Electrostatically driven charge-ordering in Fe₂OBO₃

Charge-ordering is an important phenomenon in conducting metal oxides: it leads to metal-insulator transitions in manganite perovskites (which show `colossal' magnetoresistances), and the Verwey transition in magnetite (in which the material becomes insulating at low temperatures when the conduction electrons freeze into a regular array). Charge-ordered `stripes' are found in some manganites and copper oxide superconductors; in the latter case, dynamic fluctuations of the stripes have been proposed as a mechanism of high-temperature superconductivity. But an important unresolved issue is whether the charge ordering in oxides is driven by electrostatic repulsions between the charges (Wigner crystallization), or by the strains arising from electron-lattice interactions (such as Jahn-Teller distortions) involving different localized electronic states. Here we report measurements on iron oxoborate, Fe2OBO3, that support the electrostatic repulsion charge-ordering mechanism: the system adopts a charge-ordered state below 317 K, in which Fe2+ and Fe3+ ions are equally distributed over structurally distinct Fe sites. In contrast, the isostructural manganese oxoborate, Mn2OBO3, has been previously shown to undergo charge-ordering through Jahn-Teller distortions. We therefore conclude that both mechanisms occur within the same structural arrangement

Reduction of endotoxicity in Bordetella bronchiseptica by lipid A engineering: Characterization of lpxL1 and pagP mutants

Whole-cell vaccines against Gram-negative bacteria commonly display high reactogenicity caused by the endotoxic activity of lipopolysaccharide (LPS), one of the major components of the bacterial outer membrane. Underacylation of the lipid A moiety of LPS has been related with reduced endotoxicity in several Gram-negative species. Here, we evaluated whether the inactivation of two genes encoding lipid A acylases of Bordetella bronchiseptica, i.e. pagP and lpxL1, could be used for the development of less reactogenic vaccines against this pathogen for livestock and companion animals. Inactivation of pagP resulted in the loss of the secondary palmitate chain at position 3' of lipid A, but hardly affected the potency of the LPS to activate the Toll-like receptor 4 (TLR4). Inactivation of lpxL1 resulted in the loss of the secondary 2-hydroxy laurate group present at position 2 of lipid A and, unexpectedly, in the additional loss of the glucosamines that decorate the phosphate groups at positions 1 and 4' and in an increase in LPS molecules carrying O-antigen. The resulting LPS showed greatly reduced potency to activate TLR4 in HEK-Blue reporter cells expressing human or mouse TLR4 as well as in porcine macrophages. Characterization of the lpxL1 mutant revealed many pleiotropic phenotypes, including increased resistance to SDS and rifampicin, increased susceptibility to cationic antimicrobial peptides, decreased auto-aggregation and biofilm formation, and a tendency to decreased infectivity of macrophages, which are all related to the altered LPS structure. We suggest that the lpxL1 mutant will be useful for the generation of safer vaccines

Physical Aspects of Cell Operation and Reliability

This chapter overviews the basic physical effects involved in programming and erasing of Flash memory cells, to provide the background for a deeper understanding of their operation and reliability. In particular, tunnelling and high field transport are treated and the associated phenomena in MOS-FETs and Flash cells are described by means of measurements and simulations. Device degradation induced by charge injection into thin silicon dioxide layers is also briefly discussed