No abstract

Because of possible effects from (110) twin planes and a-b anisotropy in YBa&Cu307 z, measurements on untwinned crystals are desirable. We have measured the Hall effect and resistivity simultaneously in untwinned single-crystal YBa2Cu307 -$ with a superconducting transition temperature T, =93 K, with the magnetic field along the c axis. Below T"the measured Hall resistivity reverses sign, becoming negative over a range of temperature and magnetic field, and the location and magnitude of the minimum is similar to that measured previously by others on twinned samples. These results show that the negative Hall resistivity is not caused by twin planes inducing guided vortex motion or vortex pinning. In addition, the exponent characterizing a scaling relation of the longitudinal to the Hall resistivity has a value consistent with that measured on thin films.

The technique of solution aerosol thermolysis (SAT) for the production of ceramic electrolytic films suitable for solid oxide fuels cells was investigated. The research has focused on the optimization of process parameters and characterization of the obtained films by means of X-ray diffractometry and scanning electron microscopy/energy-dispersive spectroscopy. Dense films of gadolinia-stabilized ceria of uniform thickness have been successfully produced on substrates consisting of dense disks of yttria-stabilized zirconia by SAT using nitrate salts of the precursors dissolved in an ethanol-water solvent. Substrate temperature is an important parameter and in this system the best initial values identified were of the order of 400-420°C. The interplay between initial substrate temperature, solution flow rate, and postdeposition temperature is important for a good-quality film.

We prepared silicon that exhibits green-to-red rainbow luminescence along the sample, reflecting gradients in the crystallite size. The excitation has a size-dependent feature, riding a smooth bulklike continuum. For the size-dependent contribution, we measured absorption band edges of 3.75 and 3.0–3.25 eV at the meniscus and opposite end. Excitation in the meniscus, monitored at the edge of the blue emission, isolates ultrasmall sizes, with “excitation coefficients” quadratic with energy, and emission encompassing much of the visible spectrum. Results are discussed in terms of quantum-confinement-induced restructuring of the diamondlike bonds to form radiative Si–Si surface states.

We have performed time-resolved photoluminescence measurements in the submicrosecond to microsecond time regime on porous silicon samples under several diffusion-based chemical treatments with copper ion solutions that produce varying crystallite surface conditions. Our results for short emission wavelengths emanating from high lying states indicate that Cu acts largely on the population process commencing from the top of the well at short relaxation time scales immediately after excitation, and to less extent on the radiating states in the microsecond regime.