171 research outputs found
Micro-Raman characterizations of Pompei'smortars
The ancient town of Pompei offers a unique opportunity to study in detail many aspects of the every day life during the Roman early imperial age. The application of micro-Raman spectroscopy can be of great help in performing a reasonably rapid comparative analysis of the mortars, quite useful to ascertain the degree of uniformity of the technical recipes among the various building firms and the eventual technical evolution in the time; moreover, the individuation of minerals of specific geographical origins can give useful information about the extension of commercial intercourses. An example of a micro-Raman investigation on building materials is reported in this work, concerning the analysis of the mortars coming from different points of the wall in the 'The House of the Wedding of Hercules'. Remarkable differences between ancient and modern mortars are found, allowing a discrimination that can be useful in the case of historical building which underwent several restoration works. Copyright © 2008 John Wiley & Sons, Ltd
Coupled charge and spin dynamics in high-density ensembles of nitrogen-vacancy centers in diamond
We studied the spin depolarization of ensembles of nitrogen-vacancy (NV)
centers in nitrogen-rich single crystal diamonds. We found a strong dependence
of the evolution of the polarized state in the dark on the concentration of NV
centers. At low excitation power, we observed a simple exponential decay
profile in the low-density regime and a paradoxical inverted exponential
profile in the high-density regime. At higher excitation power, we observed
complex behavior, with an initial sharp rise in luminescence signal after the
preparation pulse followed by a slower exponential decay. Magnetic field and
excitation laser power-dependent measurements suggest that the rapid initial
increase of the luminescence signal is related to recharging of the
nitrogen-vacancy centers (from neutral to negatively charged) in the dark. The
slow relaxing component corresponds to the longitudinal spin relaxation of the
NV ensemble. The shape of the decay profile reflects the interplay between two
mechanisms: the NV charge state conversion in the dark and the longitudinal
spin relaxation. These mechanisms, in turn, are influenced by ionization,
recharging and polarization dynamics during excitation. Interestingly, we found
that charge dynamics are dominant in NV-dense samples even at very feeble
excitation power. These observations may be important for the use of ensembles
of NV centers in precession magnetometry and sensing applications.Comment: 7 pages, 6 figure
Second-harmonic generation in silicon waveguides strained by silicon nitride
Silicon photonics meets the electronics requirement of increased speed and bandwidth with on-chip optical networks.
All-optical data management requires nonlinear silicon photonics. In silicon only third-order optical nonlinearities are present
owing to its crystalline inversion symmetry. Introducing a second-order nonlinearity into silicon photonics by proper material
engineering would be highly desirable. It would enable devices for wideband wavelength conversion operating at relatively low
optical powers. Here we show that a sizeable second-order nonlinearity at optical wavelengths is induced in a silicon waveguide
by using a stressing silicon nitride overlayer. We carried out second-harmonic-generation experiments and first-principle
calculations, which both yield large values of strain-induced bulk second-order nonlinear susceptibility, up to 40pm/V at
2.300 nm. We envisage that nonlinear strained silicon could provide a competing platform for a new class of integrated light
sources spanning the near- to mid-infrared spectrum from 1.2 to 10 micron
Effect of Mn doping on the growth and properties of enstatite single crystals
Millimetric Mn-doped enstatite (MgSiO3) crystals have been grown by slow cooling in MoO3, V2O5, and Li2CO3 flux. Six starting mixture with different amount of manganese were slowly cooled from 1350 °C, 1050 °C and 950 °C down to 750 °C, 650 °C and 600 °C respectively. The enstatite crystals were characterized by X-ray powder diffraction (XRPD) and scanning electron microscopy with energy-dispersive spectrometry (SEM/EDS). Mn-doped enstatite crystals were reddish in color, euhedral and elongate parallel to c-axis. The largest enstatite crystal obtained is 8.5 mm in length. The effects of growth parameters on yield and size of crystals were studied. Variations observed in crystal size were attributed to the amount of Mn doping. Further characterizations by μ-Raman spectroscopy (μ-R) and cathodoluminescence (CL) allowed to study the effect of Mn doping on some chemical/physical characteristics of the enstatite and to assess its potential in advanced technological applications
Schwartz - Zippelov teorem i neke njegove primjene
U ovom radu izložen je rezultat poznat pod nazivom Schwartz - Zippelova lema ili Schwartz - Zippelov teorem. Tema rada pripada pretežno algebri, ali ima značajne primjene u drugim matematičkim područjima kao što je na primjer teorija algoritama, te kombinatorika. Rad se sastoji od tri poglavlja. U uvodu je ukratko opisana tema i cilj rada. Prvo poglavlje sadrži kratku povijest nastanka teorema, različite oblike rezultata pojedinih autora, te kratki opis pojmova korištenih u samom radu. Drugo poglavlje sastoji se od iskaza i dokaza Schwartz - Zippelovog teorema, a u trećem poglavlju izložene su neke primjene tog teorema na probleme koji se mogu svesti na testiranje jednakosti polinoma. Takvi su, primjerice, problem postojanja savršenog sparivanja u grafu i ispitivanje svojstva asocijativnosti u grupoidu. Uz svaku primjenu navedeni su i prikladni primjeri.In this diploma thesis we present the result usually called the Schwartz-Zippel lemma or the Schwartz-Zippel theorem. The nature of this theorem is basically algebraic, but it has significant applications in other areas of mathematics, such as the theory of algorithms and combinatorial theory. The thesis consists of three chapters. The main theme and objective are briefly described in the introduction. The first chapter contains a short history of the theorem’s origins, various forms of the main results by different authors and some comments of basic concepts related to this topic. The statement and a proof of the Schwartz-Zippel theorem are given in the second chapter, together with the general outline of its applications. The third and final chapter consists of some applications to problems which can be reduced to polynomial identity testing, including the existence of a perfect matching in a graph and testing of the associativity property in a groupoi
Epiphytic metazoans on emergent macrophytes in oxbow lakes of the Krapina River, Croatia: differences related to plant species and limnological conditions
This study investigated the structure of the epiphytic metazoans on emerged macrophytes in the littoral zone of two oxbow lakes with different trophic levels. Differences in the diversity and density of the epiphytic metazoans were analyzed in relation to plant architecture (simple or complex stems), food resources (algae and detritus) and water characteristics (transparency and derived trophic state index). Asignificant negative correlation was found between detritus on plants as food resource, and diversity and density of epiphytic metazoans, indicating grazing of microphagous species. Rotifers dominated in diversity and density in the epiphyton on all habitats. Total density of metazoans, rotifers and copepods in epiphyton were significantly higher on Mentha in mesotrophic lake than on Iris in a eutrophic lake.We presume that macrophyte belt width and trophic state governed biotic interactions and consequently epiphytic assemblages more strongly than macrophyte architecture. However, a Mentha habitat showed a slightly higher density and diversity of epiphytic metazoans in relation to Iris at the same site, but these differences were not significant
Reversible Modulation of Spontaneous Emission by Strain in Silicon Nanowires
We computationally study the effect of uniaxial strain in modulating the spontaneous emission of photons in silicon nanowires. Our main finding is that a one to two orders of magnitude change in spontaneous emission time occurs due to two distinct mechanisms: (A) Change in wave function symmetry, where within the direct bandgap regime, strain changes the symmetry of wave functions, which in turn leads to a large change of optical dipole matrix element. (B) Direct to indirect bandgap transition which makes the spontaneous photon emission to be of a slow second order process mediated by phonons. This feature uniquely occurs in silicon nanowires while in bulk silicon there is no change of optical properties under any reasonable amount of strain. These results promise new applications of silicon nanowires as optoelectronic devices including a mechanism for lasing. Our results are verifiable using existing experimental techniques of applying strain to nanowires
Silicon and Germanium Nanostructures for Photovoltaic Applications: Ab-Initio Results
Actually, most of the electric energy is being produced by fossil fuels and great is the search for viable alternatives. The most appealing and promising technology is photovoltaics. It will become truly mainstream when its cost will be comparable to other energy sources. One way is to significantly enhance device efficiencies, for example by increasing the number of band gaps in multijunction solar cells or by favoring charge separation in the devices. This can be done by using cells based on nanostructured semiconductors. In this paper, we will present ab-initio results of the structural, electronic and optical properties of (1) silicon and germanium nanoparticles embedded in wide band gap materials and (2) mixed silicon-germanium nanowires. We show that theory can help in understanding the microscopic processes important for devices performances. In particular, we calculated for embedded Si and Ge nanoparticles the dependence of the absorption threshold on size and oxidation, the role of crystallinity and, in some cases, the recombination rates, and we demonstrated that in the case of mixed nanowires, those with a clear interface between Si and Ge show not only a reduced quantum confinement effect but display also a natural geometrical separation between electron and hole
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