Phonon dispersion of Mo-stabilized γ-U measured using inelastic x-ray scattering.

We have measured the room-temperature phonon spectrum of Mo-stabilized γ-U. The dispersion curves show unusual softening near the H point, q = [1/2, 1/2, 1/2], which may derive from the metastability of the γ-U phase or from strong electron-phonon coupling. Near the zone center, the dispersion curves agree well with theory, though significant differences are observed away from the zone center. The experimental phonon density of states is shifted to higher energy compared to theory and high-temperature neutron scattering. The elastic constants of γ-UMo are similar to those of body-centered cubic elemental metals.

Pressure dependence of Ce valence in CeRhIn5.

We have studied the Ce valence as a function of pressure in CeRhIn5 at 300 K and at 22 K using x-ray absorption spectroscopy in partial fluorescent yield mode. At room temperature, we found no detectable change in Ce valence greater than 0.01 up to a pressure of 5.5 GPa. At 22 K, the valence remains robust against pressure below 6 GPa, in contrast to the predicted valence crossover at P = 2.35 GPa. This work yields an upper limit for the change in Ce-valence and suggests that the critical valence fluctuation scenario, in its current form, is unlikely.

Note: Manganin foil sensor for small uniaxial stress.

We describe a simple manganin foil resistance manometer for uniaxial stress measurements. The manometer functions at low pressures and over a range of temperatures. In this design, no temperature seasoning is necessary although the manometer must be prestressed to the upper end of the desired pressure range. The prestress pressure cannot be increased arbitrarily; irreversibility arising from shear stress limits its range. Attempting larger pressures yields irreproducible resistance measurements.

Anisotropic dependence of superconductivity on uniaxial pressure in CeIrIn5.

We measure the effect of uniaxial pressure on the superconducting transition temperature T_{c} in CeIrIn5. We find a linear change in T_{c} with both a-axis and c-axis pressure, with slopes of 56 and -66 mK/kbar, respectively. By comparing results from doping studies and different types of pressure measurements, we separate the influences of hybridization and dimensionality on T_{c}. We find the true geometric influence, for constant hybridization, is partial differentialT_{c}/ partial differential(c/a)=44 K.

Packing fractions and maximum angles of stability of granular materials.

In two-dimensional rotating drum experiments, we find two separate influences of the packing fraction of a granular heap on its stability. For a fixed grain shape, the stability increases with packing fraction. However, in determining the relative stability of different grain shapes, those with the lowest average packing fractions tend to form the most stable heaps. We also show that only the configuration close to the surface of the pile figures prominently.

Influence of shape on ordering of granular systems in two dimensions.

We investigate ordering properties of two-dimensional granular materials using several shapes created by welding ball bearings together. Ordered domains form much more easily in two than in three dimensions, even when configurations lack long-range order. The onset of ordered domains occurs near a packing density of 0.8, a phenomenon observed previously for disks. One of our shapes, the trapezoid, has packings that remain disordered and near the transition density even after annealing by shaking. Although random packings are unstable for disks and many other shapes in two dimensions, trapezoid packings provide an approach to studying two-dimensional randomness. We also find that the rotational symmetry of a shape is an excellent predictor of how easily it orders, and a potential guide to identifying two-dimensional shapes that remain random after annealing.

The accumulation and assimilation of dimethylselenide by four plant species.

Plants of Agrostis tenuis Sibth., Hordeum vulgare L., Lycopersicon esculentum Mill. and Raphanus sativus L. were grown hydroponically in sealed systems and fumigated with 8 µg m(-3) [(75)Se]-dimethylselenide. The accumulation of (75)Se was measured and the shoot tissues were extracted to examine the products of the (75)Se assimilation. Characteristic differences were observed between species in the accumulation of (75)Se and the transport from shoots to roots. High-voltage electrophoresis and chromatography of extracts made with 80% aqueous ethanol revealed the presence of inorganic selenite as an assimilation product as well as the selenium analogues of glutathione and methionine. Extensive incorporation of (75)Se into protein-bound selenomethionine was observed in all plant species.