419 research outputs found
Temperature memory and resistive glassy behaviors of a perovskite manganite
This paper reports the observations of long-time relaxation, aging, and
temperature memory behaviors of resistance and magnetization in the
ferromagnetic state of a polycrystalline La0.7Ca0.3Mn0.925Ti0.075O3 compound.
The observed glassy dynamics of the electrical transport appears to be
magnetically originated and has a very close association with the magnetic
glassiness of the sample. Phase separation and strong correlation between
magnetic interactions and electronic conduction play the essential roles in
producing such a resistive glassiness. We explain the observed effects in terms
of a coexistence of two competing thermomagnetic processes, domain growth and
magnetic freezing, and propose that hole-doped perovskite manganites can be
considered as "resistive glasses".Comment: Submitted to PR
Topological Phase Diagram of a Two-Subband Electron System
We present a phase diagram for a two-dimensional electron system with two
populated subbands. Using a gated GaAs/AlGaAs single quantum well, we have
mapped out the phases of various quantum Hall states in the density-magnetic
filed plane. The experimental phase diagram shows a very different topology
from the conventional Landau fan diagram. We find regions of negative
differential Hall resistance which are interpreted as preliminary evidence of
the long sought reentrant quantum Hall transitions. We discuss the origins of
the anomalous topology and the negative differential Hall resistance in terms
of the Landau level and subband mixing.Comment: 4 pages, 4 figure
Landau Theory of the Phase Transitions in Half Doped Manganites: Interplay of Magnetic, Charge and Structural Orders
The order parameters of the magnetic, charge and structural orders at
half-doped manganites are identified. A corresponding Landau theory of the
phase transitions is formulated. Many structural and thermodynamical behaviors
are accounted for and clarified within the framework. In particular, the theory
provides a unified picture for the scenario of the phase transitions and their
nature with respect to the variation of the tolerance factor of the manganites.
It also accounts for the origin of the incommensurate nature of the orbital
order and its subsequently accompanying antiferromagnetic order.Comment: 4 pages, 3 eps figures, Revtex, Phys. Rev. B61, 200
Morphological and Bactericidal Effects of Different Antibiotics on Helicobacter pylori
Background: Helicobacter pylori (H. pylori) is a spiral Gram negative bacteria that can transform to the coccoid form in adverse conditions.
Objectives: The aim of this study was to determine the in vitro morphological and bactericidal effects of metronidazole, amoxicillin and
clarithromycin on H. pylori.
Materials and Methods: The standard strain 26695 of H. pylori was cultured on Brucella agar (BA) and the minimum inhibitory
concentrations (MICs) of three antibiotics were determined by E-test method. The bacteria were exposed to antibiotics at 1/2 MIC, MIC and
2X MIC concentrations in Brucella broth (BB). Induced coccoid forms were confirmed by Gram staining and light microscopy. The viability
of cells as well as the susceptibility of viable coccoids to antibiotics were examined using the flow cytometry method.
Results: All of the three antibiotics at sub-MIC induced coccoid forms. The highest rates of coccoids (> 90%) were induced at 0.008 μg/
mL concentration (1/2 MIC) of amoxicillin, 72 hours postexposure. Metronidazole and clarithromycin with 1/2 MIC (0.5 and 0.125 µg/mL
respectively) induced lower rates of coccoid forms (60% and 40% respectively). Potent bactericidal effects on coccoids were observed with
Metronidazole at 2X MIC and clarithromycin at MIC (0.25 µg/mL) (80 - 90%). Amoxicillin with MIC and 2X MIC had no bactericidal effect on
coccoid forms.
Conclusions: Despite the good in vitro bactericidal effect of amoxicillin on spiral forms of H. pylori, this antibiotic has little effect on
induced coccoids that may develop after the inappropriate in vivo antibacterial treatment. Hence, for successful therapy, it is essential not
only to eradicate the spiral forms, but to eliminate the viable coccoids
Realization of the farad from the dc quantum Hall effect with digitally-assisted impedance bridges
A new traceability chain for the derivation of the farad from dc quantum Hall
effect has been implemented at INRIM. Main components of the chain are two new
coaxial transformer bridges: a resistance ratio bridge, and a quadrature
bridge, both operating at 1541 Hz. The bridges are energized and controlled
with a polyphase direct-digital-synthesizer, which permits to achieve both main
and auxiliary equilibria in an automated way; the bridges and do not include
any variable inductive divider or variable impedance box. The relative
uncertainty in the realization of the farad, at the level of 1000 pF, is
estimated to be 64E-9. A first verification of the realization is given by a
comparison with the maintained national capacitance standard, where an
agreement between measurements within their relative combined uncertainty of
420E-9 is obtained.Comment: 15 pages, 11 figures, 3 table
Franck-Condon-Broadened Angle-Resolved Photoemission Spectra Predicted in LaMnO3
The sudden photohole of least energy created in the photoemission process is
a vibrationally excited state of a small polaron. Therefore the photoemission
spectrum in LaMnO3 is predicted to have multiple Franck-Condon vibrational
sidebands. This generates an intrinsic line broadening approximately 0.5 eV.
The photoemission spectral function has two peaks whose central energies
disperse with band width approximately 1.2 eV. Signatures of these phenomena
are predicted to appear in angle-resolved photoemission spectra.Comment: Revtex file 4 pages and 3 figure
Ultrasonic evidence of an uncorrelated cluster formation temperature in manganites with first-order magnetic transition at T_C
Ultrasonic attenuation and phase velocity measurements have been carried out
in the ferromagnetic perovskites La_{2/3}Ca_{1/3}MnO_3 and
La_{2/3}Sr_{1/3}MnO_3. Data show that the transition at the Curie temperature,
T_C, changes from first- to second-order as Sr replaces Ca in the perovskite.
The compound with first-order transition shows also another transition at a
temperature T* > T_C. We interpret the temperature window T_C < T < T* as a
region of coexistence of a phase separated regime of metallic and insulating
regions, in the line of recent theoretical proposals.Comment: 4 pages, 2 figure
Multi-phonon Resonant Raman Scattering Predicted in LaMnO3 from the Franck-Condon Process via Self-Trapped Excitons
Resonant behavior of the Raman process is predicted when the laser frequency
is close to the orbital excitation energy of LaMnO3 at 2 eV. The incident
photon creates a vibrationally excited self-trapped ``orbiton'' state from the
orbitally-ordered Jahn-Teller (JT) ground state. Trapping occurs by local
oxygen rearrangement. Then the Franck-Condon mechanism activates multiphonon
Raman scattering. The amplitude of the -phonon process is first order in the
electron-phonon coupling . The resonance occurs {\it via} a dipole forbidden
to transition. We previously suggested that this transition (also seen
in optical reflectivity) becomes allowed because of asymmetric oxygen
fluctuations. Here we calculate the magnitude of the corresponding matrix
element using local spin-density functional theory. This calculation agrees to
better than a factor of two with our previous value extracted from experiment.
This allows us to calculate the absolute value of the Raman tensor for
multiphonon scattering. Observation of this effect would be a direct
confirmation of the importance of the JT electron-phonon term and the presence
of self-trapped orbital excitons, or ``orbitons''.Comment: 8 pages and 3 embedded figures. The earlier short version is now
replaced by a more complete paper with a slightly different title. This
version includes a caculation by density-functional theory of the dipole
matrix element for exciting the self-trapped orbital exciton which activates
the multiphonon Raman signal
Recent acquisition of Helicobacter pylori by Baka Pygmies
Both anatomically modern humans and the gastric pathogen Helicobacter pylori originated in Africa, and both species have been associated for at least 100,000 years. Seven geographically distinct H. pylori populations exist, three of which are indigenous to Africa: hpAfrica1, hpAfrica2, and hpNEAfrica. The oldest and most divergent population, hpAfrica2, evolved within San hunter-gatherers, who represent one of the deepest branches of the human population tree. Anticipating the presence of ancient H. pylori lineages within all hunter-gatherer populations, we investigated the prevalence and population structure of H. pylori within Baka Pygmies in Cameroon. Gastric biopsies were obtained by esophagogastroduodenoscopy from 77 Baka from two geographically separated populations, and from 101 non-Baka individuals from neighboring agriculturalist populations, and subsequently cultured for H. pylori. Unexpectedly, Baka Pygmies showed a significantly lower H. pylori infection rate (20.8%) than non-Baka (80.2%). We generated multilocus haplotypes for each H. pylori isolate by DNA sequencing, but were not able to identify Baka-specific lineages, and most isolates in our sample were assigned to hpNEAfrica or hpAfrica1. The population hpNEAfrica, a marker for the expansion of the Nilo-Saharan language family, was divided into East African and Central West African subpopulations. Similarly, a new hpAfrica1 subpopulation, identified mainly among Cameroonians, supports eastern and western expansions of Bantu languages. An age-structured transmission model shows that the low H. pylori prevalence among Baka Pygmies is achievable within the timeframe of a few hundred years and suggests that demographic factors such as small population size and unusually low life expectancy can lead to the eradication of H. pylori from individual human populations. The Baka were thus either H. pylori-free or lost their ancient lineages during past demographic fluctuations. Using coalescent simulations and phylogenetic inference, we show that Baka almost certainly acquired their extant H. pylori through secondary contact with their agriculturalist neighbors
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