Ab initio simulations of liquid NaSn alloys: Zintl anions and network formation

Using the Car-Parrinello technique, ab initio molecular dynamics simulations are performed for liquid NaSn alloys in five different compositions (20, 40, 50, 57 and 80 % sodium). The obtained structure factors agree well with the data from neutron scattering experiments. The measured prepeak in the structure factor is reproduced qualitatively for most compositions. The calculated and measured positions of all peaks show the same trend as function of the composition.\\ The dynamic simulations also yield information about the formation and stability of Sn$_4$ clusters (Zintl anions) in the liquid. In our simulations of compositions with 50 and 57 % sodium we observe the formation of networks of tin atoms. Thus, isolated tin clusters are not stable in such liquids. For the composition with 20 % tin only isolated atoms or dimers of tin appear, ``octet compounds'' of one Sn atom surrounded by 4 Na atoms are not observed.Comment: 12 pages, Latex, 3 Figures on reques

Bonding, Moment Formation, and Magnetic Interactions in Ca14MnBi11 and Ba14MnBi11

The ``14-1-11'' phase compounds based on magnetic Mn ions and typified by Ca14MnBi11 and Ba14MnBi11 show unusual magnetic behavior, but the large number (104) of atoms in the primitive cell has precluded any previous full electronic structure study. Using an efficient, local orbital based method within the local spin density approximation to study the electronic structure, we find a gap between a bonding valence band complex and an antibonding conduction band continuum. The bonding bands lack one electron per formula unit of being filled, making them low carrier density p-type metals. The hole resides in the MnBi4 tetrahedral unit and partially compensates the high spin d^5 Mn moment, leaving a net spin near 4 \mu_B that is consistent with experiment. These manganites are composed of two disjoint but interpenetrating `jungle gym' networks of spin 4/2 MnBi4^{9-} units with ferromagnetic interactions within the same network, and weaker couplings between the networks whose sign and magnitude is sensitive to materials parameters. Ca14MnBi11 is calculated to be ferromagnetic as observed, while for Ba14MnBi11 (which is antiferromagnetic) the ferro- and antiferromagnetic states are calculated to be essentially degenerate. The band structure of the ferromagnetic states is very close to half metallic.Comment: 17 pages, containing 10 postscript figures and 5 tables. Two additional figures (Fig.8 and 11 of the paper) are provided in JPG format in separate files. Submitted to Phys. Rev. B on September 20th 200

Vectors of babesiosis

Babesiosis, caused by piroplasmid protozoans in the genus Babesia, is arguably the most important vector-borne disease of livestock and companion animals and is growing in importance as a zoonosis. Ixodid ticks were identified as vectors more than a hundred years ago, but the particular tick species transmitting some significant pathogens are still unknown. Moreover, it is only recently that the complexity of the pathogen-tick relationship has been revealed as a result of studies enabled by gene expression and RNA interference methodology. In this article, we provide details of demonstrated and incriminated vectors, maps of the current knowledge of vector distribution, a summary of established features of the pathogen life cycle in the vector, and an outline of molecular research on pathogen-tick relationships. The article concludes with a discussion of vector ecology and disease epidemiology in a global-change context and with suggestions for future research

An Experimental and Theoretical Study of the Variation of 4f Hybridization Across the La1-xCexIn3 Series

Crystal structures of a series of La1-xCexIn3 (x = 0.02, 0.2, 0.5, or 0.8) intermetallic compounds have been investigated by both neutron and X-ray diffraction, and their physical properties have been characterized by magnetic susceptibility and specific heat measurements. Our results emphasize atypical atomic displacement parameters (ADP) for the In and the rare-earth sites. Depending on the x value, the In ADP presents either an "ellipsoidal" elongation (La-rich compounds) or a "butterfly-like" distortion (Ce-rich compounds). These deformations have been understood by theoretical techniques based on the band theory and are the result of hybridization between conduction electrons and 4f-electrons.Comment: 7 pages, 8 figure

Magnetoelectric ordering of BiFeO3 from the perspective of crystal chemistry

In this paper we examine the role of crystal chemistry factors in creating conditions for formation of magnetoelectric ordering in BiFeO3. It is generally accepted that the main reason of the ferroelectric distortion in BiFeO3 is concerned with a stereochemical activity of the Bi lone pair. However, the lone pair is stereochemically active in the paraelectric orthorhombic beta-phase as well. We demonstrate that a crucial role in emerging of phase transitions of the metal-insulator, paraelectric-ferroelectric and magnetic disorder-order types belongs to the change of the degree of the lone pair stereochemical activity - its consecutive increase with the temperature decrease. Using the structural data, we calculated the sign and strength of magnetic couplings in BiFeO3 in the range from 945 C down to 25 C and found the couplings, which undergo the antiferromagnetic-ferromagnetic transition with the temperature decrease and give rise to the antiferromagnetic ordering and its delay in regard to temperature, as compared to the ferroelectric ordering. We discuss the reasons of emerging of the spatially modulated spin structure and its suppression by doping with La3+.Comment: 18 pages, 5 figures, 3 table

Diverse tick-borne microorganisms identified in free-living ungulates in Slovakia

Background: Free-living ungulates are hosts of ixodid ticks and reservoirs of tick-borne microorganisms in central Europe and many regions around the world. Tissue samples and engorged ticks were obtained from roe deer, red deer, fallow deer, mouflon, and wild boar hunted in deciduous forests of south-western Slovakia. DNA isolated from these samples was screened for the presence of tick-borne microorganisms by PCR-based methods. Results: Ticks were found to infest all examined ungulate species. The principal infesting tick was Ixodes ricinus, identified on 90.4% of wildlife, and included all developmental stages. Larvae and nymphs of Haemaphysalis concinna were feeding on 9.6% of wildlife. Two specimens of Dermacentor reticulatus were also identified. Ungulates were positive for A. phagocytophilum and Theileria spp. Anaplasma phagocytophilum was found to infect 96.1% of cervids, 88.9% of mouflon, and 28.2% of wild boar, whereas Theileria spp. was detected only in cervids (94.6%). Importantly, a high rate of cervids (89%) showed mixed infections with both these microorganisms. In addition to A. phagocytophilum and Theileria spp., Rickettsia helvetica, R. monacensis, unidentified Rickettsia sp., Coxiella burnetii, "Candidatus Neoehrlichia mikurensis", Borrelia burgdorferi (s.l.) and Babesia venatorum were identified in engorged I. ricinus. Furthermore, A. phagocytophilum, Babesia spp. and Theileria spp. were detected in engorged H. concinna. Analysis of 16S rRNA and groEL gene sequences revealed the presence of five and two A. phagocytophilum variants, respectively, among which sequences identified in wild boar showed identity to the sequence of the causative agent of human granulocytic anaplasmosis (HGA). Phylogenetic analysis of Theileria 18S rRNA gene sequences amplified from cervids and engorged I. ricinus ticks segregated jointly with sequences of T. capreoli isolates into a moderately supported monophyletic clade. Conclusions: The findings indicate that free-living ungulates are reservoirs for A. phagocytophilum and Theileria spp. and engorged ixodid ticks attached to ungulates are good sentinels for the presence of agents of public and veterinary concern. Further analyses of the A. phagocytophilum genetic variants and Theileria species and their associations with vector ticks and free-living ungulates are required.Fil: Kazimírová, Mária. Slovak Academy of Sciences. Institute of Zoology; EslovaquiaFil: Hamšíková, Zuzana. Slovak Academy of Sciences. Institute of Zoology; EslovaquiaFil: Spitalská, Eva. Slovak Academy of Sciences. Institute of Virology. Biomedical Research Center,; EslovaquiaFil: Minichová, Lenka. Slovak Academy of Sciences. Institute of Virology. Biomedical Research Center,; EslovaquiaFil: Mahríková, Lenka. Slovak Academy of Sciences. Institute of Zoology; EslovaquiaFil: Caban, Radoslav. Široká ; EslovaquiaFil: Sprong, Hein. National Institute for Public Health and Environment.Laboratory for Zoonoses and Environmental Microbiology; Países BajosFil: Fonville, Manoj. National Institute for Public Health and Environment.Laboratory for Zoonoses and Environmental Microbiology; Países BajosFil: Schnittger, Leonhard. Instituto Nacional de Tecnología Agropecuaria. Centro de Investigación en Ciencias Veterinarias y Agronómicas. Instituto de Patobiología; Argentina. Consejo Nacional de Investigaciones Científicas y Técnicas; ArgentinaFil: Kocianová, Elena. Slovak Academy of Sciences. Institute of Virology. Biomedical Research Center,; Eslovaqui

Rings and rigidity transitions in network glasses

Three elastic phases of covalent networks, (I) floppy, (II) isostatically rigid and (III) stressed-rigid have now been identified in glasses at specific degrees of cross-linking (or chemical composition) both in theory and experiments. Here we use size-increasing cluster combinatorics and constraint counting algorithms to study analytically possible consequences of self-organization. In the presence of small rings that can be locally I, II or III, we obtain two transitions instead of the previously reported single percolative transition at the mean coordination number $\bar r=2.4$, one from a floppy to an isostatic rigid phase, and a second one from an isostatic to a stressed rigid phase. The width of the intermediate phase $~ \bar r$ and the order of the phase transitions depend on the nature of medium range order (relative ring fractions). We compare the results to the Group IV chalcogenides, such as Ge-Se and Si-Se, for which evidence of an intermediate phase has been obtained, and for which estimates of ring fractions can be made from structures of high T crystalline phases.Comment: 29 pages, revtex, 7 eps figure

Electronic Structure of the Complex Hydride NaAlH4

Density functional calculations of the electronic structure of the complex hydride NaAlH4 and the reference systems NaH and AlH3 are reported. We find a substantially ionic electronic structure for NaAlH4, which emphasizes the importance of solid state effects in this material. The relaxed hydrogen positions in NaAlH4 are in good agreement with recent experiment. The electronic structure of AlH3 is also ionic. Implications for the binding of complex hydrides are discussed.Comment: 4 pages, 5 figure