagellar hook (flgE) of Borrelia burgdorferi sensu lato. METHOD: Three patients presenting with erythema migrans and positive epidemiological histories were recruited for the study. Blood samples were collected, and the DNA was extracted by commercial kits. RESULTS: The gene flgE was amplified from DNA of all selected patients. Upon sequencing, these positive samples revealed 99% homology to B. burgdorferi flgE. CONCLUSION: These results support the existence of borreliosis in Brazil. However, it is unclear whether this borreliosis is caused by a genetically modified B. burgdorferi sensu stricto or by a new species of Borrelia spp
Gaps and excitations in fullerides with partially filled bands : NMR study of Na2C60 and K4C60
We present an NMR study of Na2C60 and K4C60, two compounds that are related
by electron-hole symmetry in the C60 triply degenerate conduction band. In both
systems, it is known that NMR spin-lattice relaxation rate (1/T1) measurements
detect a gap in the electronic structure, most likely related to
singlet-triplet excitations of the Jahn-Teller distorted (JTD) C60^{2-} or
C60^{4-}. However, the extended temperature range of the measurements presented
here (10 K to 700 K) allows to reveal deviations with respect to this general
trend, both at high and low temperatures. Above room temperature, 1/T1 deviates
from the activated law that one would expect from the presence of the gap and
saturates. In the same temperature range, a lowering of symmetry is detected in
Na2C60 by the appearance of quadrupole effects on the 23Na spectra. In K4C60,
modifications of the 13C spectra lineshapes also indicate a structural
modification. We discuss this high temperature deviation in terms of a coupling
between JTD and local symmetry. At low temperatures, 1/TT tends to a
constant value for Na2C60, both for 13C and 23Na NMR. This indicates a residual
metallic character, which emphasizes the proximity of metallic and insulting
behaviors in alkali fullerides.Comment: 12 pages, 13 figure
First principles study of local electronic and magnetic properties in pure and electron-doped NdCuO
The local electronic structure of Nd2CuO4 is determined from ab-initio
cluster calculations in the framework of density functional theory.
Spin-polarized calculations with different multiplicities enable a detailed
study of the charge and spin density distributions, using clusters that
comprise up to 13 copper atoms in the CuO2plane. Electron doping is simulated
by two different approaches and the resulting changes in the local charge
distribution are studied in detail and compared to the corresponding changes in
hole doped La2CuO4. The electric field gradient (EFG) at the copper nucleus is
investigated in detail and good agreement is found with experimental values. In
particular the drastic reduction of the main component of the EFG in the
electron-doped material with respect to LaCuO4 is explained by a reduction of
the occupancy of the 3d3z^2-r^2 atomic orbital. Furthermore, the chemical
shieldings at the copper nucleus are determined and are compared to results
obtained from NMR measurements. The magnetic hyperfine coupling constants are
determined from the spin density distribution
Borrelia Lyme Group
Borreliaceae is a family of the phylum Spirochaetales and includes two genera, Borrelia and Cristispira genus. Borrelia genus is divided into three groups, namely Lyme group (LG), Echidna‐Reptile group (REPG) and Relapsing Fever group (RFG). All Borrelia species have an obligate parasitic lifestyle, as they depend on their hosts for most of their nutritional needs. Borreliæ are transmitted among vertebrate hosts by arthropod vectors (ticks and lice). Transtadial transmission within their carriers occurs for the Borreliæ RF Group, while this does not (or rarely occurs) for the Borreliæ Lyme Group.
Phylogenetic data demonstrated that these two groups are genetically similar but distinct, forming independent clades sharing a common ancestor. In nature, the vectors of LB belong to the genus Ixodes spp. frequently found in the Northern Hemisphere, while the vectors of RF are usually the soft-ticks (Ornithodoros spp.). Borreliae share a unique genomic structure consisting of a single highly conserved linear chromosome and several linear and circular extrachromosomal plasmids which can vary widely between strains. In addition to Lyme and RF borreliosis, an intermediate group, called Echidna-Reptile borreliosis, has recently been identified.
Lyme disease (LD) is caused by the spirochæte Borrelia burgdorferi sensu lato (s.l.) and transmitted to humans by the bite of a hard tick of the genus Ixodes, and LD reservoir are usually small rodents. LD is present in America, Eurasia, Africa, while its presence in Australia is not yet well documented.
Not all Borreliæ Lyme Groups cause this disease in humans. Of the 23 Borreliæ burgdorferi s.l. currently known only 9 have been identified in human infection, namely Borrelia burgdorferi sensu stricto, B. afzelii, B. bavarensis, B. bissettii, B. garinii, B. lusitaniae, B. spielmani, B. valaisiana, and B. mayonii. LD is an organotropic infection, but there is also a spirochætemic form, caused by Borrelia mayonii, which gives fever similarly to the Borreliosis RF Group. A third variant of LD is Baggio-Yoshinari Syndrome (BYS), which is transmitted by another hard tick, Amblyomma cajennense. This Borrelia has not been isolated in culture, therefore its membership in the Lyme Group is not yet proven. All three of these Sub-Groups can manifest early with erythema migrans. Clinical features of LD are wide and variable, with clinical manifestations linked to distinct tissue tropisms of specific Borrelia burgdorferi s.l. genospecies. The early infection is localized and, in the absence of treatment, the spirochete can spread. The organs most frequently involved are skin, joints, muscles, nervous system, heart and eyes. B. burgdorferi s.s. is more often associated with Lyme arthritis, Borrelia garinii with neuroborreliosis and B. afzelii with acrodermatitis chronica atrophicans
Role of dynamic Jahn-Teller distortions in Na2C60 and Na2CsC60 studied by NMR
Through 13C NMR spin lattice relaxation (T1) measurements in cubic Na2C60, we
detect a gap in its electronic excitations, similar to that observed in
tetragonal A4C60. This establishes that Jahn-Teller distortions (JTD) and
strong electronic correlations must be considered to understand the behaviour
of even electron systems, regardless of the structure. Furthermore, in metallic
Na2CsC60, a similar contribution to T1 is also detected for 13C and 133Cs NMR,
implying the occurence of excitations typical of JT distorted C60^{2-} (or
equivalently C60^{4-}). This supports the idea that dynamic JTD can induce
attractive electronic interactions in odd electron systems.Comment: 3 figure
Effect of Non-Magnetic Impurities (Zn,Li) in a Hole Doped Spin-Fermion Model for Cuprates
The effect of adding non-magnetic impurities (NMI), such as Zn or Li, to
high-Tc cuprates is studied applying Monte Carlo techniques to a spin-fermion
model. It is observed that adding Li is qualitatively similar to doping with
equal percentages of Sr and Zn. The mobile holes (MH) are trapped by the NMI
and the system remains insulating and commensurate with antiferromagnetic (AF)
correlations. This behavior persists in the region %NMI > %MH. On the other
hand, when %NMI < %MH magnetic and charge incommensurabilities are observed.
The vertical or horizontal hole-rich stripes, present when % NMI=0 upon hole
doping, are pinned by the NMI and tend to become diagonal, surrounding finite
AF domains. The %MH-%NMI plane is investigated. Good agreement with
experimental results is found in the small portion of this diagram where
experimental data are available. Predictions about the expected behavior in the
remaining regions are made.Comment: Four pages with four figures embedded in tex
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