Carrier relaxation, pseudogap, and superconducting gap in high-Tc cuprates: A Raman scattering study

We describe results of electronic Raman-scattering experiments in differently doped single crystals of Y-123 and Bi-2212. The comparison of AF insulating and metallic samples suggests that at least the low-energy part of the spectra originates predominantly from excitations of free carriers. We therefore propose an analysis of the data in terms of a memory function approach. Dynamical scattering rates and mass-enhancement factors for the carriers are obtained. In B2g symmetry the Raman data compare well to the results obtained from ordinary and optical transport. For underdoped materials the dc scattering rates in B1g symmetry become temperature independent and considerably larger than in B2g symmetry. This increasing anisotropy is accompanied by a loss of spectral weight in B2g symmetry in the range between the superconducting transition at Tc and a characteristic temperature T* of order room temperature which compares well with the pseudogap temperature found in other experiments. The energy range affected by the pseudogap is doping and temperature independent. The integrated spectral loss is approximately 25% in underdoped samples and becomes much weaker towards higher carrier concentration. In underdoped samples, superconductivity related features in the spectra can be observed only in B2g symmetry. The peak frequencies scale with Tc. We do not find a direct relation between the pseudogap and the superconducting gap.Comment: RevTeX, 21 pages, 24 gif figures. For PostScript with embedded eps figures, see http://www.wmi.badw-muenchen.de/~opel/k2.htm

Thermodynamic Factors Affecting UC_1-xN_xIrradiation and Synthesis

There is interest in attempting to synthesize nearly pure uranium nitride (UN) kernels for high-temperature gas-cooled reactor (HTGR) fuel. Because the proposed process involves carbothermic conversion of a urania-carbon mixture in nitrogen and because there is a complete ideal solution of uranium carbide (UC) and UN, which is written as UC{sub 1-x}N{sub x}, the practical value of x for fuel irradiation needs to be determined. Insight is to be gained by relevant thermodynamic calculations of carbide-nitride equilibria for the fuel and fission product systems. The equilibria are readily compared on the nitrogen-based Ellingham diagram, which, in turn, provides guidance in interpreting past irradiations and in synthesis of the UC{sub 1-x}N{sub x} kernels

Chemical thermodynamic representations of <PuO/sub 2-x> and <U/sub 1-z/Pu/sub z/O/sub w/>

All available oxygen potential-temperature-composition data for the calcium fluorite-structure <PuO/sub 2-x/> phase were retrieved from the literature and utilized in the development of a binary solid solution representation of the phase. The data and phase relations are found to be best described by a solution of (Pu/sub 4/3/O/sub 2/) and (PuO/sub 2/) with a temperature dependent interaction energy. The fluorite-structure <U/sub 1-z/Pu/sub z/O/sub w/> is assumed to be represented by a combination of the binaries <PuO/sub 2-x/> and <UO/sub 2+-x/>, and thus treated as a solution of (Pu/sub 4/3/O/sub 2/), (PuO/sub 2/), (UO/sub 2/), and either (U/sub 2/O/sub 4/./sub 5/) or (U/sub 3/O/sub 7/). The resulting equations well reproduce the large amount of oxygen potential-temperature-composition data for the mixed oxide system, all of which were also retrieved from the literature. These models are the first that appear to display the appropriate oxygen potential-temperature-composition and phase relation behavior over the entire range of existence for the phases. 39 refs., 10 figs., 3 tabs

Analysis of chemical failure of coated UOsub2sub 2 and other oxide fuels in the high-temperature gas-cooled reactor

The thermodynamic data for unirradiated dioxides in the U- ThPu-O system are briefly reviewed. Calculations of oxygen release during fission were made and were found to agree qualitatively with experimental measurements. Theories were developed for prediction of mass transport rates in oxide particles; these included control by either CO-- CO/sub 2/ diffusion, reactions such as disproportionation of carbon monoxide, or solid-state diffusion through the kernel. Methods were devised for the determination of the general type of rate- controlling mechanism from the quantitative analysis of inreactor and out-of- reactor experiments. Application of these methods to an in-pile experiment indicated that CO-- CO/sub 2/ diffusion did not control the rate of UO/sub 2/ or (Th,U)O/sub 2/ migration in BISO particles; instead, control by solid-state diffusion was indicated. (54 references) (auth