II-VI Type-II Quantum Dot Superlattices for Novel Applications

In this thesis, we discuss the growth procedure and the characterization results obtained for epitaxially grown submonolayer type-II quantum dot superlattices made of II-VI semiconductors. We have investigated the spin dynamics of ZnSe layers with embedded type-II ZnTe quantum dots and the use of (Zn)CdTe/ZnCdSe QDs for intermediate band solar cell (IBSC). Samples with a higher quantum dot density exhibit longer electron spin lifetimes, up to ~1 ns at low temperatures. Tellurium isoelectronic centers, which form in the ZnSe spacer regions as a result of the growth conditions, were also probed. A new growth sequence for type-II (Zn)CdTe/ZnCdSe (QDs) was developed in order to avoid the formation of a parasitic strain-inducing ZnSe interfacial layer. Elimination of the ZnSe interfacial layer allows for simplifications in the fabrication of the IBSC cell device structure based on these QDs, since different Cd compositions in the buffer layer and barriers is no longer required to grow stress free structures. Careful consideration was taken during the initiation of the growth process of QDs by migration enhanced epitaxy, in order to avoid the formation of the undesirable interfacial layer. The use of CdTe fractional monolayer QDs in a ZnCdSe host matrix was also explored for potential application in the IBSC device. This completely eliminated the formation of any interfacial layer, and also allowed for strain engineering of the QD superlattices. Simple arguments are used following continuum elastic theory to deduce the size of the dots and the strain within the superlattice from XRD data. This is further verified using PL and used in the energy calculations that yield the values of the intermediate band energy. The results suggest that the optimized materials are highly suitable for these high efficiency solar cells

Changes in flood mortality during the last 50 years in Greece.

Η παρούσα μελέτη εξετάζει την θνησιμότητα που συνδέεται με πλημμυρικά φαινόμενα στον Ελληνικό χώρο. Στο πλαίσιο αυτό αναπτύσσεται μια βάση δεδομένων 189 θανάτων, της περιόδου 1960-2010, με λεπτομερή στοιχεία για κάθε θανατηφόρο συμβάν. Με βάση το συστηματικό αυτό αρχείο εξετάζονται οι μεταβολές στον αριθμό, τη χωρική κατανομή, τις συνθήκες κάθε περιστατικού και τα δημογραφικά στοιχεία των θυμάτων. Από τα αποτελέσματα προκύπτει η υπερεκπροσώπηση των ανδρών, των νέων και των ηλικιωμένων ατόμων μεταξύ των θυμάτων, μολονότι τα άτομα μεταξύ 20 και 65 ετών καταγράφουν σημαντική αύξηση. Παρότι οι θάνατοι παρουσιάζονται μοιρασμένα σε αστικά και μη αστικά περιβάλλοντα, τα τελευταία χρόνια η πλειοψηφία των θανάτων καταγράφεται εκτός αστικού χώρου. Από την ανάλυση προκύπτει επίσης ότι οι θάνατοι που σχετίζονται με τη χρήση οχημάτων παρουσιάζουν αυξητική τάση αντίθετα με τους θανάτους πεζών και τους θανάτους εντός οικημάτων. Τα πολύνεκρα πλημμυρικά συμβάντα παρουσιάζουν σταδιακά εξαφανίζονται, μολονότι ο αριθμός των νεκρών δεν παρουσιάζει σημαντική μείωση. Τέλος, προκύπτει ότι μεταξύ των διαφόρων αιτιών θανάτου ο πνιγμός παρουσιάζει τα μεγαλύτερα ποσοστά.This work focuses on the changes that occur in the circumstances under which flood fatalities occur in Greece. To this aim, this paper develops and studies a database of 189 flood-related deaths, between 1960 and 2010, consisting of variables describing the conditions during the time of each incident. Changes in the number and the spatial distribution of fatalities, together with changes in the conditions, the surrounding  environment  and  the  demographics  of  the  victims  are  investigated.  Results showed that males, youngsters and elderly people presented an overrepresentation among the decedents, although individuals between 20 and 65 showed a significant increase among the victims, especially during the last decades of the study period. Fatal flood events were found to be approximately equally divided between urban and rural environments. However, fatal incidents appear to gradually migrate from urban centers to rural environments. Vehicle-related fatalities showed a gradual increase, whereas pedestrian victims and indoors incidents show an opposite trend. Fatalities per flood event presented a significant decline, although in absolute numbers, flood deaths did not show a respectively important decreasing trend. Among different causes of death drowning was found to be the most common throughout the study period

Discovering Attractive Products based on Influence Sets

Skyline queries have been widely used as a practical tool for multi-criteria decision analysis and for applications involving preference queries. For example, in a typical online retail application, skyline queries can help customers select the most interesting, among a pool of available, products. Recently, reverse skyline queries have been proposed, highlighting the manufacturer's perspective, i.e. how to determine the expected buyers of a given product. In this work we develop novel algorithms for two important classes of queries involving customer preferences. We first propose a novel algorithm, termed as RSA, for answering reverse skyline queries. We then introduce a new type of queries, namely the k-Most Attractive Candidates k-MAC query. In this type of queries, given a set of existing product specifications P, a set of customer preferences C and a set of new candidate products Q, the k-MAC query returns the set of k candidate products from Q that jointly maximizes the total number of expected buyers, measured as the cardinality of the union of individual reverse skyline sets (i.e., influence sets). Applying existing approaches to solve this problem would require calculating the reverse skyline set for each candidate, which is prohibitively expensive for large data sets. We, thus, propose a batched algorithm for this problem and compare its performance against a branch-and-bound variant that we devise. Both of these algorithms use in their core variants of our RSA algorithm. Our experimental study using both synthetic and real data sets demonstrates that our proposed algorithms outperform existing, or naive solutions to our studied classes of queries

Fault specific GIS based seismic hazard maps for the Attica Region, Greece

Traditional seismic hazard assessment methods are based on the historical seismic records for the calculation of an annual probability of exceedance for a particular ground motion level. A new fault specific seismic hazard assessment method is presented, in order to address problems related to the incompleteness and the inhomogeneity of the historical records and to obtain higher spatial resolution of hazard. This method is applied to the region of Attica, which is the most densely populated area in Greece, as nearly half of the country’s population lives in Athens and its surrounding suburbs, in Greater Athens Area. The methodology is based on a database of 22 active faults that could cause damage to Attica in case of seismic rupture. This database provides information about the faults slip rates, lengths and expected magnitudes. The final output of this method are four fault specific seismic hazard maps, showing the recurrence of expected intensities that each locality in the map has been shaken at. These maps offer a high spatial resolution, as they consider the surface geology. Despite the fact that almost half of the Attica region lies on the lowest seismic risk zone according to the official seismic hazard zonation of Greece, different localities have repeatedly experienced strong ground motions during the last 15 kyrs. Moreover, the maximum recurrence for each intensity occurs in different localities across Attica. Highest recurrence for intensity VII (151-156 times over 15 kyrs, or up to 96 year return period) is observed in the central part of the Athens basin. The maximum intensity VIII recurrence (114 times over 15 kyrs, or up to 131 year return period) is observed in the western part of Attica, while the maximum intensity IX (73-77/15kyrs, or 195 year return period) and X (25-29/15kyrs, or 517 year return period) recurrences are observed near the South Alkyonides fault system, which dominates the strong ground motions hazard in the western part of the Attica mainland

Comparing flood mortality in Portugal and Greece under a gender and age perspective

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Accurate Data Approximation in Constrained Environments

Several data reduction techniques have been proposed recently as methods for providing fast and fairly accurate answers to complex queries over large quantities of data. Their use has been widespread, due to the multiple benefits that they may offer in several constrained environments and applications. Compressed data representations require less space to store, less bandwidth to communicate and can provide, due to their size, very fast response times to queries. Sensor networks represent a typical constrained environment, due to the limited processing, storage and battery capabilities of the sensor nodes. Large-scale sensor networks require tight data handling and data dissemination techniques. Transmitting a full-resolution data feed from each sensor back to the base-station is often prohibitive due to (i) limited bandwidth that may not be sufficient to sustain a continuous feed from all sensors and (ii) increased power consumption due to the wireless multi-hop communication. In order to minimize the volume of the transmitted data, we can apply two well data reduction techniques: aggregation and approximation. In this dissertation we propose novel data reduction techniques for the transmission of measurements collected in sensor network environments. We first study the problem of summarizing multi-valued data feeds generated at a single sensor node, a step necessary for the transmission of large amounts of historical information collected at the node. The transmission of these measurements may either be periodic (i.e., when a certain amount of measurements has been collected), or in response to a query from the base station. We then also consider the approximate evaluation of aggregate continuous queries. A continuous query is a query that runs continuously until explicitly terminated by the user. These queries can be used to obtain a live-estimate of some (aggregated) quantity, such as the total number of moving objects detected by the sensors

New constraints on extensional tectonics and seismic hazard in northern Attica, Greece: the case of the Milesi Fault

Northern Attica in Greece is characterized by a set of north dipping, subparallel normal faults. These faults were considered to have low tectonic activity, based on historical earthquake reports, instrumental seismicity, and slip rate estimates. This study presents new data for one of these faults, the Milesi Fault. We run GIS based geomorphological analyses on fault offset distribution, field mapping of post-glacial fault scarps, and ground penetrating radar profiling to image hanging-wall deformation. The first palaeoseismological trenching in this part of Greece allowed obtaining direct data on slip rates and palaeoearthquakes. The trenching revealed downthrown and buried palaeosols, which were dated by radiocarbon. The results of our investigations show that the slip rates are higher than previously thought and that at least four palaeoearthquakes with magnitudes of around M6.2 occurred during the last 4,000-6,000 years. We calculate an average recurrence interval of 1,000-1,500 years and a maximum throw rate of ~0.4-0.45 mm/a. Based on the new geological earthquake data we developed a seismic hazard scenario, which also incorporates geological site effects. Intensities up to IX must be expected for Northern Attica and the south- eastern part of Evia. Earthquake environmental effects like liquefaction and mass movements are also likely to occur. This scenario is in contrast to the official Greek seismic hazard zonation that is based on historical records and assigns different hazard zones for municipalities that will experience the same intensity by earthquakes on the Milesi Fault. We show that the seismic hazard is likely underestimated in our study area and emphasize the need to incorporate geological information in such assessments.This is the author accepted manuscript. The final version is available from Oxford University Press via http://dx.doi.org/10.1093/gji/ggv44