Percolative phase separation induced by nonuniformly distributed excess oxygens

The zero-field $^{139}$La and $^{55}$Mn nuclear magnetic resonances were studied in $\rm La_{0.8}Ca_{0.2}MnO_{3+\delta}$ with different oxygen stoichiometry $\delta$. The signal intensity, peak frequency and line broadening of the $^{139}$La NMR spectrum show that excess oxygens have a tendency to concentrate and establish local ferromagnetic ordering around themselves. These connect the previously existed ferromagnetic clusters embedded in the antiferromagnetic host, resulting in percolative conduction paths. This phase separation is not a charge segregation type, but a electroneutral type. The magnetoresistance peak at the temperature where percolative paths start to form provides a direct evidence that phase separation is one source of colossal magnetoresistance effect.Comment: 4 pages, 5 figure

Measurement of the local Jahn-Teller distortion in LaMnO_3.006

The atomic pair distribution function (PDF) of stoichiometric LaMnO_3 has been measured. This has been fit with a structural model to extract the local Jahn-Teller distortion for an ideal Mn(3+)O_6 octahedron. These results are compared to Rietveld refinements of the same data which give the average structure. Since the local structure is being measured in the PDF there is no assumption of long-range orbital order and the real, local, Jahn-Teller distortion is measured directly. We find good agreement both with published crystallographic results and our own Rietveld refinements suggesting that in an accurately stoichiometric material there is long range orbital order as expected. The local Jahn-Teller distortion has 2 short, 2 medium and 2 long bonds.Comment: 5 pages, 3 postscript figures, minor change

Exchange coupling in CaMnO3_3 and LaMnO3_3: configuration interaction and the coupling mechanism

The equilibrium structure and exchange constants of CaMnO$_3$ and LaMnO$_3$ have been investigated using total energy unrestricted Hartree-Fock (UHF) and localised orbital configuration interaction (CI) calculations on the bulk compounds and Mn$_2$O$_{11}^{14-}$ and Mn$_2$O$_{11}^{16-}$ clusters. The predicted structure and exchange constants for CaMnO$_3$ are in reasonable agreement with estimates based on its N\'eel temperature. A series of calculations on LaMnO$_3$ in the cubic perovskite structure shows that a Hamiltonian with independent orbital ordering and exchange terms accounts for the total energies of cubic LaMnO$_3$ with various spin and orbital orderings. Computed exchange constants depend on orbital ordering. UHF calculations tend to underestimate exchange constants in LaMnO$_3$, but have the correct sign when compared with values obtained by neutron scattering; exchange constants obtained from CI calculations are in good agreement with neutron scattering data provided the Madelung potential of the cluster is appropriate. Cluster CI calculations reveal a strong dependence of exchange constants on Mn d e$_g$ orbital populations in both compounds. CI wave functions are analysed in order to determine which exchange processes are important in exchange coupling in CaMnO$_3$ and LaMnO$_3$.Comment: 25 pages and 9 postscript figure

Estimates of electronic interaction parameters for LaMMO3_3 compounds (MM=Ti-Ni) from ab-initio approaches

We have analyzed the ab-initio local density approximation band structure calculations for the family of perovskite oxides, La$M$O$_3$ with $M$=Ti-Ni within a parametrized nearest neighbor tight-binding model and extracted various interaction strengths. We study the systematics in these interaction parameters across the transition metal series and discuss the relevance of these in a many-body description of these oxides. The results obtained here compare well with estimates of these parameters obtained via analysis of electron spectroscopic results in conjunction with the Anderson impurity model. The dependence of the hopping interaction strength, t, is found to be approximately $r^{-3}$.Comment: 18 pages; 1 tex file+9 postscript files (appeared in Phys Rev B Oct 15,1996

Minimizing the Dangers of Air Pollution Using Alternative Facts: A Science Museum Case Study

A science museum exhibition about human health contains an exhibit that minimizes health impacts of air pollution. Relevant details, such as the full range of health risks; fossil fuel combustion; air quality statutes (and the local electrical utility’s violations of these statues), are omitted, while end users of electricity are blamed. The exhibit accomplishes this, not through outright falsification, but through selected “alternative facts” that change the focus and imply misleading alternate explanations. Using two classical rhetorical concepts (the practical syllogism and the enthymeme) allows for the surfacing of missing evidence and unstated directives underlying multimodal rhetoric. By stating multimedia arguments syllogistically, a technique is proposed for revealing hidden political sub-texts from beneath a putatively disinterested presentation of facts. The piece should be of interest to researchers, message designers and policy makers interested in the rhetoric of science, ecology, health and museums

COVALENCY EFFECTS IN MAGNETIC INTERACTIONS

On présente une revue des interactions magnétiques dans le but de dégager les informations sur la covalence qui peuvent en être déduites. Les interactions hyperfines sur des ligands, les champs hyperfins dans les métaux mesurés par spectrométrie Mössbauer et diffraction neutronique sont présentées en détail, en insistant sur la dernière approche qui est la plus productive en ce domaine. Les informations déjà recueillies sont confrontées aux connaissances en chimie et on essaie de dégager les axes intéressants pour de futures études.Magnetic interactions, categorised into ligand based, metal based and neutron diffraction, are reviewed with regard to the information they give on covalency. The ligand hyperfine interaction, the metal hyperfine field measured by the Mössbauer effect and neutron diffraction measurements are mentioned in some detail, with particular emphasis on the latter as being the most productive experimental technique in this field. The information already gathered is assessed with reference to chemical knowledge, and useful avenues for future investigation are suggested