Shear modulus and plasticity of a driven charge density wave

We have probed the effects of transverse variations in pinning strength on charge-density-wave (CDW) structure in NbSe3 by x-ray micro-beam diffraction. In ribbonlike crystals having a large longitudinal step in thickness, the CDW first depins on the thick side of the step, causing rotations of the CDW wave vector. By measuring these rotations as a function of position and electric field, the corresponding shear strains are determined, allowing the CDW's shear modulus to be estimated. These results demonstrate the usefulness of x-ray microdiffraction as a tool in studying collective dynamics in electronic crystals.

Microfabricated mounts for high-throughput macromolecular cryocrystallography

A new approach is described for mounting microcrystals of biological macromolecules for cryocrystallography. The sample mounts are prepared by patterning thin polyimide films by standard microfabrication techniques. The patterned structures contain a small hole for the crystal connected to a larger holeviaa drainage channel, allowing removal of excess liquid and easier manipulation in viscous solutions. These polyimide structures are wrapped around small metal rods. The resulting curvature increases their rigidity and allows a convenient scoop-like action in retrieving crystals. The polyimide contributes minimally to X-ray background and absorption, and can be treated to obtain desired hydrophobicity or hydrophilicity. The new mounts are fully compatible with existing automated sample-handling hardware for cryocrystallography. Their potential advantages include completely reproducible sample hole sizes to below 10 µm; accurate and reproducible sample positioning and good sample-to-mount contrast, simplifying alignment; more convenient manipulation of small crystals; easier removal of excess liquid and reduced background scatter; reduced thermal mass and more rapid flash-cooling; and easy design customization and mass production. They are especially well suited to data collection from the smaller crystals produced in high-throughput crystallization trials, and are suitable for automated crystal retrieval. They should be more generally useful for X-ray data collection from small organic and inorganic crystals of all types.

Structure of interfacial liquids: X-ray scattering studies

We have used synchrotron x rays to study three different liquids near solid-liquid interfaces. For either ultrathin (45-90 A) or thick ( approximately 5000 A) liquid films on silicon substrates, we find (on the basis of diffraction peaks or specular reflectivity data) that the molecules form 3-6 layers at the interface, with plane spacings close to the molecular dimensions. Rough surfaces and/or impurities reduce the density oscillation amplitudes. Making the liquid film very thin does not observably enhance the effect, which implies that layering is present even at an isolated interface (i.e., in a semi-infinite liquid). On the other hand, predeposited impurities diffuse away from the interface more easily if the liquid films are thick. The liquids studied are nonconducting, nonpolar, and nonreactive; the molecules are roughly spherical; and our substrate surface has no lateral structure. Thus our observations should apply to any liquid near a hard wall.

In situ and interrupted-growth studies of the self-assembly of octadecyltrichlorosilane monolayers

We have examined the self-assembly process of octadecyltrichlorosilane on silicon using x-ray reflectivity. By comparing the commonly used "interrupted-growth" characterization technique with results obtained in situ, we have determined that quenching the growth and then rinsing and drying the sample introduces free area into the film, presumably by removal of non-cross-linked (physisorbed) molecules. Reintroduction of a quenched and rinsed film to solvent does not restore the thickness of the film to its previous value. We have also performed in situ growth studies over a range of concentrations. For all concentrations, we observe growth of islands of vertical molecules. The growth follows Langmuir kinetics, except at short times for low concentration solutions.