Direct observation of the influence of the As-Fe-As angle on the Tc of superconducting SmFeAsO1−x_{1-x}Fx_{x}

The electrical resistivity, crystalline structure and electronic properties calculated from the experimentally measured atomic positions of the compound SmFeAsO$_{0.81}$F$_{0.19}$ have been studied up to pressures ~20GPa. The correlation between the pressure dependence of the superconducting transition temperature (Tc) and crystallographic parameters on the same sample shows clearly that a regular FeAs$_{4}$ tetrahedron maximizes Tc, through optimization of carrier transfer to the FeAs planes as indicated by the evolution of the electronic band structures.Comment: 15pages, 4 figure

Determining Pair Interactions from Structural Correlations

We examine metastable configurations of a two-dimensional system of interacting particles on a quenched random potential landscape and ask how the configurational pair correlation function is related to the particle interactions and the statistical properties of the potential landscape. Understanding this relation facilitates quantitative studies of magnetic flux line interactions in type II superconductors, using structural information available from Lorentz microscope images or Bitter decorations. Previous work by some of us supported the conjecture that the relationship between pair correlations and interactions in pinned flux line ensembles is analogous to the corresponding relationship in the theory of simple liquids. The present paper aims at a more thorough understanding of this relation. We report the results of numerical simulations and present a theory for the low density behavior of the pair correlation function which agrees well with our simulations and captures features observed in experiments. In particular, we find that the resulting description goes beyond the conjectured classical liquid type relation and we remark on the differences.Comment: 7 pages, 6 figures. See also http://rainbow.uchicago.edu/~grier

First report of Rice stripe necrosis virus infecting rice in Benin

International audienc

Photon Correlation Spectroscopy and Applications

Contains reports on two research projects.National Science Foundation (Grant DMR-84- 18718)Wan-Yuan Company, Beijing, Peoples Republic of China Contrac

High temperature superconductivity (Tc onset at 34K) in the high pressure orthorhombic phase of FeSe

We have studied the structural and superconducting properties of tetragonal FeSe under pressures up to 26GPa using synchrotron radiation and diamond anvil cells. The bulk modulus of the tetragonal phase is 28.5(3)GPa, much smaller than the rest of Fe based superconductors. At 12GPa we observe a phase transition from the tetragonal to an orthorhombic symmetry. The high pressure orthorhombic phase has a higher Tc reaching 34K at 22GPa.Comment: 15 pages, 4 figure