Quantum phases of correlated electrons in artificial molecules under magnetic fields

We investigate the stability of few-electron quantum phases in vertically coupled quantum dots under a magnetic field of arbitrary strength and direction. The orbital and spin stability diagrams of realistic devices containing up to five electrons, from strong to weak interdot coupling, is determined. Correlation effects and realistic sample geometries are fully taken into account within the full configuration interaction method. In general, the magnetic field drives the system into a strongly correlated regime by modulating the single-particle gaps. In coupled quantum dots different components of the field, either parallel or perpendicular to the tunneling direction, affect single-dot orbitals and tunneling energy, respectively. Therefore the stability of the quantum phases is related to different correlation mechanisms, depending on the field direction. Comparison of exact diagonalization results with simple models allows one to identify the specific role of correlations

Spin-Spin Interaction In Artificial Molecules With In-Plane Magnetic Field

We investigate theoretically the spin-spin interaction of two-electrons in vertically coupled QDs as a function of the angle between magnetic field and growth axis. Our numerical approach is based on a real-space description of single-particle states in realistic samples and exact diagonalization of carrier-carrier Coulomb interaction. In particular, the effect of the in-plane field component on tunneling and, therefore, spin-spin interaction will be discussed; the singlet-triplet phase diagram as a function of the field strength and direction is drawn.Comment: Proc. of EP2DS-15, Nara, Japan - 6 pages, 4 figure

Full configuration interaction approach to the few-electron problem in artificial atoms

We present a new high-performance configuration interaction code optimally designed for the calculation of the lowest energy eigenstates of a few electrons in semiconductor quantum dots (also called artificial atoms) in the strong interaction regime. The implementation relies on a single-particle representation, but it is independent of the choice of the single-particle basis and, therefore, of the details of the device and configuration of external fields. Assuming no truncation of the Fock space of Slater determinants generated from the chosen single-particle basis, the code may tackle regimes where Coulomb interaction very effectively mixes many determinants. Typical strongly correlated systems lead to very large diagonalization problems; in our implementation, the secular equation is reduced to its minimal rank by exploiting the symmetry of the effective-mass interacting Hamiltonian, including square total spin. The resulting Hamiltonian is diagonalized via parallel implementation of the Lanczos algorithm. The code gives access to both wave functions and energies of first excited states. Excellent code scalability in a parallel environment is demonstrated; accuracy is tested for the case of up to eight electrons confined in a two-dimensional harmonic trap as the density is progressively diluted and correlation becomes dominant. Comparison with previous Quantum Monte Carlo simulations in the Wigner regime demonstrates power and flexibility of the method.Comment: RevTeX 4.0, 18 pages, 6 tables, 9 postscript b/w figures. Final version with new material. Section 6 on the excitation spectrum has been added. Some material has been moved to two appendices, which appear in the EPAPS web depository in the published versio

Competing mechanisms for singlet-triplet transition in artificial molecules

We study the magnetic field induced singlet/triplet transition for two electrons in vertically coupled quantum dots by exact diagonalization of the Coulomb interaction. We identify the different mechanisms occurring in the transition, involving either in-plane correlations or localization in opposite dots, depending on the field direction. Therefore, both spin and orbital degrees of freedom can be manipulated by field strength and direction. The phase diagram of realistic devices is determined.Comment: To appear in Phys. Rev. B - Rapid Comm. - 5 pages, 3 figure

Characterization of aluminium nitride nanostructures by XANES and FTIR spectroscopies with synchrotron radiation

We investigated different AlN nano-systems using spectroscopic methods. Experiments were performed at the Synchrotron Radiation Facility of the Laboratori Nazionali di Frascati using both XANES (x-ray absorption near edge spectroscopy) and FTIR (Fourier transform infrared spectroscopy) techniques in order to investigate materials with both interesting tribological and electronic properties. Comparisons have been performed between measurements by standard x-ray diffraction (XRD) and x-ray absorption (XRS) at the K-edge of Al, a spectroscopy method sensitive to the local order and correlated to the local and empty density of states of this wide band-gap semiconductor. Preliminary XAS simulations at the Al K edge are also presented. Correlations between XRD and XAS have been drawn, since x-ray absorption reveals structural information complementary to that addressed by x-ray diffraction. Moreover, a comparison has been performed by infrared (IR) absorption both in the mid- and in the far-IR ranges between different AlN forms: namely, powders, nanoparticles and nanotubes. Data clearly show changes connected with the electronic properties and the optical phonon modes of AlN nano-systems

Pair Production of small Black Holes in Heterotic String Theories

We study pair production of small BPS BH's in heterotic strings compactified on tori and in the FHSV model. After recalling the identification of small BH's in the perturbative BPS spectrum, we compute the tree-level amplitudes for processes initiated by massless vector bosons or gravitons. We then analyze the resulting cross sections in terms of energy and angular distributions. Finally, we briefly comment on scenari with large extra dimensions and on generalizations of our results to non-BPS, non-extremal and rotating BH's.Comment: 33 page

The last glacial maximum Balearic Abyssal Plain megabed revisited

Megabeds are thick sedimentary layers extending over thousands square kilometres in deep sea basins and are thought to result from large slope failures triggered by major external events. Such deposits have been found in at least three areas of the Mediterranean Sea. Although their discovery dates back to the early 1980s, many questions remain, concerning their initiation, source area, extent, and the nature of their emplacement. One of the largest previously documented megabeds was emplaced during the Last Glacial Maximum across the Balearic Abyssal Plain with a thickness of 8-10 m in water depths of up to 2800 m. New 3.5 kHz sub-bottom profiles and sediment cores provide greater constraint on the lateral variability of the megabed and allow to map it beyond previous estimates, with a revised areal extent up to 90,000-100,000 km2. Megabed terminations show gradual pinch-out to the West and an abrupt eastward termination against the Sardinia steep margin. The megabed presents both in seismic profiles and in sediment cores a tripartite subdivision likely corresponding to changes in flow regimes across the basin with a central area of sandy facies and erosional base oriented NNE-SSW allowing renewed discussions about sources and trigger of the megabed

The real-time multiparametric network of Campi Flegrei and Vesuvius

Volcanic processes operate over a wide range of time scale that requires different instruments and techniques to be monitored. The best approach to survey a volcanic unrest is to jointly monitor all the geophysical quantities that could vary before an eruption. The monitoring techniques are sometimes peculiar for each volcano, which has its own behavior. The simultaneous investigation of all the geophysical and geochemical parameters improves the sensibility and the understanding of any variation in the volcanic system. The Osservatorio Vesuviano is the INGV division charged of the Campi Flegrei and Vesuvius monitoring, two of the highest risk volcanic complexes in the world due to the large number of people living on or close to them. Each of them have peculiarities that increase the monitoring challenge: Campi Flegrei has high anthropic noise due to people living within its numerous craters; Vesuvius has a sharp topography that complicates the data transmission and analysis. The real time monitoring of the two areas involves several geophysical fields and the data are transmitted by a wide data-communication wired or radio infrastructure to the Monitoring Centre of Osservatorio Vesuviano: - The seismic network counts of 20 station sites in Campi Flegrei and 23 in Vesuvius equipped with velocimetric, accelerometric and infrasonic sensors. Some of them are borehole stations. - The GPS network counts of 25 stations operating at Campi Flegrei caldera and 9 stations at Vesuvius volcano. All the procedures for remote stations managing (raw data downloading, data quality control and data processing) take place automatically and the computed data are shown in the Monitoring Centre. - The mareographic network counts of 4 stations in the Campi Flegrei caldera coast and 3 close to the Vesuvius that transmit to the Monitoring Centre where the data are elaborated. - The tiltmetric network consist of 10 stations distributed around Pozzuoli harbor, the area of maximum ground uplift of Campi Flegrei, evidenced since 2005, and 7 stations distributed around the Vesuvius crater. Each tiltmetric station is also equipped with a temperature and magnetic sensor. The signals recorded are sent to the Monitoring Centre. - The 4 marine multiparametric stations installed in the Pozzuoli gulf send accelerometric, broad band, hydrophonic and GPS data to the Monitoring Centre. - The geochemical network counts of 4 multiparametric stations in the fumarolic areas of Campi Flegrei and 2 stations in the Vesuvius crater (rim and bottom) with data transmission to the Monitoring Centre. They collect soil CO2 flux, temperature gradient and environmental and meteorological parameters and transmit them directly to the Monitoring Centre. - The permanent thermal infrared surveillance network (TIRNet) is composed of 6 stations distributed among Campi Flegrei and Vesuvius. The stations acquire IR scenes at night-time of highly diffuse degassing areas. IR data are processed by an automated system of IR analysis and the temperatures values are sent to the Monitoring CentrePublishedVienna, Austria1IT. Reti di monitoraggio e sorveglianz

Transcriptome sequencing and microarray development for the Manila clam, Ruditapes philippinarum: genomic tools for environmental monitoring

Abstract Background The Manila clam, Ruditapes philippinarum, is one of the major aquaculture species in the world and a potential sentinel organism for monitoring the status of marine ecosystems. However, genomic resources for R. philippinarum are still extremely limited. Global analysis of gene expression profiles is increasingly used to evaluate the biological effects of various environmental stressors on aquatic animals under either artificial conditions or in the wild. Here, we report on the development of a transcriptomic platform for global gene expression profiling in the Manila clam. Results A normalized cDNA library representing a mixture of adult tissues was sequenced using a ultra high-throughput sequencing technology (Roche 454). A database consisting of 32,606 unique transcripts was constructed, 9,747 (30%) of which could be annotated by similarity. An oligo-DNA microarray platform was designed and applied to profile gene expression of digestive gland and gills. Functional annotation of differentially expressed genes between different tissues was performed by enrichment analysis. Expression of Natural Antisense Transcripts (NAT) analysis was also performed and bi-directional transcription appears a common phenomenon in the R. philippinarum transcriptome. A preliminary study on clam samples collected in a highly polluted area of the Venice Lagoon demonstrated the applicability of genomic tools to environmental monitoring. Conclusions The transcriptomic platform developed for the Manila clam confirmed the high level of reproducibility of current microarray technology. Next-generation sequencing provided a good representation of the clam transcriptome. Despite the known limitations in transcript annotation and sequence coverage for non model species, sufficient information was obtained to identify a large set of genes potentially involved in cellular response to environmental stress.This work was partially supported by a grant from European Union-funded Network of Excellence "Marine Genomics Europe". CS wishes to acknowledge additional funding from the Ministry of Education and Science (Spain) through grant AGL2007-60049. MM had a PhD scholarship from the University of Florence, Italy. RL was recipient of PhD fellowship SFRH/BD/30112/2006, from the Portuguese Science and Technology Foundation (FCT) and LC and RL acknowledge a grant from FCT project ISOPERK (PTDC/CVT/72083/2006).Peer Reviewe