Abstract: ZnO/ZnO:Mn coreshell nanowires were studied by means of X-ray absorption spectroscopy of the Mn K- and L2,3-edges and electron energy loss spectroscopy of the O K-edge. The combination of conventional X-ray and nanofocused electron spectroscopies together with advanced theoretical analysis turned out to be fruitful for the clear identification of the Mn phase in the volume of the coreshell structures. Theoretical simulations of spectra, performed using the full-potential linear augmented plane wave approach, confirm that the shell of the nanowires, grown by the pulsed laser deposition method, is a real dilute magnetic semiconductor with Mn2+ atoms at the Zn sites, while the core is pure ZnO.

Abstract: Analytical chemistry does play a key role in the chemical characterization of the environment and it appears that X-ray spectrometry, in its many forms, is one of the most relevant analytical techniques in preventive conservation, as it is in cultural heritage research in general. X-ray spectrometry has indeed been the method of choice for the characterization of the inorganic composition of atmospheric aerosols, for a long time. We have, over the last decade, intensively used various forms of X-ray spectrometry, viz., mostly energy-dispersive X-ray fluorescence, e.g. with polarized high-energy beam excitation, and automated electron probe X-ray microanalysis, together with other techniques, to identify particle types and their sources in indoor environments, including museums, while gaseous indoor pollutants were assessed using passive diffusion samplers. In each case, both bulk aerosols and individual aerosol particles were studied. For microanalysis of single particles, we have investigated a dozen techniques, but for wide, real-life applications, automated electron probe X-ray microanalysis is the most rewarding. We have first studied atmospheric aerosols in and around the Correr Museum in Venice, many other museums in Austria, Japan and England, and in the caves with prehistoric rock paintings in Altamira, Spain. Very recently, measurements were done in the Metropolitan Museum of Art in New York and theWawel Castle in Cracow, in Italian and Polish mountain churches, in a number of museums in Belgium and the Netherlands, and in cathedrals with medieval stained glass windows. In the Correr museum, it appeared that the particles most threatening for the Bellini paintings were released by the deteriorating plaster renderings, and this could be avoided by simply improving the rendering on the museum walls. In the Wawel Castle, outdoor pollution particles, like fine soot from diesel traffic, entering via leaks in the windows and doors, and also street-deicing salts and coal burning pollution particles, brought in by visitors, mostly in winter, were found to be most worrisome. Urgent questions that are not solved at this moment pertain to the deposition processes from the atmosphere to the cultural heritage items, the critical surface interactions that take place on these items, and the establishment of suitable particle concentration standards.

Abstract: Imaging of crystalline phase distributions in heterogeneous materials, either plane projected or in virtual cross sections of the object under investigation, can be achieved by scanning X-ray powder diffraction employing X-ray micro beams and X-ray-sensitive area detectors. Software exists to convert the two-dimensional powder diffraction patterns that are recorded by these detectors to one-dimensional diffractograms, which may be analysed by the broad variety of powder diffraction software developed by the crystallography community. However, employing these tools for the construction of crystalline phase distribution maps proves to be very difficult, especially when employing micro-focused X-ray beams, as most diffraction software tools have mainly been developed having structure solution in mind and are not suitable for phase imaging purposes. XRDUA has been developed to facilitate the execution of the complete sequence of data reduction and interpretation steps required to convert large sequences of powder diffraction patterns into a limited set of crystalline phase maps in an integrated fashion.

Abstract: The structural and transport properties of YBa2Cu3O7−x films grown by pulsed laser deposition with mixed 2.5 mol% Ba2YTaO6 (BYTO) and 2.5 mol% Ba2YNbO6 (BYNO) double-perovskite secondary phases are investigated in an extended film growth rate, R = 0.02–1.8 nm s−1. The effect of R on the film microstructure analyzed by TEM techniques shows an evolution from sparse and straight to denser, thinner and splayed continuous columns, with mixed BYNO + BYTO (BYNTO) composition, as R increases from 0.02 nm s−1 to 1.2 nm s−1. This microstructure results in very efficient flux pinning at 77 K, leading to a remarkable improvement in the critical current density (J c) behaviour, with the maximum pinning force density F p(Max) = 13.5 GN m−3 and the irreversibility field in excess of 11 T. In this range, the magnetic field values at which the F p is maximized varies from 1 T to 5 T, being related to the BYNTO columnar density. The film deposited when R = 0.3 nm s−1 exhibits the best performances over the whole temperature and magnetic field ranges, achieving F p(Max) = 900 GN m−3 at 10 K and 12 T. At higher rates, R > 1.2 nm s−1, BYNTO columns show a meandering nature and are prone to form short nanorods. In addition, in the YBCO film matrix a more disordered structure with a high density of short stacking faults is observed. From the analysis of the F p(H, T) curves it emerges that in films deposited at the high R limit, the vortex pinning is no longer dominated by BYNTO columnar defects, but by a new mechanism showing the typical temperature scaling law. Even though this microstructure produces a limited improvement at 77 K, it exhibits a strong J c improvement at lower temperature with F p = 700 GN m−3 at 10 K, 12 T and 900 GN m−3 at 4.2 K, 18 T.

Abstract: We consider a classical system of two-dimensional (2D) charged particles, interacting through a repulsive Yukawa potential exp(-r/λ)/r, and confined in a parabolic channel that limits the motion of the particles in the y direction. Along the x direction, the particles are subject to a periodic potential. The ground-state configurations and the normal-mode spectra of the system are obtained as a function of the periodicity and strength of the periodic potential (V0) and density. An interesting set of tunable ground-state configurations are found, with first- or second-order structural transitions between them. A configuration with particles aligned, perpendicular to the x direction, in each minimum of the periodic potential is obtained for V0 larger than some critical value that has a power-law dependence on the density. The phonon spectrum of different configurations was also calculated. A localization of the modes into a small frequency interval is observed for sufficiently large strength of the periodic potential, and a tunable gap in the phonon spectrum is found as a function of V0.

Abstract: Recently, a renewed interest in Zener tunneling has arisen because of its increasing impact on semiconductor device performance at nanometer dimensions. In this paper we evaluate the tunnel probability under the action of a nonuniform electric field using a two-band model and arrive at significant deviations from the commonly used Kanes model, valid for weak uniform fields only. A threshold on the junction bias where Kanes model for Zener tunneling breaks down is determined. Comparison with Kanes model particularly shows that our calculation yields a higher tunnel probability for intermediate electric fields and a lower tunnel probability for high electric fields. When performing a current calculation comparing to the WKB approximation for the case of an abrupt p-n junction significant differences concerning the shape of the I-V curve are demonstrated.