Abstract: The influence of the well width fluctuations on the dependence of the binding energy of excitonic complexes in quantum wells is studied by using the path-integral Monte-Carlo technique. The results are compared with available experimental data and a good agreement is found.

Abstract: The ground-state energy of three-particle systems consisting of electrons and holes as found in semiconducting quantum wells is studied. The degree of confinement is determined by the quantum-well width and we can vary the dimensionality of the system from two to three dimensions. The energy levels of the system can further be altered by the application of an external magnetic field which is directed perpendicular to the well.

Abstract: The research on the use of obsidian in the Mediterranean is extensive but concerns almost exclusively volcanic glass from prehistoric and Bronze Age contexts. The consumption of obsidian during the Roman imperial period, however, has only occasionally received attention. Never a comprehensive account on what the Romans made in vitrum obsianum has been set up, nor have the sources exploited by them been examined. This paper provides a concise overview of the current knowledge on obsidian during the Roman imperial period and offers an introductory outline on potential research. The ancient writers inform us about the use of volcanic glass to create exclusive vessels, gemstones, mirrors and sculpture, but also about the creation of black appearing man-made glass initiated as a cheap and easier workable substitute of obsidian. The archaeological data on the other hand propose a more complex story with the occurrence of obsidian chunks in early Roman secondary glass workshops, and the bulky use of obsidian in late Antiquity to produce tesserae for the creation of wall and vault mosaics. Because it is extremely difficult to visually distinguish natural obsidian from man-made glass imitations we present in this paper data collected by means of non-destructive chemico-physical analyses SEM-EDX, portable X-ray fluorescence (p-XRF) and Raman spectroscopy to easily distinguish man-made glass from natural obsidian. In particular the use of portable instruments makes possible in situ analysis of objects in archaeological depots or museum collections to help defining distribution networks to better understand the shifting consumption patterns in Antiquity.

Abstract: The microstructure and phase transitions in the perovskite-based ferroelectric lead scandium tantalate, Pb(Sc0.5Ta0.5)O-3 have been investigated by transmission electron microscopy. The effects of ordering of Sc and Ta cations are apparent in reciprocal space as well as in direct space images. High-resolution observations allow direct structure imaging of the domain structure. The structure of the low temperature ferroelectric phase is studied by selected area electron diffraction (SAED) and electron microdiffraction. The relaxer behaviour of this paraelectric-ferroelectric transition is displayed by diffuse intensities in the SAED patterns at temperatures around the Curie point.

Abstract: By means of electron diffraction the 2 root 2 a(p) x 2 root 2 a(p) phase, well-known in YBa2Cu3O7-delta was observed in two other perovskite-based materials (Y0.75Ce0.25)(2)(Sr0.85Y0.15)(2)AlCU2O9 and Bi1.8Pb0.4Sr2Ca2Cu3O10+x. Highly correlated ordering is observed in the ab-plane, the correlation along the c-direction being weak. The plane group of the superstructure symmetry elements was determined on the basis of observed reflection conditions in diffraction patterns. Our results unambiguously rule out oxygen ordering as a possible origin of the superstructure. Experimental evidence points out that the superstructure is associated with the CuO2 layers, that are the only structural elements common to the three compounds studied. A model is proposed where the CuO2 sheet is displacively modulated. Experimental evidence suggests a correlation between adjacent CuO2 sheets. Comparison of simulated and experimental [001] zone diffraction patterns strongly supports our model.

Abstract: The perfect as well as the defect structure of several members of the superconducting family HgBa2Can-1CunO2n+2+delta have been studied in detail by electron diffraction and high-resolution electron microscopy Identification of the cation configuration is possible, even in the defect regions, with the help of computer simulations. The fine structure of several defects is analyzed in detail, the most common defect being the intergrowth of slabs corresponding to different n values in the title formula. In general, however, the crystals are of a high perfection, particularly for the lower n members. The occurrence of double (HgO)delta layers is seen occasionally, indicating the possible existence of a material with a double mercury layer. Some of these defects are possibly related to recently discovered anomalies at 250 K in these compounds.

Abstract: A new superconducting mercury oxycarbonate, Pb0.7Hg0.3Sr4Cu2CO3O7, has been synthesized. This tetragonal phase (a = 3.824 angstrom, c= 16.468 angstrom) consists of an intergrowth of two nonsuperconducting compounds, Sr2CuO2CO3 and Pb0.7Hg0.3Sr2CuO5. It exhibits after optimization a critical temperature of 70 K, with a sharp transition and a superconducting volume fraction of 50%. Its behavior can be compared to that of thallium oxycarbonates previously isolated. This study is completed by a reinvestigation of the 1212 cuprate of the system Hg-Pb-Sr-Ca-Nd-Cu. A superconducting phase with the 1212 structure, similar to that previously obtained but with a significantly different composition, Pb0.7Hg0.3Sr2+xCa0.7Nd0.3-xCu2O7, has been obtained, with a T(c onset) of 100 K. The behavior of the latter is compared with other lead-based 1212 cuprates.