102 research outputs found
Low density approach to the Kondo-lattice model
We propose a new approach to the (ferromagnetic) Kondo-lattice model in the
low density region, where the model is thought to give a reasonable frame work
for manganites with perovskite structure exhibiting the "colossal
magnetoresistance" -effect. Results for the temperature- dependent
quasiparticle density of states are presented. Typical features can be
interpreted in terms of elementary spin-exchange processes between itinerant
conduction electrons and localized moments. The approach is exact in the zero
bandwidth limit for all temperatures and at T=0 for arbitrary bandwidths,
fulfills exact high-energy expansions and reproduces correctly second order
perturbation theory in the exchange coupling.Comment: 11 pages, 7 figures, accepted by PR
Ferromagnetism within the periodic Anderson model: A new approximation scheme
We introduce a new approach to the periodic Anderson model (PAM) that allows
a detailed investigation of the magnetic properties in the Kondo as well as the
intermediate valence regime. Our method is based on an exact mapping of the PAM
onto an effective medium strong-coupling Hubbard model. For the latter, the
so-called spectral density approach (SDA) is rather well motivated since it is
based on exact results in the strong coupling limit. Besides the T=0 phase
diagram, magnetization curves and Curie temperatures are presented and
discussed with help of temperature-dependent quasiparticle densities of state.
In the intermediate valence regime, the hybridization gap plays a major role in
determining the magnetic behaviour. Furthermore, our results indicate that
ferromagnetism in this parameter regime is not induced by an effective
spin-spin interaction between the localized levels mediated by conduction
electrons as it is the case in the Kondo regime. The magnetic ordering is
rather a single band effect within an effective f-band.Comment: 13 pages, 16 figures, Phys. Stat. Sol. in pres
Measurement of the Elastic Constants of Aluminium and Copper Using a Continuous Wave Ultrasonic Technique
Nickel and chromium in wastewater - The Malaysian experience
Heavy metals in natural water bodies pose a serious problem in Malaysia.Ni and Cr have drawn particular attention owing to their prevalence in various industrial effluents. While efforts are being made by industries
to control discharges by recycling the metals, a lack of cost effective technologies appear to be a constraint, This paper discusses these issues in Malaysian and global perspectives
Carrier induced ferromagnetism in concentrated and diluted local-moment systems
For modeling the magnetic properties of concentrated and diluted magnetic
semiconductors, we use the Kondo-lattice model. The magnetic phase diagram is
derived by inspecting the static susceptibility of itinerant band electrons,
which are exchange coupled to localized magnetic moments. It turns out that
rather low band occupations favour a ferromagnetic ordering of the local moment
systems due to an indirect coupling mediated by a spin polarization of the
itinerant charge carriers. The disorder in diluted systems is treated by adding
a CPA-type concept to the theory. For almost all moment concentrations x,
ferromagnetism is possible, however, only for carrier concentrations n
distinctly smaller than x. The charge carrier compensation in real magnetic
semiconductors (in Ga_{1-x}Mn_{x}As by e.g. antisites) seems to be a necessary
condition for getting carrier induced ferromagnetism.Comment: 9 pages (REVTeX), 6 figures, to be published in Phys. Rev.
Ferromagnetism in the Periodic Anderson Model - a Modified Alloy Analogy
We introduce a new aproximation scheme for the periodic Anderson model (PAM).
The modified alloy approximation represents an optimum alloy approximation for
the strong coupling limit, which can be solved within the CPA-formalism.
Zero-temperature and finite-temperature phase diagrams are presented for the
PAM in the intermediate-valence regime. The diversity of magnetic properties
accessible by variation of the system parameters can be studied by means of
quasiparticle densities of states: The conduction band couples either ferro- or
antiferromagneticaly to the f-levels. A finite hybridization is a necessary
precondition for ferromagnetism. However, too strong hybridization generally
suppresses ferromagnetism, but can for certain system parameters also lead to a
semi-metallic state with unusual magnetic properties. By comparing with the
spectral density approximation, the influence of quasiparticle damping can be
examined.Comment: 20 pages, 13 figure
Interplay of magnetic order and Jahn-Teller distortion in a model with strongly correlated electron system
The Hubbard model has been employed successfully to understand many aspects
of correlation driven physical properties, in particular, the magnetic order in
itenerant electron systems. In some systems such as Heusler alloys, manganites
etc., it is known that, in addition to magnetic order, distortion induced by
Jahn-Teller(J-T) effect also exists. In this paper, based on two-fold
degenerate Hubbard model, the influence of magnetic order on J-T distortion is
investigated. The electron correlation is treated using a spectral density
approach and J-T interaction is added to the model. We find that magnetic order
and structural distortion coexist at low temperature for a certain range of
electron correlation strength , J-T coupling strength and band
occupation . At T=0, for a given and , magnetic order is present but
distortion appears only for a larger than a critical value. We also studied
the temperature dependence of lattice strain and magnetization choosing a
close to the critical value.Comment: 12 pages, 5 Figures. Physica- B 405 1701-1705 (2010
Characterization of Pulmonary CYP4B2, Specific Catalyst of Methyl Oxidation of 3-Methylindole
ABSTRACT The selective toxicity of chemicals to lung tissues is predominately mediated by the selective expression of certain pulmonary cytochrome P450 enzymes. This report describes the purification, cloning, and characterization of a unique enzyme, CYP4B2, from goat lung. The purified P450 enzyme was isolated by multistep ion exchange chromatography to electrophoretic homogeneity with an apparent molecular mass of 55,000 Da. Western blotting studies demonstrated that CYP4B enzymes were selectively expressed in lung tissues of rabbits, rats, and mice. Two cDNAs, CYP4B2 and CYP4B2v, were cloned from goat lung tissue. CYP4B2 was predicted to be 511 amino acids and approximately 82% similar to the four known CYP4B1 proteins. Concurrently, a variant of the known human CYP4B1 cDNA, that contained a S207 insertion, was cloned from human lung tissue. The modified recombinant goat CYP4B2 was expressed in Escherichia coli and the enzyme catalyzed the N-hydroxylation of the prototypical substrate 2AF. CYP4B2 preferentially dehydrogenated, rather than hydroxylated, the pneumotoxicant 3-methylindole (3MI) (V max ϭ 4.61 versus 0.83 nmol/nmol of P450/min, respectively). To investigate the relevance of covalent heme binding of CYP4 enzymes in CYP4B2-mediated metabolism of 3MI, a site-directed mutant (CYP4B2/A315E) was evaluated. The mutation had little effect on the V max of either dehydrogenation or hydroxylation but increased the K m , which decreased the catalytic efficiency (V/K) for 3MI. The A315E mutation shifted the absorbance maximum of the enzyme from 448 to 451 nm, suggesting that the electron density of the heme was altered. These results demonstrate that CYP4B2 is highly specific for methyl group oxidation of 3MI, without formation of ring-oxidized metabolites, and seems to be predominately responsible for the highly organ-specific toxicity of 3MI in goats
Detection of very long antisense transcripts by whole transcriptome RNA-Seq analysis of Listeria monocytogenes by semiconductor sequencing technology
The Gram-positive bacterium Listeria monocytogenes is the causative agent of listeriosis, a severe food-borne infection characterised by abortion, septicaemia, or meningoencephalitis. L. monocytogenes causes outbreaks of febrile gastroenteritis and accounts for community-acquired bacterial meningitis in humans. Listeriosis has one of the highest mortality rates (up to 30%) of all food-borne infections. This human pathogenic bacterium is an important model organism for biomedical research to investigate cell-mediated immunity. L. monocytogenes is also one of the best characterised bacterial systems for the molecular analysis of intracellular parasitism. Recently several transcriptomic studies have also made the ubiquitous distributed bacterium as a model to understand mechanisms of gene regulation from the environment to the infected host on the level of mRNA and non-coding RNAs (ncRNAs). We have used semiconductor sequencing technology for RNA-seq to investigate the repertoire of listerial ncRNAs under extra- and intracellular growth conditions. Furthermore, we applied a new bioinformatic analysis pipeline for detection, comparative genomics and structural conservation to identify ncRNAs. With this work, in total, 741 ncRNA locations of potential ncRNA candidates are now known for L. monocytogenes, of which 611 ncRNA candidates were identified by RNA-seq. 441 transcribed ncRNAs have never been described before. Among these, we identified novel long non-coding antisense RNAs with a length of up to 5,400 nt e.g. opposite to genes coding for internalins, methylases or a high-affinity potassium uptake system, namely the kdpABC operon, which were confirmed by qRT-PCR analysis. RNA-seq, comparative genomics and structural conservation of L. monocytogenes ncRNAs illustrate that this human pathogen uses a large number and repertoire of ncRNA including novel long antisense RNAs, which could be important for intracellular survival within the infected eukaryotic host
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