Concentration Dependence of Superconductivity and Order-Disorder Transition in the Hexagonal Rubidium Tungsten Bronze RbxWO3. Interfacial and bulk properties

We revisited the problem of the stability of the superconducting state in RbxWO3 and identified the main causes of the contradictory data previously published. We have shown that the ordering of the Rb vacancies in the nonstoichiometric compounds have a major detrimental effect on the superconducting temperature Tc.The order-disorder transition is first order only near x = 0.25, where it cannot be quenched effectively and Tc is reduced below 1K. We found that the high Tc's which were sometimes deduced from resistivity measurements, and attributed to compounds with .25 < x < .30, are to be ascribed to interfacial superconductivity which generates spectacular non-linear effects. We also clarified the effect of acid etching and set more precisely the low-rubidium-content boundary of the hexagonal phase.This work makes clear that Tc would increase continuously (from 2 K to 5.5 K) as we approach this boundary (x = 0.20), if no ordering would take place - as its is approximately the case in CsxWO3. This behaviour is reminiscent of the tetragonal tungsten bronze NaxWO3 and asks the same question : what mechanism is responsible for this large increase of Tc despite the considerable associated reduction of the electron density of state ? By reviewing the other available data on these bronzes we conclude that the theoretical models which are able to answer this question are probably those where the instability of the lattice plays a major role and, particularly, the model which call upon local structural excitations (LSE), associated with the missing alkali atoms.Comment: To be published in Physical Review

BONDING IN HYDROGENATED AMORPHOUS SILICON

A model for the H distribution and bonding structure in a-Si : H is presented based on new NMR and ir absorption data. The model is capable of explaining previous discrepancies in the interpretation of the ir absorption data without including oscillator strengths which vary with H density. Three types of H distributions are considered. First, the typical Si-H2 form which has a stretching mode at 2100 cm-1 and a bending mode at 890 cm-1 is included. Additionally, two distributions of Si-H bonds are included ; a dilute, random distribution which has an ir stretching mode at 2000 cm-1 and a clustered form which has its ir stretching mode near 2090 cm-l. Local H densities and sample wide average H densities for these two forms of Si-H bonds are determined from the NMR spectrum. This NMR data is correlated with the ir absorption data to calculate a fixed oscillator strength for each of the three types of H distribution

Photoconductivity in single crystal carbazole

AC photoconductivity of single crystal carbazole has been studied a photoconductive response lag was observed with respect to the optical absorption spectrum. Spectral distribution of the photoconductivity is found to be dependent on the geometrical arrangements of the electrodes

Preparation of thin film solar cells under very low pressure conditions. Final report, October 1, 1976--September 30, 1977

In this study the feasibility of fabricating backwall Schottky barrier polycrystalline solar cells under ultra-high vacuum conditions of 1 x 10/sup -10/ torr (N/sub 2/) was investigated. Thin films of electron beam vaporized silicon were deposited on cleaned metal substrates of tungsten, tantalum and hafnium. Mass spectra from the quadrapole residual gas analyzer were used to determine the partial pressure of peak heights of 13 residual gases during each processing step. During separate silicon depositions, the substrate temperature was varied between 400 and 750/sup 0/C and deposition rates between 20 and 750 A/min were used. Surface contamination and metal diffusion were monitored by in situ Auger electron spectrometry before and after cleaning, deposition and annealing. Auger depth profiling, x-ray analysis, and SEM in the topographic and channeling modes, were utilized to characterize the samples with respect to silicon-metal boundary layer, interdiffusion, silicide formation and grain size of silicon. The clean metal surface was found to enhance thin film silicide growth. Fine grain silicon films were obtained for all samples that were not completely converted to a metallic silicide. Tungsten, tantalum and hafnium were found to form silicides at temperatures as low as 600/sup 0/C