We prepared porous silicon for which the UV reflectance (3.3–6 eV) is nearly eliminated, and exhibits no features at the Si interband bulk transitions 3.3, 4.3, and 5.5 eV. Plating with a thin layer of copper is found to cause recovery of the UV bulk-like crystalline reflectance and interband resonances. This provides evidence that the loss of crystalline absorption is reversible and is not due to a permanent loss in the crystalline structure. This may relate to a recent model in which the optical activity of ultra small nanocrystallites is produced by a new Si–Si crystalline configuration (or phase), distinct from but interconnected to the diamond-like configuration by a potential barrier.

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We use high resolution cross sectional transmission electron microscopy to image the nanostructure of (100) p-type porous Si. A network of pore tracks subdivide the material into nano-islands and nanocrystallites are resolved throughout the material. With distance from the substrate, electron diffraction develops noncrystalline-like diffuse patterns that dominate the coherent scattering in the topmost luminescent layer. Also, with distance from the substrate, crystalline islands evolve such that their size drops to as small as 1 nm in the topmost luminescence material. Although the topmost luminescent layer is very rich in nanocrystallites, it has the strongest diffuse scattering of all regions. This confirms that diffuse scattering is due to size reduction effects rather than to an amorphous state.

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The effect of pressure on the critical temperature T, is studied for YBa,Cu,O,-, compounds prepared under various forming pressures. A systematic decrease in lattice parameters is observed with forming pressure. Resistivity measurements show an increase of the onset temperature T,, with forming pressure.A value of dT,,/dP = (0.070 0.010) x lo-' K/Pa is determined. A new mechanism based on the variation of the interlayer tunneling integral t , is used to analyse the experiment results. L'effet de la pression sur la temperature critique T, est etudiee pour le compose YBa,Cu,O,_, prepare sous une variett de pressions differentes. Une diminution systematique des parametres du rtseau sous l'influence de la pression a ett observke. Les mtsures de resistance montrent que la temperature regnante dans l'echantillon T,, augmente avec la pression. La valeur dT,,/dP 5 (0.070 0.010) x lo-' K/Pa et determinee. Le resultat experimental a ete analyse a l'aide d'un nouvel mecanisme base sur la variation de "tunneling" integral t , entre les couches.