First order Raman spectra of Si nanocrystals exhibit large shift and width, strongly exceeding the theoretical calculated values. The disagreement between theory and experiment is dramatically increasing for the smallest nanocrystal sizes. Here, we present a novel theoretical approach showing one-, two-, or three-dimensional quantum effects, due to the limited size of the corresponding coordinate of the quantum structure. The method can be applied to nanosized layers, quantum wires and/or quantum dots. The agreement of the present theory with experimental data is dramatically improved.

In this paper the evolution of a sandy bottom subject to a wave generated flow has been analyzed using an image acquisition technique. In particular, ripple formation was observed starting from a flat bed till a stable configuration was attained. The experimental findings showed that rolling grain ripples never appeared as a stable configuration but only as a transition toward the equilibrium, represented by vortex ripples. The latter stage was reached after about 100 cycles if lightweight sediments were used, or about 400 cycles if quartz sediments were adopted. It was also observed that when the bedforms appear their wavelength is much smaller (about one half) than that at the equilibrium stage, this result being in contrast with most of the analytical models on bedform evolution. Finally, it was observed that ripples migrate as soon as they appear. The measured velocity of migration seems to confirm, from a qualitative point of view, the theoretical findings obtained for rolling grain ripples by Blondeaux et al. [Eur. J. Mech. B/Fluids 19, 285 (2000)].