444 research outputs found
Magnetic structure of EuFe2As2 determined by single crystal neutron diffraction
Among various parent compounds of iron pnictide superconductors, EuFe2As2
stands out due to the presence of both spin density wave of Fe and
antiferromagnetic ordering (AFM) of the localized Eu2+ moment. Single crystal
neutron diffraction studies have been carried out to determine the magnetic
structure of this compound and to investigate the coupling of two magnetic
sublattices. Long range AFM ordering of Fe and Eu spins was observed below 190
K and 19 K, respectively. The ordering of Fe2+ moments is associated with the
wave vector k = (1,0,1) and it takes place at the same temperature as the
tetragonal to orthorhombic structural phase transition, which indicates the
strong coupling between structural and magnetic components. The ordering of Eu
moment is associated with the wave vector k = (0,0,1). While both Fe and Eu
spins are aligned along the long a axis as experimentally determined, our
studies suggest a weak coupling between the Fe and Eu magnetism.Comment: 7 pages, 7 figure
Field induced spin reorientation and giant spin-lattice coupling in EuFe2As2
We have studied a EuFe2As2 single crystal by neutron diffraction under
magnetic fields up to 3.5 T and temperatures down to 2 K. A field induced spin
reorientation is observed in the presence of a magnetic field along both the a
and c axes, respectively. Above critical field, the ground state
antiferromagnetic configuration of Eu moments transforms into a
ferromagnetic structure with moments along the applied field direction. The
magnetic phase diagram for Eu magnetic sublattice in EuFe2As2 is presented. A
considerable strain (0.9%) is induced by the magnetic field, caused by
the realignment of the twinning structure. Furthermore, the realignment of the
twinning structure is found to be reversible with the rebound of magnetic
field, which suggested the existence of magnetic shape-memory effect. The Eu
moment ordering exhibits close relationship with the twinning structure. We
argue that the Zeeman energy in combined with magnetic anisotropy energy is
responsible for the observed spin-lattice coupling.Comment: 5 pages, 4 figure
Synthesizing non-natural parts from natural genomic template
<p>Abstract</p> <p>Background</p> <p>The current knowledge of genes and proteins comes from 'naturally designed' coding and non-coding regions. It would be interesting to move beyond natural boundaries and make user-defined parts. To explore this possibility we made six non-natural proteins in <it>E. coli</it>. We also studied their potential tertiary structure and phenotypic outcomes.</p> <p>Results</p> <p>The chosen intergenic sequences were amplified and expressed using pBAD 202/D-TOPO vector. All six proteins showed significantly low similarity to the known proteins in the NCBI protein database. The protein expression was confirmed through Western blot. The endogenous expression of one of the proteins resulted in the cell growth inhibition. The growth inhibition was completely rescued by culturing cells in the inducer-free medium. Computational structure prediction suggests globular tertiary structure for two of the six non-natural proteins synthesized.</p> <p>Conclusion</p> <p>To our best knowledge, this is the first study that demonstrates artificial synthesis of non-natural proteins from existing genomic template, their potential tertiary structure and phenotypic outcome. The work presented in this paper opens up a new avenue of investigating fundamental biology. Our approach can also be used to synthesize large numbers of non-natural RNA and protein parts for useful applications.</p
Band structure model of magnetic coupling in semiconductors
We present a unified band structure model to explain magnetic ordering in
Mn-doped semiconductors. This model is based on the - and - level
repulsions between the Mn ions and host elements and can successfully explain
magnetic ordering observed in all Mn doped II-VI and III-V semiconductors such
as CdTe, GaAs, ZnO, and GaN. This model, therefore, provides a simple guideline
for future band structure engineering of magnetic semiconductors.Comment: 4+ pages, 5 figure
Coalgebraic Geometric Logic
Using the theory of coalgebra, we introduce a uniform framework for adding modalities to the language of propositional geometric logic. Models for this logic are based on coalgebras for an endofunctor T on some full subcategory of the category Top of topological spaces and continuous functions. We compare the notions of modal equivalence, behavioural equivalence and bisimulation on the resulting class of models, and we provide a final object for the corresponding category. Furthermore, we specify a method of lifting an endofunctor on Set, accompanied by a collection of predicate liftings, to an endofunctor on the category of topological spaces
Cytological and transcript analyses reveal fat and lazy persister-like bacilli in tuberculous sputum
As nonreplicating tubercle bacilli are tolerant to the cidal action of antibiotics and resistant to multiple stresses, identification of this persister-like population of tubercle bacilli in sputum presents exciting and tractable new opportunities to investigate both responses to chemotherapy and the transmission of tuberculosis
Nonlinear Insulator in Complex Oxides
The insulating state is one of the most basic electronic phases in condensed
matter. This state is characterised by an energy gap for electronic excitations
that makes an insulator electrically inert at low energy. However, for complex
oxides, the very concept of an insulator must be re-examined. Complex oxides
behave differently from conventional insulators such as SiO2, on which the
entire semiconductor industry is based, because of the presence of multiple
defect levels within their band gap. As the semiconductor industry is moving to
such oxides for high-dielectric (high-k) materials, we need to truly understand
the insulating properties of these oxides under various electric field
excitations. Here we report a new class of material called nonlinear insulators
that exhibits a reversible electric-field-induced metal-insulator transition.
We demonstrate this behaviour for an insulating LaAlO3 thin film in a
metal/LaAlO3/Nb-SrTiO3 heterostructure. Reproducible transitions were observed
between a low-resistance metallic state and a high-resistance non-metallic
state when applying suitable voltages. Our experimental results exclude the
possibility that diffusion of the metal electrodes or oxygen vacancies into the
LaAlO3 layer is occurring. Instead, the phenomenon is attributed to the
formation of a quasi-conduction band (QCB) in the defect states of LaAlO3 that
forms a continuum state with the conduction band of the Nb-SrTiO3. Once this
continuum (metallic) state is formed, the state remains stable even when the
voltage bias is turned off. An opposing voltage is required to deplete the
charges from the defect states. Our ability to manipulate and control these
defect states and, thus, the nonlinear insulating properties of complex oxides
will open up a new path to develop novel devices.Comment: 23 pages, 4 figure
Pressure dependence of phonon modes across the tetragonal to collapsed tetragonal phase transition in CaFe2As2
The pressure dependence of a large number of phonon modes in CaFe2As2 with
energies covering the full range of the phonon spectrum has been studied using
inelastic x-ray and neutron scattering. The observed phonon frequency changes
are in general rather small despite the sizable changes of the lattice
parameters at the phase transition. This indicates that the bonding properties
are not profoundly altered by the phase transition. The transverse acoustic
phonons propagating along the c-direction are an exception because they stiffen
very significantly in response to the large contraction of the c-axis. The
lattice parameters are found to change significantly as a function of pressure
before, during and after the first-order phase transition. However, the
frequencies change nearly uniformly with the change in the lattice parameters
due to pressure, with no regard specifically to the first-order phase
transition. Density functional theory describes the frequencies in both the
zero pressure and in the collapsed phase in a satisfactory way if based on the
respective crystal structures
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