180 research outputs found
Selective Spin-State Switch and Metal-Insulator Transition in \boldmath
Ultra-high resolution synchrotron diffraction data for
throw new light on the metal-insulator transition of Co Ba-cobaltites.
An anomalous expansion of CoO octahedra is observed at the phase transition
on heating, while CoO pyramids show the normal shrinking at the closing of
the gap. The insulator-to-metal transition is attributed to a sudden excitation
of some electrons in the octahedra ( state) into the Co band
(final state). The state in the pyramids does
not change and the structural study also rules out a
orbital ordering at .Comment: Phys. Rev. B (to appear
Transport and magnetic properties of GdBaCo_{2}O_{5+x} single crystals: A cobalt oxide with square-lattice CoO_2 planes over a wide range of electron and hole doping
Single crystals of the layered perovskite GdBaCo_{2}O_{5+x} (GBCO) have been
grown by the floating-zone method, and their transport, magnetic, and
structural properties have been studied in detail over a wide range of oxygen
contents. The obtained data are used to establish a rich phase diagram centered
at the "parent'' compound GdBaCo_{2}O_{5.5} -- an insulator with Co ions in the
3+ state. An attractive feature of GBCO is that it allows a precise and
continuous doping of CoO_{2} planes with either electrons or holes, spanning a
wide range from the charge-ordered insulator at 50% electron doping (x=0) to
the undoped band insulator (x=0.5), and further towards the heavily hole-doped
metallic state. This continuous doping is clearly manifested in the behavior of
thermoelectric power which exhibits a spectacular divergence with approaching
x=0.5, where it reaches large absolute values and abruptly changes its sign. At
low temperatures, the homogeneous distribution of doped carriers in GBCO
becomes unstable, and both the magnetic and transport properties point to an
intriguing nanoscopic phase separation. We also find that throughout the
composition range the magnetic behavior in GBCO is governed by a delicate
balance between ferromagnetic (FM) and antiferromagnetic (AF) interactions,
which can be easily affected by temperature, doping, or magnetic field,
bringing about FM-AF transitions and a giant magnetoresistance (MR) phenomenon.
An exceptionally strong uniaxial anisotropy of the Co spins, which dramatically
simplifies the possible spin arrangements, together with the possibility of
continuous ambipolar doping turn GBCO into a model system for studying the
competing magnetic interactions, nanoscopic phase separation and accompanying
magnetoresistance phenomena.Comment: 31 pages, 32 figures, submitted to Phys. Rev.
Comparison of hospital-wide and unit-specific cumulative antibiograms in hospital- and community-acquired infection
Background: Empirical antibacterial therapy in hospitals is usually guided by local epidemiologic features reflected by institutional cumulative antibiograms. We investigated additional information inferred by aggregating cumulative antibiograms by type of unit or according to the place of acquisition (i.e. community vs. hospital) of the bacteria. Materials and methods: Antimicrobial susceptibility rates of selected pathogens were collected over a 4-year period in an university-affiliated hospital. Hospital-wide antibiograms were compared with those selected by type of unit and sampling time (48h after hospital admission). Results: Strains isolated >48h after admission were less susceptible than those presumably arising from the community (48h after admission. When compared to hospital-wide antibiograms, susceptibility rates were lower in the ICU and surgical units for Escherichia coli to amoxicillin-clavulanate, enterococci to penicillin, and Pseudomonas aeruginosa to anti-pseudomonal beta-lactams, and in medical units for Staphylococcus aureus to oxacillin. In contrast, few differences were observed among strains isolated within 48h of admission. Conclusions: Hospital-wide antibiograms reflect the susceptibility pattern for a specific unit with respect to community-acquired, but not to hospital-acquired strains. Antibiograms adjusted to these parameters may be useful in guiding the choice of empirical antibacterial therap
Spin state and phase competition in TbBaCo_{2}O_{5.5} and the lanthanide series LnBaCo_{2}O_{5+\delta} (0<=\delta<=1)
A clear physics picture of TbBaCoO is revealed on the basis of
density functional theory calculations. An antiferromagnetic (AFM)
superexchange coupling between the almost high-spin Co ions competes
with a ferromagnetic (FM) interaction mediated by both p-d exchange and double
exchange, being responsible for the observed AFM-FM transition. And the
metal-insulator transition is accompanied by an xy/xz orbital-ordering
transition. Moreover, this picture can be generalized to the whole lanthanide
series, and it is predicted that a few room-temperature magnetoresistance
materials could be found in LnBaACoO
(Ln=Ho,Er,Tm,Yb,Lu; A=Sr,Ca,Mg).Comment: 13 pages, 2 figures; to be published in Phys. Rev. B on 1st Sept.
Title and Bylines are added to the revised versio
Adaptive modulation in Ni2Mn1.4In0.6 magnetic shape memory Heusler alloy
The origin of incommensurate structural modulation in Ni-Mn based Heusler
type magnetic shape memory alloys (MSMAs) is still an unresolved issue inspite
of intense focus on this due to its role in the magnetic field induced
ultra-high strains. In the archetypal MSMA Ni2MnGa, the observation of
non-uniform displacement of atoms from their mean positions in the modulated
martensite phase, premartensite phase and charge density wave as well as the
presence of phason broadening of satellite peaks have been taken in support of
the electronic instability model linked with a soft acoustic phonon. We present
here results of a combined high resolution synchrotron x-ray powder diffraction
(SXRPD) and neutron powder diffraction (NPD) study on Ni2Mn1.4In0.6 using
(3+1)D superspace group approach, which reveal not only uniform atomic
displacements in the modulated structure of the martensite phase with
physically acceptable ordered magnetic moments in the antiferromagnetic phase
at low temperatures but also the absence of any premartensite phase and phason
broadening of the satellite peaks. Our HRTEM studies and first principles
calculations of the ground state also support uniform atomic displacements
predicted by powder diffraction studies. All these observations suggest that
the structural modulation in the martensite phase of Ni2Mn1.4In0.6 MSMA can be
explained in terms of the adaptive phase model. The present study underlines
the importance of superspace group analysis using complimentary SXRPD and NPD
in understanding the physics of the origin of modulation as well as the
magnetic and the modulated ground states of the Heusler type MSMAs. Our work
also highlights the fact that the mechanism responsible for the origin of
modulated structure in different Ni-Mn based MSMAs may not be universal and it
must be investigated thoroughly in different alloy compositions
Evidence for a Low-Spin to Intermediate-Spin State Transition in LaCoO3
We present measurements of the magnetic susceptibility and of the thermal
expansion of a LaCoO single crystal. Both quantities show a strongly
anomalous temperature dependence. Our data are consistently described in terms
of a spin-state transition of the Co ions with increasing temperature
from a low-spin ground state to an intermediate-spin state without (100K -
500K) and with (>500K) orbital degeneracy. We attribute the lack of orbital
degeneracy up to 500K to (probably local) Jahn-Teller distortions of the
CoO octahedra. A strong reduction or disappearance of the Jahn-Teller
distortions seems to arise from the insulator-to-metal transition around 500 K.Comment: an error in the scaling factor of Eq.(4) and consequently 2 values of
table I have been corrected. The conclusions of the paper remain unchanged.
See also: C. Zobel et al. Phys. Rev. B 71, 019902 (2005) and J. Baier et al.
Phys. Rev. B 71, 014443 (2005
Crystal growth and superconductivity of FeSe_x
Single crystals FeSe_x have been grown in evacuated sealed quartz tube using
a NaCl/KCl flux. The products include two crystal structures of tetragon and
hexagon. The electronic transport and magnetic properties measurements of
FeSe_x single crystal exhibits a superconducting transition at about 10K.Comment: 9 pages, 4 Figure
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The recovery of European freshwater biodiversity has come to a halt
Owing to a long history of anthropogenic pressures, freshwater ecosystems are among the most vulnerable to biodiversity loss. Mitigation measures, including wastewater treatment and hydromorphological restoration, have aimed to improve environmental quality and foster the recovery of freshwater biodiversity. Here, using 1,816 time series of freshwater invertebrate communities collected across 22 European countries between 1968 and 2020, we quantified temporal trends in taxonomic and functional diversity and their responses to environmental pressures and gradients. We observed overall increases in taxon richness (0.73% per year), functional richness (2.4% per year) and abundance (1.17% per year). However, these increases primarily occurred before the 2010s, and have since plateaued. Freshwater communities downstream of dams, urban areas and cropland were less likely to experience recovery. Communities at sites with faster rates of warming had fewer gains in taxon richness, functional richness and abundance. Although biodiversity gains in the 1990s and 2000s probably reflect the effectiveness of water-quality improvements and restoration projects, the decelerating trajectory in the 2010s suggests that the current measures offer diminishing returns. Given new and persistent pressures on freshwater ecosystems, including emerging pollutants, climate change and the spread of invasive species, we call for additional mitigation to revive the recovery of freshwater biodiversity
Characterization of the translation products of the major mRNA species from rabbit lactating mammary glands and construction of bacterial recombinants containing casein and alpha-lactalbumin complementary DNA.
Total cytoplasmic polyadenylated RNA from lactating rabbit mammary glands was analysed on methylmercury hydroxide-agarose gels. The size of the most abundant mRNA species ranged between 0.5 and 5.0 kb (kilobases), with major bands at 0.55, 0.84, 0.92, 1.18 and 2.4 kb and discrete minor bands of 1.5, 1.7, 3.0 and 3.9 kb. Translation in vitro of total mRNA with [3H]leucine or [35S]methionine as precursor yielded four major bands with apparent Mr values of 16 000, 25 000, 26 000 and 29 000. The four protein bands were identified by immunoprecipitation by using specific antisera as alpha-lactalbumin and x-, kappa- and alpha-caseins, respectively. Labelling with (35S]cysteine followed by immunoprecipitation with anti-transferrin or anti-alpha-lactalbumin sera allowed the identification of two whey proteins. Translated transferrin was resolved as an 80 000-dalton band and alpha-lactalbumin appeared as a 16 000-dalton protein. A library of recombinant plasmids containing cDNA (complementary DNA) sequences representing cytoplasmic polyadenylated RNA was used to isolate clones for the major rabbit caseins and alpha-lactalbumin. A preliminary characterization of these cDNA clones was achieved by colony hybridization with enriched RNA fractions as probes. Positive clones were identified by use of hybrid-promoted translation in vitro and immunoprecipitation of the translation products. The corresponding mRNA species were further identified by hybridizing RNA blots with radioactively labelled cDNA clones. We present the restriction map of alpha-casein and kappa-casein cDNA clones
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