Study of the peak effect phenomenon in single crystals of 2H-NbSe2

The weakly pinned single crystals of the hexagonal 2H-NbSe₂ compound have emerged as prototypes for determining and characterizing the phase boundaries of the possible order-disorder transformations in the vortex matter. We present here a status report based on the ac and dc magnetization measurements of the peak effect phenomenon in three crystals of 2H-NbSe₂, in which the critical current densities vary over two orders of magnitude. We sketch the generic vortex phase diagram of a weakly pinned superconductor, which also utilizes theoretical proposals. We also establish the connection between the metastability effects and pinning.     

Shaping Solid-State Supramolecular Cavities: Chemically Induced Accordionlike Movement of gamma-Zirconium Phosphate Containing Polyethylenoxide Pillars

The hydrophilic oxygen atoms of polyethylenoxide chains inserted as pillars in gamma-zirconium phosphate form hydrogen bonds with the acid groups of the host. As a result the pillars are almost perpendicular to the gamma layers. Upon changing the pH level of the supernatant solution the hydrogen bonds are broken and the pillars become almost perpendicular to the layers (shown schematically). Thus there is a reversible enlargement-shortening of the interlayer space.     

Surface layer self diffusion in icosahedral Al-Pd-Mn quasicrystals

The self diffusion of Mn and Pd in a single grain icosahedral Al_69.9Pd_20.5Mn_9.6 quasicrystal has been determined by low energy ion scattering (LEIS). The diffusion was determined by depositing different elements (Pd, Mn) on the surface and measuring the rate of change in surface composition as a function of temperature by LEIS. The surface composition was monitored over the temperature range of 355-575 K for Mn and 440-745 K for Pd and compared to model calculations to allow the activation energy for diffusion to be determined. Activation energies of 0.20 ± 0.01 eV for Mn and 0.64 ± 0.03 eV for Pd have then been measured for self diffusion in i-Al-Pd-Mn, respectively. No deviation from Arrhenius behavior was detected in the temperature range covered by the present experiments. From the low values of activation energy we propose that this range of diffusion is phason related, reflecting the specific nature of the icosahedral structure.     

Children's Books Reviewed

Maps and Globes Jack Knowlton Hurricane Watch Franklyn M. Branley Sunshine Makes the Seasons Franklyn M. Branley Heart Disease America's No. 1 Killer Dr. Alvin and Virginia B. Silverstein Cuts, Breaks, Bruises and Burns Joanna Cole Auks, Rocks, and the Odd Dinosaur Inside Stories from the Smithsonian Institute by Peggy Thomson The Secret Life of Cosmetics: A Science Experiment Book Vicki Cobb A Children's Zoo Tana Hoban Milk Donald Carrick Wallaby Creek Joyce Powzyk Being a Twin, Having a Twin Maxine Rosenberg     

Crystal and molecular structure of tetraiodo (1,8-di-2-pyridyl-3,6-dithiaoctane-N,S,S′,N′)dimercury(II)

Summary The X-ray crystal structure of the title compound, [Hg₂(C₁₆H₂₀N₂S₂)I₄], has been determined. The crystals are monoclinic (space groupP2₁/c), with cell dimensionsa=15.016(2),b=6.201(3),c=15.273(2) Å,=115.18(1)°;V _o =1287.0(7) ų,M _r =1213.266,Z=4,D _x =6.26mg m^–3, MoK radiation (graphite crystal monochromator, ),(MoK)=336 cm^–1,F(000)=4272, andT=290°K. The final conventionalR factor=0.054, andR _w =0.062 for 2799 observed reflections. The structure was solved by Patterson and Fourier methods and refined by block-diagonal least-squares methods. The coordination around the mercury atoms is approximately tetrahedral, although with considerable distortion, with two iodine atoms and the N and S atoms of one-half of the ligand molecule.